/**
 * Pure SHA-256 (FIPS 180-4) over bytes, for verifying content-addressed
 * native blobs inside the interpretation layer's purity gate. Verified
 * against node:crypto in test/transcript/cursor.test.ts.
 */

const K = Uint32Array.from([
  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 rotr = (value: number, bits: number): number => (value >>> bits) | (value << (32 - bits));
const at = (words: Uint32Array, index: number): number => words[index] ?? 0;

export function sha256Hex(bytes: Uint8Array): string {
  const length = bytes.length;
  const padded = new Uint8Array((((length + 9 + 63) >> 6) << 6) >>> 0);
  padded.set(bytes);
  padded[length] = 0x80;
  const view = new DataView(padded.buffer);
  view.setUint32(padded.length - 8, Math.floor(length / 0x20000000));
  view.setUint32(padded.length - 4, (length * 8) >>> 0);
  const hash = Uint32Array.from([
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
  ]);
  const w = new Uint32Array(64);
  for (let block = 0; block < padded.length; block += 64) {
    for (let i = 0; i < 16; i++) w[i] = view.getUint32(block + i * 4);
    for (let i = 16; i < 64; i++) {
      const s0 = rotr(at(w, i - 15), 7) ^ rotr(at(w, i - 15), 18) ^ (at(w, i - 15) >>> 3);
      const s1 = rotr(at(w, i - 2), 17) ^ rotr(at(w, i - 2), 19) ^ (at(w, i - 2) >>> 10);
      w[i] = (at(w, i - 16) + s0 + at(w, i - 7) + s1) >>> 0;
    }
    const v = hash.slice();
    for (let i = 0; i < 64; i++) {
      const e = at(v, 4);
      const a = at(v, 0);
      const choose = (e & at(v, 5)) ^ (~e & at(v, 6));
      const t1 =
        (at(v, 7) + (rotr(e, 6) ^ rotr(e, 11) ^ rotr(e, 25)) + choose + at(K, i) + at(w, i)) >>> 0;
      const majority = (a & at(v, 1)) ^ (a & at(v, 2)) ^ (at(v, 1) & at(v, 2));
      const t2 = ((rotr(a, 2) ^ rotr(a, 13) ^ rotr(a, 22)) + majority) >>> 0;
      v.copyWithin(1, 0, 7);
      v[4] = (at(v, 4) + t1) >>> 0;
      v[0] = (t1 + t2) >>> 0;
    }
    for (let i = 0; i < 8; i++) hash[i] = (at(hash, i) + at(v, i)) >>> 0;
  }
  return Array.from(hash, (word) => word.toString(16).padStart(8, "0")).join("");
}