const byte ROWS = 4;
const byte COLS = 4;
char keys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};
byte rowPins[ROWS] = {13, 12, 14, 27};
byte colPins[COLS] = {26, 25, 33, 32};

String inputString = "";
int lastButton0Time = 0;
int lastButton1Time = 0;
int lastButton2Time = 0;
int lastButton3Time = 0;
int lastButton4Time = 0;
int lastButton5Time = 0;
int lastButton6Time = 0;
int lastButton7Time = 0;
int lastButton8Time = 0;
int lastButton9Time = 0;

char getKey() {
  for (byte row = 0; row < ROWS; ++row) {
    digitalWrite(rowPins[row], LOW);
    for (byte col = 0; col < COLS; ++col) {
      if (!digitalRead(colPins[col])) {
        delay(50);
        while (!digitalRead(colPins[col]));
        digitalWrite(rowPins[row], HIGH);
        return keys[row][col];
      }
    }
    digitalWrite(rowPins[row], HIGH);
  }
  return '\0';
}

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

  for (byte row = 0; row < ROWS; ++row) {
    pinMode(rowPins[row], OUTPUT);
    digitalWrite(rowPins[row], HIGH);
  }

  for (byte col = 0; col < COLS; ++col) {
    pinMode(colPins[col], INPUT_PULLUP);
  }
}

void loop() {
  char key = getKey();

  if (key != '\0') {
    if (key == '1') {
      int now = millis();
      if (now - lastButton1Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'A') {
            inputString.setCharAt(inputString.length() - 1, 'B');
          } else if (lastChar == 'B') {
            inputString.setCharAt(inputString.length() - 1, 'C');
          } else if (lastChar == 'C') {
            inputString.setCharAt(inputString.length() - 1, 'C');
          }
        }
      } else {
        inputString += 'A';
      }
      lastButton1Time = now;
    } else if (key == '2') {
      int now = millis();
      if (now - lastButton2Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'A') {
            inputString.setCharAt(inputString.length() - 1, 'B');
          } else if (lastChar == 'B') {
            inputString.setCharAt(inputString.length() - 1, 'C');
          } else if (lastChar == 'C') {
            inputString.setCharAt(inputString.length() - 1, '2');
          }
        }
      } else {
        inputString += 'A';
      }
      lastButton2Time = now;
    } else if (key == '3') {
      int now = millis();
      if (now - lastButton3Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'D') {
            inputString.setCharAt(inputString.length() - 1, 'E');
          } else if (lastChar == 'E') {
            inputString.setCharAt(inputString.length() - 1, 'F');
          } else if (lastChar == 'F') {
            inputString.setCharAt(inputString.length() - 1, '3');
          }
        }
      } else {
        inputString += 'D';
      }
      lastButton3Time = now;
    } else if (key == '4') {
      int now = millis();
      if (now - lastButton4Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'G') {
            inputString.setCharAt(inputString.length() - 1, 'H');
          } else if (lastChar == 'H') {
            inputString.setCharAt(inputString.length() - 1, 'I');
          } else if (lastChar == 'I') {
            inputString.setCharAt(inputString.length() - 1, '4');
          }
        }
      } else {
        inputString += 'G';
      }
      lastButton4Time = now;
    } else if (key == '5') {
      int now = millis();
      if (now - lastButton5Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'J') {
            inputString.setCharAt(inputString.length() - 1, 'K');
          } else if (lastChar == 'K') {
            inputString.setCharAt(inputString.length() - 1, 'L');
          } else if (lastChar == 'L') {
            inputString.setCharAt(inputString.length() - 1, '5');
          }
        }
      } else {
        inputString += 'J';
      }
      lastButton5Time = now;
    } else if (key == '6') {
      int now = millis();
      if (now - lastButton6Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'M') {
            inputString.setCharAt(inputString.length() - 1, 'N');
          } else if (lastChar == 'N') {
            inputString.setCharAt(inputString.length() - 1, 'O');
          } else if (lastChar == 'O') {
            inputString.setCharAt(inputString.length() - 1, '6');
          }
        }
      } else {
        inputString += 'M';
      }
      lastButton6Time = now;
    } else if (key == '7') {
      int now = millis();
      if (now - lastButton7Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'P') {
            inputString.setCharAt(inputString.length() - 1, 'Q');
          } else if (lastChar == 'Q') {
            inputString.setCharAt(inputString.length() - 1, 'R');
          } else if (lastChar == 'R') {
            inputString.setCharAt(inputString.length() - 1, 'S');
          } else if (lastChar == 'S') {
            inputString.setCharAt(inputString.length() - 1, '7');
          }
        }
      } else {
        inputString += 'P';
      }
      lastButton7Time = now;
    } else if (key == '8') {
      int now = millis();
      if (now - lastButton8Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'T') {
            inputString.setCharAt(inputString.length() - 1, 'U');
          } else if (lastChar == 'U') {
            inputString.setCharAt(inputString.length() - 1, 'V');
          } else if (lastChar == 'V') {
            inputString.setCharAt(inputString.length() - 1, '8');
          }
        }
      } else {
        inputString += 'T';
      }
      lastButton8Time = now;
    } else if (key == '9') {
      int now = millis();
      if (now - lastButton9Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == 'W') {
            inputString.setCharAt(inputString.length() - 1, 'X');
          } else if (lastChar == 'X') {
            inputString.setCharAt(inputString.length() - 1, 'Y');
          } else if (lastChar == 'Y') {
            inputString.setCharAt(inputString.length() - 1, 'Z');
          } else if (lastChar == 'Z') {
            inputString.setCharAt(inputString.length() - 1, '9');
          }
        }
      } else {
        inputString += 'W';
      }
      lastButton9Time = now;
    } else if (key == '0') {
      int now = millis();
      if (now - lastButton0Time < 500) {
        if (inputString.length() > 0) {
          char lastChar = inputString.charAt(inputString.length() - 1);
          if (lastChar == ' ') {
            inputString.setCharAt(inputString.length() - 1, '0');
          }
        }
      } else {
        inputString += ' ';
      }
      lastButton0Time = now;
    } else {
      inputString += key;
    }
    Serial.println(inputString);
  }
}