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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2021 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
//
//===----------------------------------------------------------------------===//
import Swift
@available(SwiftStdlib 5.1, *)
extension AsyncSequence {
/// Omits a specified number of elements from the base asynchronous sequence,
/// then passes through all remaining elements.
///
/// Use `dropFirst(_:)` when you want to drop the first *n* elements from the
/// base sequence and pass through the remaining elements.
///
/// In this example, an asynchronous sequence called `Counter` produces `Int`
/// values from `1` to `10`. The `dropFirst(_:)` method causes the modified
/// sequence to ignore the values `1` through `3`, and instead emit `4` through `10`:
///
/// for await number in Counter(howHigh: 10).dropFirst(3) {
/// print(number, terminator: " ")
/// }
/// // Prints "4 5 6 7 8 9 10 "
///
/// If the number of elements to drop exceeds the number of elements in the
/// sequence, the result is an empty sequence.
///
/// - Parameter count: The number of elements to drop from the beginning of
/// the sequence. `count` must be greater than or equal to zero.
/// - Returns: An asynchronous sequence that drops the first `count`
/// elements from the base sequence.
@inlinable
public __consuming func dropFirst(
_ count: Int = 1
) -> AsyncDropFirstSequence<Self> {
precondition(count >= 0,
"Can't drop a negative number of elements from an async sequence")
return AsyncDropFirstSequence(self, dropping: count)
}
}
/// An asynchronous sequence which omits a specified number of elements from the
/// base asynchronous sequence, then passes through all remaining elements.
@available(SwiftStdlib 5.1, *)
public struct AsyncDropFirstSequence<Base: AsyncSequence> {
@usableFromInline
let base: Base
@usableFromInline
let count: Int
@usableFromInline
init(_ base: Base, dropping count: Int) {
self.base = base
self.count = count
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncDropFirstSequence: AsyncSequence {
/// The type of element produced by this asynchronous sequence.
///
/// The drop-first sequence produces whatever type of element its base
/// iterator produces.
public typealias Element = Base.Element
/// The type of errors produced by this asynchronous sequence.
///
/// The drop-first sequence produces whatever type of error its base
/// sequence produces.
@available(SwiftStdlib 6.0, *)
public typealias Failure = Base.Failure
/// The type of iterator that produces elements of the sequence.
public typealias AsyncIterator = Iterator
/// The iterator that produces elements of the drop-first sequence.
public struct Iterator: AsyncIteratorProtocol {
// FIXME: Remove when $AssociatedTypeImplements is no longer needed
@available(SwiftStdlib 6.0, *)
public typealias Failure = Base.Failure
@usableFromInline
var baseIterator: Base.AsyncIterator
@usableFromInline
var count: Int
@usableFromInline
init(_ baseIterator: Base.AsyncIterator, count: Int) {
self.baseIterator = baseIterator
self.count = count
}
/// Produces the next element in the drop-first sequence.
///
/// Until reaching the number of elements to drop, this iterator calls
/// `next()` on its base iterator and discards the result. If the base
/// iterator returns `nil`, indicating the end of the sequence, this
/// iterator returns `nil`. After reaching the number of elements to
/// drop, this iterator passes along the result of calling `next()` on
/// the base iterator.
@inlinable
public mutating func next() async rethrows -> Base.Element? {
var remainingToDrop = count
while remainingToDrop > 0 {
guard try await baseIterator.next() != nil else {
count = 0
return nil
}
remainingToDrop -= 1
}
count = 0
return try await baseIterator.next()
}
/// Produces the next element in the drop-first sequence.
///
/// Until reaching the number of elements to drop, this iterator calls
/// `next(isolation:)` on its base iterator and discards the result. If the
/// base iterator returns `nil`, indicating the end of the sequence, this
/// iterator returns `nil`. After reaching the number of elements to drop,
/// this iterator passes along the result of calling `next(isolation:)` on
/// the base iterator.
@available(SwiftStdlib 6.0, *)
@inlinable
public mutating func next(isolation actor: isolated (any Actor)?) async throws(Failure) -> Base.Element? {
var remainingToDrop = count
while remainingToDrop > 0 {
guard try await baseIterator.next(isolation: actor) != nil else {
count = 0
return nil
}
remainingToDrop -= 1
}
count = 0
return try await baseIterator.next(isolation: actor)
}
}
@inlinable
public __consuming func makeAsyncIterator() -> Iterator {
return Iterator(base.makeAsyncIterator(), count: count)
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncDropFirstSequence {
/// Omits a specified number of elements from the base asynchronous sequence,
/// then passes through all remaining elements.
///
/// When you call `dropFirst(_:)` on an asynchronous sequence that is already
/// an `AsyncDropFirstSequence`, the returned sequence simply adds the new
/// drop count to the current drop count.
@inlinable
public __consuming func dropFirst(
_ count: Int = 1
) -> AsyncDropFirstSequence<Base> {
// If this is already a AsyncDropFirstSequence, we can just sum the current
// drop count and additional drop count.
precondition(count >= 0,
"Can't drop a negative number of elements from an async sequence")
return AsyncDropFirstSequence(base, dropping: self.count + count)
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncDropFirstSequence: Sendable
where Base: Sendable,
Base.Element: Sendable { }
@available(SwiftStdlib 5.1, *)
extension AsyncDropFirstSequence.Iterator: Sendable
where Base.AsyncIterator: Sendable,
Base.Element: Sendable { }
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