import { sha256 } from '@node-dlc/crypto';

export class CommitmentSecret {
  /**
   * Generic version of commitment secret generator as defined in
   * BOLT3.
   *
   * This function works by decrementing from bits to 0. If the index
   * number has the corresponding bit set, it flips the bit in the
   * secret and hashes it.
   *
   * The natural conclusion is the first secret flips every bit and
   * the last secret flips zero (and is thus the seed).
   *
   * This function can be used for generate the local per-commitment
   * secret based on the per-commitment seed and a commitment index
   * (in this case it) generates all 48 bits.
   *
   * This function can also be used to generate prior commitment
   * secrets from a newer secret, which acts as a prefix.
   *
   * @param base base secret that will be used to derive from
   * @param i secret at index I to generate
   * @param bits bits to evaluate, default is 48
   */
  public static derive(base: Buffer, i: bigint, bits = 48): Buffer {
    // intii
    let p = Buffer.from(base);

    for (let b = bits - 1; b >= 0; b--) {
      if (i & (BigInt(1) << BigInt(b))) {
        // flip the specified bit in the specific byte
        const byteIndex = Math.floor(b / 8);
        const bitIndex = b % 8;
        p[byteIndex] = p[byteIndex] ^ (1 << bitIndex);
        p = sha256(p);
      }
    }
    return p;
  }
}