initial commit

src/rnd.c

@@ -0,0 +1,121 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <time.h>
+#include <sys/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) {
+        struct timeval tv;
+        gettimeofday(&tv, NULL);
+        // srand(time(NULL));
+        srand(tv.tv_usec);
+        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>
+#include <assert.h>
+int main() {
+    int n = 10000;
+    while (--n > 0) {
+        // printf("%c", get_random_letter());
+        int r = random_number(0,10);
+        assert(r >= 0);
+        assert(r <= 10);
+    }
+    return 0;
+}
+
+#endif