/**
 * Top loading bar (NProgress-style) for the DWG/DXF load pipeline.
 *
 * Why two layers:
 *   The expensive stages — `parse_dwg_json` on the WASM heap and
 *   `Viewer2D.load()` — are SYNCHRONOUS. While they run, the main thread is
 *   blocked: no rAF ticks, no `width` transitions, no repaint. A JS-driven bar
 *   simply freezes for the seconds that matter most.
 *
 *   · `.bar`   real progress, driven by `width` (accurate, main-thread bound).
 *   · `.sweep` liveness, a `transform` keyframe animation. Transform keyframes
 *              are handed to the compositor thread, so the stripe keeps moving
 *              across a blocked main thread. That is the only feedback the page
 *              can give while the parser is inside WASM.
 *
 * The markup and CSS live in index.html (`#progress`), same as the other
 * panels; this module just drives them.
 */

/** Wait until the browser has actually painted the pending style change. */
function nextPaint(): Promise<void> {
  return new Promise<void>((resolve) => {
    let settled = false;
    const finish = () => {
      if (settled) return;
      settled = true;
      resolve();
    };
    // Two frames: the first callback still runs BEFORE paint, the second one
    // only fires once the frame carrying our change has been composited.
    requestAnimationFrame(() => requestAnimationFrame(finish));
    // Backgrounded tabs never fire rAF — don't stall the load behind it.
    setTimeout(finish, 150);
  });
}

export class TopProgress {
  private root: HTMLElement;
  private bar: HTMLElement;
  private value = 0;
  private hideTimer = 0;

  constructor(root: HTMLElement) {
    this.root = root;
    this.bar = root.querySelector<HTMLElement>('.bar') ?? root;
  }

  /** Show the bar and reset it to zero (no rewind animation). */
  start(): void {
    window.clearTimeout(this.hideTimer);
    this.root.classList.remove('err');
    this.value = 0;
    this.jumpTo(0);
    this.root.classList.add('on');
  }

  /**
   * Move to `v` (0–1). Monotonic — a lower value is ignored, so out-of-order
   * stage callbacks can never make the bar walk backwards.
   */
  set(v: number): void {
    const next = Math.max(this.value, Math.min(1, Math.max(0, v)));
    if (next === this.value) return;
    this.value = next;
    this.bar.style.width = `${(next * 100).toFixed(2)}%`;
  }

  /** Toggle the compositor-driven sweep for a stage that blocks the main thread. */
  busy(on: boolean): void {
    this.root.classList.toggle('busy', on);
  }

  /** Resolve once the pending width/class change is on screen. */
  paint(): Promise<void> {
    return nextPaint();
  }

  /** Fill to 100% and fade out. */
  done(): void {
    this.finish();
  }

  /** Fill to 100% in the error color and fade out. */
  fail(): void {
    this.root.classList.add('err');
    this.finish();
  }

  private finish(): void {
    this.busy(false);
    this.set(1);
    window.clearTimeout(this.hideTimer);
    this.hideTimer = window.setTimeout(() => {
      this.root.classList.remove('on');
      // Rewind only after the fade-out has finished, so the bar never visibly
      // slides back to zero.
      this.hideTimer = window.setTimeout(() => {
        this.root.classList.remove('err');
        this.value = 0;
        this.jumpTo(0);
      }, 320);
    }, 260);
  }

  /** Set the width with the transition suppressed for one frame. */
  private jumpTo(v: number): void {
    const prev = this.bar.style.transition;
    this.bar.style.transition = 'none';
    this.bar.style.width = `${(v * 100).toFixed(2)}%`;
    void this.bar.offsetWidth; // force reflow so the next change animates again
    this.bar.style.transition = prev;
  }
}

/** 1.4 MB / 812 kB — short byte label for the download readout. */
export function formatBytes(n: number): string {
  if (!Number.isFinite(n) || n < 0) return '?';
  if (n < 1024) return `${n} B`;
  if (n < 1024 * 1024) return `${(n / 1024).toFixed(0)} kB`;
  return `${(n / 1024 / 1024).toFixed(1)} MB`;
}

/**
 * fetch() that reports download progress.
 *
 * Falls back to a plain `arrayBuffer()` when the response has no body stream
 * or no Content-Length (then `total` is 0 and the caller should show an
 * indeterminate state). `loaded` is decompressed bytes while Content-Length is
 * the compressed size, so the ratio is clamped to 1.
 */
export async function fetchWithProgress(
  url: string,
  onProgress: (loaded: number, total: number) => void,
): Promise<ArrayBuffer> {
  const res = await fetch(url);
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  const total = Number(res.headers.get('content-length')) || 0;
  if (!res.body || !total) {
    const buf = await res.arrayBuffer();
    onProgress(buf.byteLength, buf.byteLength);
    return buf;
  }

  const reader = res.body.getReader();
  const chunks: Uint8Array[] = [];
  let loaded = 0;
  for (;;) {
    const { done, value } = await reader.read();
    if (done) break;
    if (!value) continue;
    chunks.push(value);
    loaded += value.length;
    onProgress(loaded, total);
  }

  const out = new Uint8Array(loaded);
  let offset = 0;
  for (const c of chunks) {
    out.set(c, offset);
    offset += c.length;
  }
  return out.buffer;
}