Bitwise memmove

Question

What is the best way to implement a bitwise memmove? The method should take an additional destination and source bit-offset and the count should be in bits too.

Answer 1

Here is a partial implementation (not tested). There are obvious efficiency and usability improvements.

Copy n bytes from src to dest (not overlapping src), and shift bits at dest rightwards by bit bits, 0 <= bit <= 7. This assumes that the least significant bits are at the right of the bytes

void memcpy_with_bitshift(unsigned char *dest, unsigned char *src, size_t n, int bit)
{
  int i;

  memcpy(dest, src, n);

  for (i = 0; i < n; i++) {
    dest[i] >> bit;
  }

  for (i = 0; i < n; i++) {
    dest[i+1] |= (src[i] << (8 - bit));
  }
}

Some improvements to be made:

• Don't overwrite first bit bits at beginning of dest.
• Merge loops
• Have a way to copy a number of bits not divisible by 8
• Fix for >8 bits in a char