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Merge pull request #23 from jsr-core/repeatable
feat(async): add `repeatable` helper function
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Original file line number | Diff line number | Diff line change |
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@@ -0,0 +1,40 @@ | ||
export function repeatable<T>(iterable: AsyncIterable<T>): AsyncIterable<T> { | ||
const cache: T[] = []; | ||
let buildingCache: Promise<void> | undefined = undefined; | ||
let pendingResolvers: ((value: T) => void)[] = []; | ||
let finished = false; | ||
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return { | ||
[Symbol.asyncIterator]: async function* () { | ||
yield* cache; | ||
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if (!finished) { | ||
if (!buildingCache) { | ||
buildingCache = (async () => { | ||
try { | ||
for await (const item of iterable) { | ||
cache.push(item); | ||
pendingResolvers.forEach((resolve) => resolve(item)); | ||
pendingResolvers = []; | ||
} | ||
} finally { | ||
finished = true; | ||
} | ||
})(); | ||
} | ||
} | ||
let index = cache.length; | ||
while (!finished || index < cache.length) { | ||
if (index < cache.length) { | ||
yield cache[index++]; | ||
} else { | ||
const nextItem = await new Promise<T>((resolve) => { | ||
pendingResolvers.push(resolve); | ||
}); | ||
yield nextItem; | ||
index++; | ||
} | ||
} | ||
}, | ||
}; | ||
} |
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Original file line number | Diff line number | Diff line change |
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import { test } from "@cross/test"; | ||
import { delay } from "@std/async/delay"; | ||
import { assertEquals } from "@std/assert"; | ||
import { repeatable } from "./repeatable.ts"; | ||
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async function* delayedGenerator(sideEffect?: () => void) { | ||
yield 1; | ||
await delay(100); | ||
yield 2; | ||
await delay(100); | ||
yield 3; | ||
sideEffect?.(); | ||
} | ||
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await test("repeatable should return the same sequence on multiple iterations", async () => { | ||
const input = delayedGenerator(); | ||
const it = repeatable(input); | ||
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const result1 = await Array.fromAsync(it); | ||
const result2 = await Array.fromAsync(it); | ||
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assertEquals(result1, [1, 2, 3], "First iteration"); | ||
assertEquals(result2, [1, 2, 3], "First iteration"); | ||
}); | ||
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await test("repeatable should call internal iterator only once", async () => { | ||
let called = 0; | ||
const input = delayedGenerator(() => called++); | ||
const it = repeatable(input); | ||
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const result1 = await Array.fromAsync(it); | ||
const result2 = await Array.fromAsync(it); | ||
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assertEquals(result1, [1, 2, 3], "First iteration"); | ||
assertEquals(result2, [1, 2, 3], "First iteration"); | ||
assertEquals(called, 1, "Internal iterator called only once"); | ||
}); | ||
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await test("repeatable should work correctly when consumed partially and then fully", async () => { | ||
const input = delayedGenerator(); | ||
const it = repeatable(input); | ||
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const result1: number[] = []; | ||
const firstIter = it[Symbol.asyncIterator](); | ||
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result1.push((await firstIter.next()).value); // 1 | ||
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const result2 = await Array.fromAsync(it); | ||
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result1.push((await firstIter.next()).value); // 2 | ||
result1.push((await firstIter.next()).value); // 3 | ||
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assertEquals(result1, [1, 2, 3], "First iteration"); | ||
assertEquals(result2, [1, 2, 3], "First iteration"); | ||
}); | ||
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await test("repeatable should cache values and return them immediately on subsequent iterations", async () => { | ||
const input = delayedGenerator(); | ||
const it = repeatable(input); | ||
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const start = performance.now(); | ||
const result1 = await Array.fromAsync(it); | ||
const end1 = performance.now(); | ||
const timeTaken1 = end1 - start; | ||
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const start2 = performance.now(); | ||
const result2 = await Array.fromAsync(it); | ||
const end2 = performance.now(); | ||
const timeTaken2 = end2 - start2; | ||
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assertEquals(result1, [1, 2, 3], "First iteration"); | ||
assertEquals(result2, [1, 2, 3], "Second iteration"); | ||
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console.debug("Time taken for first consume:", timeTaken1); | ||
console.debug("Time taken for second consume (with cache):", timeTaken2); | ||
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if (timeTaken2 > timeTaken1 / 10) { | ||
throw new Error( | ||
"Second consume took too long, cache might not be working.", | ||
); | ||
} | ||
}); |
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