hex 13 dec 19 bin 10011 has 3 bits
hex 7FFFFFFF dec 2147483647 bin 1111111111111111111111111111111 has 31 bits
hex 7A31 dec 31281 bin 111101000110001 has 8 bits
hex A1 dec 161 bin 10100001 has 3 bits
hex A3 dec 163 bin 10100011 has 4 bits
Idiom #84 Count bits set in integer binary representation