caesarcrypt/src/crypt.c

/*
  * Filename: crypt.c
  * Authors(s): Roland (r.weirhowell@gmail.com)
  * Description: Perform caesar cipher cryptographic operations
  *
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/.
*/

#include "crypt.h"

#include <stdbool.h>
#include <stdint.h>


/* 
  * For built in C mod operator (%) -1 % 26 == -1
  * For the requirements of this algorithm -1 mod 26 == 25 is required
  * The below functions accomplish this
*/

// Which of the two below functions is faster (find out!)
inline int mod(int a, int b)
{
	return (a % b + b) % b;
}


inline int fast_mod(int a, int b)
{
	const int result = a % b;
	return result >= 0 ? result : result + b;
}


/*
  * 65 - 90	 = ASCII 'A' - 'Z'
  * 97 - 122 = ASCII 'a' - 'z'
*/

inline bool isupper(char c)
{
	return c >= 'A' && c <= 'Z';
}


void caesar_encrypt(char* str, int64_t size, int key)
{
	for (int64_t i = 0; i < size && str[i] != '\n'; i++)
		if (str[i] != ' ')
		{
			bool upper = isupper(str[i]);
			str[i] += (char)key - (upper ? 65 : 97);
			str[i] %= 26;
			str[i] += (upper ? 65 : 97);
		}
}


void caesar_decrypt(char* str, int64_t size, int key)
{
	for (int64_t i = 0; i < size && str[i] != '\n'; i++)
		if (str[i] != ' ')
		{
			bool upper = isupper(str[i]);
			str[i] = str[i] - (char)key - (upper ? 65 : 97);
			str[i] = (char)mod(str[i], 26);
			str[i] += (upper ? 65 : 97);
		}
}