pack_unpack.c

/* from http://number-none.com/product/Packing%20Integers/index.html */

#include <stdio.h>
#include <stddef.h>
#include <stdint.h>

static uint32_t m_accumulator = 0;
static char m_buffer[17];

char* to_bitstring(uint32_t val, char buffer[], int size) {
    buffer[--size] = 0;
    while (size > 0) {
        buffer[--size] = (val % 2 ? '1' : '0');
        val = val >> 1;
    }

    return buffer; /* convenience */
}
 
void pack(uint32_t limit, uint32_t value) {
    m_accumulator = (limit * m_accumulator) + value;
    to_bitstring(m_accumulator, m_buffer, sizeof(m_buffer));
    printf("packing %d - 0x%08x [%s]\n", value, m_accumulator, m_buffer);
}
 
uint32_t unpack(uint32_t limit) {
    uint32_t quotient = m_accumulator / limit;
    uint32_t remainder = m_accumulator % limit;
    m_accumulator = quotient;
    return remainder;
}

int main(int argc, char const *argv[])
{
    int val;

    pack(10, 2);
    pack(10, 3);
    pack(10, 9);
    pack(10, 5);
    pack(10, 4);

    do {
        val = unpack(10);
        printf("%d\n", val);
    } while (val != 0);
    
    /* awesome trick from Jon Blow... */

    return 0;
}