// Code based on shiftOutCode, Hello World                                
// By  : Carlyn Maw,Tom Igoe, David A. Mellis 
// Updated by Daniel Hirschmann to offer a way to control
// cascading shift registers

int latchPin = 8;    //Pin connected to latch pin (ST_CP) of 74HC595
int clockPin = 12;   //Pin connected to clock pin (SH_CP) of 74HC595
int dataPin = 11;    //Pin connected to Data in (DS) of 74HC595

int totalLeds = 16;
int ledPos = 0;
int ledDirection = 1;
int delayPeriod = 10;

int potVal = 0;
int totalSR = 3;

void setup() {
  //set pins to output because they are addressed in the main loop
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin, OUTPUT);  
  pinMode(clockPin, OUTPUT);

  Serial.begin(9600);
  Serial.println("STARTED");
}

void loop() {

  potVal = analogRead(0)/4;
  ledPos+=ledDirection;
  if (ledPos >= totalLeds - 1) {
    ledDirection = - 1;
  } 
  else if (ledPos <= 0) {
    ledDirection = 1;
  }
  //Serial.println(ledPos);

  shiftToRegisters(ledPos,HIGH,2);  
  delay(potVal);
}

// This method sends bits to ALL the Shift Registers:
void shiftToRegisters(int whichPin, int whichState, int numberSR) {
  digitalWrite(latchPin, LOW);    //Prepares the SR to receive data

  Serial.print(" pin=");    //Debugging for our human brain's sake
  Serial.print(whichPin);
  Serial.print(" SRs=");
  Serial.print(numberSR);
  Serial.print(" vals = ");

  for (int k = numberSR-1; k >= 0; k--)    //Iterate through the SR's in the chain
  {      
    byte shiftRegisterByte;  
    //Check to see if the shift register is the one with the Night Rider LED active 
    if (whichPin - k*8 < 0) shiftRegisterByte = encodeBits(9, whichState);    //This results in a byte of 0b00000000
    else shiftRegisterByte = encodeBits(whichPin-k*8, whichState); //This results in a byte with the correct position active

    shiftOut(dataPin, clockPin, MSBFIRST, shiftRegisterByte);
    Serial.print(shiftRegisterByte,BIN);  //More debugging for the brains
    Serial.print(" ");
  }
  Serial.println();          //Go to the next line of the debugging
  digitalWrite(latchPin, HIGH);  //Tell the SR's to show us what we've just sent it!

}

//Function to encode a position value into a resulting position on the shift registers
byte encodeBits(int pos, int val)
{
  byte result = 0x01 << pos;
  return result;
}