/* Write the function htoi(s), which converts a string of hexadecimal
 * digits (including an optional 0x or 0X) into its equivalent integer
 * value. The allowable digits are 0 through 9, a through f, and
 * A through F.
 *
 * I've tried hard to restrict the solution code to use only what
 * has been presented in the book at this point (page 46). As a
 * result, the implementation may seem a little naive. Error
 * handling is a problem. I chose to adopt atoi's approach, and
 * return 0 on error. Not ideal, but the interface doesn't leave
 * me much choice.
 *
 * I've used unsigned int to keep the behaviour well-defined even
 * if overflow occurs. After all, the exercise calls for conversion
 * to 'an integer', and unsigned ints are integers!
 */

/* These two header files are only needed for the test driver */
#include <stdio.h>
#include <stdlib.h>

/* Here's a helper function to get me around the problem of not
 * having strchr
 */

int hexalpha_to_int(int c)
{
  char hexalpha[] = "aAbBcCdDeEfF";
  int i;
  int answer = 0;

  for(i = 0; answer == 0 && hexalpha[i] != '\0'; i++)
  {
    if(hexalpha[i] == c)
    {
      answer = 10 + (i / 2);
    }
  }

  return answer;
}

unsigned int htoi(const char s[])
{
  unsigned int answer = 0;
  int i = 0;
  int valid = 1;
  int hexit;

  if(s[i] == '0')
  {
    ++i;
    if(s[i] == 'x' || s[i] == 'X')
    {
      ++i;
    }
  }

  while(valid && s[i] != '\0')
  {
    answer = answer * 16;
    if(s[i] >= '0' && s[i] <= '9')
    {
      answer = answer + (s[i] - '0');
    }
    else
    {
      hexit = hexalpha_to_int(s[i]);
      if(hexit == 0)
      {
        valid = 0;
      }
      else
      {
        answer = answer + hexit;
      }
    }

    ++i;
  }

  if(!valid)
  {
    answer = 0;
  }

  return answer;
}

/* Solution finished. This bit's just a test driver, so
 * I've relaxed the rules on what's allowed.
 */

int main(void)
{
  char *endp = NULL;
  char *test[] =
  {
    "F00",
    "bar",
    "0100",
    "0x1",
    "0XA",
    "0X0C0BE",
    "abcdef",
    "123456",
    "0x123456",
    "deadbeef",
    "zog_c"
  };

  unsigned int result;
  unsigned int check;

  size_t numtests = sizeof test / sizeof test[0];

  size_t thistest;

  for(thistest = 0; thistest < numtests; thistest++)
  {
    result = htoi(test[thistest]);
    check = (unsigned int)strtoul(test[thistest], &endp, 16);

    if((*endp != '\0' && result == 0) || result == check)
    {
      printf("Testing %s. Correct. %u\n", test[thistest], result);
    }
    else
    {
      printf("Testing %s. Incorrect. %u\n", test[thistest], result);
    }
  }

  return 0;
}

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

long hchartoi (char hexdig, int pos);   /* converts a hex char to decimal knowing its 0 based place value */
long htoi (char hexstring[]);               /* converts a string of hex bits to integer ... */

int main(void)
{
  char *endp = NULL;
  char *test[] =
  {
    "F00",
    "bar",
    "0100",
    "0x1",
    "0XA",
    "0X0C0BE",
    "abcdef",
    "123456",
    "0x123456",
    "deadbeef",
    "zog_c"
  };

  long int result;
  long int check;

  size_t numtests = sizeof test / sizeof test[0];

  size_t thistest;

  for(thistest = 0; thistest < numtests; thistest++)
  {
    result = htoi(test[thistest]);
    check = strtol(test[thistest], &endp, 16);

    if((*endp != '\0' && result == -1) || result == check)
    {
      printf("Testing %s. Correct. %ld\n", test[thistest], result);
    }
    else
    {
      printf("Testing %s. Incorrect. %ld\n", test[thistest], result);
    }
  }

  return 0;
}

long htoi (char s[])
{

    char *p = &s[strlen(s)-1];
    long deci = 0, dig = 0;
    int pos = 0;

    while (p >= s) {

        if ((dig = hchartoi(*p, pos)) < 0 ) {
            printf("Error\n");
            return -1;

        }
        deci += dig;
        --p;
        ++pos;

    }
    return deci;
}

/* convert hex char to decimal value */
long hchartoi (char hexdig, int pos)
{

    char hexdigits[] = "0123456789ABCDEF";
    char *p = &hexdigits[0];
    long deci = 0;
    int i;

    while (*p != toupper(hexdig) && deci < 16) {

        ++p;
        ++deci;

    }
    if (*p == toupper(hexdig)) {
       for (i = 0; i < pos; i++)
           deci *= 16;
       return deci;

    }
    return -1;
}