// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.
/**
 * Convert the generator function into a {@linkcode TransformStream}.
 *
 * @example
 * ```ts
 * import { toTransformStream } from "https://deno.land/std@$STD_VERSION/streams/to_transform_stream.ts";
 *
 * const readable = ReadableStream.from([0, 1, 2])
 *   .pipeThrough(toTransformStream(async function* (src) {
 *     for await (const chunk of src) {
 *       yield chunk * 100;
 *     }
 *   }));
 *
 * for await (const chunk of readable) {
 *   console.log(chunk);
 * }
 * // output: 0, 100, 200
 * ```
 *
 * @param transformer A function to transform.
 * @param writableStrategy An object that optionally defines a queuing strategy for the stream.
 * @param readableStrategy An object that optionally defines a queuing strategy for the stream.
 */ export function toTransformStream(transformer, writableStrategy, readableStrategy) {
  const { writable, readable } = new TransformStream(undefined, writableStrategy);
  const iterable = transformer(readable);
  const iterator = iterable[Symbol.asyncIterator]?.() ?? iterable[Symbol.iterator]?.();
  return {
    writable,
    readable: new ReadableStream({
      async pull (controller) {
        let result;
        try {
          result = await iterator.next();
        } catch (error) {
          // Propagate error to stream from iterator
          // If the stream status is "errored", it will be thrown, but ignore.
          await readable.cancel(error).catch(()=>{});
          controller.error(error);
          return;
        }
        if (result.done) {
          controller.close();
          return;
        }
        controller.enqueue(result.value);
      },
      async cancel (reason) {
        // Propagate cancellation to readable and iterator
        if (typeof iterator.throw === "function") {
          try {
            await iterator.throw(reason);
          } catch  {
          /* `iterator.throw()` always throws on site. We catch it. */ }
        }
        await readable.cancel(reason);
      }
    }, readableStrategy)
  };
}
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