count of bits set in a long

[Richard A. O'Keefe]

In article <37545 at ut-emx.uucp>, nwagner at ut-emx.uucp (Neal R. Wagner) writes:
>   I need a fast method to count the number of bits that are set in a 32-bit

A simple way is to set up a table:
	static char n_bits_set[256] = {0, 1, 1, 2, ... };

	int bit_count(n)
	    unsigned long int n;
	    {
		return n_bits_set[(n >> 24)      ]
		     + n_bits_set[(n >> 16) & 255]
		     + n_bits_set[(n >>  8) & 255]
		     + n_bits_set[(n      ) & 255];
	    }

There's a rather nice loop that does one iteration per bit, and works
for any size, but I'll leave that for others.
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