/* This sketch is uploaded on the Arduino Uno.
 * It acts as the slave device at address 8 on the I2C bus.
 */

#include <Wire.h>

String msg = "";        // stores the message to be sent
String input = "";      // buffers the input we get from the computer to output them at once
int mode;               // indicates transmission state

// handler function for incomming data
void recv(int count) {
  char c = Wire.read();             // read the transmition state from peer and save
  if (c == 0 || c == 1) {           // '0' means the peer is listening for data, '1' means the peer is requesting the data
    mode = c;
  } else if (c == 2) {              // '2' means the peer is sending data
    while (Wire.available() > 0) {
      char c = Wire.read();         // read message and print
      Serial.write(c);
    }
  }
}

// handler function when requested data by the peer
void req() {
  if (mode == 0) {                  // '0' means the peer is waiting for data
    Wire.write(msg.length());       // send the length of the message
  } else if (mode == 1) {           // '1' means the peer is requesting data
    Wire.write(msg.c_str(), msg.length());  // send the actual message
    msg = "";                       // resets the message buffer
  }
}

void setup() {
  Serial.begin(9600);

  Wire.begin(8);                    // set the address of the device
  Wire.onReceive(recv);             // setup callback functions
  Wire.onRequest(req);
}

void loop() {
  while (Serial.available() > 0) {  // read input from the computer
    char c = Serial.read;
    msg += c;                       // buffer the message to be send when requested by peer
    input += c;                     // buffer the line we input
    if (c == '\n') {                // input end, print to the serial monitor and reset
      Serial.print("me: ");
      Serial.print(input);
      input = "";
    }
  }
}