Keypad scanner with debounce

// Example: 4x4 matrix, rows on D2-D5, columns on D6-D9
const int NUM_ROWS = 4;
const int ROW_PINS[] = {2, 3, 4, 5};
const int NUM_COLS = 4;
const int COL_PINS[] = {6, 7, 8, 9};

// Must be volatile since its values aren't guaranteed to change
volatile bool buttons[NUM_ROWS * NUM_COLS];
bool debounce[NUM_ROWS * NUM_COLS];

volatile char buf[8]; // Used to send back keycodes

void setup() {

    Serial.begin(9600);

    // Make all row pins initially inputs, but also set them low
    // A row will be pulled low by setting its pin to an output
    for (int i = 0; i < NUM_ROWS; i++) {
        pinMode(ROW_PINS[i], INPUT);
        digitalWrite(ROW_PINS[i], LOW);
    }

    // Make all column pins inputs with pullups
    for (int i = 0; i < NUM_COLS; i++) {
        pinMode(COL_PINS[i], INPUT);
    }

}

void loop() {
    pollKeys();

    // Example: Far top-left button
    if (keyPresed(0)) {
        Serial.println("Hello, world!");
    }

    delay(20);
}

// Broken off into its own function, so loop() isn't as messy
// This is especially important with more complex programs
void pollKeys() {
    for (int i = 0; i < NUM_ROWS; i++) {
        pinMode(ROW_PINS[i], OUTPUT);
        for (int j = 0; j < NUM_COLS; j++) {
            int k = i * NUM_COLS + j;
            debounce[k] = buttons[k];
            buttons[k] = !digitalRead(COL_PINS[j]);
        }
        pinMode(ROW_PINS[i], INPUT);
    }
}

// Read the state of a key using the debounce array
bool keyPressed(int i) {
    return buttons[i] && !debounce[i];
}