Educational HyperLogLog demo in C

hyperLogLog.c

#include "stdio.h"
#include "stdlib.h"

unsigned int max(unsigned int a, unsigned int b) {
    return a > b ? a : b;
}

unsigned int bucket(unsigned int val) {
    return val & 0xF0000000 >> 28;
}

unsigned int trailingZeros(unsigned int val) {
   unsigned int mask = 0x00000001;
   unsigned int cnt = 0;
   while (val) {
       val = val & mask;
       mask <<= 1;
       cnt++;
   }
   return cnt;
} 

void initBuckets(unsigned int* buckets) {
    int i;
    for (i = 0; i < 16; i++) {
        buckets[i] = 0;
    }
}

unsigned int cardinality(unsigned int* buckets) {
    int i;
    unsigned int cnt = 0;
    for (i = 0; i < 16; i++) {
        if (buckets[i]) {
            cnt += (1 << (buckets[i] - 1)); // 2^buckets[i]
        }
    }
    return cnt;

}

void diagnosticReport(unsigned int* buckets) {
    int i;
    for (i = 0; i < 16; i++) {
        printf("bucket %i: %08x\n", i, buckets[i]);
    }
    printf("Cardinality %i\n", cardinality(buckets));
}

int main() {
    /*zero 16 buckets*/
    unsigned int buckets[16];
    initBuckets(buckets);

    unsigned int inputNumber = 0;
    unsigned int tryNo = 0;
    while (1) {
        printf("%ith Value for HLL\n", tryNo);
        /*user enters a number, 
         *
         * in real HLL we would hash the number, we're sorta expecting the user 
         * to bash on keys*/
        scanf("%u", &inputNumber);

        /* 
         * hash this number to a bucket based on the <numbuckets> significant
         * digits*/
        unsigned int buck = bucket(inputNumber);
        printf("Hashed To Bucket %i\n", buck);

        /* Write the largest number of trailing 0s we've seen thus far
         *
         * This is the magic of the hyperloglog, it gets very very hard
         * to get more trailing zeros after some point
         *
         * Its much like flipping a coin
         * */
        buckets[buck] = max(buckets[buck], trailingZeros(inputNumber));

        /* Print diagnistic, including cardinality. which is just a sum of
         *  2^bucketVal over all buckets if bucketVal != 0
         * */
        diagnosticReport(buckets);
        tryNo++;
    }

}

The trailingZeros function doesn't seem to do what the function's name suggests it does. Here's an alternate version that might work for you.

unsigned int trailingZeros(unsigned int val) {
  unsigned int cnt = 0;

  while (val) {
    if ((val & 1) == 0) {
      cnt++;
    }
    val >>= 1;
  }

  return cnt;
}

POSIX support: http://linux.die.net/man/3/ffs