reverse-engineering-anything/src/desktop/DesktopSessionRequests.ts

import type { Interface } from "node:readline";

import type { SessionRequest } from "./DesktopApplicationSession.js";

const QUEUE_PAUSE = 16;
const QUEUE_RESUME = 4;

/**
 * `stop` ends the session. An empty line, `checkpoint` or `checkpoint <name>`
 * requests a checkpoint; any other line names one.
 */
const parseSessionRequest = (line: string): SessionRequest => {
  const text = line.trim();
  if (text === "stop") return { kind: "stop" };
  const name = text.replace(/^checkpoint(?:\s+|$)/u, "");
  return { kind: "checkpoint", name: name.length === 0 ? undefined : name };
};

/**
 * Requests read line by line. The end of input counts as `stop`, so a driver
 * must keep its input open between checkpoints. Reading pauses while many
 * requests are queued, so a fast driver cannot grow the queue without bound.
 */
export class SessionRequestQueue {
  readonly #requests: SessionRequest[] = [];
  #paused = false;

  constructor(readonly lines: Interface) {
    lines.on("line", (line) => {
      this.#requests.push(parseSessionRequest(line));
      if (!this.#paused && this.#requests.length >= QUEUE_PAUSE) {
        this.#paused = true;
        lines.pause();
      }
    });
    lines.once("close", () => {
      this.#requests.push({ kind: "stop" });
    });
  }

  next(): SessionRequest | undefined {
    const request = this.#requests.shift();
    if (this.#paused && this.#requests.length <= QUEUE_RESUME) {
      this.#paused = false;
      this.lines.resume();
    }
    return request;
  }

  close(): void {
    this.lines.close();
  }
}