Arduino Tachometer Using Photo-interrupter

/*
 * Arduino Tachometer Using Photo-interrupter
 *
 * learnelectronics
 * 16 OCT 2013
 *
 * www.youtube.com/c/learnelectronics
 * arduino0169@gmail.com
 */

#include <Wire.h>                                                     //Wire library for I2c display
#include <Adafruit_SSD1306.h>                                         //Adafruit OLED display library

#define OLED_RESET 4                                                  //does nothing, but needed for library

Adafruit_SSD1306 display(OLED_RESET);                                 //create instance of SSD1306 library called display


int val;                                                              //calibration value
long last=0;                                                          //used in timing - holds thelast milis count
int stat=LOW;                                                         //used in calibration
int stat2;                                                            //used in calibration
int counter=0;                                                        //used to count light/dark iterations

int sens=75;                                                          //calibration value


int slots=20;                                                         //# slots in disc

int milisecs=500;                                                     //time of sample


void setup()
{
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);                          //begin oled
  display.display();                                                  //show bufer
  delay(10);                                                          //wait 10 mS
  display.clearDisplay();                                             //clear display and buffer

  Serial.begin(9600);                                                 //serial comms for debuging
  pinMode(13,OUTPUT);                                                 //digital pin 13 set for output
}

void loop()
{
  val=analogRead(0);                                                  //light or dark?
  if(val<sens)                                                        //within calibartion value?
    stat=LOW;                                                         //set var to LOW
   else                                                               //if not
    stat=HIGH;                                                        //set var HIGH
   digitalWrite(13,stat);                                             //LED on or off


   if(stat2!=stat){                                                   //if there has been a change

     counter++;                                                       //increment counter
     stat2=stat;                                                      //remember last value
   }
   if(millis()-last>=milisecs){                                       //has time passed?
     double rps=((double)counter/slots)/2.0*1000.0/milisecs;          //do maths
     double rpm=((double)counter/slots)/2.0*60000.0/(milisecs);       //do more maths
     display.clearDisplay();                                          //clear display & buffer
     display.setTextSize(2);                                          //prepare to show stuff
     display.setTextColor(WHITE);
     display.setCursor(0,0);
     //display.print((counter/2.0));
     //display.println("  RPS ");
     //display.print(rps);
     display.print("RPM ");                                           //print out the units we are using to buffer
     display.println(rpm);                                            //print out the value returned by the maths
     display.display();                                               //show me the buffer!

     counter=0;                                                       //reset counter
     last=millis();                                                   //remember this moment

                                                                      //the time is gone, the song is over
                                                                      //I thought I'd something more to say...
   }
}