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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 error-throwing 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.
///
/// ```
/// let runningTotal = [1, 2, 3, 4].async.reductions(+)
/// print(await Array(runningTotal))
///
/// // prints [1, 3, 6, 10]
/// ```
///
/// - Parameter transform: A closure that combines the previously reduced
/// result and the next element in the receiving sequence. If the closure
/// throws an error, the sequence throws.
/// - Returns: An asynchronous sequence of the reduced elements.
@inlinable
public func reductions(
_ transform: @Sendable @escaping (Element, Element) async throws -> Element
) -> AsyncThrowingInclusiveReductionsSequence<Self> {
AsyncThrowingInclusiveReductionsSequence(self, transform: transform)
}
}
/// An asynchronous sequence containing the accumulated results of combining the
/// elements of the asynchronous sequence using a given error-throwing closure.
@frozen
public struct AsyncThrowingInclusiveReductionsSequence<Base: AsyncSequence> {
@usableFromInline
let base: Base
@usableFromInline
let transform: @Sendable (Base.Element, Base.Element) async throws -> Base.Element
@inlinable
init(_ base: Base, transform: @Sendable @escaping (Base.Element, Base.Element) async throws -> Base.Element) {
self.base = base
self.transform = transform
}
}
extension AsyncThrowingInclusiveReductionsSequence: AsyncSequence {
public typealias Element = Base.Element
/// The iterator for an `AsyncThrowingInclusiveReductionsSequence` instance.
@frozen
public struct Iterator: AsyncIteratorProtocol {
@usableFromInline
internal var iterator: Base.AsyncIterator?
@usableFromInline
internal var element: Base.Element?
@usableFromInline
internal let transform: @Sendable (Base.Element, Base.Element) async throws -> Base.Element
@inlinable
init(
_ iterator: Base.AsyncIterator,
transform: @Sendable @escaping (Base.Element, Base.Element) async throws -> Base.Element
) {
self.iterator = iterator
self.transform = transform
}
@inlinable
public mutating func next() async throws -> Base.Element? {
guard let previous = element else {
element = try await iterator?.next()
return element
}
guard let next = try await iterator?.next() else { return nil }
do {
element = try await transform(previous, next)
} catch {
iterator = nil
throw error
}
return element
}
}
@inlinable
public func makeAsyncIterator() -> Iterator {
Iterator(base.makeAsyncIterator(), transform: transform)
}
}
extension AsyncThrowingInclusiveReductionsSequence: Sendable where Base: Sendable { }
@available(*, unavailable)
extension AsyncThrowingInclusiveReductionsSequence.Iterator: Sendable { }
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