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type Gen<T, TReturn> = AsyncGenerator<T, TReturn, undefined>;
type GenPromiseMap<T, TReturn> = Map<
Gen<T, TReturn>,
Promise<{ gen: Gen<T, TReturn> } & IteratorResult<T, TReturn>>
>;
/** Merges multiple async generators into a single async generator that yields values from all of them in parallel. */
export async function* mergeAsyncGenerators<T, TReturn>(
generators: Gen<T, TReturn>[]
): Gen<T, TReturn[]> {
const promises: GenPromiseMap<T, TReturn> = new Map();
const results: Map<Gen<T, TReturn>, TReturn> = new Map();
for (const gen of generators) {
promises.set(
gen,
gen.next().then((result) => ({ gen, ...result }))
);
}
while (promises.size) {
const { gen, value, done } = await Promise.race(promises.values());
if (done) {
results.set(gen, value as TReturn);
promises.delete(gen);
} else {
promises.set(
gen,
gen.next().then((result) => ({ gen, ...result }))
);
yield value as T;
}
}
const orderedResults = generators.map((gen) => results.get(gen) as TReturn);
return orderedResults;
}
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