//VERSION=3
function setup () {
	return{
		input:["B04", "B08"],
		mosaicking: Mosaicking.ORBIT,
		output:{bands: 1}
	}		
}

function calcNDVI(sample) {
  var denom = sample.B04+sample.B08;
  return ((denom!=0) ? (sample.B08-sample.B04) / denom : 0.0);
}
function evaluatePixel(samples,scenes) {  
  var avg1 = 0;
  var count1 = 0;
  var avg2 = 0;
  var count2 = 0;
  var endMonth = scenes[0].date.getMonth(); //get month of the most recent scene
  
  for (var i=0;i<samples.length;i++) {
      var ndvi = calcNDVI(samples[i]);
      if (scenes[i].date.getMonth()==endMonth) //if the scene was acquired on the same month as most recent scene (if the scene is from the most recent month)
      {
		avg1 = avg1 + ndvi;
        count1++;
      }
      else // all other scenes (depends on filterScenes setup below - for 1 additional month, set it to 2 months)
      {      
		avg2= avg2 + ndvi;
        count2++;
      }
      
  }
  avg1 = avg1/count1;
  avg2 = avg2/count2;
  
  let NDVI_change_ratio = (avg1-avg2)/avg2
  return [NDVI_change_ratio]
}  
function filterScenes (scenes, inputMetadata) {
    return scenes.filter(function (scene) {
	  return scene.date.getTime()>=(inputMetadata.to.getTime()-2*31*24*3600*1000) ;
    });
}