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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
/// producing an asynchronous sequence consisting of the initial value followed
/// by these results.
///
/// ```
/// 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, and returns
/// the result.
/// - Returns: An asynchronous sequence of the reduced elements.
@inlinable
public func reductions(
_ transform: @Sendable @escaping (Element, Element) async -> Element
) -> AsyncInclusiveReductionsSequence<Self> {
AsyncInclusiveReductionsSequence(self, transform: transform)
}
}
/// An asynchronous sequence containing the accumulated results of combining the
/// elements of the asynchronous sequence using a given closure.
@frozen
public struct AsyncInclusiveReductionsSequence<Base: AsyncSequence> {
@usableFromInline
let base: Base
@usableFromInline
let transform: @Sendable (Base.Element, Base.Element) async -> Base.Element
@inlinable
init(_ base: Base, transform: @Sendable @escaping (Base.Element, Base.Element) async -> Base.Element) {
self.base = base
self.transform = transform
}
}
extension AsyncInclusiveReductionsSequence: AsyncSequence {
public typealias Element = Base.Element
/// The iterator for an `AsyncInclusiveReductionsSequence` 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 -> Base.Element
@inlinable
init(
_ iterator: Base.AsyncIterator,
transform: @Sendable @escaping (Base.Element, Base.Element) async -> Base.Element
) {
self.iterator = iterator
self.transform = transform
}
@inlinable
public mutating func next() async rethrows -> Base.Element? {
guard let previous = element else {
element = try await iterator.next()
return element
}
guard let next = try await iterator.next() else { return nil }
element = await transform(previous, next)
return element
}
}
@inlinable
public func makeAsyncIterator() -> Iterator {
Iterator(base.makeAsyncIterator(), transform: transform)
}
}
extension AsyncInclusiveReductionsSequence: Sendable where Base: Sendable { }
@available(*, unavailable)
extension AsyncInclusiveReductionsSequence.Iterator: Sendable { }
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