gistfile1.c

#include <stdio.h>
#include <stdlib.h>

#define COUNTER_RIGHT_BITS 24
#define COUNTER_MASK 0x000000ff
#define ADDER_RIGHT_BITS 16
#define ADDER_MASK 0x000000ff
#define SUM_MASK 0x0000ffff

#define extract_sum(v) (v & SUM_MASK)
#define add_to_sum(v, w) (extract_sum(v) + w)
#define extract_adder(v) ((v >> ADDER_RIGHT_BITS) & ADDER_MASK)
#define shift_adder(v) (v << ADDER_RIGHT_BITS)
#define extract_counter(v) ((v >> COUNTER_RIGHT_BITS) & COUNTER_MASK)
#define shift_counter(v) (v << COUNTER_RIGHT_BITS)
#define make_x(s, a, c) ((s & SUM_MASK) | shift_adder(a) | shift_counter(c))

int sum_with_one_var(int from, int to) {
    int x = make_x(0, from - 1, 0);
    while (1) {
        x = make_x(add_to_sum(x, extract_adder(x) + 1),
              extract_adder(x) + 1,
              extract_counter(x) + 1);
              
            
        if (extract_counter(x) == (to - from + 1))
          break;
    }
    
    return extract_sum(x);
}

int sum_normally(int from, int to) {
    int sum = 0;
    int adder = from - 1;
    int i;

    for (i = 0; i < (to - from + 1); i++) {
        sum += adder + 1;
        adder += 1;
    }

    return sum;
}

int main(int argc, char** argv) {
    if (argc != 3) {
        fprintf(stderr, "Bad argument\n");
        fprintf(stderr, "Usage: %s <from> <to>\n", argv[0]);
        
        exit(1);
    }
    
    int from = atoi(argv[1]);
    int to = atoi(argv[2]);
    
    printf("sum_with_one_var: %d\n", sum_with_one_var(from ,to));
    printf("sum_normally: %d\n", sum_normally(from, to));
    
    return 0;
}