Android Code Snippet Check equality of two files

Skip to Entry #3 for a more efficient way (thank you Erel)

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Edit: Code to compare two files: One slow way to compare two files:

Sub FilesAreIdentical(strFile1 As String, strFile2 As String) As Boolean
    If  File.Size("", strFile1) <> File.Size("", strFile2) Then Return False
    Dim in1 As InputStream
    Dim in2 As InputStream
    in1 = File.OpenInput("", strFile1)
    in2 = File.OpenInput("", strFile2)
    Dim buffer1(File.Size("", strFile1)) As Byte
    Dim buffer2(File.Size("", strFile2)) As Byte
    in1.ReadBytes(buffer1, 0, buffer1.length)
    in2.ReadBytes(buffer2, 0, buffer2.length)
    Dim data1(buffer1.Length) As Byte
    Dim data2(buffer2.Length) As Byte
    Dim md As MessageDigest
    data1 = md.GetMessageDigest(buffer1, "MD5")
    data2 = md.GetMessageDigest(buffer2, "MD5")
    Dim Bconv As ByteConverter
    Return ( Bconv.HexFromBytes(data1) = Bconv.HexFromBytes(data2) )
End Sub

Spoiler: Inspired by

https://www.b4x.com/android/forum/threads/fastest-file-compare.36829/#post-216919
https://www.b4x.com/android/forum/threads/how-to-get-md5-of-a-file.7332/#post-41871​

 

[MarcoRome]

Well done

 

[Erel]

I'm sorry to say but this is a very inefficient code. I wouldn't use it unless the files are small.

The best solution is to avoid reading the whole files. However a simple solution that still reads the whole files and is much more efficient:

Sub FilesAreIdentical(strFile1 As String, strFile2 As String) As Boolean
    If  File.Size("", strFile1) <> File.Size("", strFile2) Then Return False
   Dim b1() As Byte = File.ReadBytes("", strFile1)
   Dim b2() As Byte = File.ReadBytes("", strFile2)
   For i = 0 To b1.Length - 1
    If b1(i) <> b2(i) Then Return False
   Next
   Return True
End Sub

 

[MarcoRome]

Well done

 

[LucaMs]

File.ReadBytes

?

 

[LucaMs]

?

View attachment 69377

Found: it is b4j (only)


P.S.
maybe some method of StringUtils? ByteConverter? Or other libraries, I don't know, have to search

 

[MarcoRome]

?

View attachment 69377

You found this in B4A 8.30 / B4i 5.00

• New File methods: CopyAsync, Copy2Async, ReadBytes, WriteBytes and ListFilesAsync.

 

[LucaMs]

I was "certain" of having started 8.30. This may be the problem of having more than one link

 

[Erel]

If you are using an older version of B4A then you can replace File.ReadBytes with:

Bit.InputStreamToBytes(File.OpenInput(...))

@fredo I will explain the three issues that I see in your code as they are quite common mistakes:

1.

Dim data1(buffer1.Length) As Byte
Dim data2(buffer2.Length) As Byte
Dim md As MessageDigest
data1 = md.GetMessageDigest(buffer1, "MD5")
data2 = md.GetMessageDigest(buffer2, "MD5")

The first two lines above allocate two large arrays and then assign new arrays to the same variables. The previously allocated arrays are discarded. It should have been written like this:

Dim data1() As Byte = md1.GetMessageDigest(buffer1, "MD5")
...

2. GetMessageDigest is not very helpful in this case. Internally it must go over all bytes and calculate the hash. So it will be slower than the code I posted above and worst, if the test must be 100% accurate, then it can only tell us whether the files are different.
It can be useful in some cases if you cache the result and later compare it to other files. Especially if you are fine with %99+ accuracy.

At this point I see that I went over your code too fast and missed the fact that you are testing the hash and not the file content itself. Sorry!!!
Your code is fine. Though the code I posted is better

3. Point is not relevant...

 

[fredo]

...I will explain...

Thanks, Erel, for taking the time to teach us about the efficiency of the code every so often.

This almost tempts me to post all my "Quick and dirty" functions of the last 3 years here...
Just kidding. It is certainly better to take the time to build more efficient code in the first place.