twos-complement

twos-complement

Used in production by the MultiversX Go VM: mx-chain-vm-go.

Package twoscomplement: big-endian two's complement encoding of *big.Int.

math/big only offers sign-magnitude bytes (Bytes() / SetBytes() ignore the sign). This package adds the signed encoding, with the minimal number of bytes.

Encoding

• 0 is the empty byte string.
• Positive values are the magnitude, with a leading 0x00 if the top bit would otherwise be 1.
• Negative values are the bitwise negation of |x|-1, with a leading 0xFF if the top bit would otherwise be 0.

Value	Bytes
0	`` (empty)
127	7f
128	00 80
-1	ff
-128	80
-129	ff 7f

API

Function	Description
ToBytes(bi)	Minimal two's complement bytes. Does not alter the input.
ToBytesOfLength(bi, n)	Exactly n bytes, sign-extended; error if the value does not fit.
FromBytes(b)	Decode two's complement bytes to a new *big.Int. Empty input is 0.
SetBytes(z, b)	Same as FromBytes, but stores into z.
ByteLength(bi)	len(ToBytes(bi)), computed from the bit length, no allocation.
UnsignedByteLength(bi)	len(bi.Bytes()) (sign disregarded), no allocation.
CopyAlignRight(b, n)	Copy of b as n bytes, left-padded with zeroes or left-trimmed.

ByteLength and UnsignedByteLength treat nil as 0 bytes.

b := twoscomplement.ToBytes(big.NewInt(-129)) // ff 7f
x := twoscomplement.FromBytes(b)              // -129
n := twoscomplement.ByteLength(x)             // 2