Some fun with C in a functional programming style

c-sar-cipher.c

// c-sar-cipher
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

//#include "c-sar-cipher.h"
void run(char* arg, void (*cout)(char*));
void print_string(char* str);

char* apply_cipher(char* input);
char rot13(char c);
char* rot13_for_each(char* input, char* output, size_t len, size_t i);


int main(int argc, char** argv)
{
    if (argc != 2)
        return 1;
    run(argv[1], print_string);
    return 0;
}

void print_string(char* str) { printf("%s\n", str); }

void run(char* arg, void (*cout)(char*))
{
    cout(apply_cipher(arg));
}

char* apply_cipher(char* input)
{
    size_t len = strlen(input);
    char* output = (char*)malloc(len + 1);
    output = rot13_for_each(input, output, len, 0);
    output[len] = '\0';
    return output;
}

char* rot13_for_each(char* input, char* output, size_t len, size_t i)
{
    if (i == len)
        return output;
    output[i] = rot13(input[i]);
    return rot13_for_each(input, output, len, i + 1);
}

char rot13(char c)
{
    if ((c >= 'a' && c <= 'm') || (c >= 'A' && c <= 'M'))
        return c + 13;
    else if ((c >= 'n' && c <= 'z') || (c >= 'N' && c <= 'Z'))
        return c - 13;
    else
        return c;
}

gcc c-sar-cipher.c -o rot13