htoi.c

#include<assert.h>
#include<stdio.h>

/* htoi(): convert hext digit string to native int. */

/*
left-to-right recursive:
htoir("2") == 2
htoir("A") == 10
htoir("23") == 16*htoir("2") + 3
htoir("234") == 16*htoir("23") + 4 == 16*[16*htoir("2") + 3] + 4
 */

int
converth(char ch) {
  int d;
  switch (ch) {
  case '0': d = 0; break;
  case '1': d = 1; break;
  case '2': d = 2; break;
  case '3': d = 3; break;
  case '4': d = 4; break;
  case '5': d = 5; break;
  case '6': d = 6; break;
  case '7': d = 7; break;
  case '8': d = 8; break;
  case '9': d = 9; break;
  case 'A': case 'a': d = 10; break;
  case 'B': case 'b': d = 11; break;
  case 'C': case 'c': d = 12; break;
  case 'D': case 'd': d = 13; break;
  case 'E': case 'e': d = 14; break;
  case 'F': case 'f': d = 15; break;
  default: d = -1; /* error */
  }
  return d;
}

/* right-to-left iterative*/
int
htoi(char s[]) {
  int i = 0;
  int n = 0;
  int d;
  
  d = converth(s[i]);
  while (d != -1) {
    n = 16 * n + d;
    i++;
    d = converth(s[i]);
  }
  return n;
}

int
main (int argc, char *argv[]) {
  assert (htoi("0") == 0);
  assert (htoi("9") == 9);
  assert (htoi("A") == 10);
  assert (htoi("a") == 10);
  assert (htoi("00000000") == 0);
  assert (htoi("10") == 16);
  assert (htoi("19") == 25);
  assert (htoi("1A") == 26);
  assert (htoi("1a") == 26);
  assert (htoi("100") == 256);
  // printf("%d\n", htoi("FFFF"));
  // printf("%d\n", htoi("FFFFFFFF"));
  // printf("%d\n", htoi("FFFFFFFFFF")); /* overflow */
  printf("PASS\n");
  return 0;
}