add S256 back

Author: amsanghi

common/math/big.go

@@ -24,6 +24,7 @@ import (
 
 // Various big integer limit values.
 var (
+	tt255     = BigPow(2, 255)
 	tt256     = BigPow(2, 256)
 	tt256m1   = new(big.Int).Sub(tt256, big.NewInt(1))
 	MaxBig256 = new(big.Int).Set(tt256m1)
@@ -177,3 +178,17 @@ func U256(x *big.Int) *big.Int {
 func U256Bytes(n *big.Int) []byte {
 	return PaddedBigBytes(U256(n), 32)
 }
+
+// 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
+func S256(x *big.Int) *big.Int {
+	if x.Cmp(tt255) < 0 {
+		return x
+	}
+	return new(big.Int).Sub(x, tt256)
+}