mirror of
https://gitlab.nic.cz/labs/bird.git
synced 2024-12-22 09:41:54 +00:00
95 lines
2.1 KiB
C
95 lines
2.1 KiB
C
#include <stdio.h>
|
|
#include <math.h>
|
|
#include <unistd.h>
|
|
#include <stdlib.h>
|
|
|
|
int h[65536];
|
|
|
|
/*
|
|
* Probability analysis of hashing function:
|
|
*
|
|
* Let n be number of items and k number of boxes. For uniform distribution
|
|
* we get:
|
|
*
|
|
* Expected value of "item i is in given box": Xi = 1/k
|
|
* Expected number of items in given box: a = EX = E(sum Xi) = sum E(Xi) = n/k
|
|
* Expected square value: E(X^2) = E((sum Xi)^2) = E((sum_i Xi^2) + (sum_i,j i<>j XiXj)) =
|
|
* = sum_i E(Xi^2) + sum_i,j i<>j E(XiXj) =
|
|
* = sum_i E(Xi) [Xi is binary] + sum_i,j i<>j E(XiXj) [those are independent] =
|
|
* = n/k + n*(n-1)/k^2
|
|
* Variance: var X = E(X^2) - (EX)^2 = n/k + n*(n-1)/k^2 - n^2/k^2 =
|
|
* = n/k - n/k^2 = a * (1-1/k)
|
|
* Probability of fixed box being zero: Pz = ((k-1)/k)^n = (1-1/k)^n = (1-1/k)^(ak) =
|
|
* = ((1-1/k)^k)^a which we can approximate by e^-a.
|
|
*/
|
|
|
|
unsigned int hf(unsigned int n)
|
|
{
|
|
#if 0
|
|
n = (n ^ (n >> 16)) & 0xffff;
|
|
n = (n ^ (n << 8)) & 0xffff;
|
|
#elif 1
|
|
n = (n >> 16) ^ n;
|
|
n = (n ^ (n << 10)) & 0xffff;
|
|
#elif 0
|
|
n = (n >> 16) ^ n;
|
|
n *= 259309;
|
|
#elif 0
|
|
n ^= (n >> 20);
|
|
n ^= (n >> 10);
|
|
n ^= (n >> 5);
|
|
#elif 0
|
|
n = (n * 259309) + ((n >> 16) * 123479);
|
|
#else
|
|
return random();
|
|
#endif
|
|
return n;
|
|
}
|
|
|
|
int
|
|
main(int argc, char **argv)
|
|
{
|
|
int cnt=0;
|
|
int i;
|
|
|
|
int bits = atol(argv[1]);
|
|
int z = 1 << bits;
|
|
int max = atol(argv[2]);
|
|
|
|
while (max--)
|
|
{
|
|
unsigned int i, e;
|
|
if (scanf("%x/%d", &i, &e) != 2)
|
|
if (feof(stdin))
|
|
break;
|
|
else
|
|
fprintf(stderr, "BUGGG\n");
|
|
// i >>= (32-e);
|
|
// i |= (i >> e);
|
|
cnt++;
|
|
h[(hf(i) >> 1*(16 - bits)) & (z-1)]++;
|
|
}
|
|
// printf(">>> %d addresses\n", cnt);
|
|
#if 0
|
|
for(i=0; i<z; i++)
|
|
printf("%d\t%d\n", i, h[i]);
|
|
#else
|
|
{
|
|
int min=cnt, max=0, zer=0;
|
|
double delta=0;
|
|
double avg = (double) cnt / z;
|
|
double exdelta = avg*(1-1/(double)z);
|
|
double exzer = exp(-avg);
|
|
for(i=0; i<z; i++) {
|
|
if (h[i] < min) min=h[i];
|
|
if (h[i] > max) max=h[i];
|
|
delta += (h[i] - avg) * (h[i] - avg);
|
|
if (!h[i]) zer++;
|
|
}
|
|
printf("size=%5d, min=%d, max=%2d, delta=%-7.6g (%-7.6g), avg=%-5.3g, zero=%g%% (%g%%)\n", z, min, max, delta/z, exdelta, avg, zer/(double)z*100, exzer*100);
|
|
}
|
|
#endif
|
|
|
|
return 0;
|
|
}
|