kub-kar-timer/src/main.cpp

/*
     _  __     _     _  __          _____ ___
    | |/ _   _| |__ | |/ /__ _ _ __|_   _|_ _|_ __ ___   ___ _ __
    | ' | | | | '_ \| ' // _` | '__  | |  | || '_ ` _ \ / _ | '__|
    | . | |_| | |_) | . | (_| | |    | |  | || | | | | |  __| |
    |_|\_\__,_|_.__/|_|\_\__,_|_|    |_| |___|_| |_| |_|\___|_|

 */
#include "Arduino.h"

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

LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);

#define PIN_RESET 2
#define PIN_CAR1 3
#define PIN_LED LED_BUILTIN
#define DISPLAY_LINES 4

// assume all cars are on sequential digital pins
#define PIN_CAR_STARTING 3

// number of cars (4 max)
#define CAR_COUNT 4

// buffer for the LCD line
char buffer[40];

void setup()
{
  pinMode(PIN_RESET, INPUT_PULLUP);
  pinMode(PIN_LED, OUTPUT);

  // initialize each car PIN
  for(short i=0; i<CAR_COUNT; i++) {
    pinMode(PIN_CAR_STARTING + i, INPUT_PULLUP);
  }

  // initialize LCD
  lcd.begin(20, DISPLAY_LINES);
  lcd.backlight();
  lcd.setCursor(1,1);
  lcd.print("Jeff's Inept");
  lcd.setCursor(6,2);
  lcd.print("Kub Kar Timer");
}

#define STATE_READY 1
#define STATE_RUNNING 2

unsigned long startTime;         // start time in ms
unsigned short state = 0;
unsigned short place = 0;
bool finished[CAR_COUNT];

void loop()
{

  if (digitalRead(PIN_RESET) == LOW) {
    if (state != STATE_READY) {
      digitalWrite(PIN_LED, HIGH);
      state = STATE_READY;
      lcd.clear();
      lcd.setCursor(0,0);
      lcd.print("Ready...");
    }
  } else {
    if (state == STATE_READY) {
      digitalWrite(PIN_LED, LOW);
      state = STATE_RUNNING;
      lcd.setCursor(0,0);
      lcd.print("Running!");
      startTime = millis();
      for(int i=0; i<CAR_COUNT; i++) {
        finished[i] = false;
      }
      place = 0;
    }
    if (state == STATE_RUNNING) {
      unsigned long elapsedTime = millis() - startTime;
      for(short i = 0; i<CAR_COUNT; i++) {
        if (!finished[i] && digitalRead(PIN_CAR_STARTING + i) == LOW) {
          place++;
          finished[i] = true;
          if (place <= DISPLAY_LINES) {
            // NOTE: Some concern LCD writing takes enough cycles it'll potentially miss other car finishing
            // events.  Alternative is to draw only once all cars finish but then we need a timeout to handle
            // missing or non-finishing cars.  Who wants to wait for results?
            lcd.setCursor(0,place-1);
            sprintf(buffer, "%d%s: CAR%d @ %0.2fs", place, place==1?"st":(place==2?"nd":(place==3?"rd":"th")) , i+1, elapsedTime/1000.0);
            lcd.print(buffer);
          }
        }
      }

    }
  }
}