// Minimal protocol helpers in JS (match server/protocol.py)

export const PacketType = {
  JOIN: 0,
  JOIN_ACK: 1,
  JOIN_DENY: 2,
  INPUT: 3,
  INPUT_BROADCAST: 4,
  STATE_DELTA: 5,
  STATE_FULL: 6,
  PART: 7,
  CONFIG_UPDATE: 8,
  PING: 9,
  PONG: 10,
  ERROR: 11,
};

export const Direction = { UP: 0, RIGHT: 1, DOWN: 2, LEFT: 3 };

export function quicVarintEncode(n) {
  if (n <= 0x3f) return new Uint8Array([n & 0x3f]);
  if (n <= 0x3fff) {
    const v = 0x4000 | n;
    return new Uint8Array([(v >> 8) & 0xff, v & 0xff]);
  }
  if (n <= 0x3fffffff) {
    const v = 0x80000000 | n;
    return new Uint8Array([(v >>> 24) & 0xff, (v >>> 16) & 0xff, (v >>> 8) & 0xff, v & 0xff]);
  }
  throw new Error("varint too large for demo");
}

export function quicVarintDecode(view, offset) {
  const first = view.getUint8(offset);
  const prefix = first >> 6;
  if (prefix === 0) return [first & 0x3f, offset + 1];
  if (prefix === 1) return [view.getUint16(offset) & 0x3fff, offset + 2];
  if (prefix === 2) return [view.getUint32(offset) & 0x3fffffff, offset + 4];
  throw new Error("8-byte varint not supported in demo");
}

export function packHeader(ver, type, flags, seq, tick /* or null */) {
  const len = tick == null ? 5 : 7;
  const buf = new ArrayBuffer(len);
  const dv = new DataView(buf);
  dv.setUint8(0, ver & 0xff);
  dv.setUint8(1, type & 0xff);
  dv.setUint8(2, flags & 0xff);
  dv.setUint16(3, seq & 0xffff);
  if (tick != null) dv.setUint16(5, tick & 0xffff);
  return new Uint8Array(buf);
}

export function parseHeader(dv, offset = 0, expectTick = false) {
  if (dv.byteLength - offset < 5) throw new Error("small header");
  const ver = dv.getUint8(offset);
  const type = dv.getUint8(offset + 1);
  const flags = dv.getUint8(offset + 2);
  const seq = dv.getUint16(offset + 3);
  let tick = null;
  let off = offset + 5;
  if (expectTick) {
    tick = dv.getUint16(off);
    off += 2;
  }
  return [ver, type, flags, seq, tick, off];
}

export function buildJoin(ver, seq, name, preferredColor = null) {
  const enc = new TextEncoder();
  const nb = enc.encode(name);
  const nameLen = quicVarintEncode(nb.length);
  const header = packHeader(ver, PacketType.JOIN, 0, seq, null);
  const extra = preferredColor == null ? new Uint8Array([]) : new Uint8Array([preferredColor & 0xff]);
  const out = new Uint8Array(header.length + nameLen.length + nb.length + extra.length);
  out.set(header, 0);
  out.set(nameLen, header.length);
  out.set(nb, header.length + nameLen.length);
  out.set(extra, header.length + nameLen.length + nb.length);
  return out;
}

export function buildInput(ver, seq, ackSeq, inputSeq, baseTick, events /* [{rel, dir}] */) {
  const header = packHeader(ver, PacketType.INPUT, 0, seq, null);
  const evParts = [];
  evParts.push(...[ackSeq >> 8, ackSeq & 0xff, inputSeq >> 8, inputSeq & 0xff, baseTick >> 8, baseTick & 0xff]);
  const evCount = quicVarintEncode(events.length);
  const chunks = [header, new Uint8Array(evParts), evCount];
  for (const e of events) {
    const off = quicVarintEncode(e.rel);
    chunks.push(off, new Uint8Array([e.dir & 0x03]));
  }
  const size = chunks.reduce((s, a) => s + a.length, 0);
  const out = new Uint8Array(size);
  let p = 0;
  for (const c of chunks) { out.set(c, p); p += c.length; }
  return out;
}

export function parseJoinAck(dv, off) {
  const playerId = dv.getUint8(off); off += 1;
  const colorId = dv.getUint8(off); off += 1;
  const width = dv.getUint8(off); off += 1;
  const height = dv.getUint8(off); off += 1;
  const tickRate = dv.getUint8(off); off += 1;
  const wrapEdges = dv.getUint8(off) !== 0; off += 1;
  const applesPerSnake = dv.getUint8(off); off += 1;
  const applesCap = dv.getUint8(off); off += 1;
  const compressionMode = dv.getUint8(off); off += 1;
  return { playerId, colorId, width, height, tickRate, wrapEdges, applesPerSnake, applesCap, compressionMode, off };
}

export function parseStateFullBody(dv, off) {
  const snakes = [];
  // snakes count
  let [n, p] = quicVarintDecode(dv, off); off = p;
  for (let i = 0; i < n; i++) {
    const id = dv.getUint8(off); off += 1;
    const len = dv.getUint16(off); off += 2;
    const hx = dv.getUint8(off); off += 1;
    const hy = dv.getUint8(off); off += 1;
    // TLV
    let [t, p1] = quicVarintDecode(dv, off); off = p1;
    let [L, p2] = quicVarintDecode(dv, off); off = p2;
    const tStart = off;
    const tEnd = off + L;
    let dirs = [];
    if (t === 0 /* 2-bit */) {
      const needed = Math.max(0, (len - 1));
      let bits = 0, acc = 0, di = 0;
      for (let iB = off; iB < tEnd && di < needed; iB++) {
        acc |= dv.getUint8(iB) << bits;
        bits += 8;
        while (bits >= 2 && di < needed) {
          dirs.push(acc & 0x03);
          acc >>= 2; bits -= 2; di++;
        }
      }
    } else if (t === 1 /* RLE */) {
      let countAccum = 0;
      while (off < tEnd && countAccum < (len - 1)) {
        const dir = dv.getUint8(off); off += 1;
        let c; [c, off] = quicVarintDecode(dv, off);
        for (let k = 0; k < c; k++) dirs.push(dir & 0x03);
        countAccum += c;
      }
    } else {
      // chunk types not expected in full body for initial join; skip
      off = tEnd;
    }
    off = tEnd;
    snakes.push({ id, len, hx, hy, dirs });
  }
  // apples
  let m; [m, off] = quicVarintDecode(dv, off);
  const apples = [];
  for (let i = 0; i < m; i++) { apples.push([dv.getUint8(off++), dv.getUint8(off++)]); }
  return { snakes, apples, off };
}