cifra afim

afim.c

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

#define USAGE "USAGE: %s <cifra|decifra> [arquivo_entrada.txt]\n"
#define BUFFER_SIZE 2048

#define CMD_CIFRA "cifra"
#define CMD_DECIFRA "decifra"

#define N_ALF 26
#define A 7
#define B 22

void troca(int*a, int*b){
	int cup = *a;
	*a = *b;
	*b = cup;
}

int inversa_modular(int a, int b){
	int x[2] = {1, 0};
	int y[2] = {0, 1};
	int temp[2];

	if(b > a)
		troca(&a, &b);
	while(b != 0){
		int antb = b;
		int q = a/b;
		b = a%b;
		a = antb;

		temp[0] = x[0] - x[1]*q;
		temp[1] = y[0] - y[1]*q;
		x[0] = x[1]; y[0] = y[1];
		x[1] = temp[0]; y[1] = temp[1];
	}
	return (y[0] < 0)?(N_ALF+y[0]):y[0];
}

char cifra(char c){
	return (((c - 'a')*A+B)%N_ALF)+'a';
}

char decifra(char c){
	int inv = inversa_modular(A, N_ALF);
	return (((N_ALF+(c - 'a')-B)*inv)%N_ALF)+'a';
}

//ponteiro para a função
char (*func) (char);

int main (int argc, char *argv[]) {
	FILE* ifile;
	char buffer[BUFFER_SIZE];
	
	if(argc != 3 && argc != 2){
		printf(USAGE, argv[0]);
		return -1;
	}else{

		if(strcmp(argv[1], CMD_DECIFRA) == 0){
			func = decifra;
		}else{
			func = cifra;
		}

		if(argc == 3){
			//arquivo de entrada
			ifile = fopen(argv[2], "r");
		}else if(argc == 2){
			//entrada padrão
			ifile = stdin;
		}
	}

	int n_data;
	while((n_data = fread(buffer, sizeof(char), BUFFER_SIZE, ifile))){
		int i;
		//altera o buffer
		for(i=0; i<n_data; i++){
			if(buffer[i] >= 'a' && buffer[i] <= 'z')
				buffer[i] = func(buffer[i]);
		}


		for(i=0; i<n_data; i++){
			printf("%c", buffer[i]);
		}
		
	}
	
	fclose(ifile);
	return 0;
}