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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 elements from the base asynchronous sequence until a given closure
/// returns false, after which it passes through all remaining elements.
///
/// Use `drop(while:)` to omit elements from an asynchronous sequence until
/// the element received meets a condition you specify.
///
/// In this example, an asynchronous sequence called `Counter` produces `Int`
/// values from `1` to `10`. The `drop(while:)` method causes the modified
/// sequence to ignore received values until it encounters one that is
/// divisible by `3`:
///
/// let stream = Counter(howHigh: 10)
/// .drop { $0 % 3 != 0 }
/// for await number in stream {
/// print(number, terminator: " ")
/// }
/// // Prints "3 4 5 6 7 8 9 10 "
///
/// After the predicate returns `false`, the sequence never executes it again,
/// and from then on the sequence passes through elements from its underlying
/// sequence as-is.
///
/// - Parameter predicate: A closure that takes an element as a parameter and
/// returns a Boolean value indicating whether to drop the element from the
/// modified sequence.
/// - Returns: An asynchronous sequence that skips over values from the
/// base sequence until the provided closure returns `false`.
@preconcurrency
@inlinable
public __consuming func drop(
while predicate: @Sendable @escaping (Element) async -> Bool
) -> AsyncDropWhileSequence<Self> {
AsyncDropWhileSequence(self, predicate: predicate)
}
}
/// An asynchronous sequence which omits elements from the base sequence until a
/// given closure returns false, after which it passes through all remaining
/// elements.
@available(SwiftStdlib 5.1, *)
public struct AsyncDropWhileSequence<Base: AsyncSequence> {
@usableFromInline
let base: Base
@usableFromInline
let predicate: (Base.Element) async -> Bool
@usableFromInline
init(
_ base: Base,
predicate: @escaping (Base.Element) async -> Bool
) {
self.base = base
self.predicate = predicate
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncDropWhileSequence: AsyncSequence {
/// The type of element produced by this asynchronous sequence.
///
/// The drop-while sequence produces whatever type of element its base
/// sequence produces.
public typealias Element = Base.Element
/// The type of errors produced by this asynchronous sequence.
///
/// The drop-while 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-while 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 predicate: ((Base.Element) async -> Bool)?
@usableFromInline
init(
_ baseIterator: Base.AsyncIterator,
predicate: @escaping (Base.Element) async -> Bool
) {
self.baseIterator = baseIterator
self.predicate = predicate
}
/// Produces the next element in the drop-while sequence.
///
/// This iterator calls `next()` on its base iterator and evaluates the
/// result with the `predicate` closure. As long as the predicate returns
/// `true`, this method returns `nil`. After the predicate returns `false`,
/// for a value received from the base iterator, this method returns that
/// value. After that, the iterator returns values received from its
/// base iterator as-is, and never executes the predicate closure again.
@inlinable
public mutating func next() async rethrows -> Base.Element? {
while let predicate = self.predicate {
guard let element = try await baseIterator.next() else {
return nil
}
if await predicate(element) == false {
self.predicate = nil
return element
}
}
return try await baseIterator.next()
}
/// Produces the next element in the drop-while sequence.
///
/// This iterator calls `next(isolation:)` on its base iterator and
/// evaluates the result with the `predicate` closure. As long as the
/// predicate returns `true`, this method returns `nil`. After the predicate
/// returns `false`, for a value received from the base iterator, this
/// method returns that value. After that, the iterator returns values
/// received from its base iterator as-is, and never executes the predicate
/// closure again.
@available(SwiftStdlib 6.0, *)
@inlinable
public mutating func next(isolation actor: isolated (any Actor)?) async throws(Failure) -> Base.Element? {
while let predicate = self.predicate {
guard let element = try await baseIterator.next(isolation: actor) else {
return nil
}
if await predicate(element) == false {
self.predicate = nil
return element
}
}
return try await baseIterator.next(isolation: actor)
}
}
/// Creates an instance of the drop-while sequence iterator.
@inlinable
public __consuming func makeAsyncIterator() -> Iterator {
return Iterator(base.makeAsyncIterator(), predicate: predicate)
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncDropWhileSequence: @unchecked Sendable
where Base: Sendable,
Base.Element: Sendable { }
@available(SwiftStdlib 5.1, *)
extension AsyncDropWhileSequence.Iterator: @unchecked Sendable
where Base.AsyncIterator: Sendable,
Base.Element: Sendable { }
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