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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Async Algorithms open source project
//
// Copyright (c) 2022 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
//
//===----------------------------------------------------------------------===//
extension AsyncSequence {
/// Returns an asynchronous sequence containing the accumulated results of combining the
/// elements of the asynchronous sequence using the given closure.
///
/// This can be seen as applying the reduce function to each element and
/// providing the initial value followed by these results as an asynchronous sequence.
///
/// - Parameters:
/// - initial: The value to use as the initial value.
/// - transform: A closure that combines the previously-reduced result and
/// the next element in the receiving asynchronous sequence, which it returns.
/// - Returns: An asynchronous sequence of the initial value followed by the reduced
/// elements.
@inlinable
public func reductions<Result>(_ initial: Result, _ transform: @Sendable @escaping (Result, Element) async -> Result) -> AsyncExclusiveReductionsSequence<Self, Result> {
reductions(into: initial) { result, element in
result = await transform(result, element)
}
}
/// Returns an asynchronous sequence containing the accumulated results of combining the
/// elements of the asynchronous sequence using the given closure.
///
/// This can be seen as applying the reduce function to each element and
/// providing the initial value followed by these results as an asynchronous sequence.
///
/// - Parameters:
/// - initial: The value to use as the initial value.
/// - transform: A closure that combines the previously-reduced result and
/// the next element in the receiving asynchronous sequence, mutating the
/// previous result instead of returning a value.
/// - Returns: An asynchronous sequence of the initial value followed by the reduced
/// elements.
@inlinable
public func reductions<Result>(into initial: Result, _ transform: @Sendable @escaping (inout Result, Element) async -> Void) -> AsyncExclusiveReductionsSequence<Self, Result> {
AsyncExclusiveReductionsSequence(self, initial: initial, transform: transform)
}
}
/// An asynchronous sequence of applying a transform to the element of an asynchronous sequence and the
/// previously transformed result.
@frozen
public struct AsyncExclusiveReductionsSequence<Base: AsyncSequence, Element> {
@usableFromInline
let base: Base
@usableFromInline
let initial: Element
@usableFromInline
let transform: @Sendable (inout Element, Base.Element) async -> Void
@inlinable
init(_ base: Base, initial: Element, transform: @Sendable @escaping (inout Element, Base.Element) async -> Void) {
self.base = base
self.initial = initial
self.transform = transform
}
}
extension AsyncExclusiveReductionsSequence: AsyncSequence {
/// The iterator for an `AsyncExclusiveReductionsSequence` instance.
@frozen
public struct Iterator: AsyncIteratorProtocol {
@usableFromInline
var iterator: Base.AsyncIterator
@usableFromInline
var current: Element?
@usableFromInline
let transform: @Sendable (inout Element, Base.Element) async -> Void
@inlinable
init(_ iterator: Base.AsyncIterator, initial: Element, transform: @Sendable @escaping (inout Element, Base.Element) async -> Void) {
self.iterator = iterator
self.current = initial
self.transform = transform
}
@inlinable
public mutating func next() async rethrows -> Element? {
guard let result = current else { return nil }
let value = try await iterator.next()
if let value = value {
var result = result
await transform(&result, value)
current = result
return result
} else {
current = nil
return nil
}
}
}
@inlinable
public func makeAsyncIterator() -> Iterator {
Iterator(base.makeAsyncIterator(), initial: initial, transform: transform)
}
}
extension AsyncExclusiveReductionsSequence: Sendable where Base: Sendable, Element: Sendable { }
@available(*, unavailable)
extension AsyncExclusiveReductionsSequence.Iterator: Sendable { }
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