[stdlib] One-sided ranges and RangeExpression

stdlib/public/core/Policy.swift

@@ -660,6 +660,9 @@ infix operator   ^ : AdditionPrecedence
 // FIXME: is this the right precedence level for "..." ?
 infix operator  ... : RangeFormationPrecedence
 infix operator  ..< : RangeFormationPrecedence
+postfix operator ...
+prefix operator ...
+prefix operator ..<
 
 // The cast operators 'as' and 'is' are hardcoded as if they had the
 // following attributes:

stdlib/public/core/Range.swift.gyb

@@ -10,6 +10,33 @@
 //
 //===----------------------------------------------------------------------===//
 
+/// A type which can be used to slice a collection. A `RangeExpression` can
+/// convert itself to a `Range<Bound>` of indices within a given collection;
+/// the collection can then slice itself with that `Range`.
+public protocol RangeExpression {
+  associatedtype Bound: Comparable
+  /// Returns `self` expressed as a range of indices within `collection`.
+  ///
+  /// -Parameter collection: The collection `self` should be
+  ///                        relative to.
+  ///
+  /// -Returns: A `Range<Bound>` suitable for slicing `collection`.
+  ///           The return value is *not* guaranteed to be inside
+  ///           its bounds. Callers should apply the same preconditions
+  ///           to the return value as they would to a range provided
+  ///           directly by the user.
+  func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound
+
+  func contains(_ element: Bound) -> Bool
+}
+
+extension RangeExpression {
+  @_inlineable
+  public static func ~= (pattern: Self, value: Bound) -> Bool {
+    return pattern.contains(value)
+  }  
+}
+
 // FIXME(ABI)#55 (Statically Unavailable/Dynamically Available): remove this type, it creates an ABI burden
 // on the library.
 //
@@ -513,6 +540,18 @@ extension ${Self} {
   }
 }
 
+extension ${Self}: RangeExpression {
+  public func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound {
+    %   if 'Closed' in Self:
+      return Range(uncheckedBounds: 
+        (lower: lowerBound, upper: collection.index(after: self.upperBound))
+    %   else:
+      return Range(uncheckedBounds: (lower: lowerBound, upper: upperBound)
+    %   end
+    )
+  }
+}
+
 extension ${Self} : CustomStringConvertible {
   /// A textual representation of the range.
   public var description: String {
@@ -697,6 +736,201 @@ public func ..< <Bound>(
   return CountableRange(uncheckedBounds: (lower: minimum, upper: maximum))
 }
 
+@_fixed_layout
+public struct PartialRangeUpTo<Bound: Comparable>: RangeExpression {
+  public init(_ upperBound: Bound) { self.upperBound = upperBound }
+  public let upperBound: Bound
+  @_transparent
+  public func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound {
+    return collection.startIndex..<self.upperBound
+  }
+  @_transparent
+  public func contains(_ element: Bound) -> Bool {
+    return element < upperBound
+  }
+}
+
+@_fixed_layout
+public struct PartialRangeThrough<Bound: Comparable>: RangeExpression {
+  public init(_ upperBound: Bound) { self.upperBound = upperBound }
+  public let upperBound: Bound
+  @_transparent
+  public func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound {
+    return collection.startIndex..<collection.index(after: self.upperBound)
+  }
+  @_transparent
+  public func contains(_ element: Bound) -> Bool {
+    return element <= upperBound
+  }
+}
+
+@_fixed_layout
+public struct PartialRangeFrom<Bound: Comparable>: RangeExpression {
+  public init(_ lowerBound: Bound) { self.lowerBound = lowerBound }
+  public let lowerBound: Bound
+  @_transparent
+  public func relative<C: _Indexable>(to collection: C) -> Range<Bound> where C.Index == Bound {
+    return self.lowerBound..<collection.endIndex
+  }
+  @_transparent
+  public func contains(_ element: Bound) -> Bool {
+    return lowerBound <= element
+  }
+}
+
+@_fixed_layout
+public struct CountablePartialRangeFrom<
+  Bound: Strideable
+>: RangeExpression where Bound.Stride : SignedInteger  {
+  public init(_ lowerBound: Bound) { self.lowerBound = lowerBound }
+  public let lowerBound: Bound
+  @_transparent
+  public func relative<C: _Indexable>(
+    to collection: C
+  ) -> Range<Bound> where C.Index == Bound {
+    return self.lowerBound..<collection.endIndex
+  }
+  public func contains(_ element: Bound) -> Bool {
+    return lowerBound <= element
+  }
+}
+
+extension CountablePartialRangeFrom: Sequence {
+  @_fixed_layout
+  public struct Iterator: IteratorProtocol {
+    @_inlineable
+    public init(_current: Bound) { self._current = _current }
+    @_inlineable
+    public mutating func next() -> Bound? {
+      defer { _current = _current.advanced(by: 1) }
+      return _current
+    }
+    @_versioned
+    var _current: Bound
+  }
+  @_inlineable
+  public func makeIterator() -> Iterator { 
+    return Iterator(_current: lowerBound) 
+  }
+}
+
+extension Comparable {
+  /// Returns a partial range up to but not including its maximum.
+  ///
+  /// Use the partial range up to operator (`..<`) to create a partial range of
+  /// any type that conforms to the `Comparable` protocol. This example creates
+  /// a `PartialRangeUpTo<Double>` up to, but not including, 5.0.
+  ///
+  ///     let lessThanFive = ..<5.0
+  ///     print(lessThanFive.contains(3.14))  // Prints "true"
+  ///     print(lessThanFive.contains(5.0))   // Prints "false"
+  ///
+  /// Partial ranges can be used to slice types conforming to `Collection`
+  /// from the start of the collection up to, but not including, the maximum.
+  ///
+  /// let array = Array(0..<5)
+  /// print(array[..<3])      // prints [0, 1, 2]
+  ///
+  /// - Parameters:
+  ///   - maximum: The upper bound for the range.
+  @_transparent
+  public static prefix func ..<(maximum: Self) -> PartialRangeUpTo<Self> {
+    return PartialRangeUpTo(maximum)
+  }
+
+  /// Returns a partial range up to and including its maximum.
+  ///
+  /// Use the partial range up to operator (`...`) to create a partial range of
+  /// any type that conforms to the `Comparable` protocol. This example creates
+  /// a `PartialRangeThrough<Double>` up to 5.
+  ///
+  ///     let upToFive = ..<5.0
+  ///     print(upToFive.contains(4))   // Prints "true"
+  ///     print(upToFive.contains(5))   // Prints "true"
+  ///     print(upToFive.contains(6))  // Prints "false"
+  ///
+  /// Partial ranges can be used to slice types conforming to `Collection`
+  /// from the start of the collection up to the maximum.
+  ///
+  /// let array = Array(0..<5)
+  /// print(array[...3])      // prints [0, 1, 2, 3]
+  ///
+  /// - Parameters:
+  ///   - maximum: The upper bound for the range.
+  @_transparent
+  public static prefix func ...(maximum: Self) -> PartialRangeThrough<Self> {
+    return PartialRangeThrough(maximum)
+  }
+
+  /// Returns a countable partial range from a lower bound.
+  ///
+  /// Use the partial range up to operator (`...`) to create a range of any type
+  /// that conforms to the `Strideable` protocol with an associated integer
+  /// `Stride` type, such as any of the standard library's integer types. This
+  /// example creates a `CountablePartialRangeFrom<Int>` from 5 up.
+  ///
+  ///     let fiveOrMore = 5...
+  ///     print(fiveOrMore.contains(3))         // Prints "false"
+  ///     print(fiveOrMore.contains(5))         // Prints "true"
+  ///
+  /// - Parameters:
+  ///   - minimum: The lower bound for the range.
+  @_transparent
+  public static postfix func ...(minimum: Self) -> PartialRangeFrom<Self> {
+    return PartialRangeFrom(minimum)
+  }
+}
+
+extension Strideable where Stride: SignedInteger {
+  /// Returns a countable partial range from a lower bound.
+  ///
+  /// Use the partial range up to operator (`...`) to create a range of any type
+  /// that conforms to the `Strideable` protocol with an associated integer
+  /// `Stride` type, such as any of the standard library's integer types. This
+  /// example creates a `CountablePartialRangeFrom<Int>` from 5 up.
+  ///
+  ///     let fiveOrMore = 5...
+  ///     print(fiveOrMore.contains(3))         // Prints "false"
+  ///     print(fiveOrMore.contains(5))         // Prints "true"
+  ///
+  /// You can use sequence methods on these partial ranges.
+  ///
+  /// let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+  /// let asciiTable = zip(0x41..., alphabet)
+  /// for (code, letter) in asciiTable { print(code, letter) }
+  ///
+  /// Note that these sequences count up indefinitely. You should not use them
+  /// with algorithms such as `map` or `filter` that will try to read the entire
+  /// sequence eagerly. The upper limit for the sequence is determined by the
+  /// type of `Bound`. For example, `CountablePartialRangeFrom<Int>` will trap
+  /// when the sequences' next value would be above `Int.max`.
+  ///
+  /// - Parameters:
+  ///   - minimum: The lower bound for the range.
+  @_transparent
+  public static postfix func ...(minimum: Self) -> CountablePartialRangeFrom<Self> {
+      return CountablePartialRangeFrom(minimum)
+  }
+}
+
+extension _Indexable {
+  @_inlineable
+  public subscript<R: RangeExpression>(r: R) -> SubSequence where R.Bound == Index {
+    return self[r.relative(to: self)]
+  }
+}
+extension _MutableIndexable {
+  @_inlineable
+  public subscript<R: RangeExpression>(r: R) -> SubSequence where R.Bound == Index {
+    get {
+      return self[r.relative(to: self)]
+    }
+    set {
+      self[r.relative(to: self)] = newValue
+    }
+  }
+}
+
 // swift-3-indexing-model: this is not really a proper rename
 @available(*, unavailable, renamed: "IndexingIterator")
 public struct RangeGenerator<Bound> {}

stdlib/public/core/RangeReplaceableCollection.swift.gyb

@@ -890,28 +890,7 @@ extension RangeReplaceableCollection where SubSequence == Self {
   }
 }
 
-% for Range in ['CountableRange', 'ClosedRange', 'CountableClosedRange']:
-extension RangeReplaceableCollection
-%   if 'Countable' in Range:
-  where
-  Index : Strideable, Index.Stride : SignedInteger
-%   end
-{
-  /// Returns a half-open range denoting the same positions as `r`.
-  @_versioned
-  @_inlineable
-  internal func _makeHalfOpen(_ r: ${Range}<Index>) -> Range<Index> {
-    // The upperBound of the result depends on whether `r` is a closed
-    // range.
-%   if 'Closed' in Range:
-    return Range(uncheckedBounds: (
-      lower: r.lowerBound,
-      upper: index(after: r.upperBound)))
-%   else:
-    return Range(r)
-%   end
-  }
-
+extension RangeReplaceableCollection {
   /// Replaces the specified subrange of elements with the given collection.
   ///
   /// This method has the effect of removing the specified range of elements
@@ -949,11 +928,11 @@ extension RangeReplaceableCollection
   ///   contents of `newElements` to the collection, the complexity is O(*n*),
   ///   where *n* is the length of `newElements`.
   @_inlineable
-  public mutating func replaceSubrange<C>(
-    _ subrange: ${Range}<Index>,
+  public mutating func replaceSubrange<C: Collection, R: RangeExpression>(
+    _ subrange: R,
     with newElements: C
-  ) where C : Collection, C.Iterator.Element == Iterator.Element {
-    self.replaceSubrange(_makeHalfOpen(subrange), with: newElements)
+  ) where C.Iterator.Element == Iterator.Element, R.Bound == Index {
+    self.replaceSubrange(subrange.relative(to: self), with: newElements)
   }
 
   /// Removes the elements in the specified subrange from the collection.
@@ -975,11 +954,12 @@ extension RangeReplaceableCollection
   ///
   /// - Complexity: O(*n*), where *n* is the length of the collection.
   @_inlineable
-  public mutating func removeSubrange(_ bounds: ${Range}<Index>) {
-    removeSubrange(_makeHalfOpen(bounds))
+  public mutating func removeSubrange<R: RangeExpression>(
+    _ bounds: R
+  ) where R.Bound == Index  {
+    removeSubrange(bounds.relative(to: self))
   }
 }
-% end
 
 extension RangeReplaceableCollection {
   @_inlineable

stdlib/public/core/StringRangeReplaceableCollection.swift.gyb

@@ -247,7 +247,6 @@ extension String {
     }
   }
 
-% for Range in ['Range', 'ClosedRange']:
   /// Replaces the text within the specified bounds with the given characters.
   ///
   /// Calling this method invalidates any existing indices for use with this
@@ -263,7 +262,7 @@ extension String {
   ///   removes text at the end of the string, the complexity is O(*n*), where
   ///   *n* is equal to `bounds.count`.
   public mutating func replaceSubrange<C>(
-    _ bounds: ${Range}<Index>,
+    _ bounds: Range<Index>,
     with newElements: C
   ) where C : Collection, C.Iterator.Element == Character {
     withMutableCharacters {
@@ -272,27 +271,6 @@ extension String {
     }
   }
 
-  /// Replaces the text within the specified bounds with the given string.
-  ///
-  /// Calling this method invalidates any existing indices for use with this
-  /// string.
-  ///
-  /// - Parameters:
-  ///   - bounds: The range of text to replace. The bounds of the range must be
-  ///     valid indices of the string.
-  ///   - newElements: The new text to add to the string.
-  ///
-  /// - Complexity: O(*m*), where *m* is the combined length of the string and
-  ///   `newElements`. If the call to `replaceSubrange(_:with:)` simply
-  ///   removes text at the end of the string, the complexity is O(*n*), where
-  ///   *n* is equal to `bounds.count`.
-  public mutating func replaceSubrange(
-    _ bounds: ${Range}<Index>, with newElements: String
-  ) {
-    replaceSubrange(bounds, with: newElements.characters)
-  }
-% end
-
   /// Inserts a new character at the specified position.
   ///
   /// Calling this method invalidates any existing indices for use with this
@@ -357,26 +335,21 @@ extension String {
     }
   }
 
-% for Range in ['Range', 'ClosedRange']:
   /// Removes the characters in the given range.
   ///
   /// Calling this method invalidates any existing indices for use with this
   /// string.
   ///
-% if Range == 'ClosedRange':
   /// - Parameter bounds: The range of the elements to remove. The upper and
   ///   lower bounds of `bounds` must be valid indices of the string and not
   ///   equal to the string's end index.
-% else:
   /// - Parameter bounds: The range of the elements to remove. The upper and
   ///   lower bounds of `bounds` must be valid indices of the string.
-% end
-  public mutating func removeSubrange(_ bounds: ${Range}<Index>) {
+  public mutating func removeSubrange(_ bounds: Range<Index>) {
     withMutableCharacters {
       (v: inout CharacterView) in v.removeSubrange(bounds)
     }
   }
-% end
 
   /// Replaces this string with the empty string.
   ///