signExtend method

int signExtend(int startSize, int endSize)

Returns this sign-extended to endSize bits.

Sign extension is the operation of increasing the number of bits of a binary number while preserving the number's sign. This is done by appending digits to the most significant side of the number, i.e. if 6 bits are used to represent the value 00 1010 (decimal +10) and the sign extend operation increases the word length to 16 bits, the new representation is 0b0000 0000 0000 1010.

Simiarily, the 10 bit value 11 1111 0001 can be sign extended to 16-bits as 1111 1111 1111 0001. This method is provided as a convenience when converting between ints of smaller sizes to larger sizes.

Implementation

int signExtend(int startSize, int endSize) {
  if (endSize <= startSize) {
    throw RangeError.value(
      endSize,
      'endSize',
      'Expected endSize ($endSize) <= startSize ($startSize).',
    );
  }
  if (isClear(startSize)) {
    return bitRange(startSize, 0);
  } else {
    final mask = endSize == 32 ? 0xffffffff : (1 << endSize) - 1;
    final bits = this | (mask & ~((1 << startSize) - 1));
    return bits < 0 ? bits + 0x100000000 : bits;
  }
}