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

size_t hex_to_binary(unsigned char *out, const char hex[static 1]) {
  for (size_t i = 0; i < strlen(hex); ++i) {
    char c = tolower(hex[i]);
    unsigned char value;
    if (isdigit(c)) {
      value = c - '0';
    } else {
      value = 10 + c - 'a';
    }
    if (i % 2 == 0) {
      value <<= 4;
    }
    out[i / 2] += value;
  }
  return strlen(hex) / 2 + strlen(hex) % 2;
}

unsigned char *binary_to_base64(unsigned char *out,
                                const unsigned char in[static 1], size_t len) {
  static const unsigned char base64_table[65] =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

  for (size_t i = 0; i < len - 2; i += 3) {
    size_t j = i / 3 * 4;
    out[j] = base64_table[in[i] >> 2];
    out[j+1] = base64_table[((in[i] & 0x03) << 4) | (in[i+1] >> 4)];
    out[j+2] = base64_table[((in[i+1] & 0x0f) << 2) | (in[i+2] >> 6)];
    out[j+3] = base64_table[in[i+2] & 0x3f];
  }
  return out;
}

int main(int argc, char *argv[argc + 1]) {
  if (argc < 2) {
    printf("Usage: %s <hex>\n", argv[0]);
    return EXIT_FAILURE;
  }

  unsigned char buf[512] = {0};
  size_t len = hex_to_binary(buf, argv[1]);
  unsigned char out[512] = {'\0'};
  binary_to_base64(out, buf, len);
  printf("%s\n", out);

  return EXIT_SUCCESS;
}