packages/frontend/src/utils/text_line_stream.ts

// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.

interface TextLineStreamOptions {
  /** Allow splitting by solo \r */
  allowCR: boolean;
}

/** Transform a stream into a stream where each chunk is divided by a newline,
 * be it `\n` or `\r\n`. `\r` can be enabled via the `allowCR` option.
 *
 * ```ts
 * import { TextLineStream } from "https://deno.land/std@$STD_VERSION/streams/text_line_stream.ts";
 * const res = await fetch("https://example.com");
 * const lines = res.body!
 *   .pipeThrough(new TextDecoderStream())
 *   .pipeThrough(new TextLineStream());
 * ```
 */
export class TextLineStream extends TransformStream<string, string> {
  readonly #allowCR: boolean;
  #buf = '';

  constructor(options?: TextLineStreamOptions) {
    super({
      transform: (chunk, controller) => this.#handle(chunk, controller),
      flush: (controller) => {
        if (this.#buf.length > 0) {
          if (this.#allowCR && this.#buf[this.#buf.length - 1] === '\r')
            controller.enqueue(this.#buf.slice(0, -1));
          else controller.enqueue(this.#buf);
        }
      },
    });
    this.#allowCR = options?.allowCR ?? false;
  }

  #handle(chunk: string, controller: TransformStreamDefaultController<string>) {
    chunk = this.#buf + chunk;

    for (;;) {
      const lfIndex = chunk.indexOf('\n');

      if (this.#allowCR) {
        const crIndex = chunk.indexOf('\r');

        if (
          crIndex !== -1 &&
          crIndex !== chunk.length - 1 &&
          (lfIndex === -1 || lfIndex - 1 > crIndex)
        ) {
          controller.enqueue(chunk.slice(0, crIndex));
          chunk = chunk.slice(crIndex + 1);
          continue;
        }
      }

      if (lfIndex !== -1) {
        let crOrLfIndex = lfIndex;
        if (chunk[lfIndex - 1] === '\r') {
          crOrLfIndex--;
        }
        controller.enqueue(chunk.slice(0, crOrLfIndex));
        chunk = chunk.slice(lfIndex + 1);
        continue;
      }

      break;
    }

    this.#buf = chunk;
  }
}