/**
 * Frame-coalesced release of streamed agent output.
 *
 * Every network chunk used to become its own React commit, and each commit
 * re-parsed the whole partial answer. This buffer sits between the event
 * parser and `setState`: events mutate an accumulator, and at most one flush
 * happens per animation frame. Answer and thinking text are released at a
 * steady per-frame pace so a burst of chunks reads as flowing text instead of
 * a jump, while a large backlog (reconnect, slow tab) catches up in roughly a
 * dozen frames.
 *
 * Pure release arithmetic lives in exported functions so the policy is
 * testable without a DOM; scheduling is injectable for the same reason.
 */

export const STREAM_RELEASE_MIN_CHARS = 2;
export const STREAM_RELEASE_CATCHUP_DIVISOR = 12;
export const STREAM_HIDDEN_FLUSH_MS = 250;

/**
 * Characters to reveal on one frame. `backlogChars` is how much was waiting
 * when the newest text arrived: dividing that by twelve clears any burst in
 * about twelve frames, while the two-character floor keeps a slow model from
 * reading as stalled. Callers without a backlog figure pass the pending count.
 */
export function streamReleaseCount(pendingChars: number, backlogChars = pendingChars): number {
  if (pendingChars <= 0) return 0;
  return Math.min(
    pendingChars,
    Math.max(STREAM_RELEASE_MIN_CHARS, Math.ceil(backlogChars / STREAM_RELEASE_CATCHUP_DIVISOR)),
  );
}

/** Advance a released prefix toward its target by one frame's worth of text. */
export function advanceStreamRelease(released: string, target: string, backlogChars?: number): string {
  if (!target.startsWith(released)) {
    // The target was replaced rather than extended: restart from its head.
    return target.slice(0, streamReleaseCount(target.length, backlogChars ?? target.length));
  }
  const pending = target.length - released.length;
  if (pending <= 0) return target;
  return target.slice(0, released.length + streamReleaseCount(pending, backlogChars ?? pending));
}

export type StreamFrameSnapshot<Meta> = Readonly<{
  answer: string;
  thinking: string;
  meta: Meta;
  /** True when this flush released everything that had arrived. */
  settled: boolean;
}>;

export type StreamFrameScheduler = Readonly<{
  requestFrame: (callback: () => void) => number;
  cancelFrame: (handle: number) => void;
  requestTimeout: (callback: () => void, delayMs: number) => number;
  cancelTimeout: (handle: number) => void;
  hidden: () => boolean;
}>;

export type StreamFrameBufferOptions<Meta> = Readonly<{
  initialMeta: Meta;
  flush: (snapshot: StreamFrameSnapshot<Meta>) => void;
  scheduler?: StreamFrameScheduler;
}>;

export type StreamFrameBuffer<Meta> = Readonly<{
  setAnswer: (fullText: string) => void;
  setThinking: (fullText: string) => void;
  setMeta: (next: Meta | ((current: Meta) => Meta)) => void;
  /** Publish meta-only changes (timeline rows, activity) on the next frame. */
  touch: () => void;
  /** Release everything received and flush synchronously. */
  settle: () => void;
  /** Drop everything, including scheduled work, without flushing. */
  reset: (meta?: Meta) => void;
  dispose: () => void;
  /** Text released so far, for callers that persist partial output. */
  released: () => Readonly<{ answer: string; thinking: string }>;
}>;

function pendingChars(released: string, target: string): number {
  return target.startsWith(released) ? target.length - released.length : target.length;
}

function browserScheduler(): StreamFrameScheduler {
  return {
    requestFrame: (callback) => window.requestAnimationFrame(callback),
    cancelFrame: (handle) => window.cancelAnimationFrame(handle),
    requestTimeout: (callback, delayMs) => window.setTimeout(callback, delayMs),
    cancelTimeout: (handle) => window.clearTimeout(handle),
    hidden: () => typeof document !== "undefined" && document.hidden,
  };
}

export function createStreamFrameBuffer<Meta>(
  options: StreamFrameBufferOptions<Meta>,
): StreamFrameBuffer<Meta> {
  const scheduler = options.scheduler ?? browserScheduler();
  let targetAnswer = "";
  let targetThinking = "";
  let releasedAnswer = "";
  let releasedThinking = "";
  let answerBacklog = 0;
  let thinkingBacklog = 0;
  let meta = options.initialMeta;
  let disposed = false;
  let frameHandle: number | null = null;
  let timeoutHandle: number | null = null;

  const cancelScheduled = () => {
    if (frameHandle !== null) {
      scheduler.cancelFrame(frameHandle);
      frameHandle = null;
    }
    if (timeoutHandle !== null) {
      scheduler.cancelTimeout(timeoutHandle);
      timeoutHandle = null;
    }
  };

  const emit = (settled: boolean) => {
    options.flush({
      answer: releasedAnswer,
      thinking: releasedThinking,
      meta,
      settled,
    });
  };

  const step = () => {
    frameHandle = null;
    timeoutHandle = null;
    if (disposed) return;
    if (scheduler.hidden()) {
      releasedAnswer = targetAnswer;
      releasedThinking = targetThinking;
    } else {
      releasedAnswer = advanceStreamRelease(releasedAnswer, targetAnswer, answerBacklog);
      releasedThinking = advanceStreamRelease(releasedThinking, targetThinking, thinkingBacklog);
    }
    if (releasedAnswer === targetAnswer) answerBacklog = 0;
    if (releasedThinking === targetThinking) thinkingBacklog = 0;
    const caughtUp = releasedAnswer === targetAnswer && releasedThinking === targetThinking;
    emit(caughtUp);
    if (!caughtUp) schedule();
  };

  const schedule = () => {
    if (disposed || frameHandle !== null || timeoutHandle !== null) return;
    if (scheduler.hidden()) {
      timeoutHandle = scheduler.requestTimeout(step, STREAM_HIDDEN_FLUSH_MS);
    } else {
      frameHandle = scheduler.requestFrame(step);
    }
  };

  return {
    setAnswer(fullText) {
      if (disposed || fullText === targetAnswer) return;
      targetAnswer = fullText;
      answerBacklog = Math.max(answerBacklog, pendingChars(releasedAnswer, targetAnswer));
      schedule();
    },
    setThinking(fullText) {
      if (disposed || fullText === targetThinking) return;
      targetThinking = fullText;
      thinkingBacklog = Math.max(thinkingBacklog, pendingChars(releasedThinking, targetThinking));
      schedule();
    },
    setMeta(next) {
      if (disposed) return;
      meta = typeof next === "function" ? (next as (current: Meta) => Meta)(meta) : next;
      schedule();
    },
    touch() {
      if (disposed) return;
      schedule();
    },
    settle() {
      if (disposed) return;
      cancelScheduled();
      releasedAnswer = targetAnswer;
      releasedThinking = targetThinking;
      answerBacklog = 0;
      thinkingBacklog = 0;
      emit(true);
    },
    reset(nextMeta) {
      cancelScheduled();
      targetAnswer = "";
      targetThinking = "";
      releasedAnswer = "";
      releasedThinking = "";
      answerBacklog = 0;
      thinkingBacklog = 0;
      if (nextMeta !== undefined) meta = nextMeta;
    },
    dispose() {
      disposed = true;
      cancelScheduled();
    },
    released() {
      return { answer: releasedAnswer, thinking: releasedThinking };
    },
  };
}