#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <errno.h>

const char HEX_DIGITS[] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};

static inline int to_nibble(char *bin) {
    char *endptr = NULL;
    char *dup = strndup(bin, 4);
    int result = strtol(dup, &endptr, 2);
    if (dup + strlen(dup) != endptr) {
        free(dup);
        return -1;
    }
    free(dup);
    return result; 
}

char *pad_to_n(char *str, size_t n) {
    size_t len = (strlen(str) / n + 1) * n;
    char *out = calloc(len + 1, sizeof(char));
    char *ptr = out;
    for (size_t i = 0; i < len - strlen(str); i++) {
        *ptr = '0';
        ptr++;
    }
    while (*str) {
        *ptr = *str;
        ptr++;
        str++;
    }
    return out;
}

char *bin_to_hex(char *num) {
    char *padded = pad_to_n(num, 8);
    char *ptr = padded; 
    char *end = padded + strlen(padded);
    char *out = calloc(strlen(padded) / 4 + 1, sizeof(char));

    size_t i = 0;
    while (ptr < end) {
        int num = to_nibble(ptr);
        if (num == -1) {
            free(padded);
            free(out);
            return NULL;
        }
        out[i] = HEX_DIGITS[num];
        ptr += 4;
        i++;
    }
    free(padded);
    return out;
}