#include <Arduino.h>

// Define pins for LEDs
const int led1Pin = 2;
const int led2Pin = 3;

// Define pins for the shift register driving the 7-segment display
const int dsPin   = 8;  // serial data input
const int shcpPin = 9;  // shift register clock pin
const int stcpPin = 10; // storage register (latch) clock pin

// Timing intervals in milliseconds
const unsigned long ledInterval = 2000;   // 2 seconds for LED blink
const unsigned long countInterval = 4000;   // 4 seconds for countdown update

// Variables to hold the previous event times
unsigned long previousLedMillis = 0;
unsigned long previousCountMillis = 0;

// Keeps track of which LED should be on next (true means led1, false means led2)
bool ledToggle = true;

// We are using a common anode 7-seg display. In a common anode display a segment lights 
// when its pin is LOW. Thus we use active LOW patterns. 
// The following digitPatterns are standard patterns for a common anode 7-seg with segments order: 
// bit0: A, bit1: B, bit2: C, bit3: D, bit4: E, bit5: F, bit6: G, bit7: DP (we keep DP off).
// Patterns taken from many common examples:
//   0 -> 0xC0, 1 -> 0xF9, 2 -> 0xA4, 3 -> 0xB0, 4 -> 0x99, 
//   5 -> 0x92, 6 -> 0x82, 7 -> 0xF8, 8 -> 0x80, 9 -> 0x90
byte digitPatterns[10] = {
  0xC0, // 0: A B C D E F on, G off, DP off
  0xF9, // 1: B C on
  0xA4, // 2
  0xB0, // 3
  0x99, // 4
  0x92, // 5
  0x82, // 6
  0xF8, // 7
  0x80, // 8
  0x90  // 9
};

// The current number to display on the 7-seg countdown
int currentDigit = 9;

void setup() {
  // Initialize LED pins as outputs
  pinMode(led1Pin, OUTPUT);
  pinMode(led2Pin, OUTPUT);

  // Initialize shift register pins as outputs
  pinMode(dsPin, OUTPUT);
  pinMode(shcpPin, OUTPUT);
  pinMode(stcpPin, OUTPUT);
  
  // Set initial states: 
  // - LED off
  digitalWrite(led1Pin, LOW);
  digitalWrite(led2Pin, LOW);
  
  // Set initial display to 9
  updateDisplay(digitPatterns[currentDigit]);
}

void loop() {
  unsigned long currentMillis = millis();

  // LED blinking every 2 seconds (alternate between led1 and led2)
  if (currentMillis - previousLedMillis >= ledInterval) {
    previousLedMillis = currentMillis;
    
    if (ledToggle) {
      digitalWrite(led1Pin, HIGH);   // turn led1 ON
      digitalWrite(led2Pin, LOW);    // turn led2 OFF
    } else {
      digitalWrite(led1Pin, LOW);
      digitalWrite(led2Pin, HIGH);
    }
    ledToggle = !ledToggle;
  }

  // 7-segment countdown update every 4 seconds
  if (currentMillis - previousCountMillis >= countInterval) {
    previousCountMillis = currentMillis;
    
    // Decrement the displayed digit; if 0 then restart at 9.
    currentDigit = (currentDigit == 0) ? 9 : currentDigit - 1;
    
    // Update the display with the new digit's pattern by shifting out the byte.
    updateDisplay(digitPatterns[currentDigit]);
  }
}

// Function to update the 7-segment display through the shift register
// by shifting out a byte representing the segments (active LOW).
void updateDisplay(byte segPattern) {
  // Begin by setting latch low
  digitalWrite(stcpPin, LOW);
  
  // Use Arduino's shiftOut: by default, it shifts out MSB first.
  // Our connections assume that Q0 gets bit0 (segment A) etc.
  shiftOut(dsPin, shcpPin, MSBFIRST, segPattern);
  
  // Latch the output: setting stcpPin high copies the data to the outputs.
  digitalWrite(stcpPin, HIGH);
}