Run Length Encoding and Decoding for JS number arrays, with Delta Encoding and Delta Decoding.

RLE.js

function RLEencode(array) {
	// output an array of values
	// consisting of alternating "rips" and "runs"
	// a rip begins with a negative count followed by a 
	// cooresponding number of non-repeating values
	// 
	// a run begins with a positive count, followed by 
	// the value to be repeated by the count.

	var newArray=[];
	var rip=[];
	var lastValue=undefined;
	var runCount=0;

	for (var i = 1,lastValue=array[0]; i <= array.length; i++) {
		if(array[i]!==lastValue){
			if(runCount!==0){
				newArray.push(runCount+1,lastValue);
			} else {
				rip.push(lastValue);
			}
			runCount=0;
		}

		if(array[i]===lastValue || i===array.length){
			if(rip.length !== 0){
				if(rip.length){
					newArray.push(-rip.length);
					newArray=newArray.concat(rip);
				}
				rip=[];
			}
			runCount++;
		}
		lastValue=array[i];
	};
	return newArray;
}

function RLEdecode(array) {
	var newArray=[],isRip,isRun,ripCount,runCount;
	for (var i = 0; i < array.length; i++) {
		isRip=array[i]<0;
		isRun=array[i]>0;
		if(isRip){
			ripCount=Math.abs(array[i]);
			i+=1;

			newArray=newArray.concat(array.slice(i,i+ripCount));
			// console.log("rip",ripCount,array.slice(i,i+ripCount));
			i+=ripCount-1;
		}
		if(isRun){
			runCount=array[i];
			i+=1;
			for (var j = 0; j < runCount; j++) {
				newArray.push(array[i])
			};
			
		}
		
	};
	return newArray;

}

function deltaEncode (array) {
	var newArray=[];
	var lastval=0;
	var count=0;
	var i;
	for(i=0;i<array.length;i++){
		newArray[i] = array[i] - lastval;
		lastval = array[i];
	}
	return newArray;
}
function deltaDecode (array) {
	var newArray=[];
	var lastval=0;
	var count=0;
	var i;
	for(i=0;i<array.length;i++){
		lastval= newArray[i] = array[i] + lastval;
	}
	return newArray;

}