Performance improvement suggestion

[toptensoftware]

Thanks so much for this library. I've been using a highly modified version for diffing arrays and found for some large array sets with simple edits it was quite slow (eg: 2+ seconds when prepending many items).

For my use case, the edits are often single append, prepend, insert or delete operations and I found for these common cases I could speed up things considerably by trimming common items from the start/end before calling the core diff routine. With this tweak, many operations that previously took 150 - 2000ms now take 1-2 ms.

The code below shows the basic concept. As mentioned, I'm using a modified version of the library with different callbacks that just give the insert and delete operations - but it should be relatively easy to adapt it to work with this library again.

// callback(op, index, count) where op = "insert" or "delete"
export function diff(oldArray, newArray, callback, compareEqual)
{
 if (!compareEqual)
 compareEqual = function(a,b) { return a == b; }

 // Get the min and max length of the two arrays
 let minLength = Math.min(oldArray.length, newArray.length);
 let maxLength = Math.max(oldArray.length, newArray.length);

 // Work out how many matching items at the start
 let trimStart = 0;
 while (trimStart < minLength && 
 compareEqual(oldArray[trimStart], newArray[trimStart]))
 {
 trimStart++;
 }

 // Exact match?
 if (trimStart == maxLength)
 return;

 // Simple Append?
 if (trimStart == oldArray.length)
 {
 callback('insert', oldArray.length, newArray.length - oldArray.length);
 return;
 }

 // Work out how many matching items at the end
 let trimEnd = 0;
 while (trimEnd < (minLength - trimStart) && 
 compareEqual(oldArray[oldArray.length - trimEnd - 1], newArray[newArray.length - trimEnd - 1]))
 {
 trimEnd++;
 }

 // Simple prepend?
 if (trimEnd == oldArray.length)
 {
 callback('insert', 0, newArray.length - oldArray.length);
 return;
 }

 // Simple insert?
 if (trimStart + trimEnd == oldArray.length)
 {
 callback('insert', trimStart, newArray.length - oldArray.length);
 return;
 }

 // Simple delete?
 if (trimStart + trimEnd == newArray.length)
 {
 callback('delete', trimStart, oldArray.length - newArray.length);
 return;
 }

 // Untrimmed?
 if (trimStart == 0 && trimEnd == 0)
 {
 return diff_core(oldArray, newArray, callback, compareEqual);
 }

 // Trimmed diff - slice the arrays and adjust the indicies on the callbacks
 return diff_core(
 oldArray.slice(trimStart, -trimEnd),
 newArray.slice(trimStart, -trimEnd),
 (op, index, count) => callback(op, index + trimStart, count),
 compareEqual
 );
 
}