#include "../avr-i2c-slave/I2CSlave.h"
#include <util/delay.h>
#include <avr/io.h>
#include <avr/interrupt.h>

#define I2C_ADDR 0x20

volatile uint8_t challenge = 0; // Use 0 as 'challenge not sent' condition

uint8_t check_challenge(uint8_t arg) {
  return (arg >> 3) ^ (arg << 3);
}

void I2C_received(uint8_t received_data) {
  if (challenge != 0) {
    if (check_challenge(received_data) == check_challenge(challenge)) { // Check validity of response
      // Open lock
      PORTC |= 2 << 1;
      _delay_ms(5000);
      PORTC = 0;
    }
  }
  challenge = 0;
}

void I2C_requested() {
  challenge = (uint8_t)(TCNT1 & 0xff); // Use content of timer as pseudo-random number
  if (challenge == 0) // Prevent 'challenge' from being 0, which has a special meaning
    challenge = 1;
  I2C_transmitByte(challenge);
}

void setup() {
  // set received/requested callbacks
  I2C_setCallbacks(I2C_received, I2C_requested);
  // init I2C
  I2C_init(I2C_ADDR);
  // init output gpio
  DDRC = 0b00000100; // PC2
  TCNT1 = 0;
  TCCR1B |= (1 << CS10) | (1 << CS11); // No prescaling
}

int main() {
  setup();
  while (1); // All the work is done in the callbacks
}