Soundex.cs

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// 
//  Copyright 2010-2015 Deveel
// 
//    Licensed under the Apache License, Version 2.0 (the "License");
//    you may not use this file except in compliance with the License.
//    You may obtain a copy of the License at
// 
//        http://www.apache.org/licenses/LICENSE-2.0
// 
//    Unless required by applicable law or agreed to in writing, software
//    distributed under the License is distributed on an "AS IS" BASIS,
//    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//    See the License for the specific language governing permissions and
//    limitations under the License.
//

using System;
using System.Globalization;
using System.Text;

namespace Deveel.Data.Text {
    public abstract class Soundex {
        public static Soundex Default = new DefaultSoundex();

        public virtual int Difference(string s1, string s2) {
            throw new NotSupportedException();
        }

        public abstract string Compute(string s);

        protected virtual string EncodeChar(char c) {
            switch (Char.ToUpperInvariant(c)) {
                case 'B':
                case 'F':
                case 'P':
                case 'V':
                    return "1";
                case 'C':
                case 'G':
                case 'J':
                case 'K':
                case 'Q':
                case 'S':
                case 'X':
                case 'Z':
                    return "2";
                case 'D':
                case 'T':
                    return "3";
                case 'L':
                    return "4";
                case 'M':
                case 'N':
                    return "5";
                case 'R':
                    return "6";
                default:
                    return string.Empty;
            }
        }

        #region DefaultSoundex

        private class DefaultSoundex : Soundex {
            public override string Compute(string s) {
                if (String.IsNullOrEmpty(s)) 
                    return String.Empty;

                int startIndex;
                for (startIndex = 0; startIndex < s.Length && !char.IsLetter(s[startIndex]); startIndex++) {
                }

                if (startIndex >= s.Length) 
                    return String.Empty;

                var output = new StringBuilder();

                output.Append(Char.ToUpperInvariant(s[startIndex]));

                // Stop at a maximum of 4 characters.
                for (int i = startIndex + 1; i < s.Length && output.Length < 4; i++) {
                    string c = EncodeChar(s[i]);

                    // Ignore duplicated chars.
                    if (c != EncodeChar(s[i - 1])) {
                        output.Append(c);
                    }
                }

                // Pad with zeros.
                output.Append(new String('0', 4 - output.Length));

                return output.ToString();
            }
        }

        #endregion
    }
}