#include <stdarg.h>
#include <stdint.h>
#include <sys/types.h>

enum TYPE {
  TYPE_INT      = 1,
  TYPE_UINT     = 2,
  TYPE_DEC      = 3,
  TYPE_OCT      = 4,
  TYPE_HEX      = 5,
  TYPE_CHAR     = 6,
  TYPE_STRING   = 7,
  TYPE_PERCENT  = 8
};

enum FLAGS {
  FL_ZERO     = 1, /* zero padded */
  FL_ALT      = 2, /* alternate form (effect is type dependent) */
  FL_UPPER    = 4  /* uppercase */
};

static void num(char **str, char flags, int base, int fwidth, int x) {
  int i, y, len = 0;
  char c;
  char *s = NULL;

  /* print the prefix, if the alt flag is set */
  if(x && (flags & FL_ALT) && base == 8)
    s = "0";
  if(x && (flags & FL_ALT) && base == 16)
    s = (flags & FL_UPPER) ? "0X" : "0x";
  while(s && *s)
    *(*str)++ = *s++;

  /* calculate the number of digits */
  y = x;
  while(y) {
    y /= base;
    len++;
  }

  /* print the padding characters (if any) */
  for(i = 0; i < fwidth - len; i++)
    *((*str)++) = (flags & FL_ZERO) ? '0' : ' ';

  *str += len;
  s = *str;
  while(len--) {
    c = (x % base) + '0';
    if(c > '9')
      c += (flags & FL_UPPER) ? 7 : 39;

    *(--s) = c;

    x /= base;
  }
}

int vsprintf(char *str, char *fmt, va_list ap) {
  int x, fwidth;
  char flags;
  char *s;
  char *start = str;
  uint8_t type;

  while(*fmt) {
    if(*fmt != '%') {
      *str++ = *fmt++;
      continue;
    }

    type = 0;
    flags = 0;
    fwidth = 0;

    fmt++;
    for(;;) {
      switch(*fmt) {
        case '#':
          flags |= FL_ALT;
          break;
        case '%':
          type = TYPE_PERCENT;
          goto done;
        case 'c':
          type = TYPE_CHAR;
          goto done;
        case 'd':
          type = TYPE_DEC;
          goto done;
        case 'o':
          type = TYPE_OCT;
          goto done;
        case 's':
          type = TYPE_STRING;
          goto done;
        case 'x':
          type = TYPE_HEX;
          goto done;
        case 'X':
          type = TYPE_HEX;
          flags |= FL_UPPER;
          goto done;
        default:
          /* the first zero enables zero-padding */
          if(*fmt == '0' && !fwidth) {
            flags |= FL_ZERO;
            goto next;
          }

          /* subsequent numbers indicate field width */
          if(*fmt >= '0' && *fmt <= '9') {
            fwidth = (fwidth * 10) + (*fmt - '0');
            goto next;
          }

          goto done;
      }
next:
      fmt++;
    }

done:
    fmt++;

    switch(type) {
      case TYPE_STRING:
        s = va_arg(ap, char*);
        while(*s)
          *str++ = *s++;
        break;
      case TYPE_CHAR:
        *str++ = va_arg(ap, char);
        break;
      case TYPE_DEC:
        x = va_arg(ap, int);
        num(&str, flags, 10, fwidth, x);
        break;
      case TYPE_OCT:
        x = va_arg(ap, int);
        num(&str, flags, 8, fwidth, x);
        break;
      case TYPE_HEX:
        x = va_arg(ap, int);
        num(&str, flags, 16, fwidth, x);
        break;
      case TYPE_PERCENT:
        *str++ = '%';
        break;
    }
  }

  *str = 0;

  return (str - start);
}