self.onmessage = async (event) => {
  const data = event.data || {};
  const payload = data.payload || "";
  const difficulty = Number(data.difficulty || 4);
  const maxMs = Number(data.maxMs || 3000);
  const start = Date.now();
  const zeroPrefix = "0".repeat(Math.max(1, difficulty));
  let counter = 0;

  while (Date.now() - start < maxMs) {
    const message = `${payload}:${counter}`;
    const hash = await sha256Hex(message);
    if (hash.startsWith(zeroPrefix)) {
      self.postMessage({
        ok: true,
        counter,
        hash,
        tookMs: Date.now() - start,
      });
      return;
    }
    counter += 1;
  }

  self.postMessage({ ok: false, reason: "timeout", tookMs: Date.now() - start });
};

async function sha256Hex(input) {
  const bytes = encodeUtf8(input);
  if (hasWebCryptoSha256()) {
    const digest = await self.crypto.subtle.digest("SHA-256", bytes);
    const arr = Array.from(new Uint8Array(digest));
    return arr.map((b) => b.toString(16).padStart(2, "0")).join("");
  }
  return sha256HexFallback(bytes);
}

function hasWebCryptoSha256() {
  return (
    typeof self !== "undefined" &&
    !!self.crypto &&
    !!self.crypto.subtle &&
    typeof self.crypto.subtle.digest === "function"
  );
}

function encodeUtf8(input) {
  if (typeof TextEncoder === "function") {
    return new TextEncoder().encode(String(input || ""));
  }

  const str = unescape(encodeURIComponent(String(input || "")));
  const out = new Uint8Array(str.length);
  for (let i = 0; i < str.length; i += 1) {
    out[i] = str.charCodeAt(i);
  }
  return out;
}

function sha256HexFallback(bytes) {
  const K = [
    0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
    0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
    0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
    0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
    0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
    0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
    0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
    0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2,
  ];
  const H = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a,
    0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
  ];

  const chunkLength = (((bytes.length + 9) + 63) >> 6) << 6;
  const msg = new Uint8Array(chunkLength);
  msg.set(bytes);
  msg[bytes.length] = 0x80;

  let bitLen = bytes.length * 8;
  for (let i = 0; i < 8; i += 1) {
    msg[msg.length - 1 - i] = bitLen & 0xff;
    bitLen = Math.floor(bitLen / 256);
  }

  const w = new Uint32Array(64);
  for (let offset = 0; offset < msg.length; offset += 64) {
    for (let i = 0; i < 16; i += 1) {
      const j = offset + i * 4;
      w[i] = (
        (msg[j] << 24) |
        (msg[j + 1] << 16) |
        (msg[j + 2] << 8) |
        msg[j + 3]
      ) >>> 0;
    }

    for (let i = 16; i < 64; i += 1) {
      w[i] = (smallSigma1(w[i - 2]) + w[i - 7] + smallSigma0(w[i - 15]) + w[i - 16]) >>> 0;
    }

    let a = H[0];
    let b = H[1];
    let c = H[2];
    let d = H[3];
    let e = H[4];
    let f = H[5];
    let g = H[6];
    let h = H[7];

    for (let i = 0; i < 64; i += 1) {
      const t1 = (h + bigSigma1(e) + ch(e, f, g) + K[i] + w[i]) >>> 0;
      const t2 = (bigSigma0(a) + maj(a, b, c)) >>> 0;
      h = g;
      g = f;
      f = e;
      e = (d + t1) >>> 0;
      d = c;
      c = b;
      b = a;
      a = (t1 + t2) >>> 0;
    }

    H[0] = (H[0] + a) >>> 0;
    H[1] = (H[1] + b) >>> 0;
    H[2] = (H[2] + c) >>> 0;
    H[3] = (H[3] + d) >>> 0;
    H[4] = (H[4] + e) >>> 0;
    H[5] = (H[5] + f) >>> 0;
    H[6] = (H[6] + g) >>> 0;
    H[7] = (H[7] + h) >>> 0;
  }

  return Array.from(H).map((n) => n.toString(16).padStart(8, "0")).join("");
}

function rotr(x, n) {
  return ((x >>> n) | (x << (32 - n))) >>> 0;
}

function ch(x, y, z) {
  return (x & y) ^ (~x & z);
}

function maj(x, y, z) {
  return (x & y) ^ (x & z) ^ (y & z);
}

function bigSigma0(x) {
  return rotr(x, 2) ^ rotr(x, 13) ^ rotr(x, 22);
}

function bigSigma1(x) {
  return rotr(x, 6) ^ rotr(x, 11) ^ rotr(x, 25);
}

function smallSigma0(x) {
  return rotr(x, 7) ^ rotr(x, 18) ^ (x >>> 3);
}

function smallSigma1(x) {
  return rotr(x, 17) ^ rotr(x, 19) ^ (x >>> 10);
}

