// Copyright (C) 2017 Marius Schellenberger

#include <stdint.h>
#include <avr/interrupt.h>
#include <avr/wdt.h>

// data
#define ALPHA_MAX 36
char _itoa[ALPHA_MAX];

void init_itoa(void) {
  for (int i = 0; i < 10; i++)
    _itoa[i] = (char)48 + i;
  for (int i = 10; i < ALPHA_MAX; i++)
    _itoa[i] = (char)55 + i;
}

// random
#define CHAR_MAX 256
volatile unsigned char _r_sample = 0;
volatile char _r_new = 0;
unsigned char _r = 0;
const unsigned char _r_max = CHAR_MAX - (CHAR_MAX % ALPHA_MAX);

ISR(WDT_vect) {
  _r_sample = TCNT1L;
  _r_new = 1;
}

void _r_init(void) {
  cli();
  MCUSR = 0;
  WDTCSR |= _BV(WDCE) | _BV(WDE);
  WDTCSR = _BV(WDIE);
  sei();
}

char _r_rotl_one(const char v) {
  int shift = 1;
  if ((shift &= sizeof(v) * 8 - 1) == 0)
    return v;
  return (v << shift) | (v >> (sizeof(v) * 8 - shift));
}

char _r_char(void) {
  char i = 0;
  while (1) {
    if (_r_new) {
      _r_new = 0;
      _r = _r_rotl_one(_r);
      _r ^= _r_sample;
      if (i++ > 7) {
        if (_r < _r_max)
          return _itoa[_r % ALPHA_MAX];
        else
          i = 0;
      }
    }
  }
}

void setup(void) {
  Serial.begin(115200);
  _r_init();
  init_itoa();
}

void loop(void) {
  Serial.write(_r_char());
}