/*
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.IO;
using System.Text;
using System.Collections;

namespace Lucene.Net.Analysis.De
{
	/// <summary>
	/// A stemmer for German words. The algorithm is based on the report
	/// "A Fast and Simple Stemming Algorithm for German Words" by Jörg
	/// Caumanns (joerg.caumanns@isst.fhg.de).
	/// </summary>
	public class GermanStemmer
	{
		/// <summary>
		/// Buffer for the terms while stemming them. 
		/// </summary>
		private StringBuilder sb = new StringBuilder();

		/// <summary>
		/// Amount of characters that are removed with <tt>Substitute()</tt> while stemming.
		/// </summary>
		private int substCount = 0;

		/// <summary>
		/// Stemms the given term to an unique <tt>discriminator</tt>.
		/// </summary>
		/// <param name="term">The term that should be stemmed.</param>
		/// <returns>Discriminator for <tt>term</tt></returns>
		internal String Stem( String term )
		{
			// Use lowercase for medium stemming.
			term = term.ToLower();
			if ( !IsStemmable( term ) )
				return term;
			// Reset the StringBuilder.
			sb.Remove(0, sb.Length);
			sb.Insert(0, term);
			// Stemming starts here...
			Substitute( sb );
			Strip( sb );
			Optimize( sb );
			Resubstitute( sb );
			RemoveParticleDenotion( sb );
			return sb.ToString();
		}

		/// <summary>
		/// Checks if a term could be stemmed.
		/// </summary>
		/// <param name="term"></param>
		/// <returns>true if, and only if, the given term consists in letters.</returns>
		private bool IsStemmable( String term )
		{
			for ( int c = 0; c < term.Length; c++ ) 
			{
				if ( !Char.IsLetter(term[c])) return false;
			}
			return true;
		}

		/// <summary>
		/// Suffix stripping (stemming) on the current term. The stripping is reduced
		/// to the seven "base" suffixes "e", "s", "n", "t", "em", "er" and * "nd",
		/// from which all regular suffixes are build of. The simplification causes
		/// some overstemming, and way more irregular stems, but still provides unique.
		/// discriminators in the most of those cases.
		/// The algorithm is context free, except of the length restrictions.
		/// </summary>
		/// <param name="buffer"></param>
		private void Strip( StringBuilder buffer )
		{
			bool doMore = true;
			while ( doMore && buffer.Length > 3 ) 
			{
				if ( ( buffer.Length + substCount > 5 ) &&
					buffer.ToString().Substring(buffer.Length - 2, 2).Equals( "nd" ) )
				{
					buffer.Remove( buffer.Length - 2, 2 );
				}
				else if ( ( buffer.Length + substCount > 4 ) &&
					buffer.ToString().Substring( buffer.Length - 2, 2).Equals( "em" ) ) 
				{
					buffer.Remove( buffer.Length - 2, 2 );
				}
				else if ( ( buffer.Length + substCount > 4 ) &&
					buffer.ToString().Substring( buffer.Length - 2, 2).Equals( "er" ) ) 
				{
					buffer.Remove( buffer.Length - 2, 2 );
				}
				else if ( buffer[buffer.Length - 1] == 'e' ) 
				{
					buffer.Remove(buffer.Length - 1, 1);
				}
				else if ( buffer[buffer.Length - 1] == 's' ) 
				{
					buffer.Remove(buffer.Length - 1, 1);
				}
				else if ( buffer[buffer.Length - 1] == 'n' ) 
				{
					buffer.Remove(buffer.Length - 1, 1);
				}
					// "t" occurs only as suffix of verbs.
				else if ( buffer[buffer.Length - 1] == 't') 
				{
					buffer.Remove(buffer.Length - 1, 1);
				}
				else 
				{
					doMore = false;
				}
			}
		}

		/// <summary>
		/// Does some optimizations on the term. This optimisations are contextual.
		/// </summary>
		/// <param name="buffer"></param>
		private void Optimize( StringBuilder buffer )
		{
			// Additional step for female plurals of professions and inhabitants.
			if ( buffer.Length > 5 && buffer.ToString().Substring(buffer.Length - 5, 5).Equals( "erin*" )) 
			{
				buffer.Remove(buffer.Length - 1, 1);
				Strip(buffer);
			}
			// Additional step for irregular plural nouns like "Matrizen -> Matrix".
			if ( buffer[buffer.Length - 1] == ('z') ) 
			{
				buffer[buffer.Length - 1] = 'x';
			}
		}

		/// <summary>
		/// Removes a particle denotion ("ge") from a term.
		/// </summary>
		/// <param name="buffer"></param>
		private void RemoveParticleDenotion( StringBuilder buffer )
		{
			if ( buffer.Length > 4 ) 
			{
				for ( int c = 0; c < buffer.Length - 3; c++ ) 
				{
					if ( buffer.ToString().Substring( c, 4 ).Equals( "gege" ) ) 
					{
						buffer.Remove(c, 2);
						return;
					}
				}
			}
		}

		/// <summary>
		/// Do some substitutions for the term to reduce overstemming:
		///
		/// - Substitute Umlauts with their corresponding vowel: äöü -> aou,
		///   "ß" is substituted by "ss"
		/// - Substitute a second char of a pair of equal characters with
		/// an asterisk: ?? -> ?*
		/// - Substitute some common character combinations with a token:
		///   sch/ch/ei/ie/ig/st -> $/В§/%/&/#/!
		/// </summary>
		private void Substitute( StringBuilder buffer )
		{
			substCount = 0;
			for ( int c = 0; c < buffer.Length; c++ ) 
			{
				// Replace the second char of a pair of the equal characters with an asterisk
				if ( c > 0 && buffer[c] == buffer[c - 1]) 
				{
					buffer[c] = '*';
				}
					// Substitute Umlauts.
				else if ( buffer[c] == 'ä' ) 
				{
					buffer[c] = 'a';
				}
				else if ( buffer[c] == 'ö' ) 
				{
					buffer[c] = 'o';
				}
				else if ( buffer[c] == 'ü' ) 
				{
					buffer[c] = 'u';
				}
				// Fix bug so that 'ß' at the end of a word is replaced.
				else if ( buffer[c] == 'ß' ) 
				{
				
					buffer[c] = 's';
					buffer.Insert(c + 1, 's');
					substCount++;
				}
				// Take care that at least one character is left left side from the current one
				if ( c < buffer.Length - 1 ) 
				{
					// Masking several common character combinations with an token
					if ( ( c < buffer.Length - 2 ) && buffer[c] == 's' &&
						buffer[c + 1] == 'c' && buffer[c + 2] == 'h' )
					{
						buffer[c] = '$';
						buffer.Remove(c + 1, 2);
						substCount =+ 2;
					}
					else if ( buffer[c] == 'c' && buffer[c + 1] == 'h' ) 
					{
						buffer[c] = '§';
						buffer.Remove(c + 1, 1);
						substCount++;
					}
					else if ( buffer[c] == 'e' && buffer[c + 1] == 'i' ) 
					{
						buffer[c] = '%';
						buffer.Remove(c + 1, 1);
						substCount++;
					}
					else if ( buffer[c] == 'i' && buffer[c + 1] == 'e' ) 
					{
						buffer[c] = '&';
						buffer.Remove(c + 1, 1);
						substCount++;
					}
					else if ( buffer[c] == 'i' && buffer[c + 1] == 'g' ) 
					{
						buffer[c] = '#';
						buffer.Remove(c + 1, 1);
						substCount++;
					}
					else if ( buffer[c] == 's' && buffer[c + 1] == 't' ) 
					{
						buffer[c] = '!';
						buffer.Remove(c + 1, 1);
						substCount++;
					}
				}
			}
		}

		/// <summary>
		/// Undoes the changes made by Substitute(). That are character pairs and
		/// character combinations. Umlauts will remain as their corresponding vowel,
		/// as "?" remains as "ss".
		/// </summary>
		/// <param name="buffer"></param>
		private void Resubstitute( StringBuilder buffer )
		{
			for ( int c = 0; c < buffer.Length; c++ ) 
			{
				if ( buffer[c] == '*' ) 
				{
					char x = buffer[c - 1];
					buffer[c] = x;
				}
				else if ( buffer[c] == '$' ) 
				{
					buffer[c] = 's';
					buffer.Insert( c + 1, new char[]{'c', 'h'}, 0, 2);
				}
				else if ( buffer[c] == '§' ) 
				{
					buffer[c] = 'c';
					buffer.Insert( c + 1, 'h' );
				}
				else if ( buffer[c] == '%' ) 
				{
					buffer[c] = 'e';
					buffer.Insert( c + 1, 'i' );
				}
				else if ( buffer[c] == '&' ) 
				{
					buffer[c] = 'i';
					buffer.Insert( c + 1, 'e' );
				}
				else if ( buffer[c] == '#' ) 
				{
					buffer[c] = 'i';
					buffer.Insert( c + 1, 'g' );
				}
				else if ( buffer[c] == '!' ) 
				{
					buffer[c] = 's';
					buffer.Insert( c + 1, 't' );
				}
			}
		}
	}
}