Hash function collision test

test.c

// Hash function collision test
// * https://www.andreinc.net/2021/10/02/implementing-hash-tables-in-c-part-1
// * https://redd.it/q88m49
// This is free and unencumbered software released into the public domain.
#include <stdint.h>
#include <stdio.h>

#define INIT 5381
#define MULT 33

uint32_t hashf_djb2(char *str) 
{
    uint32_t hash = INIT;
    char c;
    while ((c = *str++)) {
        hash = ((hash << 5) + hash) + c;
    }
    return hash;
}

uint32_t hashf_djb2_m(char *str) 
{
    uint32_t hash = INIT;
    char c;
    while ((c = *str++)) {
        hash = hash * MULT + c;
    }
    return hash;
}

uint32_t hashf_sdbm(char *str) 
{
    uint32_t hash = 0;
    char c;
    while ((c = *str++)) {
        hash = c + (hash << 6) + (hash << 16) - hash;
    }
    return hash;
}

uint32_t finalizer(uint32_t h)
{
    h ^= h >> 16;
    h *= 0x3243f6a9U;
    h ^= h >> 16;
    return h;
}

int test(uint32_t (*f)(char *), _Bool finalize)
{
    int collisions = 0;
    char seen[1<<14] = {0};
    for (int i = 0; i < 10000; i++) {
        char buf[16];
        snprintf(buf, sizeof(buf), "%d", i);
        uint32_t h = f(buf);

        if (finalize) {
            h = finalizer(h);
        }

        h %= sizeof(seen);
        if (seen[h]) {
            collisions++;
        }
        seen[h] = 1;
    }
    return collisions;
}

int main(void)
{
    printf("       collisions finalized\n");
    printf("djb2   %10d%10d\n", test(hashf_djb2,   0), test(hashf_djb2,   1));
    printf("djb2_m %10d%10d\n", test(hashf_djb2_m, 0), test(hashf_djb2_m, 1));
    printf("sdbm   %10d%10d\n", test(hashf_sdbm,   0), test(hashf_sdbm,   1));
}