#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>

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

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

static void hex(char **str, char flags, int fwidth, int size, int x) {
  bool seen;
  int i, y, len;
  char c;
  char *prefl = "0x";
  char *prefu = "0X";

  /* print the prefix, if the alt flag is set */
  if(flags & FL_ALT) {
    if(flags & FL_UPPER) {
      while(*prefu)
        *(*str)++ = *prefu++;
    } else {
      while(*prefl)
        *(*str)++ = *prefl++;
    }
  }

  /* calculate the number of digits */
  len = 0;
  y = x;
  seen = false;
  for(i = 0; i < sizeof(y) << 1; i++) {
    c = ((unsigned int) y >> 28);
    if(c)
      seen = true;
    if(c || seen)
      len++;
    y = y << 4;
  }

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

  /* actually print the digits */
  seen = false;
  for(i = 0; i < sizeof(x) << 1; i++) {
    c = ((unsigned int) x >> 28) + '0';
    if(c > '9') {
      c += 0x27;
      if(flags & FL_UPPER)
        c -= 0x20;
    }

    if(c != '0')
      seen = true;
    if(c != '0' || seen)
      *(*str)++ = c;

    x = x << 4;
  }
}

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 '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_HEX:
        x = va_arg(ap, int);
        hex(&str, flags, fwidth, 4, x);
        break;
      case TYPE_PERCENT:
        *str++ = '%';
        break;
    }
  }

  *str = 0;

  return (str - start);
}