#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include <math.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

// HC-SR04
#define TRIG_PIN 5
#define ECHO_PIN 18

// Inventory settings
#define MAX_STOCK 10

const float FULL_DISTANCE = 5.0;
const float EMPTY_DISTANCE = 30.0;


float getDistance() {

  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);

  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);

  digitalWrite(TRIG_PIN, LOW);

  long duration = pulseIn(ECHO_PIN, HIGH, 30000);

  if (duration == 0) {
    return -1;
  }

  return duration * 0.0343 / 2;
}


int calculateStock(float distance) {

  // Sensor error
  if (distance < 0) {
    return -1;
  }

  // If distance is smaller than our full-stock distance
  if (distance <= FULL_DISTANCE) {
    return MAX_STOCK;
  }

  // If distance is larger than our empty distance
  if (distance >= EMPTY_DISTANCE) {
    return 0;
  }

  // Convert distance to stock percentage
  float percentage =
    (EMPTY_DISTANCE - distance) /
    (EMPTY_DISTANCE - FULL_DISTANCE);

  // Convert percentage to number of products
  int stock = round(percentage * MAX_STOCK);

  return stock;
}


void setup() {

  Serial.begin(115200);

  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);

  lcd.init();
  lcd.backlight();

  lcd.setCursor(0, 0);
  lcd.print("SMART INVENTORY");

  delay(2000);

  lcd.clear();
}


void loop() {

  float distance = getDistance();

  int stock = calculateStock(distance);

  // Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.print(" cm");

  Serial.print(" | Stock: ");
  Serial.println(stock);


  // LCD
  lcd.clear();

  lcd.setCursor(0, 0);
  lcd.print("STOCK: ");

  if (stock < 0) {
    lcd.print("ERROR");
  }
  else {
    lcd.print(stock);
    lcd.print("/");
    lcd.print(MAX_STOCK);
  }


  lcd.setCursor(0, 1);

  if (stock < 0) {
    lcd.print("Sensor Error");
  }
  else if (stock == 0) {
    lcd.print("OUT OF STOCK!");
  }
  else if (stock <= 2) {
    lcd.print("LOW STOCK!");
  }
  else {
    lcd.print("STOCK OK");
  }


  delay(1000);
}