/**
 * Inject Google "Spherical Video V1" metadata into an MP4 so that any player
 * (VLC, YouTube, this app) recognises the file as an equirectangular 360 video.
 *
 * ffmpeg drops the spherical side data when re-encoding, so we add the uuid
 * box back ourselves: it lives at the end of the video `trak`, and because
 * moov usually precedes mdat (faststart) every chunk offset after the
 * insertion point has to be shifted by the box size.
 */
import fs from 'node:fs/promises';

const SPHERICAL_UUID = Buffer.from('ffcc8263f8554a938814587a02521fdd', 'hex');
const XML =
  '<?xml version="1.0"?><rdf:SphericalVideo\n' +
  'xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"\n' +
  'xmlns:GSpherical="http://ns.google.com/videos/1.0/spherical/">' +
  '<GSpherical:Spherical>true</GSpherical:Spherical>' +
  '<GSpherical:Stitched>true</GSpherical:Stitched>' +
  '<GSpherical:StitchingSoftware>360 Player</GSpherical:StitchingSoftware>' +
  '<GSpherical:ProjectionType>equirectangular</GSpherical:ProjectionType>' +
  '</rdf:SphericalVideo>';

function makeUuidBox() {
  const payload = Buffer.from(XML, 'utf8');
  const box = Buffer.alloc(8 + 16 + payload.length);
  box.writeUInt32BE(box.length, 0);
  box.write('uuid', 4, 'latin1');
  SPHERICAL_UUID.copy(box, 8);
  payload.copy(box, 24);
  return box;
}

/** Iterate the boxes inside buf[start, end). */
function* boxes(buf, start, end) {
  let p = start;
  while (p + 8 <= end) {
    let size = buf.readUInt32BE(p);
    const type = buf.toString('latin1', p + 4, p + 8);
    let headerSize = 8;
    if (size === 1) { size = Number(buf.readBigUInt64BE(p + 8)); headerSize = 16; }
    else if (size === 0) size = end - p;
    if (size < headerSize || p + size > end) throw new Error(`box ${type} @${p} 大小不正確`);
    yield { pos: p, size, type, headerSize, end: p + size };
    p += size;
  }
}
const kids = (buf, b) => boxes(buf, b.pos + b.headerSize, b.end);
function child(buf, b, type) {
  for (const c of kids(buf, b)) if (c.type === type) return c;
  return null;
}
function descend(buf, b, ...types) {
  for (const t of types) { b = b && child(buf, b, t); }
  return b;
}

async function copyRange(src, dst, from, to) {
  const chunk = Buffer.alloc(4 * 1024 * 1024);
  let pos = from;
  while (pos < to) {
    const { bytesRead } = await src.read(chunk, 0, Math.min(chunk.length, to - pos), pos);
    if (!bytesRead) throw new Error('讀取檔案時提前結束');
    await dst.write(chunk, 0, bytesRead);
    pos += bytesRead;
  }
}

/**
 * Write a copy of `file` with spherical metadata to `outFile`.
 * Returns { injected: true } or { injected: false, reason } (outFile untouched).
 */
export async function injectSphericalV1(file, outFile) {
  let fh = await fs.open(file, 'r');
  try {
    const { size: fileSize } = await fh.stat();

    // Top-level scan for moov.
    let moov = null;
    const hdr = Buffer.alloc(16);
    for (let pos = 0; pos + 8 <= fileSize;) {
      const { bytesRead } = await fh.read(hdr, 0, 16, pos);
      if (bytesRead < 8) break;
      let size = hdr.readUInt32BE(0);
      const type = hdr.toString('latin1', 4, 8);
      let headerSize = 8;
      if (size === 1) { size = Number(hdr.readBigUInt64BE(8)); headerSize = 16; }
      else if (size === 0) size = fileSize - pos;
      if (size < headerSize) break;
      if (type === 'moov') { moov = { pos, size, headerSize }; break; }
      pos += size;
    }
    if (!moov) return { injected: false, reason: '找不到 moov' };
    if (moov.headerSize !== 8) return { injected: false, reason: 'moov 使用 64-bit size' };
    if (moov.size > 512 * 1024 * 1024) return { injected: false, reason: 'moov 過大' };

    const buf = Buffer.alloc(moov.size);
    await fh.read(buf, 0, moov.size, moov.pos);
    const moovBox = { pos: 0, size: moov.size, type: 'moov', headerSize: 8, end: moov.size };

    const traks = [...kids(buf, moovBox)].filter(c => c.type === 'trak');
    const videoTrak = traks.find(t => {
      const hdlr = descend(buf, t, 'mdia', 'hdlr');
      return hdlr && buf.toString('latin1', hdlr.pos + 16, hdlr.pos + 20) === 'vide';
    });
    if (!videoTrak) return { injected: false, reason: '找不到視訊 trak' };
    if (videoTrak.headerSize !== 8) return { injected: false, reason: 'trak 使用 64-bit size' };
    for (const c of kids(buf, videoTrak)) {
      if (c.type === 'uuid' && buf.subarray(c.pos + 8, c.pos + 24).equals(SPHERICAL_UUID)) {
        return { injected: false, reason: '已經有 spherical metadata' };
      }
    }

    const uuid = makeUuidBox();
    const delta = uuid.length;
    const insertAt = moov.pos + videoTrak.end;   // absolute file offset of the insertion

    // Shift every chunk offset that points past the insertion point.
    for (const t of traks) {
      const stbl = descend(buf, t, 'mdia', 'minf', 'stbl');
      if (!stbl) continue;
      for (const c of kids(buf, stbl)) {
        if (c.type === 'stco') {
          const n = buf.readUInt32BE(c.pos + 12);
          for (let i = 0; i < n; i++) {
            const o = c.pos + 16 + i * 4;
            const v = buf.readUInt32BE(o);
            if (v >= insertAt) {
              if (v + delta > 0xffffffff) return { injected: false, reason: 'stco 偏移量溢位' };
              buf.writeUInt32BE(v + delta, o);
            }
          }
        } else if (c.type === 'co64') {
          const n = buf.readUInt32BE(c.pos + 12);
          const at = BigInt(insertAt), d = BigInt(delta);
          for (let i = 0; i < n; i++) {
            const o = c.pos + 16 + i * 8;
            const v = buf.readBigUInt64BE(o);
            if (v >= at) buf.writeBigUInt64BE(v + d, o);
          }
        }
      }
    }

    buf.writeUInt32BE(moov.size + delta, 0);
    buf.writeUInt32BE(videoTrak.size + delta, videoTrak.pos);
    const newMoov = Buffer.concat([buf.subarray(0, videoTrak.end), uuid, buf.subarray(videoTrak.end)]);

    const out = await fs.open(outFile, 'w');
    try {
      await copyRange(fh, out, 0, moov.pos);
      await out.write(newMoov);
      await copyRange(fh, out, moov.pos + moov.size, fileSize);
    } finally {
      await out.close();
    }
    return { injected: true };
  } finally {
    await fh.close();
  }
}