URL shortner implementation in C. This is just an example how it could be implemented. Ideally the lookup table should be pre-generated and not hardcoded.

url-shortner.c

/**
 * Author: Felipe Ferreri Tonello <eu@felipetonello.com>
 *
 * This url-shortner it only works with ASCII characters. It encodes and
 * decodes ids.
 * You can change base_x as you wish.
 *
 * It runs at least 20 times faster then a Python implementation.
 *
 * $ time python url-shortner.py -s I7
 * 1123
 *
 * real    0m0.020s
 * user    0m0.015s
 * sys     0m0.001s
 * $ time ./url-shortner -d I7
 * 1123
 *
 * real    0m0.001s
 * user    0m0.000s
 * sys     0m0.000s
 *
 * But this gain it's not really useful.
 *
 * TODO: use big numbers instead of unsigned long long
 * Compile instructions: gcc url-shortner.c -o url-shortner -lm
 */

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <math.h>
#include <string.h>

#define BUFFER_SIZE 50
#define BASE_NUM (sizeof(base_x) / sizeof(*(base_x)))
#define HASH_BASE_X_SIZE (sizeof(char) * (0xff + 1))

/* NOTE: Ideally you want to generate the lookup table
   and not hard code it. */

/* Base X lookup table */
static char base_x[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

/* Inverted base X lookup table */
static size_t base_x_to_dec[HASH_BASE_X_SIZE];

/* This is a work around. You should use
   a pre generated lookup table */
static void init_hash_base_x()
{
	size_t i, j;

	for (i = 0; i < HASH_BASE_X_SIZE; ++i)
		for (j = 0; j < BASE_NUM; ++j)
			if (i == (size_t) base_x[j]) {
				base_x_to_dec[i] = j;
				break;
			}
}

inline static swap(char *a, char *b)
{
	char tmp = *a;
	*a = *b;
	*b = tmp;
}

static void strrev(char *str, size_t len)
{
	size_t i, mid = len / 2;

	for (i = 0; i < mid; ++i)
		swap(&str[i], &str[len - i - 1]);
}

unsigned long long decode(char *str)
{
	unsigned long long sum = 0;
	size_t i = 0;

	strrev(str, strlen(str));

	while (*str) {
		sum += base_x_to_dec[(size_t)*str] * pow(BASE_NUM, i);
		str++, i++;
	}

	return sum;
}

char * encode(unsigned long long id)
{
	char *buf = malloc(sizeof(char) * BUFFER_SIZE);
	size_t i = 0;

	while (id > 0) {
		int mod = id % BASE_NUM;
		buf[i] = base_x[mod];
		id /= BASE_NUM;
		++i;
	}

	buf[i] = '\0';

	strrev(buf, i);

	return buf;
}

void print_usage_and_exit(char *argv0)
{
	fprintf(stderr, "usage: %s option value\n"
	        "Options:\n"
	        "  -e\tEnconde\n"
	        "  -d\tDecode\n"
	        "  -h\tDisplay this information\n", argv0);

	exit(-1);
}

int main(int argc, char *argv[])
{
	if (argc != 3)
		print_usage_and_exit(argv[0]);

	if(strcmp(argv[1], "-e") == 0) {
		char *buf = encode(strtoull(argv[2], NULL, 10));
		printf("%s\n", buf);
		free(buf);

	} else if(strcmp(argv[1], "-d") == 0) {
		unsigned long long dec;

		init_hash_base_x();

		dec = decode(argv[2]);

		printf("%llu\n", dec);

	} else {
		print_usage_and_exit(argv[0]);
	}

	return 0;
}