Quick Rot13 impl

rot13.c

#include "stdio.h"
#include "stdlib.h"
#include "string.h"

const char upperAlpha[26] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const char lowerAlpha[26] = "abcdefghijklmnopqrstuvwxyz";

int charPos(const char *alphabet, char letter) {
  for (int i = 0; i < 26; ++i) {
    if (letter == alphabet[i])
      return i;
  }
  return -1;
}

void rot13(char *src, char *dst, size_t size) {
  for (int i = 0; i < (int)size; ++i) {
    if (charPos(upperAlpha, src[i]) != -1) {
      int pos = charPos(upperAlpha, src[i]);
      pos = (pos + 13) % 26;
      dst[i] = upperAlpha[pos];
    } else if (charPos(lowerAlpha, src[i]) != -1) {
      int pos = charPos(lowerAlpha, src[i]);
      pos = (pos + 13) % 26;
      dst[i] = lowerAlpha[pos];
    } else {
      dst[i] = src[i];
    }
  }
}

int main() {
  char *src = "Hello World!";
  char *dst = malloc(strlen(src));
  printf("%s \n", src);
  rot13(src, dst, strlen(src));
  printf("%s \n", dst);
  rot13(dst, dst, strlen(dst));
  printf("%s \n", dst);
}