#include <Keypad.h>
#define multi_tap_threshold 1000
#define RESET_MTP '~'

const byte ROWS = 4; //four rows
const byte COLS = 4; //three columns
int pointer;
char keys[ROWS][COLS] = {
  {'1','2','3'},
  {'4','5','6'},
  {'7','8','9'},
  {'*','0','#'}
};
byte rowPins[ROWS] = {1,15,2,4}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {16,17,5,18}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );

void setup(){
  Serial.begin(9600);
}
 
void loop()
{
  int mtKey=0;
  byte key=keypad.getKey(); // Get a key press from the keypad
  mtKey=multi_tap(key); // Feed the key press to the multi_tap function.
  if(!(mtKey&B10000000)) Serial.print(mtKey);
}

int multi_tap(byte key)
{
  static boolean upperCase=true;
  static byte prevKeyPress=NO_KEY,cyclicPtr=0;
  static unsigned long prevKeyMillis=0;
  static const char multi_tap_mapping[10][5]={{'0','#','.','?'},{'1','+','-','^','/'},{'A','B','C','2','!'},{'D','E','F','3','%'},{'G','H','I','4','('},{'J','K','L','5',')'},{'M','N','O','6','@'},{'P','Q','R','S','7'},{'T','U','V','8',','},{'W','X','Y','Z','9'}};
  if (key==RESET_MTP) // Received reset command. Flush everything and get ready for restart.
  {
    upperCase=true;
    prevKeyPress=NO_KEY;
    cyclicPtr=0;
    return 0;
  }
  if (key!=NO_KEY) // A key is pressed at this iteration.
  {
    if ((key>'9')||(key<'0')) // Function keys
    {
      if ((key==1)||(key=='#')) // Up for case change
      {
        upperCase=!upperCase;
        return 0;
      }
      else // Other function keys. These keys produce characters so they need to terminate the last keypress.
      {
        if (prevKeyPress!=NO_KEY)
        {
          char temp1=multi_tap_mapping[prevKeyPress-'0'][cyclicPtr];
          if ((!upperCase)&&(temp1>='A')&&(temp1<='Z')) temp1+='a'-'A';
          cyclicPtr=0;
          prevKeyMillis=0;
          switch (key)
          {
            case 2:
            // Call symbol list
            return 0; // Clear the buffer.
            break;
            case 3:
            prevKeyPress='\b';
            break;
            case 4:
            case '^':
            prevKeyPress=' ';
            break;
            case 5:
            prevKeyPress='\n';
            break;
            case 6:
            prevKeyPress=NO_KEY; // Clear the buffer.
            break;
          }
          return(256+(unsigned int)(temp1));
        }
        else
        {
          prevKeyPress=NO_KEY;
          cyclicPtr=0;
          prevKeyMillis=0;
          switch (key)
          {
            case 2:
            // Call symbol list
            return 0; // Clear the buffer.
            break;
            case 3:
            return (256+(unsigned int)('\b'));
            break;
            case 4:
            return (256+(unsigned int)(' '));
            break;
            case 5:
            return (256+(unsigned int)('\n'));
            break;
            case 6:
            return 0; // Clear the buffer.
            break;
          }
        }
      }
   
    }
    if (prevKeyPress!=NO_KEY)
    {
      if (prevKeyPress==key)
      {
        char temp1;
        cyclicPtr++;
        if ((multi_tap_mapping[key-'0'][cyclicPtr]==0)||(cyclicPtr==5)) cyclicPtr=0; //Cycle key
        prevKeyMillis=millis();
        temp1=multi_tap_mapping[key-'0'][cyclicPtr];
        if ((!upperCase)&&(temp1>='A')&&(temp1<='Z')) temp1+='a'-'A';
        return ((unsigned int)(temp1));
      }
      else
      {
        char temp1=multi_tap_mapping[prevKeyPress-'0'][cyclicPtr];
        if ((!upperCase)&&(temp1>='A')&&(temp1<='Z')) temp1+='a'-'A';
        prevKeyPress=key;
        cyclicPtr=0;
        prevKeyMillis=millis();
        //Print key on cursor+1
        return(256+(unsigned int)(temp1));
      }
    }
    else
    {
      char temp1=multi_tap_mapping[key-'0'][cyclicPtr];
      if ((!upperCase)&&(temp1>='A')&&(temp1<='Z')) temp1+='a'-'A';
      prevKeyPress=key;
      prevKeyMillis=millis();
      cyclicPtr=0;
      return ((unsigned int)(temp1));
    }
 
  }
  else // No key is pressed at this iteration.
  {
    if (prevKeyPress==NO_KEY) return 0; // No key was previously pressed.
    else if (millis()-prevKeyMillis<multi_tap_threshold) // Key was pressed previously but within threshold
    {
      char temp1=multi_tap_mapping[prevKeyPress-'0'][cyclicPtr];
      if ((!upperCase)&&(temp1>='A')&&(temp1<='Z')) temp1+='a'-'A';
      return((unsigned int)(temp1));
    }
    else // Key was pressed previously and threshold has passed
    {
      char temp1=multi_tap_mapping[prevKeyPress-'0'][cyclicPtr];
      if ((!upperCase)&&(temp1>='A')&&(temp1<='Z')) temp1+='a'-'A';
      prevKeyPress=NO_KEY;
      cyclicPtr=0;
      return(256+(unsigned int)(temp1));
    }
  }
  return 0;
}