Re: Is there a faster way to fetch data from stl map?

James Kanze <>
Sun, 28 Dec 2008 05:38:49 -0800 (PST)
On Dec 27, 8:24 pm, (blargg) wrote:

tni wrote:

[...]> struct hash {

     size_t operator()(int a) const {
         a = (a ^ 61) ^ (a >> 16);
         a = a + (a << 3);
         a = a ^ (a >> 4);
         a = a * 0x27d4eb2d;
         a = a ^ (a >> 15);
         return a;


Your hash function would be more portable if you used unsigned
int, since signed int can overflow in the expressions you use.

More to the point: how are the integer values distributed. If
they're dense, then just using an std::vector would be even
better than a hash table. If they're more or less randomly
distributed, just returning the integral value, converted to
size_t, would be a lot faster than his code, and just as
effective. (His function is really a good example of how not to
implement a hash function.)

James Kanze (GABI Software)
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