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si = %A_LoopField%

si := A_LoopField

MsgBox % Levenshtein_distance( "A H K", "A H K" )
MsgBox % Levenshtein_distance( "AHK", "" )
MsgBox % Levenshtein_distance( "He", "Ben" )
MsgBox % Levenshtein_distance( "Toralf", "esromneb" )
MsgBox % Levenshtein_distance( "Toralf", "RalfLaDuce" )

Levenshtein_distance( s, t )
{
   n := StrLen(s)
   IfEqual n,0, Return m
   m := StrLen(t)
   IfEqual m,0, Return n

   d0_0 = 0
   Loop %n%
      d0_%A_Index% := A_Index
   Loop %m%
      d%A_Index%_0 := A_Index

   Loop Parse, s
   {
      i  = %A_Index%
      i1:= i-1
      si := A_LoopField
      Loop Parse, t
      {
         j1 := A_Index - 1
         d%A_Index%_%i% := min( d%A_Index%_%i1%+1, d%j1%_%i%+1, d%j1%_%i1%+!(si == A_LoopField) )
      }
   }
   Return d%m%_%n%
}

min( a, b, c )
{
   IfLess b, %a%, SetEnv a, %b%
   IfLess c, %a%, Return c
   Return a
}

TextDifference2(String1,String2)
{
 Distance := 0, (String1Len := StrLen(String1)) < (String2Len := StrLen(String2)) ? (ShorterString := String1, LongerString := String2) : (ShorterString := String2, LongerString := String1)
 Loop, Parse, ShorterString
  SubStr(LongerString,A_Index,1) <> A_LoopField ? Distance ++
 Return, Distance + Abs(String1Len - String2Len)
}