///
/// input hex string then convert to binary string output as file
///

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

int main(int argc, char* argv[])
{
  // get input hex string to convert to binary
  if (argc != 2)
  {
    printf("Usage: hextobin <input-hexstring>\n");
    return -1;
  }

  // get input hex string
  char* hexstring = argv[1];
  char* hexstr_ptr = hexstring;

  // find string length
  int len = strlen(hexstring);
  // form hex string
  char temp_hexstr[4+1];
  memset(temp_hexstr, 0, sizeof(temp_hexstr));

  // result file content is half of the input string's length
  unsigned char bytes_result[len / 2];

  // result index to store result back to our array
  int ri = 0;
  // loop through each byte in hexstring
  for (int i=0; i<len; i+=2)
  {
    // form hexstring
    strncpy(temp_hexstr+2, hexstr_ptr, 2);
    // prefix with 0x
    temp_hexstr[0] = '0';
    temp_hexstr[1] = 'x';
    // proceed hexstring pointer
    hexstr_ptr += 2;

    // convert hexstring to value
    unsigned char byte = strtol(temp_hexstr, NULL, 0);
    // store result in our result array
    bytes_result[ri++] = byte;

    //printf("%s => 0x%X\n", temp_hexstr, byte);

    // zero out temp hexstr
    memset(temp_hexstr, 0, sizeof(temp_hexstr));
  }

  //for (int i=0; i<ri; i++)
  //{
  //  printf("bytes_result[%i] = 0x%X\n", i, bytes_result[i]);
  //}

  // open file for writing
  FILE* file = fopen("output.txt", "wb");
  if (file == NULL)
  {
    printf("Cannot open file for writing");
    return -1;
  }

  // write all at once to result file
  int result_obj_written = 0;
  result_obj_written = fwrite(bytes_result, sizeof(bytes_result), 1, file);

  if (result_obj_written != 1)
  {
    printf("Something wrong in writing the file");

    // close the file
    fclose(file);
    file = NULL;
    return -1;
  }

  // close the file
  fclose(file);
  file = NULL;

  return 0;
}