#include <Wire.h>

// Arduino I2C address

#define SLAVE_ADDR  0x04

// pseudo register addresses

#define READ_A0     0x05
#define WRITE_D2    0x0A

uint8_t reg = 0;

void read_analog_pin (){

    switch (reg){
        case READ_A0: {
            __read_analog(A0);
            break;
        }
    }
}

void __read_analog (int pin){
    
    int val = analogRead(pin);

    uint8_t buf[2];

    // reverse endian so we're little endian going out

    buf[0] = (val >> 8) & 0xFF;
    buf[1] = val & 0xFF;
    
    Wire.write(buf, 2);
}

void write_digital_pin (int num_bytes){

    reg = Wire.read(); // global register addr
  
    while(Wire.available()){

        uint8_t state = Wire.read();
        
        switch (reg){
            case WRITE_D2: {
                digitalWrite(2, state);
                break;
            }
        }
    }
}

void setup(){
    Wire.begin(SLAVE_ADDR);

    // set up the I2C callbacks
    
    Wire.onReceive(write_digital_pin);
    Wire.onRequest(read_analog_pin);

    // set up the pins
    
    pinMode(2, OUTPUT);
    pinMode(A0, INPUT);
}

void loop(){
    delay(10000);
}