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