1+3+5+... benchmark source code

#include <stdio.h>

#ifndef TEST
#    define TEST 1
#endif

#define SQUARE(a) \
    ((a) * (a))

unsigned long
oddsum_On(unsigned long n)
{
    unsigned long total = 0;

    for(unsigned long i = 1; i <= n; i += 2)
        total += i;

    return total;
}

unsigned long
oddsum_O2n(unsigned long n)
{
    unsigned long total = 0;

    for(unsigned long i = 0; i <= n; ++i)
        if(i % 2 == 1)
            total += i;

    return total;
}

unsigned long
oddsum_O1(unsigned long n)
{
    return SQUARE( (n + 1) / 2 );
}

int
main(void)
{
#if TEST == 1
    // verify that the functions work correctly
    printf("%lu\n", oddsum_O1(111));
    printf("%lu\n", oddsum_On(111));
    printf("%lu\n", oddsum_O2n(111));
#elif TEST == 2
    // benchmark O(1) solution
    oddsum_O1(4000000001L);
#elif TEST == 3
    // benchmark O(n) solution
    oddsum_On(4000000001L);
#elif TEST == 4
    // benchmark O(2n) solution
    oddsum_O2n(4000000001L);
#endif
    // example:
    // cc odds.c -DTEST=2 && time ./a.out
    // this benchmarks the O(1) solution
}