#include "common.h"

void putchar(char ch);

void printf(const char *fmt, ...) {
  va_list vargs;
  va_start(vargs, fmt);

  while (*fmt) {
    if (*fmt == '%') {
      fmt++; // Skip '%'
      switch (*fmt) { // Read the next character
        case '\0': // '%' at the end of the format string
          putchar('%');
          goto end;
        case '%': // Print '%'
          putchar('%');
          break;
        case 's': { // Print a NULL-termintated string
          const char *s = va_arg(vargs, const char *);
          while (*s) {
            putchar(*s);
            s++;
          }
          break;
        }
        case 'd': { // Print an integer in decimal
          int value = va_arg(vargs, int);
          unsigned magnitude = value;
          if (value < 0) {
            putchar('-');
            magnitude = -magnitude;
          }

          unsigned divisor = 1;
          while (magnitude / divisor > 9)
            divisor *= 10;

          while (divisor > 0) {
            putchar('0' + magnitude / divisor);
            magnitude %= divisor;
            divisor /= 10;
          }

          break;
        }
        case 'x': { // Print an integer in hexademical
          unsigned value = va_arg(vargs, unsigned);
          for (int i = 7; i >= 0; i--) {
            unsigned nibble = (value >> (i * 4)) & 0xf;
            putchar("0123456789abcdef"[nibble]);
          }
        }
      }
    } else {
      putchar(*fmt);
    }

    fmt++;
  }

end:
  va_end(vargs);
}

void *memcpy(void *dst, const void *src, size_t n) {
  uint8_t *d = (uint8_t *) dst;
  const uint8_t *s = (const uint8_t *) src;
  while (n--)
    *d++ = *s++;
  return dst;
}

void *memset(void *buf, char c, size_t n) {
  uint8_t *p = (uint8_t *) buf;
  while (n--)
    *p++ = c;
  return buf;
}

char *strcpy(char *dst, const char *src) {
  char *d = dst;
  while (*src)
    *d++ = *src++;
  *d = '\0';
  return dst;
} // The strcpy function continues copying even if
  // src is longer than the memory area of dst. 
  // This can easily lead to bugs and vulnerabilities

int strcmp(const char *s1, const char *s2) {
  while (*s1 && *s2) {
    if (*s1 != *s2)
      break;
    s1++;
    s2++;
  }

  return *(unsigned char *)s1 - *(unsigned char *)s2;
}