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Added AsyncBufferedIterator
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‎Sources/AsyncSequenceReader/AsyncBufferedIterator.swift‎

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//
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// AsyncBufferedIterator.swift
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// AsyncSequenceReader
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//
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// Created by Dimitri Bouniol on 2021-11-17.
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// Copyright © 2021 Mochi Development, Inc. All rights reserved.
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//
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#if compiler(>=5.5) && canImport(_Concurrency)
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/// An iterator that wraps and can buffer another iterator, allowing safe reading ahead.
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public struct AsyncBufferedIterator<BaseIterator: AsyncIteratorProtocol>: AsyncIteratorProtocol {
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@usableFromInline
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var baseIterator: BaseIterator
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/// The unconsumed buffer, including all reads that have been made before the user specifically requested them.
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///
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/// - Note:Ideally, this should be implemented using some sort of cyclical buffer like a Deque, but in practice, it will only ever have one entry.
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@usableFromInline
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var unconsumedBuffer: [BaseIterator.Element] = []
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@usableFromInline
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init(_ baseIterator: BaseIterator) {
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self.baseIterator = baseIterator
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}
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@inlinable
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public mutating func next() async rethrows -> BaseIterator.Element? {
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guard unconsumedBuffer.isEmpty else {
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return unconsumedBuffer.removeFirst()
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}
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return try await baseIterator.next()
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}
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/// Read ahead, and store the value for later, or throw if the base iterator also throws.
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/// - Returns: The read-ahead value.
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@usableFromInline
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mutating func nextUnconsumed() async rethrows -> BaseIterator.Element? {
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let next = try await baseIterator.next()
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if let value = next {
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unconsumedBuffer.append(value)
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}
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return next
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}
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/// Returns if the iterator has more elements to consume.
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///
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/// If it does, the iterator saves the elements, and will deliver them immediately on the next call to `next()`
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/// - Returns: A Bool indicating if there is more to consume or not.
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@inlinable
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public mutating func hasMoreData() async rethrows -> Bool {
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guard unconsumedBuffer.isEmpty else {
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return true
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}
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return try await nextUnconsumed() != nil
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}
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}
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#endif

‎Tests/AsyncSequenceReaderTests/AsyncBufferedIteratorTests.swift‎

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//
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// AsyncBufferedIteratorTests.swift
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// AsyncSequenceReader
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//
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// Created by Dimitri Bouniol on 2021-11-16.
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// Copyright © 2021 Mochi Development, Inc. All rights reserved.
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//
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import XCTest
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@testable import AsyncSequenceReader
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final class AsyncBufferedIteratorTests: XCTestCase {
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func testBufferIteratorFromStream() async throws {
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let testStream = AsyncStream<Int> { continuation in
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for value in 0..<10 {
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continuation.yield(value)
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}
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continuation.finish()
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}
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var iterator = testStream.makeAsyncIterator()
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await AsyncXCTAssertEqual(await iterator.next(), 0)
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await AsyncXCTAssertEqual(await iterator.next(), 1)
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iterator = testStream.makeAsyncIterator()
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await AsyncXCTAssertEqual(await iterator.next(), 2)
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await AsyncXCTAssertEqual(await iterator.next(), 3)
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var bufferedIterator = AsyncBufferedIterator(iterator)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 4)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 5)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 6)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 7)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 8)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 9)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), nil)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), nil)
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}
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func testBufferIteratorFromTestSequence() async throws {
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let testStream = TestSequence(base: 0..<10)
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var iterator = testStream.makeAsyncIterator()
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await AsyncXCTAssertEqual(await iterator.next(), 0)
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await AsyncXCTAssertEqual(await iterator.next(), 1)
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iterator = testStream.makeAsyncIterator()
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await AsyncXCTAssertEqual(await iterator.next(), 0)
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await AsyncXCTAssertEqual(await iterator.next(), 1)
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var bufferedIterator = AsyncBufferedIterator(iterator)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 2)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 3)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 4)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 5)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 6)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 7)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 8)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), 9)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), nil)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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await AsyncXCTAssertEqual(await bufferedIterator.next(), nil)
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}
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func testReadSequenceFromThrowingTestSequence() async throws {
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let testStream = ThrowingTestSequence(base: 0..<10)
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var iterator = testStream.makeAsyncIterator()
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try await AsyncXCTAssertEqual(await iterator.next(), 0)
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try await AsyncXCTAssertEqual(await iterator.next(), 1)
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iterator = testStream.makeAsyncIterator()
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try await AsyncXCTAssertEqual(await iterator.next(), 0)
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try await AsyncXCTAssertEqual(await iterator.next(), 1)
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var bufferedIterator = AsyncBufferedIterator(iterator)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 2)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 3)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 4)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 5)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 6)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 7)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 8)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), true)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), 9)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), nil)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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try await AsyncXCTAssertEqual(await bufferedIterator.hasMoreData(), false)
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try await AsyncXCTAssertEqual(await bufferedIterator.next(), nil)
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}
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}

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