Function S256

utils/big.go:200–205

S256 interprets x as a two's complement number. x must not exceed 256 bits (the result is undefined if it does) and is not modified. S256(0) = 0 S256(1) = 1 S256(2**255) = -2**255 S256(2**256-1) = -1.

(x *big.Int)

//   S256(2**255)   = -2**255
//   S256(2**256-1) = -1.
func S256(x *big.Int) *big.Int {
	if x.Cmp(tt255) < 0 {
		return x
	}
	return new(big.Int).Sub(x, tt256)
}

// Exp implements exponentiation by squaring.
// Exp returns a newly-allocated big integer and does not change