// Reference https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki
// Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S]
// NOTE: SIGHASH byte ignored AND restricted, truncate before use

export function check(buffer: Buffer): boolean {
  if (buffer.length < 8) return false;
  if (buffer.length > 72) return false;
  if (buffer[0] !== 0x30) return false;
  if (buffer[1] !== buffer.length - 2) return false;
  if (buffer[2] !== 0x02) return false;

  const lenR = buffer[3];
  if (lenR === 0) return false;
  if (5 + lenR >= buffer.length) return false;
  if (buffer[4 + lenR] !== 0x02) return false;

  const lenS = buffer[5 + lenR];
  if (lenS === 0) return false;
  if (6 + lenR + lenS !== buffer.length) return false;

  if (buffer[4] & 0x80) return false;
  if (lenR > 1 && buffer[4] === 0x00 && !(buffer[5] & 0x80)) return false;

  if (buffer[lenR + 6] & 0x80) return false;
  if (lenS > 1 && buffer[lenR + 6] === 0x00 && !(buffer[lenR + 7] & 0x80))
    return false;
  return true;
}

export function decode(buffer: Buffer): { r: Buffer; s: Buffer } {
  if (buffer.length < 8) throw new Error('DER sequence length is too short');
  if (buffer.length > 72) throw new Error('DER sequence length is too long');
  if (buffer[0] !== 0x30) throw new Error('Expected DER sequence');
  if (buffer[1] !== buffer.length - 2)
    throw new Error('DER sequence length is invalid');
  if (buffer[2] !== 0x02) throw new Error('Expected DER integer');

  const lenR = buffer[3];
  if (lenR === 0) throw new Error('R length is zero');
  if (5 + lenR >= buffer.length) throw new Error('R length is too long');
  if (buffer[4 + lenR] !== 0x02) throw new Error('Expected DER integer (2)');

  const lenS = buffer[5 + lenR];
  if (lenS === 0) throw new Error('S length is zero');
  if (6 + lenR + lenS !== buffer.length) throw new Error('S length is invalid');

  if (buffer[4] & 0x80) throw new Error('R value is negative');
  if (lenR > 1 && buffer[4] === 0x00 && !(buffer[5] & 0x80))
    throw new Error('R value excessively padded');

  if (buffer[lenR + 6] & 0x80) throw new Error('S value is negative');
  if (lenS > 1 && buffer[lenR + 6] === 0x00 && !(buffer[lenR + 7] & 0x80))
    throw new Error('S value excessively padded');

  // non-BIP66 - extract R, S values
  return {
    r: buffer.slice(4, 4 + lenR),
    s: buffer.slice(6 + lenR),
  };
}

/*
 * Expects r and s to be positive DER integers.
 *
 * The DER format uses the most significant bit as a sign bit (& 0x80).
 * If the significant bit is set AND the integer is positive, a 0x00 is prepended.
 *
 * Examples:
 *
 *      0 =>     0x00
 *      1 =>     0x01
 *     -1 =>     0xff
 *    127 =>     0x7f
 *   -127 =>     0x81
 *    128 =>   0x0080
 *   -128 =>     0x80
 *    255 =>   0x00ff
 *   -255 =>   0xff01
 *  16300 =>   0x3fac
 * -16300 =>   0xc054
 *  62300 => 0x00f35c
 * -62300 => 0xff0ca4
 */
export function encode(r: Buffer, s: Buffer): Buffer {
  const lenR = r.length;
  const lenS = s.length;
  if (lenR === 0) throw new Error('R length is zero');
  if (lenS === 0) throw new Error('S length is zero');
  if (lenR > 33) throw new Error('R length is too long');
  if (lenS > 33) throw new Error('S length is too long');
  if (r[0] & 0x80) throw new Error('R value is negative');
  if (s[0] & 0x80) throw new Error('S value is negative');
  if (lenR > 1 && r[0] === 0x00 && !(r[1] & 0x80))
    throw new Error('R value excessively padded');
  if (lenS > 1 && s[0] === 0x00 && !(s[1] & 0x80))
    throw new Error('S value excessively padded');

  const signature = Buffer.allocUnsafe(6 + lenR + lenS);

  // 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S]
  signature[0] = 0x30;
  signature[1] = signature.length - 2;
  signature[2] = 0x02;
  signature[3] = r.length;
  r.copy(signature, 4);
  signature[4 + lenR] = 0x02;
  signature[5 + lenR] = s.length;
  s.copy(signature, 6 + lenR);

  return signature;
}