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| 1 | +//===----------------------------------------------------------------------===// |
| 2 | +// |
| 3 | +// This source file is part of the Swift Async Algorithms open source project |
| 4 | +// |
| 5 | +// Copyright (c) 2022 Apple Inc. and the Swift project authors |
| 6 | +// Licensed under Apache License v2.0 with Runtime Library Exception |
| 7 | +// |
| 8 | +// See https://swift.org/LICENSE.txt for license information |
| 9 | +// |
| 10 | +//===----------------------------------------------------------------------===// |
| 11 | + |
| 12 | +struct Merge2StateMachine<Base1: AsyncSequence, Base2: AsyncSequence>: Sendable where Base1.AsyncIterator: Sendable, Base2.AsyncIterator: Sendable, Base1.Element: Sendable, Base2.Element: Sendable { |
| 13 | + typealias Element1 = Base1.Element |
| 14 | + typealias Element2 = Base2.Element |
| 15 | + |
| 16 | + let iter1TerminatesOnNil : Bool |
| 17 | + let iter2terminatesOnNil : Bool |
| 18 | + |
| 19 | + enum Partial: @unchecked Sendable { |
| 20 | + case first(Result<Element1?, Error>, Base1.AsyncIterator) |
| 21 | + case second(Result<Element2?, Error>, Base2.AsyncIterator) |
| 22 | + } |
| 23 | + |
| 24 | + enum Either { |
| 25 | + case first(Base1.Element) |
| 26 | + case second(Base2.Element) |
| 27 | + } |
| 28 | + |
| 29 | + var state: (PartialIteration<Base1.AsyncIterator, Partial>, PartialIteration<Base2.AsyncIterator, Partial>) |
| 30 | + |
| 31 | + init(_ iterator1: Base1.AsyncIterator, terminatesOnNil iter1TerminatesOnNil: Bool = false, _ iterator2: Base2.AsyncIterator, terminatesOnNil iter2terminatesOnNil: Bool = false) { |
| 32 | + self.iter1TerminatesOnNil = iter1TerminatesOnNil |
| 33 | + self.iter2terminatesOnNil = iter2terminatesOnNil |
| 34 | + state = (.idle(iterator1), .idle(iterator2)) |
| 35 | + } |
| 36 | + |
| 37 | + mutating func apply(_ task1: Task<Merge2StateMachine<Base1, Base2>.Partial, Never>?, _ task2: Task<Merge2StateMachine<Base1, Base2>.Partial, Never>?) async rethrows -> Either? { |
| 38 | + switch await Task.select([task1, task2].compactMap ({ $0 })).value { |
| 39 | + case .first(let result, let iterator): |
| 40 | + do { |
| 41 | + guard let value = try state.0.resolve(result, iterator) else { |
| 42 | + if iter1TerminatesOnNil { |
| 43 | + state.1.cancel() |
| 44 | + return nil |
| 45 | + } |
| 46 | + return try await next() |
| 47 | + } |
| 48 | + return .first(value) |
| 49 | + } catch { |
| 50 | + state.1.cancel() |
| 51 | + throw error |
| 52 | + } |
| 53 | + case .second(let result, let iterator): |
| 54 | + do { |
| 55 | + guard let value = try state.1.resolve(result, iterator) else { |
| 56 | + if iter2terminatesOnNil { |
| 57 | + state.0.cancel() |
| 58 | + return nil |
| 59 | + } |
| 60 | + return try await next() |
| 61 | + } |
| 62 | + return .second(value) |
| 63 | + } catch { |
| 64 | + state.0.cancel() |
| 65 | + throw error |
| 66 | + } |
| 67 | + } |
| 68 | + } |
| 69 | + |
| 70 | + func first(_ iterator1: Base1.AsyncIterator) -> Task<Partial, Never> { |
| 71 | + Task { |
| 72 | + var iter = iterator1 |
| 73 | + do { |
| 74 | + let value = try await iter.next() |
| 75 | + return .first(.success(value), iter) |
| 76 | + } catch { |
| 77 | + return .first(.failure(error), iter) |
| 78 | + } |
| 79 | + } |
| 80 | + } |
| 81 | + |
| 82 | + func second(_ iterator2: Base2.AsyncIterator) -> Task<Partial, Never> { |
| 83 | + Task { |
| 84 | + var iter = iterator2 |
| 85 | + do { |
| 86 | + let value = try await iter.next() |
| 87 | + return .second(.success(value), iter) |
| 88 | + } catch { |
| 89 | + return .second(.failure(error), iter) |
| 90 | + } |
| 91 | + } |
| 92 | + } |
| 93 | + |
| 94 | + /// Advances to the next element and returns it or `nil` if no next element exists. |
| 95 | + mutating func next() async rethrows -> Either? { |
| 96 | + if Task.isCancelled { |
| 97 | + state.0.cancel() |
| 98 | + state.1.cancel() |
| 99 | + return nil |
| 100 | + } |
| 101 | + switch state { |
| 102 | + case (.idle(let iterator1), .idle(let iterator2)): |
| 103 | + let task1 = first(iterator1) |
| 104 | + let task2 = second(iterator2) |
| 105 | + state = (.pending(task1), .pending(task2)) |
| 106 | + return try await apply(task1, task2) |
| 107 | + case (.idle(let iterator1), .pending(let task2)): |
| 108 | + let task1 = first(iterator1) |
| 109 | + state = (.pending(task1), .pending(task2)) |
| 110 | + return try await apply(task1, task2) |
| 111 | + case (.pending(let task1), .idle(let iterator2)): |
| 112 | + let task2 = second(iterator2) |
| 113 | + state = (.pending(task1), .pending(task2)) |
| 114 | + return try await apply(task1, task2) |
| 115 | + case (.idle(var iterator1), .terminal): |
| 116 | + do { |
| 117 | + if let value = try await iterator1.next() { |
| 118 | + state = (.idle(iterator1), .terminal) |
| 119 | + return .first(value) |
| 120 | + } else { |
| 121 | + state = (.terminal, .terminal) |
| 122 | + return nil |
| 123 | + } |
| 124 | + } catch { |
| 125 | + state = (.terminal, .terminal) |
| 126 | + throw error |
| 127 | + } |
| 128 | + case (.terminal, .idle(var iterator2)): |
| 129 | + do { |
| 130 | + if let value = try await iterator2.next() { |
| 131 | + state = (.terminal, .idle(iterator2)) |
| 132 | + return .second(value) |
| 133 | + } else { |
| 134 | + state = (.terminal, .terminal) |
| 135 | + return nil |
| 136 | + } |
| 137 | + } catch { |
| 138 | + state = (.terminal, .terminal) |
| 139 | + throw error |
| 140 | + } |
| 141 | + case (.terminal, .pending(let task2)): |
| 142 | + return try await apply(nil, task2) |
| 143 | + case (.pending(let task1), .pending(let task2)): |
| 144 | + return try await apply(task1, task2) |
| 145 | + case (.pending(let task1), .terminal): |
| 146 | + return try await apply(task1, nil) |
| 147 | + case (.terminal, .terminal): |
| 148 | + return nil |
| 149 | + } |
| 150 | + } |
| 151 | +} |
| 152 | + |
| 153 | +extension Merge2StateMachine.Either where Base1.Element == Base2.Element { |
| 154 | + var value : Base1.Element { |
| 155 | + switch self { |
| 156 | + case .first(let val): |
| 157 | + return val |
| 158 | + case .second(let val): |
| 159 | + return val |
| 160 | + } |
| 161 | + } |
| 162 | +} |
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