wordsearch/src/rnd.c

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

#include "rnd.h"

 /* https://www.math.cornell.edu/~mec/2003-2004/cryptography/subs/frequencies.html */

#define ALPHABET_SIZE 26
#define RND_MAXINT 10000

struct letter_frequency {
    int p; /* pegged to 10000 rather than 1 */
    char c;
} letter_frequencies[] = {
    {7,    'Z'},
    {10,   'J'},
    {11,   'Q'},
    {17,   'X'},
    {69,   'K'},
    {111,  'V'},
    {149,  'B'},
    {182,  'P'},
    {203,  'G'},
    {209,  'W'},
    {211,  'Y'},
    {230,  'F'},
    {261,  'M'},
    {271,  'C'},
    {288,  'U'},
    {398,  'L'},
    {432,  'D'},
    {592,  'H'},
    {602,  'R'},
    {628,  'S'},
    {695,  'N'},
    {731,  'I'},
    {768,  'O'},
    {812,  'A'},
    {910,  'T'},
    {1202, 'E'},
};

/* from http://stackoverflow.com/questions/822323/how-to-generate-a-random-number-in-c */
int random_number(int min_num, int max_num) {
	static int initialised = 0;
    int result  = 0;
    int low_num = 0;
    int hi_num  = 0;
    if(min_num < max_num) {
        low_num = min_num;
        hi_num = max_num + 1; // this is done to include max_num in output.
    } else {
        low_num = max_num + 1;// this is done to include max_num in output.
        hi_num = min_num;
    }
    if(!initialised) {
    	srand(time(NULL));
    	initialised = 1;
    }
    result = ( rand() % (hi_num - low_num) ) + low_num;
    return result;
}

char get_random_letter() {
	int rnd = random_number(0, RND_MAXINT);
	for(int i=0; i < ALPHABET_SIZE; i++) {
		if(rnd < letter_frequencies[i].p) {
			return letter_frequencies[i].c;
		}
		rnd -= letter_frequencies[i].p;
	}
    return '\0';
}

#ifdef DEBUG_RND_MAIN
#include <stdio.h>

int main() {
	int n = 32;
	while(--n > 0) {
		printf("%c", get_random_letter());
	}
	return 0;
}

#endif