SimilarityTool.cs

// This class implements string comparison algorithm based on character pair similarity
// Source: http://www.catalysoft.com/articles/StrikeAMatch.html
// From: http://stackoverflow.com/questions/653157/a-better-similarity-ranking-algorithm-for-variable-length-strings

public class SimilarityTool {
    // Compares the two strings based on letter pair matches
    // Returns the percentage match from 0.0 to 1.0 where 1.0 is 100%
    public double CompareStrings( string str1, string str2 ) {
        List<string> pairs1 = WordLetterPairs( str1.ToUpper() );
        List<string> pairs2 = WordLetterPairs( str2.ToUpper() );
        int intersection = 0;
        int union = pairs1.Count + pairs2.Count;

        for ( int i = 0 ; i < pairs1.Count ; i++ ) {
            for ( int j = 0 ; j < pairs2.Count ; j++ ) {
                if ( pairs1[i] == pairs2[j] ) {
                    intersection++;
                    pairs2.RemoveAt( j ); //Must remove the match to prevent "GGGG" from appearing to match "GG" with 100% success
                    break;
                }
            }
        }

        return ( 2.0 * intersection ) / union;
    }

    // Gets all letter pairs for each individual word in the string
    private List<string> WordLetterPairs( string str ) {
        List<string> AllPairs = new List<string>();
        string[] Words = Regex.Split( str, @"\s" ); // Tokenize the string and put the tokens/words into an array

        // For each word
        for ( int w = 0 ; w < Words.Length ; w++ ) {
            if ( !string.IsNullOrEmpty( Words[w] ) ) {
                // Find the pairs of characters
                String[] PairsInWord = LetterPairs( Words[w] );

                for ( int p = 0 ; p < PairsInWord.Length ; p++ ) {
                    AllPairs.Add( PairsInWord[p] );
                }
            }
        }

        return AllPairs;
    }

    // Generates an array containing every two consecutive letters in the input string
    private string[] LetterPairs( string str ) {
        int numPairs = str.Length - 1;
        string[] pairs = new string[numPairs];

        for ( int i = 0 ; i < numPairs ; i++ ) {
            pairs[i] = str.Substring( i, 2 );
        }

        return pairs;
    }
}