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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2018 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 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.")
/// }
/// }
///
/// func index(after i: Int) -> Int {
/// precondition(i < endIndex, "Can't advance beyond endIndex")
/// return i + 1
/// }
/// }
///
/// Because `CollectionOfTwo` doesn't define its own `makeIterator()`
/// method or `Iterator` associated type, it uses the default iterator type,
/// `IndexingIterator`. 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"
@_fixed_layout
public struct IndexingIterator<Elements : Collection> {
@usableFromInline
internal let _elements: Elements
@usableFromInline
internal var _position: Elements.Index
@inlinable
@inline(__always)
/// Creates an iterator over the given collection.
public /// @testable
init(_elements: Elements) {
self._elements = _elements
self._position = _elements.startIndex
}
@inlinable
@inline(__always)
/// Creates an iterator over the given collection.
public /// @testable
init(_elements: Elements, _position: Elements.Index) {
self._elements = _elements
self._position = _position
}
}
extension IndexingIterator: IteratorProtocol, Sequence {
public typealias Element = Elements.Element
public typealias Iterator = IndexingIterator<Elements>
public typealias SubSequence = AnySequence<Element>
/// 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, all
/// subsequent calls return `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`.
@inlinable
@inline(__always)
public mutating func next() -> Elements.Element? {
if _position == _elements.endIndex { return nil }
let element = _elements[_position]
_elements.formIndex(after: &_position)
return element
}
}
/// A sequence whose elements can be traversed multiple times,
/// nondestructively, and accessed by an indexed subscript.
///
/// Collections are used extensively throughout the standard library. When you
/// use arrays, dictionaries, and other collections, you benefit from the
/// operations that the `Collection` protocol declares and implements. In
/// addition to the operations that collections inherit from the `Sequence`
/// protocol, you gain access to methods that depend on accessing an element
/// at a specific position in a collection.
///
/// For example, if you want to print only the first word in a string, you can
/// search for the index of the first space, and then create a substring up to
/// that position.
///
/// let text = "Buffalo buffalo buffalo buffalo."
/// if let firstSpace = text.firstIndex(of: " ") {
/// print(text[..<firstSpace])
/// }
/// // Prints "Buffalo"
///
/// The `firstSpace` constant is an index into the `text` string---the position
/// of the first space in the string. 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.
///
/// Accessing Individual Elements
/// =============================
///
/// You can access an element of a collection through its subscript by using
/// any valid index except the collection's `endIndex` property. This property
/// is 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[text.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.first)
/// // Prints "Optional("B")"
///
/// 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 Slices of a Collection
/// ================================
///
/// You can access a slice of a collection through its ranged subscript or by
/// calling methods like `prefix(while:)` or `suffix(_:)`. A slice of a
/// collection can contain zero or more of the original collection's elements
/// and shares the original collection's semantics.
///
/// The following example creates a `firstWord` constant by using the
/// `prefix(while:)` method to get a slice of the `text` string.
///
/// let firstWord = text.prefix(while: { $0 != " " })
/// print(firstWord)
/// // Prints "Buffalo"
///
/// You can retrieve the same slice using the string's ranged subscript, which
/// takes a range expression.
///
/// if let firstSpace = text.firstIndex(of: " ") {
/// print(text[..<firstSpace]
/// // Prints "Buffalo"
/// }
///
/// The retrieved slice of `text` is equivalent in each of these cases.
///
/// Slices Share Indices
/// --------------------
///
/// A collection and its slices share the same indices. An element of a
/// collection is located under the same index in a slice as in the base
/// collection, as long as neither the collection nor the slice has been
/// mutated since the slice was created.
///
/// For example, suppose you have an array holding the number of absences from
/// each class during a session.
///
/// var absences = [0, 2, 0, 4, 0, 3, 1, 0]
///
/// You're tasked with finding the day with the most absences in the second
/// half of the session. To find the index of the day in question, follow
/// these steps:
///
/// 1) Create a slice of the `absences` array that holds the second half of the
/// days.
/// 2) Use the `max(by:)` method to determine the index of the day with the
/// most absences.
/// 3) Print the result using the index found in step 2 on the original
/// `absences` array.
///
/// Here's an implementation of those steps:
///
/// let secondHalf = absences.suffix(absences.count / 2)
/// if let i = secondHalf.indices.max(by: { secondHalf[$0] < secondHalf[$1] }) {
/// print("Highest second-half absences: \(absences[i])")
/// }
/// // Prints "Highest second-half absences: 3"
///
/// Slices Inherit Collection Semantics
/// -----------------------------------
///
/// A slice inherits the value or reference semantics of its base collection.
/// That is, when working with a slice of a mutable collection that has value
/// semantics, such as an array, mutating the original collection triggers a
/// copy of that collection and does not affect the contents of the slice.
///
/// For example, if you update the last element of the `absences` array from
/// `0` to `2`, the `secondHalf` slice is unchanged.
///
/// absences[7] = 2
/// print(absences)
/// // Prints "[0, 2, 0, 4, 0, 3, 1, 2]"
/// print(secondHalf)
/// // Prints "[0, 3, 1, 0]"
///
/// Traversing a Collection
/// =======================
///
/// Although a sequence can be consumed as it is traversed, a collection is
/// guaranteed to be *multipass*: Any element can be repeatedly accessed by
/// saving its index. Moreover, a collection's indices form a finite range of
/// the positions of the collection's elements. The fact that all collections
/// are finite guarantees the safety of many sequence operations, such as
/// using the `contains(_:)` method to test whether a collection includes 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 {
/// print(character)
/// }
/// // Prints "S"
/// // Prints "w"
/// // Prints "i"
/// // Prints "f"
/// // Prints "t"
///
/// for i in word.indices {
/// print(word[i])
/// }
/// // Prints "S"
/// // Prints "w"
/// // Prints "i"
/// // Prints "f"
/// // Prints "t"
///
/// Conforming to the Collection Protocol
/// =====================================
///
/// If you create a custom sequence that can provide repeated access to its
/// elements, make sure that its type conforms to the `Collection` protocol in
/// order to give a more useful and more efficient interface for sequence and
/// collection operations. To add `Collection` conformance to your type, you
/// must declare at least the following requirements:
///
/// - The `startIndex` and `endIndex` properties
/// - A subscript that provides at least read-only access to your type's
/// elements
/// - The `index(after:)` method for advancing an index into your collection
///
/// 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 this 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, can
/// 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: Sequence where SubSequence: Collection {
// FIXME: ideally this would be in MigrationSupport.swift, but it needs
// to be on the protocol instead of as an extension
@available(*, deprecated/*, obsoleted: 5.0*/, message: "all index distances are now of type Int")
typealias IndexDistance = Int
// FIXME(ABI): Associated type inference requires this.
associatedtype Element
/// 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.
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---that is, the position 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.firstIndex(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 }
/// A type that provides the collection's iteration interface and
/// encapsulates its iteration state.
///
/// By default, a collection conforms to the `Sequence` protocol by
/// supplying `IndexingIterator` as its associated `Iterator`
/// type.
associatedtype Iterator = IndexingIterator<Self>
// FIXME(ABI)#179 (Type checker): 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 = Slice<Self> where SubSequence.Index == Index
/// Accesses the element at the specified position.
///
/// The following example accesses 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.
///
/// - Complexity: O(1)
subscript(position: Index) -> Element { get }
/// Accesses a contiguous subrange of the collection's elements.
///
/// For example, using a `PartialRangeFrom` range expression with an array
/// accesses the subrange from the start of the range expression until the
/// end of the array.
///
/// let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
/// let streetsSlice = streets[2..<5]
/// print(streetsSlice)
/// // ["Channing", "Douglas", "Evarts"]
///
/// The accessed slice uses the same indices for the same elements as the
/// original collection. This example searches `streetsSlice` for one of the
/// strings in the slice, and then uses that index in the original array.
///
/// let index = streetsSlice.firstIndex(of: "Evarts")! // 4
/// print(streets[index])
/// // "Evarts"
///
/// Always use the slice's `startIndex` property instead of assuming that its
/// indices start at a particular value. Attempting to access an element by
/// using an index outside the bounds of the slice may result in a runtime
/// error, even if that index is valid for the original collection.
///
/// print(streetsSlice.startIndex)
/// // 2
/// print(streetsSlice[2])
/// // "Channing"
///
/// print(streetsSlice[0])
/// // error: Index out of bounds
///
/// - Parameter bounds: A range of the collection's indices. The bounds of
/// the range must be valid indices of the collection.
///
/// - Complexity: O(1)
subscript(bounds: Range<Index>) -> SubSequence { get }
/// A type that represents the indices that are valid for subscripting the
/// collection, in ascending order.
associatedtype Indices : Collection = DefaultIndices<Self>
where Indices.Element == Index,
Indices.Index == Index,
Indices.SubSequence == Indices
/// The indices that are valid for subscripting the collection, in ascending
/// order.
///
/// A collection's `indices` property can hold a strong reference to the
/// collection itself, causing the collection to be nonuniquely referenced.
/// If you mutate the collection while iterating over its indices, a strong
/// reference can result in an unexpected copy of the collection. To avoid
/// the unexpected copy, use the `index(after:)` method starting with
/// `startIndex` to produce indices instead.
///
/// var c = MyFancyCollection([10, 20, 30, 40, 50])
/// var i = c.startIndex
/// while i != c.endIndex {
/// c[i] /= 5
/// i = c.index(after: i)
/// }
/// // c == MyFancyCollection([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`. The following example searches for the index of the number `40`
/// in an array of integers, and then prints the prefix of the array up to,
/// but not including, that index:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.firstIndex(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 "[]"
///
/// Using the `prefix(upTo:)` method is equivalent to using a partial
/// half-open range as the collection's subscript. The subscript notation is
/// preferred over `prefix(upTo:)`.
///
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers[..<i])
/// }
/// // Prints "[10, 20, 30]"
///
/// - 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)
__consuming func prefix(upTo end: Index) -> SubSequence
/// Returns a subsequence from the specified position to the end of the
/// collection.
///
/// The following example searches for the index of the number `40` in an
/// array of integers, and then prints the suffix of the array starting at
/// that index:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.firstIndex(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 "[]"
///
/// Using the `suffix(from:)` method is equivalent to using a partial range
/// from the index as the collection's subscript. The subscript notation is
/// preferred over `suffix(from:)`.
///
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers[i...])
/// }
/// // Prints "[40, 50, 60]"
///
/// - 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.
///
/// - Complexity: O(1)
__consuming 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`.
/// The following example searches for the index of the number `40` in an
/// array of integers, and then prints the prefix of the array up to, and
/// including, that index:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers.prefix(through: i))
/// }
/// // Prints "[10, 20, 30, 40]"
///
/// Using the `prefix(through:)` method is equivalent to using a partial
/// closed range as the collection's subscript. The subscript notation is
/// preferred over `prefix(through:)`.
///
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers[...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)
__consuming 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.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.
///
/// To check whether a collection is empty, use its `isEmpty` property
/// instead of comparing `count` to zero. Unless the collection guarantees
/// random-access performance, calculating `count` can be an O(*n*)
/// operation.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length
/// of the collection.
var count: Int { get }
// The following requirements enable dispatching for firstIndex(of:) and
// lastIndex(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*), where *n* is the length of the collection.
func _customIndexOfEquatableElement(_ element: Element) -> Index??
/// Customization point for `Collection.lastIndex(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: Hopefully less than O(`count`).
func _customLastIndexOfEquatableElement(_ element: Element) -> Index??
// FIXME(move-only types): `first` might not be implementable by collections
// with move-only elements, since they would need to be able to somehow form
// a temporary `Optional<Element>` value from a nonoptional Element without
// modifying the collection.
/// 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: Element? { get }
/// Returns an index that is the specified distance from the given index.
///
/// The following example obtains an index advanced four positions from a
/// string's starting index and then prints the character at that position.
///
/// let s = "Swift"
/// let i = s.index(s.startIndex, offsetBy: 4)
/// print(s[i])
/// // Prints "t"
///
/// The value passed as `distance` must not offset `i` beyond the bounds of
/// the collection.
///
/// - Parameters:
/// - i: A valid index of the collection.
/// - distance: The distance to offset `i`. `distance` must not be negative
/// unless the collection conforms to the `BidirectionalCollection`
/// protocol.
/// - Returns: An index offset by `distance` from the index `i`. If
/// `distance` is positive, this is the same value as the result of
/// `distance` calls to `index(after:)`. If `distance` is negative, this
/// is the same value as the result of `abs(distance)` calls to
/// `index(before:)`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute
/// value of `distance`.
func index(_ i: Index, offsetBy distance: Int) -> Index
/// Returns an index that is the specified distance from the given index,
/// unless that distance is beyond a given limiting index.
///
/// The following example obtains an index advanced four positions from a
/// string's starting index and then prints the character at that position.
/// The operation doesn't require going beyond the limiting `s.endIndex`
/// value, so it succeeds.
///
/// let s = "Swift"
/// if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
/// print(s[i])
/// }
/// // Prints "t"
///
/// The next example attempts to retrieve an index six positions from
/// `s.startIndex` but fails, because that distance is beyond the index
/// passed as `limit`.
///
/// let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
/// print(j)
/// // Prints "nil"
///
/// The value passed as `distance` must not offset `i` beyond the bounds of
/// the collection, unless the index passed as `limit` prevents offsetting
/// beyond those bounds.
///
/// - Parameters:
/// - i: A valid index of the collection.
/// - distance: The distance to offset `i`. `distance` must not be negative
/// unless the collection conforms to the `BidirectionalCollection`
/// protocol.
/// - limit: A valid index of the collection to use as a limit. If
/// `distance > 0`, a limit that is less than `i` has no effect.
/// Likewise, if `distance < 0`, a limit that is greater than `i` has no
/// effect.
/// - Returns: An index offset by `distance` from the index `i`, unless that
/// index would be beyond `limit` in the direction of movement. In that
/// case, the method returns `nil`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute
/// value of `distance`.
func index(
_ i: Index, offsetBy distance: Int, limitedBy limit: Index
) -> Index?
/// Returns the distance between two indices.
///
/// Unless the collection conforms to the `BidirectionalCollection` protocol,
/// `start` must be less than or equal to `end`.
///
/// - Parameters:
/// - start: A valid index of the collection.
/// - end: Another valid index of the collection. If `end` is equal to
/// `start`, the result is zero.
/// - Returns: The distance between `start` and `end`. The result can be
/// negative only if the collection conforms to the
/// `BidirectionalCollection` protocol.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the
/// resulting distance.
func distance(from start: Index, to end: Index) -> Int
/// 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>)
func _failEarlyRangeCheck(_ index: Index, bounds: ClosedRange<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 the given index.
///
/// The successor of an index must be well defined. For an index `i` into a
/// collection `c`, calling `c.index(after: i)` returns the same index every
/// time.
///
/// - Parameter i: A valid index of the collection. `i` must be less than
/// `endIndex`.
/// - Returns: The index value immediately after `i`.
func index(after i: Index) -> Index
/// Replaces the given index with its successor.
///
/// - Parameter i: A valid index of the collection. `i` must be less than
/// `endIndex`.
func formIndex(after i: inout Index)
}
/// Default implementation for forward collections.
extension Collection {
/// Replaces the given index with its successor.
///
/// - Parameter i: A valid index of the collection. `i` must be less than
/// `endIndex`.
@inlinable // protocol-only
@inline(__always)
public func formIndex(after i: inout Index) {
i = index(after: i)
}
@inlinable
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")
}
@inlinable
public func _failEarlyRangeCheck(_ index: Index, bounds: ClosedRange<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")
}
@inlinable
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")
}
/// Returns an index that is the specified distance from the given index.
///
/// The following example obtains an index advanced four positions from a
/// string's starting index and then prints the character at that position.
///
/// let s = "Swift"
/// let i = s.index(s.startIndex, offsetBy: 4)
/// print(s[i])
/// // Prints "t"
///
/// The value passed as `distance` must not offset `i` beyond the bounds of
/// the collection.
///
/// - Parameters:
/// - i: A valid index of the collection.
/// - distance: The distance to offset `i`. `distance` must not be negative
/// unless the collection conforms to the `BidirectionalCollection`
/// protocol.
/// - Returns: An index offset by `distance` from the index `i`. If
/// `distance` is positive, this is the same value as the result of
/// `distance` calls to `index(after:)`. If `distance` is negative, this
/// is the same value as the result of `abs(distance)` calls to
/// `index(before:)`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute
/// value of `distance`.
@inlinable
public func index(_ i: Index, offsetBy distance: Int) -> Index {
return self._advanceForward(i, by: distance)
}
/// Returns an index that is the specified distance from the given index,
/// unless that distance is beyond a given limiting index.
///
/// The following example obtains an index advanced four positions from a
/// string's starting index and then prints the character at that position.
/// The operation doesn't require going beyond the limiting `s.endIndex`
/// value, so it succeeds.
///
/// let s = "Swift"
/// if let i = s.index(s.startIndex, offsetBy: 4, limitedBy: s.endIndex) {
/// print(s[i])
/// }
/// // Prints "t"
///
/// The next example attempts to retrieve an index six positions from
/// `s.startIndex` but fails, because that distance is beyond the index
/// passed as `limit`.
///
/// let j = s.index(s.startIndex, offsetBy: 6, limitedBy: s.endIndex)
/// print(j)
/// // Prints "nil"
///
/// The value passed as `distance` must not offset `i` beyond the bounds of
/// the collection, unless the index passed as `limit` prevents offsetting
/// beyond those bounds.
///
/// - Parameters:
/// - i: A valid index of the collection.
/// - distance: The distance to offset `i`. `distance` must not be negative
/// unless the collection conforms to the `BidirectionalCollection`
/// protocol.
/// - limit: A valid index of the collection to use as a limit. If
/// `distance > 0`, a limit that is less than `i` has no effect.
/// Likewise, if `distance < 0`, a limit that is greater than `i` has no
/// effect.
/// - Returns: An index offset by `distance` from the index `i`, unless that
/// index would be beyond `limit` in the direction of movement. In that
/// case, the method returns `nil`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute
/// value of `distance`.
@inlinable
public func index(
_ i: Index, offsetBy distance: Int, limitedBy limit: Index
) -> Index? {
return self._advanceForward(i, by: distance, limitedBy: limit)
}
/// Offsets the given index by the specified distance.
///
/// The value passed as `distance` must not offset `i` beyond the bounds of
/// the collection.
///
/// - Parameters:
/// - i: A valid index of the collection.
/// - distance: The distance to offset `i`. `distance` must not be negative
/// unless the collection conforms to the `BidirectionalCollection`
/// protocol.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute
/// value of `distance`.
@inlinable
public func formIndex(_ i: inout Index, offsetBy distance: Int) {
i = index(i, offsetBy: distance)
}
/// Offsets the given index by the specified distance, or so that it equals
/// the given limiting index.
///
/// The value passed as `distance` must not offset `i` beyond the bounds of
/// the collection, unless the index passed as `limit` prevents offsetting
/// beyond those bounds.
///
/// - Parameters:
/// - i: A valid index of the collection.
/// - distance: The distance to offset `i`. `distance` must not be negative
/// unless the collection conforms to the `BidirectionalCollection`
/// protocol.
/// - limit: A valid index of the collection to use as a limit. If
/// `distance > 0`, a limit that is less than `i` has no effect.
/// Likewise, if `distance < 0`, a limit that is greater than `i` has no
/// effect.
/// - Returns: `true` if `i` has been offset by exactly `distance` steps
/// without going beyond `limit`; otherwise, `false`. When the return
/// value is `false`, the value of `i` is equal to `limit`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the absolute
/// value of `distance`.
@inlinable
public func formIndex(
_ i: inout Index, offsetBy distance: Int, limitedBy limit: Index
) -> Bool {
if let advancedIndex = index(i, offsetBy: distance, limitedBy: limit) {
i = advancedIndex
return true
}
i = limit
return false
}
/// Returns the distance between two indices.
///
/// Unless the collection conforms to the `BidirectionalCollection` protocol,
/// `start` must be less than or equal to `end`.
///
/// - Parameters:
/// - start: A valid index of the collection.
/// - end: Another valid index of the collection. If `end` is equal to
/// `start`, the result is zero.
/// - Returns: The distance between `start` and `end`. The result can be
/// negative only if the collection conforms to the
/// `BidirectionalCollection` protocol.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the
/// resulting distance.
@inlinable
public func distance(from start: Index, to end: Index) -> Int {
_precondition(start <= end,
"Only BidirectionalCollections can have end come before start")
var start = start
var count = 0
while start != end {
count = count + 1
formIndex(after: &start)
}
return count
}
/// Returns a random element of the collection, using the given generator as
/// a source for randomness.
///
/// Call `randomElement(using:)` to select a random element from an array or
/// another collection when you are using a custom random number generator.
/// This example picks a name at random from an array:
///
/// let names = ["Zoey", "Chloe", "Amani", "Amaia"]
/// let randomName = names.randomElement(using: &myGenerator)!
/// // randomName == "Amani"
///
/// - Parameter generator: The random number generator to use when choosing
/// a random element.
/// - Returns: A random element from the collection. If the collection is
/// empty, the method returns `nil`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length of
/// the collection.
@inlinable
public func randomElement<T: RandomNumberGenerator>(
using generator: inout T
) -> Element? {
guard !isEmpty else { return nil }
let random = generator.next(upperBound: UInt(count))
let index = self.index(
startIndex,
offsetBy: numericCast(random)
)
return self[index]
}
/// Returns a random element of the collection.
///
/// Call `randomElement()` to select a random element from an array or
/// another collection. This example picks a name at random from an array:
///
/// let names = ["Zoey", "Chloe", "Amani", "Amaia"]
/// let randomName = names.randomElement()!
/// // randomName == "Amani"
///
/// This method is equivalent to calling the version that takes a generator,
/// passing in the system's default random generator.
///
/// - Returns: A random element from the collection. If the collection is
/// empty, the method returns `nil`.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length of
/// the collection.
@inlinable
public func randomElement() -> Element? {
var g = SystemRandomNumberGenerator()
return randomElement(using: &g)
}
/// Do not use this method directly; call advanced(by: n) instead.
@inlinable
@inline(__always)
internal func _advanceForward(_ i: Index, by n: Int) -> 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.
@inlinable
@inline(__always)
internal func _advanceForward(
_ i: Index, by n: Int, 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.
@inlinable // trivial-implementation
@inline(__always)
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.firstIndex(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.
///
/// - Complexity: O(1)
@inlinable
public subscript(bounds: Range<Index>) -> Slice<Self> {
_failEarlyRangeCheck(bounds, bounds: startIndex..<endIndex)
return Slice(base: self, bounds: bounds)
}
}
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)
@inlinable
public mutating func popFirst() -> Element? {
// TODO: swift-3-indexing-model - review the following
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.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)
@inlinable
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"
@inlinable
public var first: Element? {
@inline(__always)
get {
// 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: Element? {
// return isEmpty ? nil : self[startIndex]
// }
/// 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.
@inlinable
public var underestimatedCount: Int {
// TODO: swift-3-indexing-model - review the following
return count
}
/// The number of elements in the collection.
///
/// To check whether a collection is empty, use its `isEmpty` property
/// instead of comparing `count` to zero. Unless the collection guarantees
/// random-access performance, calculating `count` can be an O(*n*)
/// operation.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length
/// of the collection.
@inlinable
public var count: Int {
return distance(from: startIndex, to: endIndex)
}
// TODO: swift-3-indexing-model - rename the following to _customIndexOfEquatable(element)?
/// Customization point for `Collection.firstIndex(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: Hopefully less than O(`count`).
@inlinable
public // dispatching
func _customIndexOfEquatableElement(_: Element) -> Index?? {
return nil
}
/// Customization point for `Collection.lastIndex(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: Hopefully less than O(`count`).
@inlinable
public // dispatching
func _customLastIndexOfEquatableElement(_ element: Element) -> Index?? {
return nil
}
}
//===----------------------------------------------------------------------===//
// Default implementations for Collection
//===----------------------------------------------------------------------===//
extension Collection {
/// Returns an array containing the results of mapping the given closure
/// over the sequence's elements.
///
/// In this example, `map` is used first to convert the names in the array
/// to lowercase strings and then to count their characters.
///
/// let cast = ["Vivien", "Marlon", "Kim", "Karl"]
/// let lowercaseNames = cast.map { $0.lowercased() }
/// // 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
/// let letterCounts = cast.map { $0.count }
/// // 'letterCounts' == [6, 6, 3, 4]
///
/// - Parameter transform: A mapping closure. `transform` accepts an
/// element of this sequence as its parameter and returns a transformed
/// value of the same or of a different type.
/// - Returns: An array containing the transformed elements of this
/// sequence.
@inlinable
public func map<T>(
_ transform: (Element) throws -> T
) rethrows -> [T] {
// TODO: swift-3-indexing-model - review the following
let n = self.count
if n == 0 {
return []
}
var result = ContiguousArray<T>()
result.reserveCapacity(n)
var i = self.startIndex
for _ in 0..<n {
result.append(try transform(self[i]))
formIndex(after: &i)
}
_expectEnd(of: self, is: i)
return Array(result)
}
/// Returns a subsequence containing all but the given number of initial
/// elements.
///
/// If the number of elements to drop exceeds the number of elements in
/// the collection, the result is an empty subsequence.
///
/// let numbers = [1, 2, 3, 4, 5]
/// print(numbers.dropFirst(2))
/// // Prints "[3, 4, 5]"
/// print(numbers.dropFirst(10))
/// // Prints "[]"
///
/// - Parameter k: The number of elements to drop from the beginning of
/// the collection. `k` must be greater than or equal to zero.
/// - Returns: A subsequence starting after the specified number of
/// elements.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the number of
/// elements to drop from the beginning of the collection.
@inlinable
public func dropFirst(_ k: Int) -> SubSequence {
_precondition(k >= 0, "Can't drop a negative number of elements from a collection")
let start = index(startIndex,
offsetBy: k, limitedBy: endIndex) ?? endIndex
return self[start..<endIndex]
}
/// Returns a subsequence containing all but the specified number of final
/// elements.
///
/// If the number of elements to drop exceeds the number of elements in the
/// collection, the result is an empty subsequence.
///
/// let numbers = [1, 2, 3, 4, 5]
/// print(numbers.dropLast(2))
/// // Prints "[1, 2, 3]"
/// print(numbers.dropLast(10))
/// // Prints "[]"
///
/// - Parameter k: The number of elements to drop off the end of the
/// collection. `k` must be greater than or equal to zero.
/// - Returns: A subsequence that leaves off the specified number of elements
/// at the end.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length of
/// the collection.
@inlinable
public func dropLast(_ k: Int) -> SubSequence {
_precondition(
k >= 0, "Can't drop a negative number of elements from a collection")
let amount = Swift.max(0, count - k)
let end = index(startIndex,
offsetBy: amount, limitedBy: endIndex) ?? endIndex
return self[startIndex..<end]
}
/// Returns a subsequence by skipping elements while `predicate` returns
/// `true` and returning the remaining elements.
///
/// - Parameter predicate: A closure that takes an element of the
/// sequence as its argument and returns `true` if the element should
/// be skipped or `false` if it should be included. Once the predicate
/// returns `false` it will not be called again.
///
/// - Complexity: O(*n*), where *n* is the length of the collection.
@inlinable
public func drop(
while predicate: (Element) throws -> Bool
) rethrows -> SubSequence {
var start = startIndex
while try start != endIndex && predicate(self[start]) {
formIndex(after: &start)
}
return self[start..<endIndex]
}
/// Returns a subsequence, up to the specified maximum length, containing
/// the initial elements of the collection.
///
/// If the maximum length exceeds the number of elements in the collection,
/// the result contains all the elements in the collection.
///
/// let numbers = [1, 2, 3, 4, 5]
/// print(numbers.prefix(2))
/// // Prints "[1, 2]"
/// print(numbers.prefix(10))
/// // Prints "[1, 2, 3, 4, 5]"
///
/// - Parameter maxLength: The maximum number of elements to return.
/// `maxLength` must be greater than or equal to zero.
/// - Returns: A subsequence starting at the beginning of this collection
/// with at most `maxLength` elements.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the number of
/// elements to select from the beginning of the collection.
@inlinable
public func prefix(_ maxLength: Int) -> SubSequence {
_precondition(
maxLength >= 0,
"Can't take a prefix of negative length from a collection")
let end = index(startIndex,
offsetBy: maxLength, limitedBy: endIndex) ?? endIndex
return self[startIndex..<end]
}
/// Returns a subsequence containing the initial elements until `predicate`
/// returns `false` and skipping the remaining elements.
///
/// - Parameter predicate: A closure that takes an element of the
/// sequence as its argument and returns `true` if the element should
/// be included or `false` if it should be excluded. Once the predicate
/// returns `false` it will not be called again.
///
/// - Complexity: O(*n*), where *n* is the length of the collection.
@inlinable
public func prefix(
while predicate: (Element) throws -> Bool
) rethrows -> SubSequence {
var end = startIndex
while try end != endIndex && predicate(self[end]) {
formIndex(after: &end)
}
return self[startIndex..<end]
}
/// Returns a subsequence, up to the given maximum length, containing the
/// final elements of the collection.
///
/// If the maximum length exceeds the number of elements in the collection,
/// the result contains all the elements in the collection.
///
/// let numbers = [1, 2, 3, 4, 5]
/// print(numbers.suffix(2))
/// // Prints "[4, 5]"
/// print(numbers.suffix(10))
/// // Prints "[1, 2, 3, 4, 5]"
///
/// - Parameter maxLength: The maximum number of elements to return. The
/// value of `maxLength` must be greater than or equal to zero.
/// - Returns: A subsequence terminating at the end of the collection with at
/// most `maxLength` elements.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*n*), where *n* is the length of
/// the collection.
@inlinable
public func suffix(_ maxLength: Int) -> SubSequence {
_precondition(
maxLength >= 0,
"Can't take a suffix of negative length from a collection")
let amount = Swift.max(0, count - maxLength)
let start = index(startIndex,
offsetBy: amount, limitedBy: endIndex) ?? endIndex
return self[start..<endIndex]
}
/// 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`. The following example searches for the index of the number `40`
/// in an array of integers, and then prints the prefix of the array up to,
/// but not including, that index:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.firstIndex(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 "[]"
///
/// Using the `prefix(upTo:)` method is equivalent to using a partial
/// half-open range as the collection's subscript. The subscript notation is
/// preferred over `prefix(upTo:)`.
///
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers[..<i])
/// }
/// // Prints "[10, 20, 30]"
///
/// - 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)
@inlinable
public func prefix(upTo end: Index) -> SubSequence {
return self[startIndex..<end]
}
/// Returns a subsequence from the specified position to the end of the
/// collection.
///
/// The following example searches for the index of the number `40` in an
/// array of integers, and then prints the suffix of the array starting at
/// that index:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.firstIndex(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 "[]"
///
/// Using the `suffix(from:)` method is equivalent to using a partial range
/// from the index as the collection's subscript. The subscript notation is
/// preferred over `suffix(from:)`.
///
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers[i...])
/// }
/// // Prints "[40, 50, 60]"
///
/// - 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.
///
/// - Complexity: O(1)
@inlinable
public func suffix(from start: Index) -> SubSequence {
return self[start..<endIndex]
}
/// Returns a subsequence from the start of the collection through the
/// specified position.
///
/// The resulting subsequence *includes* the element at the position `end`.
/// The following example searches for the index of the number `40` in an
/// array of integers, and then prints the prefix of the array up to, and
/// including, that index:
///
/// let numbers = [10, 20, 30, 40, 50, 60]
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers.prefix(through: i))
/// }
/// // Prints "[10, 20, 30, 40]"
///
/// Using the `prefix(through:)` method is equivalent to using a partial
/// closed range as the collection's subscript. The subscript notation is
/// preferred over `prefix(through:)`.
///
/// if let i = numbers.firstIndex(of: 40) {
/// print(numbers[...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)
@inlinable
public func prefix(through position: Index) -> SubSequence {
return prefix(upTo: index(after: position))
}
/// Returns the longest possible subsequences of the collection, in order,
/// that don't contain elements satisfying the given predicate.
///
/// The resulting array consists of at most `maxSplits + 1` subsequences.
/// Elements that are used to split the sequence are not returned as part of
/// any subsequence.
///
/// The following examples show the effects of the `maxSplits` and
/// `omittingEmptySubsequences` parameters when splitting a string using a
/// closure that matches spaces. The first use of `split` returns each word
/// that was originally separated by one or more spaces.
///
/// let line = "BLANCHE: I don't want realism. I want magic!"
/// print(line.split(whereSeparator: { $0 == " " }))
/// // Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
///
/// The second example passes `1` for the `maxSplits` parameter, so the
/// original string is split just once, into two new strings.
///
/// print(line.split(maxSplits: 1, whereSeparator: { $0 == " " }))
/// // Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
///
/// The final example passes `false` for the `omittingEmptySubsequences`
/// parameter, so the returned array contains empty strings where spaces
/// were repeated.
///
/// print(line.split(omittingEmptySubsequences: false, whereSeparator: { $0 == " " }))
/// // Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
///
/// - Parameters:
/// - maxSplits: The maximum number of times to split the collection, or
/// one less than the number of subsequences to return. If
/// `maxSplits + 1` subsequences are returned, the last one is a suffix
/// of the original collection containing the remaining elements.
/// `maxSplits` must be greater than or equal to zero. The default value
/// is `Int.max`.
/// - omittingEmptySubsequences: If `false`, an empty subsequence is
/// returned in the result for each pair of consecutive elements
/// satisfying the `isSeparator` predicate and for each element at the
/// start or end of the collection satisfying the `isSeparator`
/// predicate. The default value is `true`.
/// - isSeparator: A closure that takes an element as an argument and
/// returns a Boolean value indicating whether the collection should be
/// split at that element.
/// - Returns: An array of subsequences, split from this collection's
/// elements.
///
/// - Complexity: O(*n*), where *n* is the length of the collection.
@inlinable
public func split(
maxSplits: Int = Int.max,
omittingEmptySubsequences: Bool = true,
whereSeparator isSeparator: (Element) throws -> Bool
) rethrows -> [SubSequence] {
// TODO: swift-3-indexing-model - review the following
_precondition(maxSplits >= 0, "Must take zero or more splits")
var result: [SubSequence] = []
var subSequenceStart: Index = startIndex
func appendSubsequence(end: Index) -> Bool {
if subSequenceStart == end && omittingEmptySubsequences {
return false
}
result.append(self[subSequenceStart..<end])
return true
}
if maxSplits == 0 || isEmpty {
_ = appendSubsequence(end: endIndex)
return result
}
var subSequenceEnd = subSequenceStart
let cachedEndIndex = endIndex
while subSequenceEnd != cachedEndIndex {
if try isSeparator(self[subSequenceEnd]) {
let didAppend = appendSubsequence(end: subSequenceEnd)
formIndex(after: &subSequenceEnd)
subSequenceStart = subSequenceEnd
if didAppend && result.count == maxSplits {
break
}
continue
}
formIndex(after: &subSequenceEnd)
}
if subSequenceStart != cachedEndIndex || !omittingEmptySubsequences {
result.append(self[subSequenceStart..<cachedEndIndex])
}
return result
}
}
extension Collection where Element : Equatable {
/// Returns the longest possible subsequences of the collection, in order,
/// around elements equal to the given element.
///
/// The resulting array consists of at most `maxSplits + 1` subsequences.
/// Elements that are used to split the collection are not returned as part
/// of any subsequence.
///
/// The following examples show the effects of the `maxSplits` and
/// `omittingEmptySubsequences` parameters when splitting a string at each
/// space character (" "). The first use of `split` returns each word that
/// was originally separated by one or more spaces.
///
/// let line = "BLANCHE: I don't want realism. I want magic!"
/// print(line.split(separator: " "))
/// // Prints "["BLANCHE:", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
///
/// The second example passes `1` for the `maxSplits` parameter, so the
/// original string is split just once, into two new strings.
///
/// print(line.split(separator: " ", maxSplits: 1))
/// // Prints "["BLANCHE:", " I don\'t want realism. I want magic!"]"
///
/// The final example passes `false` for the `omittingEmptySubsequences`
/// parameter, so the returned array contains empty strings where spaces
/// were repeated.
///
/// print(line.split(separator: " ", omittingEmptySubsequences: false))
/// // Prints "["BLANCHE:", "", "", "I", "don\'t", "want", "realism.", "I", "want", "magic!"]"
///
/// - Parameters:
/// - separator: The element that should be split upon.
/// - maxSplits: The maximum number of times to split the collection, or
/// one less than the number of subsequences to return. If
/// `maxSplits + 1` subsequences are returned, the last one is a suffix
/// of the original collection containing the remaining elements.
/// `maxSplits` must be greater than or equal to zero. The default value
/// is `Int.max`.
/// - omittingEmptySubsequences: If `false`, an empty subsequence is
/// returned in the result for each consecutive pair of `separator`
/// elements in the collection and for each instance of `separator` at
/// the start or end of the collection. If `true`, only nonempty
/// subsequences are returned. The default value is `true`.
/// - Returns: An array of subsequences, split from this collection's
/// elements.
///
/// - Complexity: O(*n*), where *n* is the length of the collection.
@inlinable
public func split(
separator: Element,
maxSplits: Int = Int.max,
omittingEmptySubsequences: Bool = true
) -> [SubSequence] {
// TODO: swift-3-indexing-model - review the following
return split(
maxSplits: maxSplits,
omittingEmptySubsequences: omittingEmptySubsequences,
whereSeparator: { $0 == separator })
}
}
extension Collection where SubSequence == Self {
/// Removes and returns the first element of the collection.
///
/// The collection must not be empty.
///
/// - Returns: The first element of the collection.
///
/// - Complexity: O(1)
@inlinable
@discardableResult
public mutating func removeFirst() -> Element {
// TODO: swift-3-indexing-model - review the following
_precondition(!isEmpty, "Can't remove items from an empty collection")
let element = first!
self = self[index(after: startIndex)..<endIndex]
return element
}
/// Removes the specified number of elements from the beginning of the
/// collection.
///
/// - Parameter k: The number of elements to remove. `k` must be greater than
/// or equal to zero, and must be less than or equal to the number of
/// elements in the collection.
///
/// - Complexity: O(1) if the collection conforms to
/// `RandomAccessCollection`; otherwise, O(*k*), where *k* is the specified
/// number of elements.
@inlinable
public mutating func removeFirst(_ k: Int) {
if k == 0 { return }
_precondition(k >= 0, "Number of elements to remove should be non-negative")
_precondition(count >= k,
"Can't remove more items from a collection than it contains")
self = self[index(startIndex, offsetBy: k)..<endIndex]
}
}
extension Collection {
@inlinable
public func _preprocessingPass<R>(
_ preprocess: () throws -> R
) rethrows -> R? {
return try preprocess()
}
}