From 357befc6ef4e53098056a3a61be3a8b775b5fb22 Mon Sep 17 00:00:00 2001
From: Seyed Mojtaba Hosseini Zeidabadi <chenzook+github@gmail.com>
Date: Tue, 11 Nov 2025 20:09:57 +0330
Subject: [PATCH 1/3] Fix: attributed string typos (#1588)

* fix: discontiguous attributed string

* fix: not divisable

* fix: item delimiter

* fix: contiguous sequence
---
 .../AttributedString/AttributedString+IndexValidity.swift     | 4 ++--
 .../AttributedString/AttributedString+_InternalRuns.swift     | 2 +-
 .../AttributedString/FoundationAttributes.swift               | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift
index 8b617aaab..13b604fb0 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+IndexValidity.swift
@@ -56,7 +56,7 @@ extension AttributedString.Index {
     
     /// Indicates whether the index is valid for use with the provided discontiguous attributed string.
     /// - Parameter text: A discontiguous attributed string used to validate the index.
-    /// - Returns: `true` when the index is valid for use with the provided discontiguous attributed string; otherwise, false. An index is valid if it is both within the bounds of the discontigous attributed string and was produced from the provided string without any intermediate mutations.
+    /// - Returns: `true` when the index is valid for use with the provided discontiguous attributed string; otherwise, false. An index is valid if it is both within the bounds of the discontiguous attributed string and was produced from the provided string without any intermediate mutations.
     public func isValid(within text: DiscontiguousAttributedSubstring) -> Bool {
         self._version == text._guts.version &&
         text._indices.contains(self._value)
@@ -100,7 +100,7 @@ extension RangeSet<AttributedString.Index> {
     }
     
     /// Indicates whether the range set is valid for use with the provided discontiguous attributed string.
-    /// - Parameter text: A discontigious attributed string used to validate the range set.
+    /// - Parameter text: A discontiguous attributed string used to validate the range set.
     /// - Returns: `true` when the range set is valid for use with the provided discontiguous attributed string; otherwise, false. A range set is valid if each of its ranges are valid in the discontiguous attributed string.
     public func isValid(within text: DiscontiguousAttributedSubstring) -> Bool {
         self.ranges.allSatisfy {
diff --git a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift
index 0c0e3c6fe..0fa265828 100644
--- a/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift
+++ b/Sources/FoundationEssentials/AttributedString/AttributedString+_InternalRuns.swift
@@ -44,7 +44,7 @@ extension AttributedString {
 extension AttributedString._InternalRuns {
     /// A metric that assigns each run a size of 1; i.e., the metric corresponding to run offsets.
     ///
-    /// Runs are not divisable under this metric.
+    /// Runs are not divisible under this metric.
     struct RunMetric: RopeMetric {
         typealias Element = AttributedString._InternalRun
         typealias Summary = Element.Summary
diff --git a/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift b/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift
index 2799e00bd..7af1c3c46 100644
--- a/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift
+++ b/Sources/FoundationEssentials/AttributedString/FoundationAttributes.swift
@@ -470,7 +470,7 @@ extension AttributeScopes.FoundationAttributes {
             guard text.count == 1 else {
                 throw DecodingError.dataCorrupted(DecodingError.Context(
                     codingPath: container.codingPath,
-                    debugDescription: "List item delimeter encoded value must contain only one character / grapheme cluster"
+                    debugDescription: "List item delimiter encoded value must contain only one character / grapheme cluster"
                 ))
             }
             return text[text.startIndex]
@@ -902,7 +902,7 @@ extension AttributedString {
     /// The writing direction of a piece of text.
     ///
     /// Writing direction defines the base direction in which bidirectional text
-    /// lays out its directional runs. A directional run is a contigous sequence
+    /// lays out its directional runs. A directional run is a contiguous sequence
     /// of characters that all have the same effective directionality, which can
     /// be determined using the Unicode BiDi algorithm. The ``leftToRight``
     /// writing direction puts the directional run that is placed first in the

From b7be5355eb8a3198055c5f5ff0c0c17f7111fc81 Mon Sep 17 00:00:00 2001
From: Tina L <49205802+itingliu@users.noreply.github.com>
Date: Tue, 11 Nov 2025 10:32:10 -0800
Subject: [PATCH 2/3] Revert "128-bit Date (#3475)" (#3549) (#1581)

Reverting this from main for now to figure out test breakages
---
 Sources/FoundationEssentials/CMakeLists.txt   |   1 -
 .../Calendar/Calendar_Gregorian.swift         |  29 +--
 .../Calendar/Calendar_Recurrence.swift        |  17 +-
 Sources/FoundationEssentials/Date.swift       |  97 ++-------
 .../FoundationEssentials/DateInterval.swift   |  49 ++---
 .../FoundationEssentials/DoubleDouble.swift   | 186 ------------------
 .../GregorianCalendarTests.swift              |  27 +--
 .../CalendarTests.swift                       |   2 +-
 .../ParseStrategy+RegexComponentTests.swift   |   5 +-
 ...ianCalendarInternationalizationTests.swift |  76 +++----
 10 files changed, 97 insertions(+), 392 deletions(-)
 delete mode 100644 Sources/FoundationEssentials/DoubleDouble.swift

diff --git a/Sources/FoundationEssentials/CMakeLists.txt b/Sources/FoundationEssentials/CMakeLists.txt
index 00d41bbfe..d009cde11 100644
--- a/Sources/FoundationEssentials/CMakeLists.txt
+++ b/Sources/FoundationEssentials/CMakeLists.txt
@@ -21,7 +21,6 @@ add_library(FoundationEssentials
     ComparisonResult.swift
     Date.swift
     DateInterval.swift
-    DoubleDouble.swift
     FoundationEssentials.swift
     IndexPath.swift
     LockedState.swift
diff --git a/Sources/FoundationEssentials/Calendar/Calendar_Gregorian.swift b/Sources/FoundationEssentials/Calendar/Calendar_Gregorian.swift
index 4f5cc4885..b7809b9b0 100644
--- a/Sources/FoundationEssentials/Calendar/Calendar_Gregorian.swift
+++ b/Sources/FoundationEssentials/Calendar/Calendar_Gregorian.swift
@@ -1448,39 +1448,30 @@ internal final class _CalendarGregorian: _CalendarProtocol, @unchecked Sendable
     }
 
     func dateInterval(of component: Calendar.Component, for date: Date) -> DateInterval? {
-        let approximateTime = date._time.head
+        let time = date.timeIntervalSinceReferenceDate
         var effectiveUnit = component
         switch effectiveUnit {
         case .calendar, .timeZone, .isLeapMonth, .isRepeatedDay:
             return nil
         case .era:
-            if approximateTime < -63113904000.0 {
+            if time < -63113904000.0 {
                 return DateInterval(start: Date(timeIntervalSinceReferenceDate: -63113904000.0 - inf_ti), duration: inf_ti)
             } else {
                 return DateInterval(start: Date(timeIntervalSinceReferenceDate: -63113904000.0), duration: inf_ti)
             }
 
         case .hour:
-            // Local hours may not be aligned to GMT hours, so we have to apply
-            // the time zone adjustment before rounding down, then unapply it.
-            let offset = Double(timeZone.secondsFromGMT(for: date))
-            let start = ((date._time + offset)/3600).floor() * 3600 - offset
-            return DateInterval(
-                start: Date(start),
-                duration: 3600
-            )
+            let ti = Double(timeZone.secondsFromGMT(for: date))
+            var fixedTime = time + ti // compute local time
+            fixedTime = floor(fixedTime / 3600.0) * 3600.0
+            fixedTime = fixedTime - ti // compute GMT
+            return DateInterval(start: Date(timeIntervalSinceReferenceDate: fixedTime), duration: 3600.0)
         case .minute:
-            return DateInterval(
-                start: Date((date._time/60).floor() * 60),
-                duration: 60
-            )
+            return DateInterval(start: Date(timeIntervalSinceReferenceDate: floor(time / 60.0) * 60.0), duration: 60.0)
         case .second:
-            return DateInterval(start: Date(date._time.floor()), duration: 1)
+            return DateInterval(start: Date(timeIntervalSinceReferenceDate: floor(time)), duration: 1.0)
         case .nanosecond:
-            return DateInterval(
-                start: Date((date._time*1e9).floor() / 1e9),
-                duration: 1e-9
-            )
+            return DateInterval(start: Date(timeIntervalSinceReferenceDate: floor(time * 1.0e+9) * 1.0e-9), duration: 1.0e-9)
         case .year, .yearForWeekOfYear, .quarter, .month, .day, .dayOfYear, .weekOfMonth, .weekOfYear:
             // Continue to below
             break
diff --git a/Sources/FoundationEssentials/Calendar/Calendar_Recurrence.swift b/Sources/FoundationEssentials/Calendar/Calendar_Recurrence.swift
index 8d9a32abc..dd8740236 100644
--- a/Sources/FoundationEssentials/Calendar/Calendar_Recurrence.swift
+++ b/Sources/FoundationEssentials/Calendar/Calendar_Recurrence.swift
@@ -93,8 +93,8 @@ extension Calendar {
             /// value is used as a lower bound for ``nextBaseRecurrenceDate()``.
             let rangeLowerBound: Date?
             
-            /// The start date's fractional seconds component
-            let fractionalSeconds: TimeInterval
+            /// The start date's nanoseconds component
+            let startDateNanoseconds: TimeInterval
             
             /// How many occurrences have been found so far
             var resultsFound = 0
@@ -131,10 +131,7 @@ extension Calendar {
                 }
                 self.recurrence = recurrence
                 
-                // round start down to whole seconds, set aside fraction.
-                let wholeSeconds = start._time.floor()
-                fractionalSeconds = (start._time - wholeSeconds).head
-                self.start = Date(wholeSeconds)
+                self.start = start
                 self.range = range
                 
                 let frequency = recurrence.frequency
@@ -236,7 +233,9 @@ extension Calendar {
                     case .monthly:  [.second, .minute, .hour, .day]
                     case .yearly:   [.second, .minute, .hour, .day, .month, .isLeapMonth]
                 }
-                var componentsForEnumerating = recurrence.calendar._dateComponents(components, from: start)
+                var componentsForEnumerating = recurrence.calendar._dateComponents(components, from: start) 
+                
+                startDateNanoseconds = start.timeIntervalSinceReferenceDate.truncatingRemainder(dividingBy: 1)
                 
                 let expansionChangesDay = dayOfYearAction == .expand || dayOfMonthAction == .expand || weekAction == .expand || weekdayAction == .expand
                 let expansionChangesMonth = dayOfYearAction == .expand || monthAction == .expand || weekAction == .expand
@@ -428,11 +427,11 @@ extension Calendar {
                     recurrence._limitTimeComponent(.second, dates: &dates, anchor: anchor)
                 }
                 
-                if fractionalSeconds != 0 {
+                if startDateNanoseconds > 0 {
                     // `_dates(startingAfter:)` above returns whole-second dates,
                     // so we need to restore the nanoseconds value present in the original start date.
                     for idx in dates.indices {
-                        dates[idx] += fractionalSeconds
+                        dates[idx] += startDateNanoseconds
                     }
                 }
                 dates = dates.filter { $0 >= self.start }
diff --git a/Sources/FoundationEssentials/Date.swift b/Sources/FoundationEssentials/Date.swift
index 2fefd10fc..df240ed2f 100644
--- a/Sources/FoundationEssentials/Date.swift
+++ b/Sources/FoundationEssentials/Date.swift
@@ -34,64 +34,12 @@ public typealias TimeInterval = Double
  A `Date` is independent of a particular calendar or time zone. To represent a `Date` to a user, you must interpret it in the context of a `Calendar`.
 */
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
-public struct Date: Comparable, Hashable, Equatable, Sendable {
-    /* Date is internally represented as a sum of two Doubles.
-    
-     Previously Date was backed by a single Double measuring time since
-     Jan 1 2001 in seconds. Because Double's precision is non-uniform, this
-     means that times within a hundred days of the epoch are represented
-     with approximately nanosecond precision, but as you get farther away
-     from that date the precision decreases. For times close to the time
-     at which this comment was written, accuracy has been reduced to about
-     100ns.
-     
-     The obvious thing would be to adopt an integer-based representation
-     similar to C's timespec (32b nanoseconds, 64b seconds) or Swift's
-     Duration (128b attoseconds). These representations suffer from a few
-     difficulties:
-     
-     - Existing API on Date takes and produces `TimeInterval` (aka Double).
-       Making Date use an integer representation internally would mean that
-       existing users of the public API would suddently start getting
-       different results for computations that were previously exact; even
-       though we could add new API, and the overall system would be more
-       precise, this would be a surprisingly subtle change for users to
-       navigate.
-     
-     - We have been told that some software interprets the raw bytes of Date
-       as a Double for the purposes of fast serialization. These packages
-       formally violate Foundation's API boundaries, but that doesn't help
-       users of those packages who would abruptly be broken by switching to
-       an integer representation.
-     
-     Using DoubleDouble instead navigates these problems fairly elegantly.
-     
-     - Because DoubleDouble is still a floating-point type, it still suffers
-       from non-uniform precision. However, because DoubleDouble is so
-       fantastically precise, it can represent dates out to ±2.5 quadrillion
-       years at ~nanosecond or better precision, so in practice this won't
-       be much of an issue.
-     
-     - Existing API on Date will produce exactly the same result as it did
-       previously in cases where those results were exact, minimizing
-       surprises. In cases where the existing API was not exact, it will
-       produce much more accurate results, even if users do not adopt new
-       API, because its internal calculations are now more precise.
-     
-     - Software that (incorrectly) interprets the raw bytes of Date as a
-       Double will get at least as accurate of a value as it did previously
-       (and often a more accurate value).                                     */
-    internal var _time: DoubleDouble
-    
-    internal init(_ time: DoubleDouble) {
-        self._time = time
-    }
-}
+public struct Date : Comparable, Hashable, Equatable, Sendable {
+
+    internal var _time : TimeInterval
 
-@available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
-extension Date {
     /// The number of seconds from 1 January 1970 to the reference date, 1 January 2001.
-    public static let timeIntervalBetween1970AndReferenceDate: TimeInterval = 978307200.0
+    public static let timeIntervalBetween1970AndReferenceDate : TimeInterval = 978307200.0
 
     /// The number of seconds from 1 January 1601 to the reference date, 1 January 2001.
     internal static let timeIntervalBetween1601AndReferenceDate: TimeInterval = 12622780800.0
@@ -103,23 +51,17 @@ extension Date {
 
     /// Returns a `Date` initialized to the current date and time.
     public init() {
-        _time = .init(uncheckedHead: Self.getCurrentAbsoluteTime(), tail: 0)
+        _time = Self.getCurrentAbsoluteTime()
     }
 
     /// Returns a `Date` initialized relative to the current date and time by a given number of seconds.
     public init(timeIntervalSinceNow: TimeInterval) {
-        self.init(.sum(
-            Self.getCurrentAbsoluteTime(),
-            timeIntervalSinceNow
-        ))
+        self.init(timeIntervalSinceReferenceDate: timeIntervalSinceNow + Self.getCurrentAbsoluteTime())
     }
 
     /// Returns a `Date` initialized relative to 00:00:00 UTC on 1 January 1970 by a given number of seconds.
     public init(timeIntervalSince1970: TimeInterval) {
-        self.init(.sum(
-            timeIntervalSince1970,
-            -Date.timeIntervalBetween1970AndReferenceDate
-        ))
+        self.init(timeIntervalSinceReferenceDate: timeIntervalSince1970 - Date.timeIntervalBetween1970AndReferenceDate)
     }
 
     /**
@@ -129,12 +71,12 @@ extension Date {
     - Parameter date: The reference date.
     */
     public init(timeInterval: TimeInterval, since date: Date) {
-        self.init(date._time + timeInterval)
+        self.init(timeIntervalSinceReferenceDate: date.timeIntervalSinceReferenceDate + timeInterval)
     }
 
     /// Returns a `Date` initialized relative to 00:00:00 UTC on 1 January 2001 by a given number of seconds.
     public init(timeIntervalSinceReferenceDate ti: TimeInterval) {
-        _time = .init(uncheckedHead: ti, tail: 0)
+        _time = ti
     }
 
     /**
@@ -143,7 +85,7 @@ extension Date {
     This property's value is negative if the date object is earlier than the system's absolute reference date (00:00:00 UTC on 1 January 2001).
     */
     public var timeIntervalSinceReferenceDate: TimeInterval {
-        return _time.head
+        return _time
     }
 
     /**
@@ -158,7 +100,7 @@ extension Date {
     - SeeAlso: `timeIntervalSinceReferenceDate`
     */
     public func timeIntervalSince(_ date: Date) -> TimeInterval {
-        return (self._time - date._time).head
+        return self.timeIntervalSinceReferenceDate - date.timeIntervalSinceReferenceDate
     }
 
     /**
@@ -231,9 +173,9 @@ extension Date {
 
     /// Compare two `Date` values.
     public func compare(_ other: Date) -> ComparisonResult {
-        if _time < other._time {
+        if _time < other.timeIntervalSinceReferenceDate {
             return .orderedAscending
-        } else if _time > other._time {
+        } else if _time > other.timeIntervalSinceReferenceDate {
             return .orderedDescending
         } else {
             return .orderedSame
@@ -242,27 +184,27 @@ extension Date {
 
     /// Returns true if the two `Date` values represent the same point in time.
     public static func ==(lhs: Date, rhs: Date) -> Bool {
-        return lhs._time == rhs._time
+        return lhs.timeIntervalSinceReferenceDate == rhs.timeIntervalSinceReferenceDate
     }
 
     /// Returns true if the left hand `Date` is earlier in time than the right hand `Date`.
     public static func <(lhs: Date, rhs: Date) -> Bool {
-        return lhs._time < rhs._time
+        return lhs.timeIntervalSinceReferenceDate < rhs.timeIntervalSinceReferenceDate
     }
 
     /// Returns true if the left hand `Date` is later in time than the right hand `Date`.
     public static func >(lhs: Date, rhs: Date) -> Bool {
-        return lhs._time > rhs._time
+        return lhs.timeIntervalSinceReferenceDate > rhs.timeIntervalSinceReferenceDate
     }
 
     /// Returns a `Date` with a specified amount of time added to it.
     public static func +(lhs: Date, rhs: TimeInterval) -> Date {
-        return Date(lhs._time + rhs)
+        return Date(timeIntervalSinceReferenceDate: lhs.timeIntervalSinceReferenceDate + rhs)
     }
 
     /// Returns a `Date` with a specified amount of time subtracted from it.
     public static func -(lhs: Date, rhs: TimeInterval) -> Date {
-        return Date(lhs._time - rhs)
+        return Date(timeIntervalSinceReferenceDate: lhs.timeIntervalSinceReferenceDate - rhs)
     }
 
     /// Add a `TimeInterval` to a `Date`.
@@ -278,6 +220,7 @@ extension Date {
     public static func -=(lhs: inout Date, rhs: TimeInterval) {
         lhs = lhs - rhs
     }
+
 }
 
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
@@ -395,7 +338,7 @@ extension Date : ReferenceConvertible, _ObjectiveCBridgeable {
 
     @_semantics("convertToObjectiveC")
     public func _bridgeToObjectiveC() -> NSDate {
-        return NSDate(timeIntervalSinceReferenceDate: _time.head)
+        return NSDate(timeIntervalSinceReferenceDate: _time)
     }
 
     public static func _forceBridgeFromObjectiveC(_ x: NSDate, result: inout Date?) {
diff --git a/Sources/FoundationEssentials/DateInterval.swift b/Sources/FoundationEssentials/DateInterval.swift
index 835157808..04d2c55d1 100644
--- a/Sources/FoundationEssentials/DateInterval.swift
+++ b/Sources/FoundationEssentials/DateInterval.swift
@@ -12,37 +12,30 @@
 
 /// DateInterval represents a closed date interval in the form of [startDate, endDate].  It is possible for the start and end dates to be the same with a duration of 0.  DateInterval does not support reverse intervals i.e. intervals where the duration is less than 0 and the end date occurs earlier in time than the start date.
 @available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
-public struct DateInterval: Comparable, Hashable, Codable, Sendable {
+public struct DateInterval : Comparable, Hashable, Codable, Sendable {
 
     /// The start date.
-    public var start: Date
-    
-    /// Underlying storage for `duration`
-    internal var _duration: DoubleDouble
+    public var start : Date
 
     /// The end date.
     ///
     /// - precondition: `end >= start`
-    public var end: Date {
+    public var end : Date {
         get {
-            return Date(start._time + _duration)
+            return start + duration
         }
         set {
             precondition(newValue >= start, "Reverse intervals are not allowed")
-            _duration = (newValue._time - start._time)
+            duration = newValue.timeIntervalSinceReferenceDate - start.timeIntervalSinceReferenceDate
         }
     }
-    
-    /// The duration
+
+    /// The duration.
     ///
     /// - precondition: `duration >= 0`
-    public var duration: TimeInterval {
-        get {
-            _duration.head
-        }
-        set {
+    public var duration : TimeInterval {
+        willSet {
             precondition(newValue >= 0, "Negative durations are not allowed")
-            _duration = DoubleDouble(uncheckedHead: newValue, tail: 0)
         }
     }
 
@@ -50,7 +43,7 @@ public struct DateInterval: Comparable, Hashable, Codable, Sendable {
     public init() {
         let d = Date()
         start = d
-        _duration = .zero
+        duration = 0
     }
 
     /// Initialize a `DateInterval` with the specified start and end date.
@@ -59,7 +52,7 @@ public struct DateInterval: Comparable, Hashable, Codable, Sendable {
     public init(start: Date, end: Date) {
         precondition(end >= start, "Reverse intervals are not allowed")
         self.start = start
-        _duration = end._time - start._time
+        duration = end.timeIntervalSince(start)
     }
 
     /// Initialize a `DateInterval` with the specified start date and duration.
@@ -68,7 +61,7 @@ public struct DateInterval: Comparable, Hashable, Codable, Sendable {
     public init(start: Date, duration: TimeInterval) {
         precondition(duration >= 0, "Negative durations are not allowed")
         self.start = start
-        _duration = DoubleDouble(uncheckedHead: duration, tail: 0)
+        self.duration = duration
     }
 
     /**
@@ -169,24 +162,6 @@ public struct DateInterval: Comparable, Hashable, Codable, Sendable {
     public static func <(lhs: DateInterval, rhs: DateInterval) -> Bool {
         return lhs.compare(rhs) == .orderedAscending
     }
-    
-    enum CodingKeys: String, CodingKey {
-        case start = "start"
-        case duration = "duration"
-    }
-    
-    public init(from decoder: Decoder) throws {
-        let container = try decoder.container(keyedBy: CodingKeys.self)
-        let start = try container.decode(Date.self, forKey: .start)
-        let duration = try container.decode(TimeInterval.self, forKey: .duration)
-        self.init(start: start, duration: duration)
-    }
-    
-    public func encode(to encoder: Encoder) throws {
-        var container = encoder.container(keyedBy: CodingKeys.self)
-        try container.encode(start, forKey: .start)
-        try container.encode(duration, forKey: .duration)
-    }
 }
 
 @available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
diff --git a/Sources/FoundationEssentials/DoubleDouble.swift b/Sources/FoundationEssentials/DoubleDouble.swift
deleted file mode 100644
index b8d58ddf4..000000000
--- a/Sources/FoundationEssentials/DoubleDouble.swift
+++ /dev/null
@@ -1,186 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// This source file is part of the Swift.org open source project
-//
-// Copyright (c) 2025 Apple Inc. and the Swift project authors
-// Licensed under Apache License v2.0 with Runtime Library Exception
-//
-// See https://swift.org/LICENSE.txt for license information
-// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
-//
-//===----------------------------------------------------------------------===//
-
-/// A numeric type that uses two Double values as its representation, providing
-/// about 106 bits of precision with the same exponent range as Double.
-///
-/// This type conforms to AdditiveArithmetic, Hashable and Comparable, but does
-/// not conform to FloatingPoint or Numeric; it implements only the API surface
-/// that is necessary to serve as an internal implementation detail of Date.
-internal struct DoubleDouble {
-    
-    private let storage: (Double, Double)
-    
-    /// A double-double value constructed by specifying the head and tail.
-    ///
-    /// This is an unchecked operation because it does not enforce the
-    /// invariant that head + tail == head in release builds, which is
-    /// necessary for subsequent arithmetic operations to behave correctly.
-    @_transparent
-    init(uncheckedHead head: Double, tail: Double) {
-        assert(!head.isFinite || head + tail == head)
-        storage = (head, tail)
-    }
-    
-    /// The high-order Double.
-    ///
-    /// This property does not have a setter because `head` should pretty much
-    /// never be set independently of `tail`, so as to maintain the invariant
-    /// that `head + tail == head`. You can use `init(uncheckedHead:tail:)`
-    /// to directly construct DoubleDouble values, which will enforce the
-    /// invariant in debug builds.
-    @_transparent
-    var head: Double { storage.0 }
-    
-    /// The low-order Double.
-    ///
-    /// This property does not have a setter because `tail` should pretty much
-    /// never be set independently of `head`, so as to maintain the invariant
-    /// that `head + tail == head`. You can use `init(uncheckedHead:tail:)`
-    /// to directly construct DoubleDouble values, which will enforce the
-    /// invariant in debug builds.
-    @_transparent
-    var tail: Double { storage.1 }
-    
-    /// `a + b` represented as a normalized DoubleDouble.
-    ///
-    /// Computed via the [2Sum algorithm](https://en.wikipedia.org/wiki/2Sum).
-    @inlinable
-    static func sum(_ a: Double, _ b: Double) -> DoubleDouble {
-        let head = a + b
-        let x = head - b
-        let y = head - x
-        let tail = (a - x) + (b - y)
-        return DoubleDouble(uncheckedHead: head, tail: tail)
-    }
-    
-    /// `a + b` represented as a normalized DoubleDouble.
-    ///
-    /// Computed via the [Fast2Sum algorithm](https://en.wikipedia.org/wiki/2Sum).
-    ///
-    /// - Precondition:
-    /// `large` and `small` must be such that `sum(large:small:)`
-    /// produces the same result as `sum(_:_:)` would. A sufficient condition
-    /// is that `|large| >= |small|`, but this is not necessary, so we do not
-    /// enforce it via an assert. Instead this function asserts that the result
-    /// is the same as that produced by `sum(_:_:)` in Debug builds. This is
-    /// unchecked in Release.
-    @inlinable
-    static func sum(large a: Double, small b: Double) -> DoubleDouble {
-        let head = a + b
-        let tail = a - head + b
-        let result = DoubleDouble(uncheckedHead: head, tail: tail)
-        assert(!head.isFinite || result == sum(a, b))
-        return result
-    }
-    
-    /// `a * b` represented as a normalized DoubleDouble.
-    @inlinable
-    static func product(_ a: Double, _ b: Double) -> DoubleDouble {
-        let head = a * b
-        let tail = (-head).addingProduct(a, b)
-        return DoubleDouble(uncheckedHead: head, tail: tail)
-    }
-}
-
-extension DoubleDouble: Comparable {
-    @_transparent
-    static func ==(a: Self, b: Self) -> Bool {
-        a.head == b.head && a.tail == b.tail
-    }
-    
-    @_transparent
-    static func <(a: Self, b: Self) -> Bool {
-        a.head < b.head || a.head == b.head && a.tail < b.tail
-    }
-}
-
-extension DoubleDouble: Hashable {
-    @_transparent
-    func hash(into hasher: inout Hasher) {
-        hasher.combine(head)
-        hasher.combine(tail)
-    }
-}
-
-extension DoubleDouble: AdditiveArithmetic {
-    @inlinable
-    static var zero: DoubleDouble {
-        Self(uncheckedHead: 0, tail: 0)
-    }
-    
-    @inlinable
-    static func +(a: DoubleDouble, b: DoubleDouble) -> DoubleDouble {
-        let heads = sum(a.head, b.head)
-        let tails = sum(a.tail, b.tail)
-        let first = sum(large: heads.head, small: heads.tail + tails.head)
-        return sum(large: first.head, small: first.tail + tails.tail)
-    }
-    
-    /// Equivalent to `a + DoubleDouble(uncheckedHead: b, tail: 0)` but
-    /// computed more efficiently.
-    @inlinable
-    static func +(a: DoubleDouble, b: Double) -> DoubleDouble {
-        let heads = sum(a.head, b)
-        let first = sum(large: heads.head, small: heads.tail + a.tail)
-        return sum(large: first.head, small: first.tail)
-    }
-    
-    @inlinable
-    prefix static func -(a: DoubleDouble) -> DoubleDouble {
-        DoubleDouble(uncheckedHead: -a.head, tail: -a.tail)
-    }
-    
-    @inlinable
-    static func -(a: DoubleDouble, b: DoubleDouble) -> DoubleDouble {
-        a + (-b)
-    }
-    
-    /// Equivalent to `a - DoubleDouble(uncheckedHead: b, tail: 0)` but
-    /// computed more efficiently.
-    @inlinable
-    static func -(a: DoubleDouble, b: Double) -> DoubleDouble {
-        a + (-b)
-    }
-}
-
-extension DoubleDouble {
-    @inlinable
-    static func *(a: DoubleDouble, b: Double) -> DoubleDouble {
-        let tmp = product(a.head, b)
-        return DoubleDouble(
-            uncheckedHead: tmp.head,
-            tail: tmp.tail.addingProduct(a.tail, b)
-        )
-    }
-    
-    @inlinable
-    static func /(a: DoubleDouble, b: Double) -> DoubleDouble {
-        let head = a.head/b
-        let residual = a.head.addingProduct(-head, b) + a.tail
-        return .sum(large: head, small: residual/b)
-    }
-}
-
-extension DoubleDouble {
-    // This value rounded down to an integer.
-    @inlinable
-    func floor() -> DoubleDouble {
-        let approx = head.rounded(.down)
-        // If head was already an integer, round tail down and renormalize.
-        if approx == head {
-            return .sum(large: head, small: tail.rounded(.down))
-        }
-        // Head was not an integer; we can simply discard tail.
-        return DoubleDouble(uncheckedHead: approx, tail: 0)
-    }
-}
diff --git a/Tests/FoundationEssentialsTests/GregorianCalendarTests.swift b/Tests/FoundationEssentialsTests/GregorianCalendarTests.swift
index 31377c924..9e3d3c05f 100644
--- a/Tests/FoundationEssentialsTests/GregorianCalendarTests.swift
+++ b/Tests/FoundationEssentialsTests/GregorianCalendarTests.swift
@@ -277,13 +277,7 @@ private struct GregorianCalendarTests {
         func test(addField field: Calendar.Component, value: Int, to addingToDate: Date, wrap: Bool, expectedDate: Date, sourceLocation: SourceLocation = #_sourceLocation) {
             let components = DateComponents(component: field, value: value)!
             let result = gregorianCalendar.date(byAdding: components, to: addingToDate, wrappingComponents: wrap)!
-            // These tests were written when Date used a 64b representation;
-            // we'll add new tests that validate the low-word of the 128b
-            // Date, but these old tests should continue passing if we only
-            // look at the high word as vended by tISRD.
-            #expect(result.timeIntervalSinceReferenceDate ==
-                    expectedDate.timeIntervalSinceReferenceDate,
-                    sourceLocation: sourceLocation)
+            #expect(result == expectedDate, sourceLocation: sourceLocation)
         }
 
         date = Date(timeIntervalSince1970: 825723300.0)
@@ -405,13 +399,7 @@ private struct GregorianCalendarTests {
         func test(addField field: Calendar.Component, value: Int, to addingToDate: Date, wrap: Bool, expectedDate: Date, sourceLocation: SourceLocation = #_sourceLocation) {
             let components = DateComponents(component: field, value: value)!
             let result = gregorianCalendar.date(byAdding: components, to: addingToDate, wrappingComponents: wrap)!
-          // These tests were written when Date used a 64b representation;
-          // we'll add new tests that validate the low-word of the 128b
-          // Date, but these old tests should continue passing if we only
-          // look at the high word as vended by tISRD.
-            #expect(result.timeIntervalSinceReferenceDate ==
-                    expectedDate.timeIntervalSinceReferenceDate,
-                    sourceLocation: sourceLocation)
+            #expect(result == expectedDate, sourceLocation: sourceLocation)
         }
 
         date = Date(timeIntervalSince1970: 62135596800.0) // 3939-01-01
@@ -838,11 +826,7 @@ private struct GregorianCalendarTests {
             let new_end = new?.end
 
             #expect(new_start == start, "interval start did not match", sourceLocation: sourceLocation)
-            // These tests were written when Date used a 64b representation;
-            // we'll add new tests that validate the low-word of the 128b
-            // Date, but these old tests should continue passing if we only
-            // look at the high word as vended by tISRD.
-            #expect(new_end?.timeIntervalSinceReferenceDate == end?.timeIntervalSinceReferenceDate, "interval end did not match", sourceLocation: sourceLocation)
+            #expect(new_end == end, "interval end did not match", sourceLocation: sourceLocation)
         }
 
         var date: Date
@@ -854,10 +838,7 @@ private struct GregorianCalendarTests {
         test(.hour, date, expectedStart: Date(timeIntervalSince1970: 820454400.0), end: Date(timeIntervalSince1970: 820458000.0))
         test(.minute, date, expectedStart: Date(timeIntervalSince1970: 820454400.0), end: Date(timeIntervalSince1970: 820454460.0))
         test(.second, date, expectedStart: Date(timeIntervalSince1970: 820454400.0), end: Date(timeIntervalSince1970: 820454401.0))
-        // Legacy test from 64b Date; expected end is the same as start due to rounding.
         test(.nanosecond, date, expectedStart: Date(timeIntervalSince1970: 820454400.0), end: Date(timeIntervalSince1970: 820454400.0))
-        // Updated test for 128b Date to benefit from improved accuracy:
-        #expect(calendar.dateInterval(of: .nanosecond, for: date)?.end == Date(timeInterval: 1e-9, since: date))
         test(.weekday, date, expectedStart: Date(timeIntervalSince1970: 820396800.0), end: Date(timeIntervalSince1970: 820483200.0))
         test(.weekdayOrdinal, date, expectedStart: Date(timeIntervalSince1970: 820396800.0), end: Date(timeIntervalSince1970: 820483200.0))
         test(.quarter, date, expectedStart: Date(timeIntervalSince1970: 812534400.0), end: Date(timeIntervalSince1970: 820483200.0))
@@ -888,7 +869,7 @@ private struct GregorianCalendarTests {
         test(.hour, date, expectedStart: Date(timeIntervalSince1970: -62135769600.0), end: Date(timeIntervalSince1970: -62135766000.0))
         test(.minute, date, expectedStart: Date(timeIntervalSince1970: -62135769600.0), end: Date(timeIntervalSince1970: -62135769540.0))
         test(.second, date, expectedStart: Date(timeIntervalSince1970: -62135769600.0), end: Date(timeIntervalSince1970: -62135769599.0))
-        test(.nanosecond, date, expectedStart: date, end: Date(timeInterval: 1e-9, since: date))
+        test(.nanosecond, date, expectedStart: Date(timeIntervalSince1970: -62135769600.00001), end: Date(timeIntervalSince1970: -62135769600.00001))
         test(.weekday, date, expectedStart: Date(timeIntervalSince1970: -62135827200.0), end: Date(timeIntervalSince1970: -62135740800.0))
         test(.weekdayOrdinal, date, expectedStart: Date(timeIntervalSince1970: -62135827200.0), end: Date(timeIntervalSince1970: -62135740800.0))
         test(.quarter, date, expectedStart: Date(timeIntervalSince1970: -62143689600.0), end: Date(timeIntervalSince1970: -62135740800.0))
diff --git a/Tests/FoundationInternationalizationTests/CalendarTests.swift b/Tests/FoundationInternationalizationTests/CalendarTests.swift
index 6ce160945..8602f9a26 100644
--- a/Tests/FoundationInternationalizationTests/CalendarTests.swift
+++ b/Tests/FoundationInternationalizationTests/CalendarTests.swift
@@ -1420,7 +1420,7 @@ private struct CalendarTests {
         func test(_ start: Date, _ end: Date) throws {
             let components = c.dateComponents([.year, .month, .day, .hour, .minute, .second, .nanosecond, .weekOfMonth], from: start, to: end)
             let added = try #require(c.date(byAdding: components, to: start))
-            #expect(added.timeIntervalSinceReferenceDate == end.timeIntervalSinceReferenceDate, "actual: \(s.format(added)), expected: \(s.format(end))")
+            #expect(added == end, "actual: \(s.format(added)), expected: \(s.format(end))")
         }
 
         // 2024-03-09T02:34:36-0800, 2024-03-17T03:34:36-0700, 10:34:36 UTC
diff --git a/Tests/FoundationInternationalizationTests/Formatting/ParseStrategy+RegexComponentTests.swift b/Tests/FoundationInternationalizationTests/Formatting/ParseStrategy+RegexComponentTests.swift
index f9370c88d..6e684a957 100644
--- a/Tests/FoundationInternationalizationTests/Formatting/ParseStrategy+RegexComponentTests.swift
+++ b/Tests/FoundationInternationalizationTests/Formatting/ParseStrategy+RegexComponentTests.swift
@@ -62,6 +62,8 @@ private struct ParseStrategyMatchTests {
         #expect(res.output.1 == expectedDate)
     }
 
+// https://github.com/apple/swift-foundation/issues/60
+#if FOUNDATION_FRAMEWORK
     @Test func apiStatement() {
 
         let statement = """
@@ -211,8 +213,9 @@ DEBIT    Mar 31/20    March Payment to BoA         -USD 52,249.98
         #expect(match.output.1 == "MergeableSetTests")
         #expect(match.output.2 == "started")
         // dateFormatter.date(from: "2021-07-08 10:19:35.418")!
-        #expect(match.output.3.timeIntervalSinceReferenceDate == 647432375.418)
+        #expect(match.output.3 == Date(timeIntervalSinceReferenceDate: 647432375.418))
     }
+#endif
 
     @Test func variousDatesAndTimes() {
         func verify(_ str: String, _ strategy: Date.ParseStrategy, _ expected: String?, sourceLocation: SourceLocation = #_sourceLocation) {
diff --git a/Tests/FoundationInternationalizationTests/GegorianCalendarInternationalizationTests.swift b/Tests/FoundationInternationalizationTests/GegorianCalendarInternationalizationTests.swift
index 3d389281c..195c18912 100644
--- a/Tests/FoundationInternationalizationTests/GegorianCalendarInternationalizationTests.swift
+++ b/Tests/FoundationInternationalizationTests/GegorianCalendarInternationalizationTests.swift
@@ -1461,8 +1461,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828262987.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 829468987.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828262987.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828867787.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828867787.0))
         
         date = Date(timeIntervalSince1970: 828871387.0) // 1996-04-07T03:03:07-0700
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828871387.0))
@@ -1489,8 +1489,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828270187.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 829476187.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828270187.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828871387.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828871387.0))
         
         date = Date(timeIntervalSince1970: 828874987.0) // 1996-04-07T04:03:07-0700
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828874987.0))
@@ -1517,8 +1517,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828273787.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 829479787.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828273787.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828874987.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828874987.0))
         
         date = Date(timeIntervalSince1970: 846403387.0) // 1996-10-27T01:03:07-0700
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846403387.0))
@@ -1547,8 +1547,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845798587.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 847011787.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845798587.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846403387.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846403387.0))
         
         date = Date(timeIntervalSince1970: 846406987.0) // 1996-10-27T01:03:07-0800
         // Previously this returns 1996-10-27T01:03:07-0700
@@ -1578,8 +1578,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845798587.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 847011787.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845798587.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846406987.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846406987.0))
         
         date = Date(timeIntervalSince1970: 846410587.0) // 1996-10-27T02:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846410587.0))
@@ -1606,8 +1606,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845802187.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 847015387.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845802187.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846410587.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846410587.0))
         
         date = Date(timeIntervalSince1970: 846414187.0) // 1996-10-27T03:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846414187.0))
@@ -1634,8 +1634,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845805787.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 847018987.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845805787.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846414187.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846414187.0))
         
         date = Date(timeIntervalSince1970: 814953787.0) // 1995-10-29T01:03:07-0700
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814953787.0))
@@ -1664,8 +1664,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814348987.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 815562187.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814348987.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814953787.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814953787.0))
         
         date = Date(timeIntervalSince1970: 814957387.0) // 1995-10-29T01:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814957387.0))
@@ -1692,8 +1692,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814348987.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 815562187.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814348987.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814957387.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814957387.0))
         
         date = Date(timeIntervalSince1970: 814960987.0) // 1995-10-29T02:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814960987.0))
@@ -1720,8 +1720,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814352587.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 815565787.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814352587.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814960987.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814960987.0))
         
         date = Date(timeIntervalSince1970: 814964587.0) // 1995-10-29T03:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814964587.0))
@@ -1748,8 +1748,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814356187.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 815569387.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814356187.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 814964587.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 814964587.0))
     }
     
     @Test func add_Wrap_DST() {
@@ -1789,8 +1789,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828262987.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 829468987.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 830851387.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828867787.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828867787.0))
         
         date = Date(timeIntervalSince1970: 828871387.0) // 1996-04-07T03:03:07-0700
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828871387.0))
@@ -1817,8 +1817,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828270187.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 829476187.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 830858587.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828871387.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828871387.0))
         
         date = Date(timeIntervalSince1970: 828874987.0) // 1996-04-07T04:03:07-0700
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828874987.0))
@@ -1845,8 +1845,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828273787.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 829479787.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 830862187.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 828874987.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 828874987.0))
         
         date = Date(timeIntervalSince1970: 846406987.0) // 1996-10-27T01:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846406987.0))
@@ -1873,8 +1873,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845798587.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846752587.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845798587.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846406987.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846406987.0))
         
         date = Date(timeIntervalSince1970: 846410587.0) // 1996-10-27T02:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846410587.0))
@@ -1901,8 +1901,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845802187.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846756187.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845802187.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846410587.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846410587.0))
         
         date = Date(timeIntervalSince1970: 846414187.0) // 1996-10-27T03:03:07-0800
         test(addField: .era, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846414187.0))
@@ -1929,8 +1929,8 @@ private struct GregorianCalendarInternationalizationTests {
         test(addField: .weekOfYear, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845805787.0))
         test(addField: .weekOfMonth, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846759787.0))
         test(addField: .weekOfMonth, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 845805787.0))
-        test(addField: .nanosecond, value: 1, to: date, expectedDate: date.addingTimeInterval(1e-9))
-        test(addField: .nanosecond, value: -1, to: date, expectedDate: date.addingTimeInterval(-1e-9))
+        test(addField: .nanosecond, value: 1, to: date, expectedDate: Date(timeIntervalSince1970: 846414187.0))
+        test(addField: .nanosecond, value: -1, to: date, expectedDate: Date(timeIntervalSince1970: 846414187.0))
     }
     
     @Test func ordinality_DST() {
@@ -2448,21 +2448,21 @@ private struct GregorianCalendarInternationalizationTests {
         dc_customCalendarAndTimeZone.timeZone = .init(secondsFromGMT: 28800)
         // calendar.timeZone = UTC+0, dc.calendar.timeZone = UTC-7, dc.timeZone = UTC+8
         // expect local time in dc.timeZone (UTC+8)
-        #expect(gregorianCalendar.date(from: dc_customCalendarAndTimeZone)!.timeIntervalSinceReferenceDate == 679024975.891) // 2022-07-09T02:02:55Z
+        #expect(gregorianCalendar.date(from: dc_customCalendarAndTimeZone)! == Date(timeIntervalSinceReferenceDate: 679024975.891)) // 2022-07-09T02:02:55Z
         
         var dc_customCalendar = dc
         dc_customCalendar.calendar = dcCalendar
         dc_customCalendar.timeZone = nil
         // calendar.timeZone = UTC+0, dc.calendar.timeZone = UTC-7, dc.timeZone = nil
         // expect local time in calendar.timeZone (UTC+0)
-        #expect(gregorianCalendar.date(from: dc_customCalendar)!.timeIntervalSinceReferenceDate == 679053775.891) // 2022-07-09T10:02:55Z
+        #expect(gregorianCalendar.date(from: dc_customCalendar)! == Date(timeIntervalSinceReferenceDate: 679053775.891)) // 2022-07-09T10:02:55Z
         
         var dc_customTimeZone = dc_customCalendarAndTimeZone
         dc_customTimeZone.calendar = nil
         dc_customTimeZone.timeZone = .init(secondsFromGMT: 28800)
         // calendar.timeZone = UTC+0, dc.calendar = nil, dc.timeZone = UTC+8
         // expect local time in dc.timeZone (UTC+8)
-        #expect(gregorianCalendar.date(from: dc_customTimeZone)!.timeIntervalSinceReferenceDate == 679024975.891) // 2022-07-09T02:02:55Z
+        #expect(gregorianCalendar.date(from: dc_customTimeZone)! == Date(timeIntervalSinceReferenceDate: 679024975.891)) // 2022-07-09T02:02:55Z
         
         let dcCalendar_noTimeZone = Calendar(identifier: .japanese, locale: Locale(identifier: ""), timeZone: .gmt, firstWeekday: 1, minimumDaysInFirstWeek: 1, gregorianStartDate: nil)
         var dc_customCalendarNoTimeZone_customTimeZone = dc
@@ -2470,7 +2470,7 @@ private struct GregorianCalendarInternationalizationTests {
         dc_customCalendarNoTimeZone_customTimeZone.timeZone = .init(secondsFromGMT: 28800)
         // calendar.timeZone = UTC+0, dc.calendar.timeZone = nil, dc.timeZone = UTC+8
         // expect local time in dc.timeZone (UTC+8)
-        #expect(gregorianCalendar.date(from: dc_customCalendarNoTimeZone_customTimeZone)!.timeIntervalSinceReferenceDate == 679024975.891) // 2022-07-09T02:02:55Z
+        #expect(gregorianCalendar.date(from: dc_customCalendarNoTimeZone_customTimeZone)! == Date(timeIntervalSinceReferenceDate: 679024975.891)) // 2022-07-09T02:02:55Z
     }
     
     @Test func dateFromComponents_componentsTimeZoneConversion() {

From d8533a2e24a7684fa8c0ed846439aa4a62b4c80d Mon Sep 17 00:00:00 2001
From: YOCKOW <YOCKOW@users.noreply.github.com>
Date: Wed, 12 Nov 2025 08:39:18 +0900
Subject: [PATCH 3/3] SF-0033: Implement `String.Encoding.ianaName` and
 `String.Encoding(ianaName:)`. (#1286)

* Import implementation for String Encoding Names from other repo.

- source: https://github.com/YOCKOW/SF-StringEncodingNameImpl

* Import tests for String Encoding Names from other repo.

- source: https://github.com/YOCKOW/SF-StringEncodingNameImpl/blob/0.4.0/Tests/StringEncodingNameImplTests/StringEncodingNameParserTests.swift

* Remove dead code in terms of the current proposal.

* Use `Testing` for String Encoding Names tests.

* NFC: Fix indentation in "String+Encoding+Names.swift".

* SF-0033: Adjust comments/attributes to match the accepted proposal.

* Auto-generate Swift source code for IANA Charset names.

* Remove unnecessary `@inlinable`.

* Simplify `String.init(ianaName:)`.

* Add new files related to SF-0033 to CMakeLists.txt.

* Rewrite script in Swift instead of Python.

In response to: https://github.com/swiftlang/swift-foundation/pull/1286#discussion_r2437753438

* Simplify logic to parse IANA Charset names.

In response to:
- https://github.com/swiftlang/swift-foundation/pull/1286#discussion_r2441497400
- https://github.com/swiftlang/swift-foundation/pull/1286#discussion_r2441505001
- https://github.com/swiftlang/swift-foundation/pull/1286#discussion_r2441546727

* Fix spelling of functions for "case-insensitively".

In response to:
- https://github.com/swiftlang/swift-foundation/pull/1286#discussion_r2457914338

* Remove redundant nested function in `String.Encoding(ianaName:)`.

In response to:
- https://github.com/swiftlang/swift-foundation/pull/1286#discussion_r2457939067
---
 .../String/CMakeLists.txt                     |   2 +
 .../String/IANACharsetNames.swift             | 213 ++++++++++++++++++
 .../String/String+Encoding+Names.swift        | 155 +++++++++++++
 .../StringTests.swift                         |  63 ++++++
 utils/update-iana-charset-names               |  62 +++++
 utils/update-iana-charset-names-impl.swift    | 195 ++++++++++++++++
 6 files changed, 690 insertions(+)
 create mode 100644 Sources/FoundationEssentials/String/IANACharsetNames.swift
 create mode 100644 Sources/FoundationEssentials/String/String+Encoding+Names.swift
 create mode 100755 utils/update-iana-charset-names
 create mode 100755 utils/update-iana-charset-names-impl.swift

diff --git a/Sources/FoundationEssentials/String/CMakeLists.txt b/Sources/FoundationEssentials/String/CMakeLists.txt
index 720eb218d..6cc7994d7 100644
--- a/Sources/FoundationEssentials/String/CMakeLists.txt
+++ b/Sources/FoundationEssentials/String/CMakeLists.txt
@@ -15,10 +15,12 @@
 target_sources(FoundationEssentials PRIVATE
     BidirectionalCollection.swift
     BuiltInUnicodeScalarSet.swift
+    IANACharsetNames.swift
     RegexPatternCache.swift
     String+Bridging.swift
     String+Comparison.swift
     String+Encoding.swift
+    String+Encoding+Names.swift
     String+EndianAdaptorSequence.swift
     String+Essentials.swift
     String+IO.swift
diff --git a/Sources/FoundationEssentials/String/IANACharsetNames.swift b/Sources/FoundationEssentials/String/IANACharsetNames.swift
new file mode 100644
index 000000000..8f3e88f09
--- /dev/null
+++ b/Sources/FoundationEssentials/String/IANACharsetNames.swift
@@ -0,0 +1,213 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+
+// WARNING: DO NOT EDIT THIS FILE DIRECTLY.
+//          This is auto-generated by `update-iana-charset-names`.
+
+
+extension IANACharset {
+    /// IANA Charset `US-ASCII`.
+    static let usASCII = IANACharset(
+        preferredMIMEName: "US-ASCII",
+        name: "US-ASCII",
+        aliases: [
+            "iso-ir-6",
+            "ANSI_X3.4-1968",
+            "ANSI_X3.4-1986",
+            "ISO_646.irv:1991",
+            "ISO646-US",
+            "US-ASCII",
+            "us",
+            "IBM367",
+            "cp367",
+            "csASCII",
+        ]
+    )
+
+    /// IANA Charset `ISO-8859-1`.
+    static let iso8859_1 = IANACharset(
+        preferredMIMEName: "ISO-8859-1",
+        name: "ISO_8859-1:1987",
+        aliases: [
+            "iso-ir-100",
+            "ISO_8859-1",
+            "ISO-8859-1",
+            "latin1",
+            "l1",
+            "IBM819",
+            "CP819",
+            "csISOLatin1",
+        ]
+    )
+
+    /// IANA Charset `ISO-8859-2`.
+    static let iso8859_2 = IANACharset(
+        preferredMIMEName: "ISO-8859-2",
+        name: "ISO_8859-2:1987",
+        aliases: [
+            "iso-ir-101",
+            "ISO_8859-2",
+            "ISO-8859-2",
+            "latin2",
+            "l2",
+            "csISOLatin2",
+        ]
+    )
+
+    /// IANA Charset `Shift_JIS`.
+    static let shiftJIS = IANACharset(
+        preferredMIMEName: "Shift_JIS",
+        name: "Shift_JIS",
+        aliases: [
+            "MS_Kanji",
+            "csShiftJIS",
+        ]
+    )
+
+    /// IANA Charset `EUC-JP`.
+    static let eucJP = IANACharset(
+        preferredMIMEName: "EUC-JP",
+        name: "Extended_UNIX_Code_Packed_Format_for_Japanese",
+        aliases: [
+            "csEUCPkdFmtJapanese",
+            "EUC-JP",
+        ]
+    )
+
+    /// IANA Charset `ISO-2022-JP`.
+    static let iso2022JP = IANACharset(
+        preferredMIMEName: "ISO-2022-JP",
+        name: "ISO-2022-JP",
+        aliases: [
+            "csISO2022JP",
+        ]
+    )
+
+    /// IANA Charset `UTF-8`.
+    static let utf8 = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-8",
+        aliases: [
+            "csUTF8",
+        ]
+    )
+
+    /// IANA Charset `UTF-16BE`.
+    static let utf16BE = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-16BE",
+        aliases: [
+            "csUTF16BE",
+        ]
+    )
+
+    /// IANA Charset `UTF-16LE`.
+    static let utf16LE = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-16LE",
+        aliases: [
+            "csUTF16LE",
+        ]
+    )
+
+    /// IANA Charset `UTF-16`.
+    static let utf16 = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-16",
+        aliases: [
+            "csUTF16",
+        ]
+    )
+
+    /// IANA Charset `UTF-32`.
+    static let utf32 = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-32",
+        aliases: [
+            "csUTF32",
+        ]
+    )
+
+    /// IANA Charset `UTF-32BE`.
+    static let utf32BE = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-32BE",
+        aliases: [
+            "csUTF32BE",
+        ]
+    )
+
+    /// IANA Charset `UTF-32LE`.
+    static let utf32LE = IANACharset(
+        preferredMIMEName: nil,
+        name: "UTF-32LE",
+        aliases: [
+            "csUTF32LE",
+        ]
+    )
+
+    /// IANA Charset `macintosh`.
+    static let macintosh = IANACharset(
+        preferredMIMEName: nil,
+        name: "macintosh",
+        aliases: [
+            "mac",
+            "csMacintosh",
+        ]
+    )
+
+    /// IANA Charset `windows-1250`.
+    static let windows1250 = IANACharset(
+        preferredMIMEName: nil,
+        name: "windows-1250",
+        aliases: [
+            "cswindows1250",
+        ]
+    )
+
+    /// IANA Charset `windows-1251`.
+    static let windows1251 = IANACharset(
+        preferredMIMEName: nil,
+        name: "windows-1251",
+        aliases: [
+            "cswindows1251",
+        ]
+    )
+
+    /// IANA Charset `windows-1252`.
+    static let windows1252 = IANACharset(
+        preferredMIMEName: nil,
+        name: "windows-1252",
+        aliases: [
+            "cswindows1252",
+        ]
+    )
+
+    /// IANA Charset `windows-1253`.
+    static let windows1253 = IANACharset(
+        preferredMIMEName: nil,
+        name: "windows-1253",
+        aliases: [
+            "cswindows1253",
+        ]
+    )
+
+    /// IANA Charset `windows-1254`.
+    static let windows1254 = IANACharset(
+        preferredMIMEName: nil,
+        name: "windows-1254",
+        aliases: [
+            "cswindows1254",
+        ]
+    )
+}
diff --git a/Sources/FoundationEssentials/String/String+Encoding+Names.swift b/Sources/FoundationEssentials/String/String+Encoding+Names.swift
new file mode 100644
index 000000000..1407e5ae1
--- /dev/null
+++ b/Sources/FoundationEssentials/String/String+Encoding+Names.swift
@@ -0,0 +1,155 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+
+// MARK: - Private extensions for parsing encoding names
+
+private extension UTF8.CodeUnit {
+    func _isASCIICaseInsensitivelyEqual(to other: UTF8.CodeUnit) -> Bool {
+        return switch self {
+        case other, other._uppercased, other._lowercased: true
+        default: false
+        }
+    }
+}
+
+private extension String {
+    func _isASCIICaseInsensitivelyEqual(to other: String) -> Bool {
+        let (myUTF8, otherUTF8) = (self.utf8, other.utf8)
+        var (myIndex, otherIndex) = (myUTF8.startIndex, otherUTF8.startIndex)
+        while myIndex < myUTF8.endIndex && otherIndex < otherUTF8.endIndex {
+            guard myUTF8[myIndex]._isASCIICaseInsensitivelyEqual(to: otherUTF8[otherIndex]) else {
+                return false
+            }
+
+            myUTF8.formIndex(after: &myIndex)
+            otherUTF8.formIndex(after: &otherIndex)
+        }
+        return myIndex == myUTF8.endIndex && otherIndex == otherUTF8.endIndex
+    }
+}
+
+
+// MARK: - IANA Charset Names
+
+/// Info about IANA Charset.
+internal struct IANACharset {
+    /// Preferred MIME Name
+    let preferredMIMEName: String?
+
+    /// The name of this charset
+    let name: String
+
+    /// The aliases of this charset
+    let aliases: Array<String>
+
+    var representativeName: String {
+        return preferredMIMEName ?? name
+    }
+
+    init(preferredMIMEName: String?, name: String, aliases: Array<String>) {
+        self.preferredMIMEName = preferredMIMEName
+        self.name = name
+        self.aliases = aliases
+    }
+
+    func matches(_ string: String) -> Bool {
+        if let preferredMIMEName = self.preferredMIMEName,
+           preferredMIMEName._isASCIICaseInsensitivelyEqual(to: string) {
+            return true
+        }
+        if name._isASCIICaseInsensitivelyEqual(to: string) {
+            return true
+        }
+        for alias in aliases {
+            if alias._isASCIICaseInsensitivelyEqual(to: string) {
+                return true
+            }
+        }
+        return false
+    }
+}
+
+
+// MARK: - `String.Encoding` Names
+
+extension String.Encoding {
+    private var _ianaCharset: IANACharset? {
+        switch self {
+        case .utf8: .utf8
+        case .ascii: .usASCII
+        case .japaneseEUC: .eucJP
+        case .isoLatin1: .iso8859_1
+        case .shiftJIS: .shiftJIS
+        case .isoLatin2: .iso8859_2
+        case .unicode: .utf16
+        case .windowsCP1251: .windows1251
+        case .windowsCP1252: .windows1252
+        case .windowsCP1253: .windows1253
+        case .windowsCP1254: .windows1254
+        case .windowsCP1250: .windows1250
+        case .iso2022JP: .iso2022JP
+        case .macOSRoman: .macintosh
+        case .utf16BigEndian: .utf16BE
+        case .utf16LittleEndian: .utf16LE
+        case .utf32: .utf32
+        case .utf32BigEndian: .utf32BE
+        case .utf32LittleEndian: .utf32LE
+        default: nil
+        }
+    }
+
+    /// The name of this encoding that is compatible with the one of the IANA registry "charset".
+    @available(FoundationPreview 6.3, *)
+    public var ianaName: String? {
+        return _ianaCharset?.representativeName
+    }
+
+    /// Creates an instance from the name of the IANA registry "charset".
+    ///
+    /// - Note: The given name is compared to each IANA "charset" name
+    ///         with ASCII case-insensitive collation
+    ///         to determine which encoding is suitable.
+    @available(FoundationPreview 6.3, *)
+    public init?(ianaName charsetName: String) {
+        let possibilities: [String.Encoding] = [
+            .utf8,
+            .ascii,
+            .japaneseEUC,
+            .isoLatin1,
+            .shiftJIS,
+            .isoLatin2,
+            .unicode, // .utf16
+            .windowsCP1251,
+            .windowsCP1252,
+            .windowsCP1253,
+            .windowsCP1254,
+            .windowsCP1250,
+            .iso2022JP,
+            .macOSRoman,
+            .utf16BigEndian,
+            .utf16LittleEndian,
+            .utf32,
+            .utf32BigEndian,
+            .utf32LittleEndian,
+        ]
+
+        for encoding in possibilities {
+            if encoding._ianaCharset!.matches(charsetName) {
+                self = encoding
+                return
+            }
+        }
+        return nil
+    }
+}
+
diff --git a/Tests/FoundationEssentialsTests/StringTests.swift b/Tests/FoundationEssentialsTests/StringTests.swift
index 26286be15..6a6781874 100644
--- a/Tests/FoundationEssentialsTests/StringTests.swift
+++ b/Tests/FoundationEssentialsTests/StringTests.swift
@@ -1397,6 +1397,69 @@ private struct StringTests {
             "abcd🎺efgh"
         ])
     }
+
+    @Test func encodingNames() {
+        // Encoding to Name
+        #expect(String.Encoding.ascii.ianaName == "US-ASCII")
+        #expect(String.Encoding.nextstep.ianaName == nil)
+        #expect(String.Encoding.japaneseEUC.ianaName == "EUC-JP")
+        #expect(String.Encoding.utf8.ianaName == "UTF-8")
+        #expect(String.Encoding.isoLatin1.ianaName == "ISO-8859-1")
+        #expect(String.Encoding.symbol.ianaName == nil)
+        #expect(String.Encoding.nonLossyASCII.ianaName == nil)
+        #expect(String.Encoding.shiftJIS.ianaName == "Shift_JIS")
+        #expect(String.Encoding.isoLatin2.ianaName == "ISO-8859-2")
+        #expect(String.Encoding.unicode.ianaName == "UTF-16")
+        #expect(String.Encoding.windowsCP1251.ianaName == "windows-1251")
+        #expect(String.Encoding.windowsCP1252.ianaName == "windows-1252")
+        #expect(String.Encoding.windowsCP1253.ianaName == "windows-1253")
+        #expect(String.Encoding.windowsCP1254.ianaName == "windows-1254")
+        #expect(String.Encoding.windowsCP1250.ianaName == "windows-1250")
+        #expect(String.Encoding.iso2022JP.ianaName == "ISO-2022-JP")
+        #expect(String.Encoding.macOSRoman.ianaName == "macintosh")
+        #expect(String.Encoding.utf16BigEndian.ianaName == "UTF-16BE")
+        #expect(String.Encoding.utf16LittleEndian.ianaName == "UTF-16LE")
+        #expect(String.Encoding.utf32.ianaName == "UTF-32")
+        #expect(String.Encoding.utf32BigEndian.ianaName == "UTF-32BE")
+        #expect(String.Encoding.utf32LittleEndian.ianaName == "UTF-32LE")
+        #expect(String.Encoding(rawValue: .max).ianaName == nil)
+
+        // Name to Encoding
+        #expect(String.Encoding(ianaName: "us-ascii") == .ascii)
+        #expect(String.Encoding(ianaName: "iso-ir-2") == nil)
+        #expect(String.Encoding(ianaName: "x-nextstep") == nil)
+        #expect(String.Encoding(ianaName: "euc-jp") == .japaneseEUC)
+        #expect(String.Encoding(ianaName: "CP51932") == nil)
+        #expect(String.Encoding(ianaName: "utf-8") == .utf8)
+        #expect(String.Encoding(ianaName: "iso_8859-1") == .isoLatin1)
+        #expect(String.Encoding(ianaName: "x-mac-symbol") == nil)
+        #expect(String.Encoding(ianaName: "Adobe-symbol-encoding") == nil)
+        #expect(String.Encoding(ianaName: "cp932") == nil)
+        #expect(String.Encoding(ianaName: "shift_jis") == .shiftJIS)
+        #expect(String.Encoding(ianaName: "windows-31j") == nil)
+        #expect(String.Encoding(ianaName: "iso_8859-2") == .isoLatin2)
+        #expect(String.Encoding(ianaName: "utf-16") == .utf16)
+        #expect(String.Encoding(ianaName: "iso-10646-ucs-2") == nil)
+        #expect(String.Encoding(ianaName: "unicode-1-1") == nil)
+        #expect(String.Encoding(ianaName: "windows-1251") == .windowsCP1251)
+        #expect(String.Encoding(ianaName: "windows-1252") == .windowsCP1252)
+        #expect(String.Encoding(ianaName: "ISO-8859-1-Windows-3.0-Latin-1") == nil)
+        #expect(String.Encoding(ianaName: "ISO-8859-1-Windows-3.1-Latin-1") == nil)
+        #expect(String.Encoding(ianaName: "windows-1253") == .windowsCP1253)
+        #expect(String.Encoding(ianaName: "windows-1254") == .windowsCP1254)
+        #expect(String.Encoding(ianaName: "iso-8859-9-windows-Latin-5") == nil)
+        #expect(String.Encoding(ianaName: "windows-1250") == .windowsCP1250)
+        #expect(String.Encoding(ianaName: "iso-8859-2-windows-Latin-2") == nil)
+        #expect(String.Encoding(ianaName: "iso-2022-jp") == .iso2022JP)
+        #expect(String.Encoding(ianaName: "macintosh") == .macOSRoman)
+        #expect(String.Encoding(ianaName: "utf-16be") == .utf16BigEndian)
+        #expect(String.Encoding(ianaName: "utf-16le") == .utf16LittleEndian)
+        #expect(String.Encoding(ianaName: "utf-32") == .utf32)
+        #expect(String.Encoding(ianaName: "iso-10646-ucs-4") == nil)
+        #expect(String.Encoding(ianaName: "utf-32be") == .utf32BigEndian)
+        #expect(String.Encoding(ianaName: "utf-32le") == .utf32LittleEndian)
+        #expect(String.Encoding(ianaName: "foo-bar-baz") == nil)
+    }
 }
 
 // MARK: - Helper functions
diff --git a/utils/update-iana-charset-names b/utils/update-iana-charset-names
new file mode 100755
index 000000000..e56e972b1
--- /dev/null
+++ b/utils/update-iana-charset-names
@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+##===----------------------------------------------------------------------===##
+##
+## This source file is part of the Swift.org open source project
+##
+## Copyright (c) 2025 Apple Inc. and the Swift project authors
+## Licensed under Apache License v2.0 with Runtime Library Exception
+##
+## See https://swift.org/LICENSE.txt for license information
+## See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+##
+##===----------------------------------------------------------------------===##
+
+# This is a shell script that generates a Swift source code file which contains
+# the list of IANA "Character Sets".
+
+set -eu
+
+declare -r commandName="$(basename "$0")"
+declare -r utilsDir="$(cd "$(dirname "$0")" && pwd)"
+declare -r foundationRepoDir="$(cd "${utilsDir}/.." && pwd)"
+declare -r targetSwiftFileRelativePath="Sources/FoundationEssentials/String/IANACharsetNames.swift"
+
+declare -r copyrightYear=$(
+  currentYear=$(date +%Y)
+  if [[ $currentYear -eq 2025 ]]; then
+    echo 2025
+  else
+    echo 2025-${currentYear}
+  fi
+)
+declare -r swiftLicenseHeader="
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) ${copyrightYear} Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+"
+declare -r warningComment="
+// WARNING: DO NOT EDIT THIS FILE DIRECTLY.
+//          This is auto-generated by \`${commandName}\`.
+
+"
+
+echo "Generating Swift source code..." 1>&2
+declare generatedCode
+generatedCode=$(
+  echo "${swiftLicenseHeader##$'\n'}"
+  echo "$warningComment"
+  swift -D PRINT_CODE "${utilsDir}/${commandName}-impl.swift"
+)
+
+echo "Writing the code to '${targetSwiftFileRelativePath}'..." 1>&2
+echo "$generatedCode" >"${foundationRepoDir}/${targetSwiftFileRelativePath}"
+
+echo "Done." 1>&2
diff --git a/utils/update-iana-charset-names-impl.swift b/utils/update-iana-charset-names-impl.swift
new file mode 100755
index 000000000..c7e83e0d8
--- /dev/null
+++ b/utils/update-iana-charset-names-impl.swift
@@ -0,0 +1,195 @@
+#!/usr/bin/env swift -D PRINT_CODE
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/*
+
+This is a Swift script that converts an XML file containing the list of IANA
+"Character Sets" to Swift source code.
+This script generates minimum code and is intended to be executed by other shell
+script.
+
+ */
+
+import Foundation
+#if canImport(FoundationXML)
+import FoundationXML
+#endif
+
+// MARK: - Constants
+
+let requiredCharsetNames = [
+    "UTF-8",
+    "US-ASCII",
+    "EUC-JP",
+    "ISO-8859-1",
+    "Shift_JIS",
+    "ISO-8859-2",
+    "UTF-16",
+    "windows-1251",
+    "windows-1252",
+    "windows-1253",
+    "windows-1254",
+    "windows-1250",
+    "ISO-2022-JP",
+    "macintosh",
+    "UTF-16BE",
+    "UTF-16LE",
+    "UTF-32",
+    "UTF-32BE",
+    "UTF-32LE",
+]
+let charsetsXMLURL = URL(
+    string: "https://www.iana.org/assignments/character-sets/character-sets.xml"
+)!
+let charsetsXMLNamespace = "http://www.iana.org/assignments"
+let swiftCodeIndent = "    "
+
+
+// MARK: - Implementation
+
+enum CodeGenerationError: Swift.Error {
+    case missingName
+    case missingAliasValue
+    case noRootElement
+}
+
+/// Representation of <record> element in 'character-sets.xml'
+///
+/// The structure of <record> element is as blow:
+/// ```xml
+/// <record>
+///     <name>US-ASCII</name>
+///     <xref type="rfc" data="rfc2046"/>
+///     <value>3</value>
+///     <description>ANSI X3.4-1986</description>
+///     <alias>iso-ir-6</alias>
+///     <alias>ANSI_X3.4-1968</alias>
+///     <alias>ANSI_X3.4-1986</alias>
+///     <alias>ISO_646.irv:1991</alias>
+///     <alias>ISO646-US</alias>
+///     <alias>US-ASCII</alias>
+///     <alias>us</alias>
+///     <alias>IBM367</alias>
+///     <alias>cp367</alias>
+///     <alias>csASCII</alias>
+///     <preferred_alias>US-ASCII</preferred_alias>
+/// </record>
+/// ```
+struct IANACharsetNameRecord {
+    /// Preferred MIME Name
+    let preferredMIMEName: String?
+
+    /// The name of this charset
+    let name: String
+
+    /// The aliases of this charset
+    let aliases: Array<String>
+
+    var representativeName: String {
+        return preferredMIMEName ?? name
+    }
+
+    var swiftCodeLines: [String] {
+        var lines: [String] = []
+        lines.append("/// IANA Charset `\(representativeName)`.")
+        lines.append("static let \(representativeName._camelcased()) = IANACharset(")
+        lines.append("\(swiftCodeIndent)preferredMIMEName: \(preferredMIMEName.map { #""\#($0)""# } ?? "nil"),")
+        lines.append("\(swiftCodeIndent)name: \"\(name)\",")
+        lines.append("\(swiftCodeIndent)aliases: [")
+        for alias in aliases {
+            lines.append("\(swiftCodeIndent)\(swiftCodeIndent)\"\(alias)\",")
+        }
+        lines.append("\(swiftCodeIndent)]")
+        lines.append(")")
+        return lines
+    }
+
+    init(_ node: XMLNode) throws {
+        guard let name = try node.nodes(forXPath: "./name").first?.stringValue else {
+            throw CodeGenerationError.missingName
+        }
+        self.name = name
+        self.preferredMIMEName = try node.nodes(forXPath: "./preferred_alias").first?.stringValue
+        self.aliases = try node.nodes(forXPath: "./alias").map {
+            guard let alias = $0.stringValue else {
+                throw CodeGenerationError.missingAliasValue
+            }
+            return alias
+        }
+    }
+}
+
+func generateSwiftCode() throws -> String {
+    let charsetsXMLDocument = try XMLDocument(contentsOf: charsetsXMLURL)
+    guard let charsetsXMLRoot = charsetsXMLDocument.rootElement() else {
+        throw CodeGenerationError.noRootElement
+    }
+    let charsetsXMLRecordElements = try charsetsXMLRoot.nodes(forXPath: "./registry/record")
+
+    var result = "extension IANACharset {"
+
+    for record in try charsetsXMLRecordElements.map({
+        try IANACharsetNameRecord($0)
+    }) where requiredCharsetNames.contains(record.representativeName) {
+        result += "\n"
+        result += record.swiftCodeLines.map({ swiftCodeIndent + $0 }).joined(separator: "\n")
+        result += "\n"
+    }
+
+    result += "}\n"
+    return result
+}
+
+#if PRINT_CODE
+print(try generateSwiftCode())
+#endif
+
+// MARK: - Extensions
+
+extension UTF8.CodeUnit {
+    var _isASCIINumeric: Bool { (0x30...0x39).contains(self) }
+    var _isASCIIUppercase: Bool { (0x41...0x5A).contains(self) }
+    var _isASCIILowercase: Bool { (0x61...0x7A).contains(self) }
+}
+
+extension String {
+    func _camelcased() -> String {
+        var result = ""
+        var previousWord: Substring.UTF8View? = nil
+        for wordUTF8 in self.utf8.split(whereSeparator: {
+            !$0._isASCIINumeric &&
+            !$0._isASCIIUppercase &&
+            !$0._isASCIILowercase
+        }) {
+            defer {
+                previousWord = wordUTF8
+            }
+            let word = String(Substring(wordUTF8))
+            guard let previousWord else {
+                result += word.lowercased()
+                continue
+            }
+            if previousWord.last!._isASCIINumeric && wordUTF8.first!._isASCIINumeric {
+                result += "_"
+            }
+            if let firstNonNumericIndex = wordUTF8.firstIndex(where: { !$0._isASCIINumeric }),
+               wordUTF8[firstNonNumericIndex...].allSatisfy({ $0._isASCIIUppercase }) {
+                result += word
+            } else {
+                result += word.capitalized(with: nil)
+            }
+
+        }
+        return result
+    }
+}