Question

I am trying to compare two long bytearrays in VB.NET and have run into a snag. Comparing two 50 megabyte files takes almost two minutes, so I’m clearly doing something wrong. I’m on an x64 machine with tons of memory so there are no issues there. Here is the code that I’m using at the moment and would like to change.

_Bytes and item.Bytes are the two different arrays to compare and are already the same length.

For Each B In item.Bytes    If B <> _Bytes(I) Then         Mismatch = True         Exit For    End If    I += 1 Next 

I need to be able to compare as fast as possible files that are potentially hundreds of megabytes and even possibly a gigabyte or two. Any suggests or algorithms that would be able to do this faster?

Item.bytes is an object taken from the database/filesystem that is returned to compare, because its byte length matches the item that the user wants to add. By comparing the two arrays I can then determine if the user has added something new to the DB and if not then I can just map them to the other file and not waste hard disk drive space.

[Update]

I converted the arrays to local variables of Byte() and then did the same comparison, same code and it ran in like one second (I have to benchmark it still and compare it to others), but if you do the same thing with local variables and use a generic array it becomes massively slower. I’m not sure why, but it raises a lot more questions for me about the use of arrays.

Answer 1

What is the _Bytes(I) call doing? It’s not loading the file each time, is it? Even with buffering, that would be bad news!

There will be plenty of ways to micro-optimise this in terms of looking at longs at a time, potentially using unsafe code etc – but I’d just concentrate on getting reasonable performance first. Clearly there’s something very odd going on.

I suggest you extract the comparison code into a separate function which takes two byte arrays. That way you know you won’t be doing anything odd. I’d also use a simple For loop rather than For Each in this case – it’ll be simpler. Oh, and check whether the lengths are correct first 🙂

EDIT: Here’s the code (untested, but simple enough) that I’d use. It’s in C# for the minute – I’ll convert it in a sec:

public static bool Equals(byte[] first, byte[] second) {     if (first == second)     {         return true;     }     if (first == null || second == null)     {         return false;     }     if (first.Length != second.Length)     {         return false;     }     for (int i=0; i < first.Length; i++)     {         if (first[i] != second[i])                         {             return false;         }     }     return true; } 

EDIT: And here’s the VB:

Public Shared Function ArraysEqual(ByVal first As Byte(), _                                    ByVal second As Byte()) As Boolean     If (first Is second) Then         Return True     End If      If (first Is Nothing OrElse second Is Nothing) Then         Return False     End If     If  (first.Length <> second.Length) Then          Return False     End If      For i as Integer = 0 To first.Length - 1         If (first(i) <> second(i)) Then             Return False         End If     Next i     Return True End Function