#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int encrypt(int key, char ch)
{
	return (ch + key) % 256;
}

int decrypt(int key, char ch)
{
	return (ch - key) % 256;
}

typedef enum
{
	action_unknown,
	action_encrypt,
	action_decrypt
} Action;

int process_file(Action action, int key, FILE* source, FILE* target)
{
	int ch = 0;
	while ( (ch = fgetc(source)) != EOF )
		fputc( action == action_encrypt ? encrypt(key, ch) : decrypt(key, ch), target);

	return 0;
}

int main(int argc, char* argv[])
{
	if ( argc < 3 )
	{
		// argv[]:     0      1   2   3        4           5
		printf("Usage: caesar -ed key [SOURCE] [-W|TARGET]\n");
		return 0;
	}

	Action action = action_encrypt;
	if ( strcmp(argv[1], "-d") == 0 ) action = action_decrypt;

	int key = atoi(argv[2]);

	FILE* source = stdin;
	if ( argc >= 4 )
	{
		source = fopen(argv[3], "r");
		if ( ! source ) return fprintf(stderr, "caesar: could not open %s\n", argv[3]);
	}

	bool overwrite = false;
	FILE* target = stdout;
	if ( argc >= 5 )
	{
		const char* filename = argv[4];
		if ( strcmp(argv[4], "-W") == 0 )
			overwrite = filename = "caesar.temp";

		target = fopen(filename, "w");
		if ( ! target )
		{
			fclose(source);
			return fprintf(stderr, "caesar: could not write %s\n", argv[4]);
		}
	}

	process_file(action, key, source, target);
	fclose(source);
	fclose(target);

	if ( overwrite)
	{
		remove(argv[3]);
		rename("caesar.temp", argv[3]);
	}

	return 0;
}