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POPerformance difference between two implementations of the same algorithm
 

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  1. POPerformance difference between two implementations of the same algorithm
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    <p>I'm working on an application that will require the Levenshtein algorithm to calculate the similarity of two strings.</p> <p>Along time ago I adapted a C# version (which can be easily found floating around in the internet) to VB.NET and it looks like this:</p> <pre><code>Public Function Levenshtein1(s1 As String, s2 As String) As Double Dim n As Integer = s1.Length Dim m As Integer = s2.Length Dim d(n, m) As Integer Dim cost As Integer Dim s1c As Char For i = 1 To n d(i, 0) = i Next For j = 1 To m d(0, j) = j Next For i = 1 To n s1c = s1(i - 1) For j = 1 To m If s1c = s2(j - 1) Then cost = 0 Else cost = 1 End If d(i, j) = Math.Min(Math.Min(d(i - 1, j) + 1, d(i, j - 1) + 1), d(i - 1, j - 1) + cost) Next Next Return (1.0 - (d(n, m) / Math.Max(n, m))) * 100 End Function </code></pre> <p>Then, trying to tweak it and improve its performance, I ended with version:</p> <pre><code>Public Function Levenshtein2(s1 As String, s2 As String) As Double Dim n As Integer = s1.Length Dim m As Integer = s2.Length Dim d(n, m) As Integer Dim s1c As Char Dim cost As Integer For i = 1 To n d(i, 0) = i s1c = s1(i - 1) For j = 1 To m d(0, j) = j If s1c = s2(j - 1) Then cost = 0 Else cost = 1 End If d(i, j) = Math.Min(Math.Min(d(i - 1, j) + 1, d(i, j - 1) + 1), d(i - 1, j - 1) + cost) Next Next Return (1.0 - (d(n, m) / Math.Max(n, m))) * 100 End Function </code></pre> <p>Basically, I thought that the array of distances d(,) could be initialized inside of the main for cycles, instead of requiring two initial (and additional) cycles. I really thought this would be a huge improvement... unfortunately, not only does not improve over the original, it actually runs slower!</p> <p>I have already tried to analyze both versions by looking at the generated IL code but I just can't understand it.</p> <p>So, I was hoping that someone could shed some light on this issue and explain why the second version (even when it has fewer for cycles) runs slower than the original?</p> <p>NOTE: The time difference is about 0.15 nano seconds. This don't look like much but when you have to check thousands of millions of strings... the difference becomes quite notable.</p>
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