fiatScalarSub function

void fiatScalarSub(List<BigInt> out1, List<BigInt> arg1, List<BigInt> arg2)

fiatScalarSub subtracts two field elements in the Montgomery domain.

Preconditions: 0 ≤ eval arg1 < m 0 ≤ eval arg2 < m Postconditions: eval (from_montgomery out1) mod m = (eval (from_montgomery arg1) - eval (from_montgomery arg2)) mod m 0 ≤ eval out1 < m

Implementation

void fiatScalarSub(List<BigInt> out1, List<BigInt> arg1, List<BigInt> arg2) {
  final (BigInt x1, int x2) = Bits.sub64(arg1[0], arg2[0], 0);

  final (BigInt x3, int x4) = Bits.sub64(arg1[1], arg2[1], x2);

  final (BigInt x5, int x6) = Bits.sub64(arg1[2], arg2[2], x4);

  final (BigInt x7, int x8) = Bits.sub64(arg1[3], arg2[3], x6);

  final x9 = fiatScalarCmovznzU64(
      x8.toBigInt, BigInt.zero, '18446744073709551615'.toBigInt());

  final (BigInt x10, int x11) =
      Bits.add64(x1, (x9 & '6346243789798364141'.toBigInt()), 0);

  final (BigInt x12, int x13) =
      Bits.add64(x3, (x9 & '1503914060200516822'.toBigInt()), x11);

  final (BigInt x14, int x15) = Bits.add64(x5, BigInt.zero, x13);

  final (BigInt x16, int _) =
      Bits.add64(x7, (x9 & '1152921504606846976'.toBigInt()), x15);

  out1[0] = x10;
  out1[1] = x12;
  out1[2] = x14;
  out1[3] = x16;
}