Statistics collector that calculates the median and average request response times for a 7 days dataset.

statsCollector.js

/*
* Our application servers receive approximately 20 000
* http requests  per second. Response timeout is 19000ms.
* Implement a statistics collector that calculates the
* median and average request response times for a 7 day
* dataset.
*/

'use strict';

// Stats collector
class StatsCollector {
	constructor(/*void*/) {
    this.responseTimeMsList = [];
  }

  /*
  * Pushes a value to the responseTimeMsList
  * @param {Number} responseTimeMs A response time value
  * @throw {Error} Will throw "invalid response time value" error
  */
	pushValue(responseTimeMs /*number*/) /*void*/ {
    //checking if the value a valid response time
  	if(this.isInvalidResponseTime(responseTimeMs)){
    	throw new Error("invalid response time value")
    }else{
    	//a valid value is pushed to the array
			this.responseTimeMsList.push(parseFloat(responseTimeMs));
    }
  }

  /*
  * Calculates median of the response time values
  *	Even number of entries: the meidan will be averge of the two mid entries
  * Odd number of entires: the median will be the middle entry
  * @returns {Number} The median of response time values
  * @throws {Error} Will throw 'the response time list is empty' error
  */
	getMedian() /*number*/ {
    var responseTimeMedian = -1;
    if(this.responseTimeMsList.length == 0){
    	throw new Error("the response time list is empty")
    }else if(this.responseTimeMsList.length == 1){//if it has only one value then that is the median
      responseTimeMedian = this.responseTimeMsList[0];     
    }else if(this.responseTimeMsList.length == 2){ // incase of two values no need to do sorting etc. the mean of both is the median.
      responseTimeMedian = (this.responseTimeMsList[0]+this.responseTimeMsList[1])/2;
    }else{
      this.sort();
      //even or odd
      if(this.isEvenNumber(this.responseTimeMsList.length)){
        var halfLength = this.responseTimeMsList.length/2;
        responseTimeMedian = (this.responseTimeMsList[halfLength] + this.responseTimeMsList[halfLength-1])/2;
      }else{
        var halfLength = Math.floor(this.responseTimeMsList.length/2);
        responseTimeMedian = this.responseTimeMsList[halfLength];
      }  
    }
    return responseTimeMedian;
  }
	
  /*
  * Sorts the response times in an ascending order 
  */
  sort(){
    var compareNumbers = function(a, b){
    	return a - b;
		}
    this.responseTimeMsList.sort(compareNumbers);  	
  }
  
  /*
  * Checks if a number is a Even number
  * @param {Number} number The number
  * @returns {Boolean} True when even number otherwise false
  */
  isEvenNumber(number) /* boolean */{
  	// not meant for negative numbers
    if (number < 0){
    	return false;
    }
    
    var fraction = number/2;
  	
    if (fraction == Math.floor(fraction)){
    	return true;
    }
    
    return false;
  } 
  
  /*
  * Checks if a value is a valid response time
  * @param {Number} responseTime The response time value
  * @returns {Boolean} True if a valid response time value otherwise false
  */
  isInvalidResponseTime(responseTime) {
  	if(responseTime<0 || responseTime==null || responseTime == undefined || isNaN(parseFloat(responseTime)) || !isFinite(responseTime)){
    	return true;
    }
    
    return false;
	}
  
  /*
  * Averages the response time values
  * @returns {Number} The average of the response time values
  * @throws {Error} Will throw "the response time list is empty" error
  */
	getAverage() {
  
  	var responseTimeSum=0, responseTimeAverage=0;
    if(this.responseTimeMsList.length == 0){
    	throw new Error("the response time list is empty")
 		}else if(this.responseTimeMsList.length == 1){
    	responseTimeAverage = this.responseTimeMsList[0];
    }else{
      this.responseTimeMsList.forEach((function(responseTime){
        responseTimeSum +=responseTime;
      }).bind(this));

      if(responseTimeSum > 0) {
        responseTimeAverage =  responseTimeSum / this.responseTimeMsList.length
      }
		}
    
    return responseTimeAverage;
  }

}

// Configure Mocha, telling both it and chai to use BDD-style tests.
mocha.setup("bdd");
chai.should();

describe('StatsCollector', function(){
  
  //------ Valid/invalid response time values -----------------
  it('Test001: Populating response time array with valid values.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(200);
    statsCollector.pushValue('200');
    statsCollector.pushValue(200.123);
    statsCollector.pushValue(0);
    statsCollector.responseTimeMsList.should.have.length(4);
  });
  it('Test002: Populating response time array with invalid values.', function(){
  	var statsCollector = new StatsCollector();
    (function(){statsCollector.pushValue('ff');}).should.throw(Error,"invalid response time value");  
    (function(){statsCollector.pushValue(null);}).should.throw(Error,"invalid response time value");  
    (function(){statsCollector.pushValue(1/0);}).should.throw(Error,"invalid response time value"); 
    (function(){statsCollector.pushValue('');}).should.throw(Error,"invalid response time value");  
    

  });
  
  //----------- AVERAGE TESTS ------------
  it('Test003: Average of an empty list.', function(){
  	var statsCollector = new StatsCollector();
    (function(){statsCollector.getAverage();}).should.throw(Error, "the response time list is empty");
  });
  it('Test004: Average of single response time entry.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(200);
    statsCollector.getAverage().should.equal(200);
  });
  it('Test005: Average of two response time entries.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(140);
    statsCollector.pushValue(160);
    statsCollector.getAverage().should.equal(150);
  });
  it('Test006: Averaging whole numbers.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(190);
    statsCollector.pushValue(200);
    statsCollector.pushValue(210); 
    statsCollector.pushValue(220); 
    statsCollector.getAverage().should.equal(205);
  });
  it('Test007: Averaging with decimal point.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(190.145);
    statsCollector.pushValue(190.145);
    statsCollector.pushValue(190.145);    
    statsCollector.getAverage().should.equal(190.145);
  });
  //----------- MEDIAN TESTS ------------
  it('Test008: Median of an empty list.', function(){
  	var statsCollector = new StatsCollector();
    (function(){statsCollector.getMedian();}).should.throw(Error, "the response time list is empty");
  });
  it('Test009: Median of single response time entry.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(200);
    statsCollector.getMedian().should.equal(200);
  });
  it('Test010: Median of two response time entries.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(190);
    statsCollector.pushValue(110);
    statsCollector.getMedian().should.equal(150);
  });
  it('Test011: Median of odd number of entries.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(200);
    statsCollector.pushValue(150);
    statsCollector.pushValue(180);
    statsCollector.pushValue(170);
    statsCollector.pushValue(140);
    statsCollector.getMedian().should.equal(170);
  });
  it('Test012: Median of even number of entries.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(200);
    statsCollector.pushValue(150);
    statsCollector.pushValue(180);
    statsCollector.pushValue(170);
    statsCollector.pushValue(140);
    statsCollector.pushValue(120);
    statsCollector.getMedian().should.equal(160);
  });
  it('Test013: Median of even number of entries containing decimal point values.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(200);
    statsCollector.pushValue(150.10);
    statsCollector.pushValue(180);
    statsCollector.pushValue(170.30);
    statsCollector.pushValue(140);
    statsCollector.pushValue(120);
    statsCollector.getMedian().should.equal(160.20);
  });
  
 
  it('Test014: Average & median of all zero value entries.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue(0);
    statsCollector.pushValue(0);
    statsCollector.pushValue(0);    
    statsCollector.getAverage().should.equal(0);
    statsCollector.getMedian().should.equal(0);
  });
  it('Test015: Average & median of numbers pushed as strings.', function(){
  	var statsCollector = new StatsCollector();
    statsCollector.pushValue('200');
    statsCollector.pushValue('150');
    statsCollector.pushValue('180');
    statsCollector.pushValue('170');
    statsCollector.pushValue('140');
    statsCollector.getAverage().should.equal(168);
    statsCollector.getMedian().should.equal(170);
  });
  //----------- Stress test ---------------------
  //Can't run, browswer is crashing
  /*it('Test016: Stress test, pushing 7 days data.', function(){
  	var statsCollector = new StatsCollector();
    var requestsIn7Days=20000*60*60*24*7;
    for(var i=1; i<=requestsIn7Days; i++){
    	statsCollector.pushValue('200');
    }
    statsCollector.responseTimeMsList.should.have.length(4);
  });*/

});

// Run all our test suites.  Only necessary in the browser.
mocha.run();