Fuzzy Bitap Algorithm
This is a fuzzy string matching version of bitap algorithm. The bitap algorithm (also known as the shift-or, shift-and or Baeza-Yates–Gonnet algorithm) is an approximate string matching algorithm. The algorithm tells whether a given text contains a substring which is "approximately equal" to a given pattern, where approximate equality is defined in terms of Levenshtein distance — if the substring and pattern are within a given distance k of each other, then the algorithm considers them equal. The algorithm begins by precomputing a set of bitmasks containing one bit for each element of the pattern. Then it is able to do most of the work with bitwise operations, which are extremely fast.
Public Shared Function SearchString(text As String, pattern As String, k As Integer) As Integer
Dim result As Integer = -1
Dim m As Integer = pattern.Length
Dim R As Integer()
Dim patternMask As Integer() = New Integer(127) {}
Dim i As Integer, d As Integer
If String.IsNullOrEmpty(pattern) Then
Return 0
End If
If m > 31 Then
Return -1 'Error: The pattern is too long!
End If
R = New Integer((k + 1) * 4 - 1) {}
For i = 0 To k
R(i) = Not 1
Next
For i = 0 To 127
patternMask(i) = Not 0
Next
For i = 0 To m - 1
patternMask(Convert.ToInt32(pattern(i))) = patternMask(Convert.ToInt32(pattern(i))) And Not (1 << i)
Next
For i = 0 To text.Length - 1
Dim oldRd1 As Integer = R(0)
R(0) = R(0) Or patternMask(Convert.ToInt32(text(i)))
R(0) <<= 1
For d = 1 To k
Dim tmp As Integer = R(d)
R(d) = (oldRd1 And (R(d) Or patternMask(Convert.ToInt32(text(i))))) << 1
oldRd1 = tmp
Next
If 0 = (R(k) And (1 << m)) Then
result = (i - m) + 1
Exit For
End If
Next
Return result
End Function
Example
Dim index As Integer = SearchString("The quick brown foax jumps over the lazy dog", "fox", 1)
Output
index: 16