hex2bin.c

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//
//
//  Author:     Lasse Collin
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
 
#include "sysdefs.h"
#include <stdio.h>
#include <ctype.h>
 
 
static int
getbin(int x)
{
    if (x >= '0' && x <= '9')
        return x - '0';
 
    if (x >= 'A' && x <= 'F')
        return x - 'A' + 10;
 
    return x - 'a' + 10;
}
 
 
int
main(void)
{
    while (true) {
        int byte = getchar();
        if (byte == EOF)
            return 0;
        if (!isxdigit(byte))
            continue;
 
        const int digit = getchar();
        if (digit == EOF || !isxdigit(digit)) {
            fprintf(stderr, "Invalid input\n");
            return 1;
        }
 
        byte = (getbin(byte) << 4) | getbin(digit);
        if (putchar(byte) == EOF) {
            perror(NULL);
            return 1;
        }
    }
}