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Collection.swift
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Collection.swift
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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
/// A type that provides subscript access to its elements, with forward
/// index traversal.
///
/// In most cases, it's best to ignore this protocol and use the `Collection`
/// protocol instead, because it has a more complete interface.
public protocol IndexableBase {
// FIXME(ABI)(compiler limitation): there is no reason for this protocol
// to exist apart from missing compiler features that we emulate with it.
//
// This protocol is almost an implementation detail of the standard
// library; it is used to deduce things like the `SubSequence` and
// `Iterator` type from a minimal collection, but it is also used in
// exposed places like as a constraint on `IndexingIterator`.
/// A type that represents a position in the collection.
///
/// Valid indices consist of the position of every element and a
/// "past the end" position that's not valid for use as a subscript
/// argument.
///
/// - SeeAlso: endIndex
associatedtype Index : Comparable
/// The position of the first element in a nonempty collection.
///
/// If the collection is empty, `startIndex` is equal to `endIndex`.
var startIndex: Index { get }
/// The collection's "past the end" position, or one greater than the last
/// valid subscript argument.
///
/// When you need a range that includes the last element of a collection, use
/// the half-open range operator (`..<`) with `endIndex`. The `..<` operator
/// creates a range that doesn't include the upper bound, so it's always
/// safe to use with `endIndex`. For example:
///
/// let numbers = [10, 20, 30, 40, 50]
/// if let index = numbers.index(of: 30) {
/// print(numbers[index ..< numbers.endIndex])
/// }
/// // Prints "[30, 40, 50]"
///
/// If the collection is empty, `endIndex` is equal to `startIndex`.
var endIndex: Index { get }
// The declaration of _Element and subscript here is a trick used to
// break a cyclic conformance/deduction that Swift can't handle. We
// need something other than a Collection.Iterator.Element that can
// be used as IndexingIterator<T>'s Element. Here we arrange for
// the Collection itself to have an Element type that's deducible from
// its subscript. Ideally we'd like to constrain this Element to be the same
// as Collection.Iterator.Element (see below), but we have no way of
// expressing it today.
associatedtype _Element
/// Accesses the element at the specified position.
///
/// For example, access an element of an array through its subscript to
/// print its value:
///
/// var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
/// print(streets[1])
/// // Prints "Bryant"
///
/// You can subscript a collection with any valid index other than the
/// collection's end index. The end index refers to the position one
/// past the last element of a collection, so it doesn't correspond with an
/// element.
///
/// - Parameter position: The position of the element to access. `position`
/// must be a valid index of the collection that is not equal to the
/// `endIndex` property.
subscript(position: Index) -> _Element { get }
// WORKAROUND: rdar://25214066
/// A `Sequence` that can represent a contiguous subrange of `self`'s
/// elements.
associatedtype SubSequence
/// Accesses the subsequence bounded by `bounds`.
///
/// - Complexity: O(1)
///
/// - Precondition: `(startIndex...endIndex).contains(bounds.lowerBound)`
/// and `(startIndex...endIndex).contains(bounds.upperBound)`
subscript(bounds: Range<Index>) -> SubSequence { get }
/// Performs a range check in O(1), or a no-op when a range check is not
/// implementable in O(1).
///
/// The range check, if performed, is equivalent to:
///
/// precondition(bounds.contains(index))
///
/// Use this function to perform a cheap range check for QoI purposes when
/// memory safety is not a concern. Do not rely on this range check for
/// memory safety.
///
/// The default implementation for forward and bidirectional indices is a
/// no-op. The default implementation for random access indices performs a
/// range check.
///
/// - Complexity: O(1).
func _failEarlyRangeCheck(_ index: Index, bounds: Range<Index>)
/// Performs a range check in O(1), or a no-op when a range check is not
/// implementable in O(1).
///
/// The range check, if performed, is equivalent to:
///
/// precondition(
/// bounds.contains(range.lowerBound) ||
/// range.lowerBound == bounds.upperBound)
/// precondition(
/// bounds.contains(range.upperBound) ||
/// range.upperBound == bounds.upperBound)
///
/// Use this function to perform a cheap range check for QoI purposes when
/// memory safety is not a concern. Do not rely on this range check for
/// memory safety.
///
/// The default implementation for forward and bidirectional indices is a
/// no-op. The default implementation for random access indices performs a
/// range check.
///
/// - Complexity: O(1).
func _failEarlyRangeCheck(_ range: Range<Index>, bounds: Range<Index>)
/// Returns the position immediately after `i`.
///
/// - Precondition: `(startIndex..<endIndex).contains(i)`
@warn_unused_result
func index(after i: Index) -> Index
/// Replaces `i` with its successor.
func formIndex(after i: inout Index)
}
public protocol Indexable : IndexableBase {
/// A type that can represent the number of steps between pairs of
/// `Index` values where one value is reachable from the other.
///
/// Reachability is defined by the ability to produce one value from
/// the other via zero or more applications of `index(after: i)`.
associatedtype IndexDistance : SignedInteger = Int
/// Returns the result of advancing `i` by `n` positions.
///
/// - Returns:
/// - If `n > 0`, the `n`th successor of `i`.
/// - If `n < 0`, the `n`th predecessor of `i`.
/// - Otherwise, `i` unmodified.
///
/// - Precondition: `n >= 0` unless `Self` conforms to
/// `BidirectionalCollection`.
/// - Precondition:
/// - If `n > 0`, `n <= self.distance(from: i, to: self.endIndex)`
/// - If `n < 0`, `n >= self.distance(from: i, to: self.startIndex)`
///
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`abs(n)`) otherwise.
@warn_unused_result
func index(_ i: Index, offsetBy n: IndexDistance) -> Index
/// Returns the result of advancing `i` by `n` positions, or `nil`
/// if doing so would pass `limit`.
///
/// - Returns:
/// - `nil` if `(limit > i) == (n > 0) && abs(distance(i, limit)) < abs(n)`
/// - Otherwise, `index(i, offsetBy: n)`
///
/// - Precondition: `n >= 0` unless `Self` conforms to
/// `BidirectionalCollection`.
///
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`abs(n)`) otherwise.
@warn_unused_result
func index(
_ i: Index, offsetBy n: IndexDistance, limitedBy limit: Index
) -> Index?
/// Advances `i` by `n` positions.
///
/// - Precondition: `n >= 0` unless `Self` conforms to
/// `BidirectionalCollection`.
/// - Precondition:
/// - If `n > 0`, `n <= self.distance(from: i, to: self.endIndex)`
/// - If `n < 0`, `n >= self.distance(from: i, to: self.startIndex)`
///
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`abs(n)`) otherwise.
func formIndex(_ i: inout Index, offsetBy n: IndexDistance)
/// Advances `i` by `n` positions, or until it equals `limit`.
///
/// - Returns `true` if index has been advanced by exactly `n` steps without
/// passing the `limit`, and `false` otherwise.
///
/// - Precondition: `n >= 0` unless `Self` conforms to
/// `BidirectionalCollection`.
///
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`abs(n)`) otherwise.
func formIndex(
_ i: inout Index, offsetBy n: IndexDistance, limitedBy limit: Index
) -> Bool
/// Returns the distance between `start` and `end`.
///
/// - Precondition: `start <= end` unless `Self` conforms to
/// `BidirectionalCollection`.
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`n`) otherwise, where `n` is the method's result.
@warn_unused_result
func distance(from start: Index, to end: Index) -> IndexDistance
}
/// A type that iterates over a collection using its indices.
///
/// The `IndexingIterator` type is the default iterator for any collection that
/// doesn't declare its own. It acts as an iterator by using a collection's
/// indices to step over each value in the collection. Most collections in the
/// standard library use `IndexingIterator` as their iterator.
///
/// By default, any custom collection type you create will inherit a
/// `makeIterator()` method that returns an `IndexingIterator` instance,
/// making it unnecessary to declare your own. When creating a custom
/// collection type, add the minimal requirements of the `Collection`
/// protocol: starting and ending indices and a subscript for accessing
/// elements. With those elements defined, the inherited `makeIterator()`
/// method satisfies the requirements of the `Sequence` protocol.
///
/// Here's an example of a type that declares the minimal requirements for a
/// collection. The `CollectionOfTwo` structure is a fixed-size collection
/// that always holds two elements of a specific type.
///
/// struct CollectionOfTwo<Element>: Collection {
/// let elements: (Element, Element)
///
/// init(_ first: Element, _ second: Element) {
/// self.elements = (first, second)
/// }
///
/// var startIndex: Int { return 0 }
/// var endIndex: Int { return 2 }
///
/// subscript(index: Int) -> Element {
/// switch index {
/// case 0: return elements.0
/// case 1: return elements.1
/// default: fatalError("Index out of bounds.")
/// }
/// }
/// }
///
/// The `CollectionOfTwo` type uses the default iterator type,
/// `IndexingIterator`, because it doesn't define its own `makeIterator()`
/// method or `Iterator` associated type. This example shows how a
/// `CollectionOfTwo` instance can be created holding the values of a point,
/// and then iterated over using a `for`-`in` loop.
///
/// let point = CollectionOfTwo(15.0, 20.0)
/// for element in point {
/// print(element)
/// }
/// // Prints "15.0"
/// // Prints "20.0"
public struct IndexingIterator<
Elements : IndexableBase
// FIXME(compiler limitation):
// Elements : Collection
> : IteratorProtocol, Sequence {
/// Creates an iterator over the given collection.
public /// @testable
init(_elements: Elements) {
self._elements = _elements
self._position = _elements.startIndex
}
/// Advances to the next element and returns it, or `nil` if no next element
/// exists.
///
/// Repeatedly calling this method returns all the elements of the underlying
/// sequence in order. As soon as the sequence has run out of elements, the
/// `next()` method returns `nil`.
///
/// You must not call this method if it has previously returned `nil`.
///
/// This example shows how an iterator can be used explicitly to emulate a
/// `for`-`in` loop. First, retrieve a sequence's iterator, and then call
/// the iterator's `next()` method until it returns `nil`.
///
/// let numbers = [2, 3, 5, 7]
/// var numbersIterator = numbers.makeIterator()
///
/// while let num = numbersIterator.next() {
/// print(num)
/// }
/// // Prints "2"
/// // Prints "3"
/// // Prints "5"
/// // Prints "7"
///
/// - Returns: The next element in the underlying sequence if a next element
/// exists; otherwise, `nil`.
public mutating func next() -> Elements._Element? {
if _position == _elements.endIndex { return nil }
let element = _elements[_position]
_elements.formIndex(after: &_position)
return element
}
internal let _elements: Elements
internal var _position: Elements.Index
}
/// A sequence whose elements can be traversed multiple times,
/// nondestructively, and accessed by indexed subscript.
///
/// Collections are used extensively throughout the standard library. When
/// you use arrays, dictionaries, views of a string's contents and other
/// types, you benefit from the operations that the `Collection` protocol
/// declares and implements.
///
/// In addition to the methods that collections inherit from the `Sequence`
/// protocol, you gain access to methods that depend on accessing an element
/// at a specific position when using a collection.
///
/// For example, if you want to print only the first word in a string,
/// search for the index of the first space, and then create a subsequence up
/// to that position.
///
/// let text = "Buffalo buffalo buffalo buffalo."
/// if let firstSpace = text.characters.index(of: " ") {
/// print(String(text.characters.prefix(upTo: firstSpace)))
/// }
/// // Prints "Buffalo"
///
/// The `firstSpace` constant is an index into the `text.characters`
/// collection. `firstSpace` is the position of the first space in the
/// collection. You can store indices in variables, and pass them to
/// collection algorithms or use them later to access the corresponding
/// element. In the example above, `firstSpace` is used to extract the prefix
/// that contains elements up to that index.
///
/// You can pass only valid indices to collection operations. You can find a
/// complete set of a collection's valid indices by starting with the
/// collection's `startIndex` property and finding every successor up to, and
/// including, the `endIndex` property. All other values of the `Index` type,
/// such as the `startIndex` property of a different collection, are invalid
/// indices for this collection.
///
/// Saved indices may become invalid as a result of mutating operations; for
/// more information about index invalidation in mutable collections, see the
/// reference for the `MutableCollection` and `RangeReplaceableCollection`
/// protocols, as well as for the specific type you're using.
///
/// Accessing Individual Elements
/// =============================
///
/// You can access an element of a collection through its subscript with any
/// valid index except the collection's `endIndex` property, a "past-the-end"
/// index that does not correspond with any element of the collection.
///
/// Here's an example of accessing the first character in a string through its
/// subscript:
///
/// let firstChar = text.characters[text.characters.startIndex]
/// print(firstChar)
/// // Prints "B"
///
/// The `Collection` protocol declares and provides default implementations
/// for many operations that depend on elements being accessible by their
/// subscript. For example, you can also access the first character of
/// `text` using the `first` property, which has the value of the first
/// element of the collection, or `nil` if the collection is empty.
///
/// print(text.characters.first)
/// // Prints "Optional("B")"
///
/// Traversing a Collection
/// =======================
///
/// While a sequence may be consumed as it is traversed, a collection is
/// guaranteed to be multi-pass: Any element may be repeatedly accessed by
/// saving its index. Moreover, a collection's indices form a finite range
/// of the positions of the collection's elements. This guarantees the
/// safety of operations that depend on a sequence being finite, such as
/// checking to see whether a collection contains an element.
///
/// Iterating over the elements of a collection by their positions yields the
/// same elements in the same order as iterating over that collection using
/// its iterator. This example demonstrates that the `characters` view of a
/// string returns the same characters in the same order whether the view's
/// indices or the view itself is being iterated.
///
/// let word = "Swift"
/// for character in word.characters {
/// print(character)
/// }
/// // Prints "S"
/// // Prints "w"
/// // Prints "i"
/// // Prints "f"
/// // Prints "t"
///
/// for i in word.characters.indices {
/// print(word.characters[i])
/// }
/// // Prints "S"
/// // Prints "w"
/// // Prints "i"
/// // Prints "f"
/// // Prints "t"
///
/// Conforming to the Collection Protocol
/// =====================================
///
/// If you create a custom type that can provide repeated access to its
/// elements, conformance to the `Collection` protocol gives your
/// custom type a more useful and more efficient interface for sequence and
/// collection operations. To add conformance to your type, declare
/// `startIndex` and `endIndex` properties and a subscript that provides at
/// least read-only access to your type's elements.
///
/// Expected Performance
/// ====================
///
/// Types that conform to `Collection` are expected to provide the
/// `startIndex` and `endIndex` properties and subscript access to elements
/// as O(1) operations. Types that are not able to guarantee that expected
/// performance must document the departure, because many collection operations
/// depend on O(1) subscripting performance for their own performance
/// guarantees.
///
/// The performance of some collection operations depends on the type of index
/// that the collection provides. For example, a random-access collection,
/// which can measure the distance between two indices in O(1) time, will be
/// able to calculate its `count` property in O(1) time. Conversely, because a
/// forward or bidirectional collection must traverse the entire collection to
/// count the number of contained elements, accessing its `count` property is
/// an O(N) operation.
public protocol Collection : Indexable, Sequence {
/// A type that can represent the number of steps between pairs of
/// `Index` values where one value is reachable from the other.
///
/// Reachability is defined by the ability to produce one value from
/// the other via zero or more applications of `index(after:)`.
associatedtype IndexDistance : SignedInteger = Int
/// A type that provides the sequence's iteration interface and
/// encapsulates its iteration state.
///
/// By default, a `Collection` satisfies `Sequence` by
/// supplying a `IndexingIterator` as its associated `Iterator`
/// type.
associatedtype Iterator : IteratorProtocol = IndexingIterator<Self>
// FIXME: Needed here so that the `Iterator` is properly deduced from
// a custom `makeIterator()` function. Otherwise we get an
// `IndexingIterator`. <rdar://problem/21539115>
/// Returns an iterator over the elements of the collection.
func makeIterator() -> Iterator
/// A sequence that represents a contiguous subrange of the collection's
/// elements.
///
/// This associated type appears as a requirement in the `Sequence`
/// protocol, but it is restated here with stricter constraints. In a
/// collection, the subsequence should also conform to `Collection`.
associatedtype SubSequence : IndexableBase, Sequence = Slice<Self>
// FIXME(compiler limitation):
// associatedtype SubSequence : Collection
// where
// Iterator.Element == SubSequence.Iterator.Element,
// SubSequence.Index == Index,
// SubSequence.Indices == Indices,
// SubSequence.SubSequence == SubSequence
//
// (<rdar://problem/20715009> Implement recursive protocol
// constraints)
//
// These constraints allow processing collections in generic code by
// repeatedly slicing them in a loop.
/// Accesses the element at the specified position.
///
/// For example, access an element of an array through its subscript to
/// print its value:
///
/// var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
/// print(streets[1])
/// // Prints "Bryant"
///
/// You can subscript a collection with any valid index other than the
/// collection's end index. The end index refers to the position one
/// past the last element of a collection, so it doesn't correspond with an
/// element.
///
/// - Parameter position: The position of the element to access. `position`
/// must be a valid index of the collection that is not equal to the
/// `endIndex` property.
subscript(position: Index) -> Iterator.Element { get }
/// Accesses a contiguous subrange of the collection's elements.
///
/// The accessed slice uses the same indices for the same elements as the
/// original collection. Always use the slice's `startIndex` property
/// instead of assuming that its indices start at a particular value.
///
/// This example demonstrates getting a slice of an array of strings, finding
/// the index of one of the strings in the slice, and then using that index
/// in the original array.
///
/// let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
/// let streetsSlice = streets[2 ..< streets.endIndex]
/// print(streetsSlice)
/// // Prints "["Channing", "Douglas", "Evarts"]"
///
/// let index = streetsSlice.index(of: "Evarts") // 4
/// print(streets[index!])
/// // Prints "Evarts"
///
/// - Parameter bounds: A range of the collection's indices. The bounds of
/// the range must be valid indices of the collection.
subscript(bounds: Range<Index>) -> SubSequence { get }
/// A collection type whose elements are the indices of `self` that
/// are valid for subscripting, in ascending order.
associatedtype Indices : IndexableBase, Sequence = DefaultIndices<Self>
// FIXME(compiler limitation):
// associatedtype Indices : Collection
// where
// Indices.Iterator.Element == Index,
// Indices.Index == Index,
// Indices.SubSequence == Indices
// = DefaultIndices<Self>
/// The indices that are valid for subscripting `self`, in ascending order.
///
/// - Note: `indices` can hold a strong reference to the collection itself,
/// causing the collection to be non-uniquely referenced. If you need to
/// mutate the collection while iterating over its indices, use the
/// `index(after:)` method starting with `startIndex` to produce indices
/// instead.
///
/// ```
/// var c = [10, 20, 30, 40, 50]
/// var i = c.startIndex
/// while i != c.endIndex {
/// c[i] /= 5
/// i = c.index(after: i)
/// }
/// // c == [2, 4, 6, 8, 10]
/// ```
var indices: Indices { get }
/// Returns a subsequence from the start of the collection up to, but not
/// including, the specified position.
///
/// The resulting subsequence *does not include* the element at the
/// position `end`.
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.index(of: 40) {
/// print(numbers.prefix(upTo: i))
/// }
/// // Prints "[10, 20, 30]"
///
/// Passing the collection's starting index as the `end` parameter results in
/// an empty subsequence.
///
/// print(numbers.prefix(upTo: numbers.startIndex))
/// // Prints "[]"
///
/// - Parameter end: The "past-the-end" index of the resulting subsequence.
/// `end` must be a valid index of the collection.
/// - Returns: A subsequence up to, but not including, the `end` position.
///
/// - Complexity: O(1)
/// - SeeAlso: `prefix(through:)`
@warn_unused_result
func prefix(upTo end: Index) -> SubSequence
/// Returns a subsequence from the specified position to the end of the
/// collection.
///
/// For example:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.index(of: 40) {
/// print(numbers.suffix(from: i))
/// }
/// // Prints "[40, 50, 60]"
///
/// Passing the collection's `endIndex` as the `start` parameter results in
/// an empty subsequence.
///
/// print(numbers.suffix(from: numbers.endIndex))
/// // Prints "[]"
///
/// - Parameter start: The index at which to start the resulting subsequence.
/// `start` must be a valid index of the collection.
/// - Returns: A subsequence starting at the `start` position.
///
/// - Precondition: `start >= self.startIndex && start <= self.endIndex`
/// - Complexity: O(1)
@warn_unused_result
func suffix(from start: Index) -> SubSequence
/// Returns a subsequence from the start of the collection through the
/// specified position.
///
/// The resulting subsequence *includes* the element at the position `end`.
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.index(of: 40) {
/// print(numbers.prefix(through: i))
/// }
/// // Prints "[10, 20, 30, 40]"
///
/// - Parameter end: The index of the last element to include in the
/// resulting subsequence. `end` must be a valid index of the collection
/// that is not equal to the `endIndex` property.
/// - Returns: A subsequence up to, and including, the `end` position.
///
/// - Complexity: O(1)
/// - SeeAlso: `prefix(upTo:)`
@warn_unused_result
func prefix(through position: Index) -> SubSequence
/// A Boolean value indicating whether the collection is empty.
///
/// When you need to check whether your collection is empty, use the
/// `isEmpty` property instead of checking that the `count` property is
/// equal to zero. For collections that don't conform to
/// `RandomAccessCollection`, accessing the `count` property iterates
/// through the elements of the collection.
///
/// let horseName = "Silver"
/// if horseName.characters.isEmpty {
/// print("I've been through the desert on a horse with no name.")
/// } else {
/// print("Hi ho, \(horseName)!")
/// }
/// // Prints "Hi ho, Silver!")
///
/// - Complexity: O(1)
var isEmpty: Bool { get }
/// The number of elements in the collection.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length
/// of the collection.
var count: IndexDistance { get }
// The following requirement enables dispatching for index(of:) when
// the element type is Equatable.
/// Returns `Optional(Optional(index))` if an element was found
/// or `Optional(nil)` if an element was determined to be missing;
/// otherwise, `nil`.
///
/// - Complexity: O(N).
@warn_unused_result
func _customIndexOfEquatableElement(_ element: Iterator.Element) -> Index??
/// The first element of the collection.
///
/// If the collection is empty, the value of this property is `nil`.
///
/// let numbers = [10, 20, 30, 40, 50]
/// if let firstNumber = numbers.first {
/// print(firstNumber)
/// }
/// // Prints "10"
var first: Iterator.Element? { get }
/// Returns the result of advancing `i` by `n` positions.
///
/// - Returns:
/// - If `n > 0`, the `n`th successor of `i`.
/// - If `n < 0`, the `n`th predecessor of `i`.
/// - Otherwise, `i` unmodified.
///
/// - Precondition: `n >= 0` unless `Self` conforms to
/// `BidirectionalCollection`.
/// - Precondition:
/// - If `n > 0`, `n <= self.distance(from: i, to: self.endIndex)`
/// - If `n < 0`, `n >= self.distance(from: i, to: self.startIndex)`
///
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`abs(n)`) otherwise.
@warn_unused_result
func index(_ i: Index, offsetBy n: IndexDistance) -> Index
// FIXME: swift-3-indexing-model: Should this mention preconditions on `n`?
/// Returns the result of advancing `i` by `n` positions, or `nil`
/// if doing so would pass `limit`.
///
/// - Returns:
/// - `nil` if `(limit > i) == (n > 0) && abs(distance(i, limit)) < abs(n)`
/// - Otherwise, `index(i, offsetBy: n)`
///
/// - Precondition: `n >= 0` unless `Self` conforms to
/// `BidirectionalCollection`.
///
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`abs(n)`) otherwise.
@warn_unused_result
func index(
_ i: Index, offsetBy n: IndexDistance, limitedBy limit: Index
) -> Index?
/// Returns the distance between `start` and `end`.
///
/// - Precondition: `start <= end` unless `Self` conforms to
/// `BidirectionalCollection`.
/// - Complexity:
/// - O(1) if `Self` conforms to `RandomAccessCollection`.
/// - O(`n`) otherwise, where `n` is the method's result.
@warn_unused_result
func distance(from start: Index, to end: Index) -> IndexDistance
}
/// Default implementation for forward collections.
extension Indexable {
@inline(__always)
public func formIndex(after i: inout Index) {
// FIXME: swift-3-indexing-model: tests.
i = index(after: i)
}
public func _failEarlyRangeCheck(_ index: Index, bounds: Range<Index>) {
// FIXME: swift-3-indexing-model: tests.
_precondition(
bounds.lowerBound <= index,
"out of bounds: index < startIndex")
_precondition(
index < bounds.upperBound,
"out of bounds: index >= endIndex")
}
public func _failEarlyRangeCheck(_ range: Range<Index>, bounds: Range<Index>) {
// FIXME: swift-3-indexing-model: tests.
_precondition(
bounds.lowerBound <= range.lowerBound,
"out of bounds: range begins before startIndex")
_precondition(
range.lowerBound <= bounds.upperBound,
"out of bounds: range ends after endIndex")
_precondition(
bounds.lowerBound <= range.upperBound,
"out of bounds: range ends before bounds.lowerBound")
_precondition(
range.upperBound <= bounds.upperBound,
"out of bounds: range begins after bounds.upperBound")
}
@warn_unused_result
public func index(_ i: Index, offsetBy n: IndexDistance) -> Index {
// FIXME: swift-3-indexing-model: tests.
return self._advanceForward(i, by: n)
}
@warn_unused_result
public func index(
_ i: Index, offsetBy n: IndexDistance, limitedBy limit: Index
) -> Index? {
// FIXME: swift-3-indexing-model: tests.
return self._advanceForward(i, by: n, limitedBy: limit)
}
public func formIndex(_ i: inout Index, offsetBy n: IndexDistance) {
i = index(i, offsetBy: n)
}
public func formIndex(
_ i: inout Index, offsetBy n: IndexDistance, limitedBy limit: Index
) -> Bool {
if let advancedIndex = index(i, offsetBy: n, limitedBy: limit) {
i = advancedIndex
return true
}
i = limit
return false
}
@warn_unused_result
public func distance(from start: Index, to end: Index) -> IndexDistance {
// FIXME: swift-3-indexing-model: tests.
_precondition(start <= end,
"Only BidirectionalCollections can have end come before start")
var start = start
var count: IndexDistance = 0
while start != end {
count = count + 1
formIndex(after: &start)
}
return count
}
/// Do not use this method directly; call advanced(by: n) instead.
@inline(__always)
@warn_unused_result
internal func _advanceForward(_ i: Index, by n: IndexDistance) -> Index {
_precondition(n >= 0,
"Only BidirectionalCollections can be advanced by a negative amount")
var i = i
for _ in stride(from: 0, to: n, by: 1) {
formIndex(after: &i)
}
return i
}
/// Do not use this method directly; call advanced(by: n, limit) instead.
@inline(__always)
@warn_unused_result
internal
func _advanceForward(
_ i: Index, by n: IndexDistance, limitedBy limit: Index
) -> Index? {
_precondition(n >= 0,
"Only BidirectionalCollections can be advanced by a negative amount")
var i = i
for _ in stride(from: 0, to: n, by: 1) {
if i == limit {
return nil
}
formIndex(after: &i)
}
return i
}
}
/// Supply the default `makeIterator()` method for `Collection` models
/// that accept the default associated `Iterator`,
/// `IndexingIterator<Self>`.
extension Collection where Iterator == IndexingIterator<Self> {
/// Returns an iterator over the elements of the collection.
public func makeIterator() -> IndexingIterator<Self> {
return IndexingIterator(_elements: self)
}
}
/// Supply the default "slicing" `subscript` for `Collection` models
/// that accept the default associated `SubSequence`, `Slice<Self>`.
extension Collection where SubSequence == Slice<Self> {
/// Accesses a contiguous subrange of the collection's elements.
///
/// The accessed slice uses the same indices for the same elements as the
/// original collection. Always use the slice's `startIndex` property
/// instead of assuming that its indices start at a particular value.
///
/// This example demonstrates getting a slice of an array of strings, finding
/// the index of one of the strings in the slice, and then using that index
/// in the original array.
///
/// let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
/// let streetsSlice = streets[2 ..< streets.endIndex]
/// print(streetsSlice)
/// // Prints "["Channing", "Douglas", "Evarts"]"
///
/// let index = streetsSlice.index(of: "Evarts") // 4
/// print(streets[index!])
/// // Prints "Evarts"
///
/// - Parameter bounds: A range of the collection's indices. The bounds of
/// the range must be valid indices of the collection.
public subscript(bounds: Range<Index>) -> Slice<Self> {
_failEarlyRangeCheck(bounds, bounds: startIndex..<endIndex)
return Slice(base: self, bounds: bounds)
}
}
// TODO: swift-3-indexing-model - review the following
extension Collection where SubSequence == Self {
/// Removes and returns the first element of the collection.
///
/// - Returns: The first element of the collection if the collection is
/// not empty; otherwise, `nil`.
///
/// - Complexity: O(1)
@warn_unused_result
public mutating func popFirst() -> Iterator.Element? {
guard !isEmpty else { return nil }
let element = first!
self = self[index(after: startIndex)..<endIndex]
return element
}
}
/// Default implementations of core requirements
extension Collection {
/// A Boolean value indicating whether the collection is empty.
///
/// When you need to check whether your collection is empty, use the
/// `isEmpty` property instead of checking that the `count` property is
/// equal to zero. For collections that don't conform to
/// `RandomAccessCollection`, accessing the `count` property iterates
/// through the elements of the collection.
///
/// let horseName = "Silver"
/// if horseName.characters.isEmpty {
/// print("I've been through the desert on a horse with no name.")
/// } else {
/// print("Hi ho, \(horseName)!")
/// }
/// // Prints "Hi ho, Silver!")
///
/// - Complexity: O(1)
public var isEmpty: Bool {
return startIndex == endIndex
}
/// The first element of the collection.
///
/// If the collection is empty, the value of this property is `nil`.
///
/// let numbers = [10, 20, 30, 40, 50]
/// if let firstNumber = numbers.first {
/// print(firstNumber)
/// }
/// // Prints "10"
public var first: Iterator.Element? {
// NB: Accessing `startIndex` may not be O(1) for some lazy collections,
// so instead of testing `isEmpty` and then returning the first element,
// we'll just rely on the fact that the iterator always yields the
// first element first.
var i = makeIterator()
return i.next()
}
// TODO: swift-3-indexing-model - uncomment and replace above ready (or should we still use the iterator one?)
/// Returns the first element of `self`, or `nil` if `self` is empty.
///
/// - Complexity: O(1)
// public var first: Iterator.Element? {
// return isEmpty ? nil : self[startIndex]
// }
// TODO: swift-3-indexing-model - review the following
/// A value less than or equal to the number of elements in the collection.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length
/// of the collection.
public var underestimatedCount: Int {
return numericCast(count)
}
/// The number of elements in the collection.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length
/// of the collection.
public var count: IndexDistance {
return distance(from: startIndex, to: endIndex)
}
// TODO: swift-3-indexing-model - rename the following to _customIndexOfEquatable(element)?
/// Customization point for `Sequence.index(of:)`.
///
/// Define this method if the collection can find an element in less than
/// O(N) by exploiting collection-specific knowledge.
///
/// - Returns: `nil` if a linear search should be attempted instead,
/// `Optional(nil)` if the element was not found, or
/// `Optional(Optional(index))` if an element was found.
///
/// - Complexity: O(`count`).
@warn_unused_result
public // dispatching
func _customIndexOfEquatableElement(_: Iterator.Element) -> Index?? {
return nil
}
}