stirling.c: first order Stirling factorial approximation calculator

stirling.c

/* Copyright (c) 2012 Kyle Gorman
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to 
 * deal in the Software without restriction, including without limitation the 
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or 
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING 
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 * Stirling's approximation calculator as an exponent on 2
 * Kyle Gorman <kgorman@ling.upenn.edu>
 */

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

// compute two-sided test p-value
double stirling_base_2(int n) {
    return (n * log(n) - n) * M_LOG2E;
}

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

    if (argc != 2) {
        fprintf(stderr, "USAGE:\n\n\tstirling N\n\nAborting.\n");
        return 1;
    }

    int n = atoi(argv[1]);
    if (n < 1) {
        fprintf(stderr, "Invalid value.\n");
        return 1;
    }
    printf("2 ^ %.03f\n", stirling_base_2(n));

    return 0;
}