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