Unsigned Integer to BCD conversion?

Question

I know you can use this table to convert decimal to BCD:

0 0000

1 0001

2 0010

3 0011

4 0100

5 0101

6 01

Answer 1

If you want two decimal digits per byte, and "unsigned" is half the size of "unsigned long" (use uint32 and uint64 typedefs if you want):

unsigned long bcd(unsigned x) {
  unsigned long ret=0;
  while(x>0) {
    unsigned d=x/10;
    ret=(ret<<4)|(x-d*10);
    x=d;
  }
  return ret;
}

This leaves you with the least significant (unit) decimal digit in the least significant half-byte. You can also execute the loop a fixed number (10 for uint32) of times, not stopping early when only 0 bits are left, which would allow the optimizer to unroll it, but that's slower if your numbers are often slow.