What's the most efficient way to merge multiple arrays of sorted objects in JavaScript?

Question

Consider four arrays of objects all sorted individually by date

Object: { id: userId, date : date }

Using JavaScript how should I sort these lists into one merged list by date?

Here's a fiddle with the four lists individually pre sorted.

Answer 1

If you'll definitely have the arrays passed to you sorted, the most efficient thing I can think of is to make your own merge algorithm. It would be something like this:

var merged = [];
var arrIndex1 = 0;
var arrIndex2 = 0;
var arrIndex3 = 0;
var arrIndex4 = 0;

while (arrIndex1 < arr1.length || arrIndex2 < arr2.length || arrIndex3 < arr3.length || arrIndex4 < arr4.length) {
     var val1 = arrIndex1 < arr1.length ? arr1[arrIndex1].date : Number.POSITIVE_INFINITY;
     var val2 = arrIndex2 < arr1.length ? arr2[arrIndex2].date : Number.POSITIVE_INFINITY;
     var val3 = arrIndex3 < arr1.length ? arr3[arrIndex3].date : Number.POSITIVE_INFINITY;
     var val4 = arrIndex4 < arr1.length ? arr4[arrIndex4].date : Number.POSITIVE_INFINITY;

     if (val1 < val2 && val1 < val3 && val1 < val4) {
          merged.push(arr1[arrIndex1++]); 
     } else if (val2 < val2 && val1 < val3 && val1 < val4) {
          merged.push(arr2[arrIndex2++]); 
     } else if (val3 < val2 && val1 < val3 && val1 < val4) {
          merged.push(arr3[arrIndex3++]); 
     } else {
          merged.push(arr4[arrIndex4++]); 
     }
}

This would be the fastest way. However, the easiest way to code it - if you're not concerned about it being the fastest - is simply to splice the four arrays together, and run them through your activites.sort() function.

Answer 2

Because I've got something hard to do and need to procrastinate :-) here's a function "mergeSortedArrays".

It can take any number of parameters, so you call it as:

var resultSorted = mergeSortedArrays(resultA, resultB, resultC, resultD);

It's not as good as it could be, because it only merges two arrays at a time. Perhaps the best implementation would simultaneously merge all the arrays at once. (I'm not sure how that would compare though)

The implementation:

function mergeSortedArrays() {
    function merge(arrayOne, arrayTwo) {        
        var totalLength = arrayOne.length + arrayTwo.length;
        var returnArray = new Array(totalLength);
        var iResult = 0;
        var iOne = 0;
        var iTwo = 0;
        for(var i = 0; i < totalLength; ++i) {
            if(iTwo < arrayTwo.length) {
                if(iOne >= arrayOne.length) {
                    returnArray[i] = arrayTwo[iTwo++];                    
                } else if (arrayOne[iOne].date < arrayTwo[iTwo].date) {
                    returnArray[i] = arrayOne[iOne++];
                } else {
                    returnArray[i] = arrayTwo[iTwo++];
                }  
            } else {
                returnArray[i] = arrayOne[iOne++];
            }
        }
        return returnArray;
    }
    var sortedArray = [];
    for(var i = 0; i < arguments.length; ++i) {
        sortedArray = merge(sortedArray, arguments[i]);
    }
    return sortedArray;
}

Here's the jsFiddle