Slave_imperial.cpp
// The library for I2C communication
#include "Wire.h"

// Slave address
#define I2C_DEV_ADDR 0x55
// Buzzer PIN
#define BUZZER_PIN 5

// NOTES FOR THE IMPERIAL MARCH
#define NOTE_B0  31
#define NOTE_C1  33
#define NOTE_CS1 35
#define NOTE_D1  37
#define NOTE_DS1 39
#define NOTE_E1  41
#define NOTE_F1  44
#define NOTE_FS1 46
#define NOTE_G1  49
#define NOTE_GS1 52
#define NOTE_A1  55
#define NOTE_AS1 58
#define NOTE_B1  62
#define NOTE_C2  65
#define NOTE_CS2 69
#define NOTE_D2  73
#define NOTE_DS2 78
#define NOTE_E2  82
#define NOTE_F2  87
#define NOTE_FS2 93
#define NOTE_G2  98
#define NOTE_GS2 104
#define NOTE_A2  110
#define NOTE_AS2 117
#define NOTE_B2  123
#define NOTE_C3  131
#define NOTE_CS3 139
#define NOTE_D3  147
#define NOTE_DS3 156
#define NOTE_E3  165
#define NOTE_F3  175
#define NOTE_FS3 185
#define NOTE_G3  196
#define NOTE_GS3 208
#define NOTE_A3  220
#define NOTE_AS3 233
#define NOTE_B3  247
#define NOTE_C4  262
#define NOTE_CS4 277
#define NOTE_D4  294
#define NOTE_DS4 311
#define NOTE_E4  330
#define NOTE_F4  349
#define NOTE_FS4 370
#define NOTE_G4  392
#define NOTE_GS4 415
#define NOTE_A4  440
#define NOTE_AS4 466
#define NOTE_B4  494
#define NOTE_C5  523
#define NOTE_CS5 554
#define NOTE_D5  587
#define NOTE_DS5 622
#define NOTE_E5  659
#define NOTE_F5  698
#define NOTE_FS5 740
#define NOTE_G5  784
#define NOTE_GS5 831
#define NOTE_A5  880
#define NOTE_AS5 932
#define NOTE_B5  988
#define NOTE_C6  1047
#define NOTE_CS6 1109
#define NOTE_D6  1175
#define NOTE_DS6 1245
#define NOTE_E6  1319
#define NOTE_F6  1397
#define NOTE_FS6 1480
#define NOTE_G6  1568
#define NOTE_GS6 1661
#define NOTE_A6  1760
#define NOTE_AS6 1865
#define NOTE_B6  1976
#define NOTE_C7  2093
#define NOTE_CS7 2217
#define NOTE_D7  2349
#define NOTE_DS7 2489
#define NOTE_E7  2637
#define NOTE_F7  2794
#define NOTE_FS7 2960
#define NOTE_G7  3136
#define NOTE_GS7 3322
#define NOTE_A7  3520
#define NOTE_AS7 3729
#define NOTE_B7  3951
#define NOTE_C8  4186
#define NOTE_CS8 4435
#define NOTE_D8  4699
#define NOTE_DS8 4978
#define REST      0

// How the song goes
int melody[] = {
  NOTE_A4,-4, NOTE_A4,-4, NOTE_A4,16, NOTE_A4,16, NOTE_A4,16, NOTE_A4,16, NOTE_F4,8, REST,8,
  NOTE_A4,-4, NOTE_A4,-4, NOTE_A4,16, NOTE_A4,16, NOTE_A4,16, NOTE_A4,16, NOTE_F4,8, REST,8,
  NOTE_A4,4, NOTE_A4,4, NOTE_A4,4, NOTE_F4,-8, NOTE_C5,16,
  NOTE_A4,4, NOTE_F4,-8, NOTE_C5,16, NOTE_A4,2,
  NOTE_E5,4, NOTE_E5,4, NOTE_E5,4, NOTE_F5,-8, NOTE_C5,16,
  NOTE_A4,4, NOTE_F4,-8, NOTE_C5,16, NOTE_A4,2,
  NOTE_A5,4, NOTE_A4,-8, NOTE_A4,16, NOTE_A5,4, NOTE_GS5,-8, NOTE_G5,16,
  NOTE_DS5,16, NOTE_D5,16, NOTE_DS5,8, REST,8, NOTE_A4,8, NOTE_DS5,4, NOTE_D5,-8, NOTE_CS5,16,
  NOTE_C5,16, NOTE_B4,16, NOTE_C5,16, REST,8, NOTE_F4,8, NOTE_GS4,4, NOTE_F4,-8, NOTE_A4,-16,
  NOTE_C5,4, NOTE_A4,-8, NOTE_C5,16, NOTE_E5,2,
  NOTE_A5,4, NOTE_A4,-8, NOTE_A4,16, NOTE_A5,4, NOTE_GS5,-8, NOTE_G5,16,
  NOTE_DS5,16, NOTE_D5,16, NOTE_DS5,8, REST,8, NOTE_A4,8, NOTE_DS5,4, NOTE_D5,-8, NOTE_CS5,16,
  NOTE_C5,16, NOTE_B4,16, NOTE_C5,16, REST,8, NOTE_F4,8, NOTE_GS4,4, NOTE_F4,-8, NOTE_A4,-16,
  NOTE_A4,4, NOTE_F4,-8, NOTE_C5,16, NOTE_A4,2,
};

// It divides the value of the array and the first item of the array
// and it is divided by 2, because it stores the value of the note and the duration.
// So the total notes of the melody is half of the elements in the array.
int notes = sizeof(melody) / sizeof(melody[0]) / 2;
// How fast is the song goes
int tempo = 120;
// It calculates the duration of one whole note in milliseconds.
int wholenote = (60000 * 4) / tempo;
// divider: It is used to obtain the duration of each note of the melody.
// noteDuration: It stores the duration in milliseconds of each individual note, 
// after the calculus.
int divider = 0, noteDuration = 0;

// Variable to control the state of the song.
volatile bool playSong = false;

void onReceive(int len) {
  // It waits till something is available
  while (Wire.available()) {
    // It stores the 1 in a char variable
    char c = Wire.read();
    if (c == '1') {
      // It changes the state of the song 
      // so it can start or stop
      playSong = !playSong;
      // Serial prints to check if the song is playing
      Serial.print("playSong: ");
      Serial.println(playSong ? "ON" : "OFF");
    }
  }
}

void setup() {
  // It initializes the serial port
  Serial.begin(115200);
  // It initializes the I2C communication
  Wire.begin(I2C_DEV_ADDR);
  Wire.onReceive(onReceive);
  // It initializes the buzzer as output
  pinMode(BUZZER_PIN, OUTPUT);
}

void loop() {
  // It plays the song playSong is true
  if (playSong) {
    // A for loop to play the song
    for (int thisNote = 0; thisNote < notes * 2; thisNote = thisNote + 2) {
      // If playSong is false it stops playing
      if (!playSong) break;

      // It obtains the duration of the actual 
      // note in the melody
      divider = melody[thisNote + 1];
      // It checks if divider es greater than 0,
      // which says that it is a normal note.
      if (divider > 0) {
        noteDuration = (wholenote) / divider;
        // It checks if divider is less than 0, 
        // which says that it is a dotted note.
      } else if (divider < 0) {
        // It calculates the duration of the dotted note.
        noteDuration = (wholenote) / abs(divider);
        // It in increases the duration of the dotted note.
        noteDuration *= 1.5;
      }

      // It generates a tone for the buzzer with the 
      // frequency of the actual note.
      tone(BUZZER_PIN, melody[thisNote], noteDuration * 0.9);
      // It waits the whole note duration before the next one.
      delay(noteDuration);
      // It stops the generation of the tone.
      noTone(BUZZER_PIN);
    }
  } else {
    // Small delay so it waits for the next instruction.
    delay(100);
  }
}