N=SomeValue;
int *ptr=new int[N];
// initialize table, could be constant, but that's a lot to type out
int numBits[256] = { };
for(int i = 0; i < 256; ++i) {
numBits =
((i & 0x01) ? 1 : 0) +
((i & 0x02) ? 1 : 0) +
((i & 0x04) ? 1 : 0) +
((i & 0x08) ? 1 : 0) +
((i & 0xF0) ? 1 : 0) +
((i & 0xF1) ? 1 : 0) +
((i & 0xF2) ? 1 : 0) +
((i & 0xF4) ? 1 : 0) +
((i & 0xF8) ? 1 : 0);
}
int *ptr=new int[N];
int totalNumBits;
for(int i = 0; i < N; ++i) {
union {
int integer;
char charArray[4];
} int2char;
int2char.integer = ptr;
totalNumBits += numBits[int2char.charArray[0]] + numBits[int2char.charArray[1]] + numBits[int2char.charArray[2]] + numBits[int2char.charArray[3]];
}
Written in Notepad, it may or may not compile.
Also it's not tested, it may or may not be faster/slower, that's up to you to test.
unsigned popcount(unsigned x) {
x -= (x >> 1) & 0x55555555u;
x = (x & 0x33333333u) + ((x >> 2) & 0x33333333u);
x = (x + (x >> 4)) & 0x0f0f0f0fu;
return (x * 0x01010101u) >> 24; // assumes unsigned is 32 bits
}
gcc also has a function called __builtin_popcount, but on my computer the one above is three times faster. You can do it even faster using 64-bit arithmetic.
