C++ fast division/mod by 10^x

Question

In my program I use a lot of integer division by 10^x and integer mod function of power 10.

For example:

unsigned __int64 a = 12345;
a = a / 100;
...         


        

Answer 1

This is great for environments that lack any div operation and its only ~2x slower than native division on my i7 (optimizations off, naturally).

Here's a slightly faster version of the algorithm, though there are still some nasty rounding errors with negative numbers.

static signed Div10(signed n)
{
    n = (n >> 1) + (n >> 2);
    n += n < 0 ? 9 : 2;
    n = n + (n >> 4);
    n = n + (n >> 8);
    n = n + (n >> 16);
    n = n >> 3;
    return n;
}

Since this method is for 32-bit integer precision, you can optimize away most of these shifts if you're working in an 8-bit or 16-bit environment.