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Statistical parsing of English sentences

, 13 Dec 2006
Shows how to generate parse trees for English language sentences, using a C# port of OpenNLP, a statistical natural language parsing library.
englishparsing_bin.zip
Lithium.dll
ModelConverter.exe
OpenNLP Tools.chm
OpenNLP.dll
ParseTree.exe
SharpEntropy.dll
ToolsExample.exe
englishparsing_net2_0_bin.zip
ToolsExample.exe
Lithium.dll
ModelConverter.exe
OpenNLP Tools.chm
OpenNLP.dll
ParseTree.exe
SharpEntropy.dll
englishparsing_net2_0_src.zip
Lithium
Collections
Delegates
Enums
Interfaces
IO
Lithium.csproj.vspscc
LithiumControl.bmp
Shapes
UI
Visitors
ModelConverter
App.ico
ModelConverter.csproj.vspscc
ParseTree
App.ico
ParseTree.csproj.vspscc
ToolsExample
App.ico
ToolsExample.csproj.vspscc
OpenNLP
OpenNLP.csproj.vspscc
SharpEntropy.dll
Tools
Chunker
NameFind
Parser
PosTagger
SentenceDetect
Tokenize
Util
englishparsing_src.zip
Lithium.csproj.user
LithiumControl.bmp
App.ico
ModelConverter.csproj.user
OpenNLP.csproj.user
SharpEntropy.dll
vssver.scc
vssver.scc
vssver.scc
vssver.scc
vssver.scc
vssver.scc
vssver.scc
App.ico
ParseTree.csproj.user
App.ico
ToolsExample.csproj.user
//Copyright (C) 2005 Richard J. Northedge
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

//This file is based on the SDContextGenerator.java source file found in the
//original java implementation of OpenNLP.  That source file contains the following header:

// Copyright (C) 2002 Jason Baldridge and Gann Bierner
// 
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
// 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Lesser General Public License for more details.
// 
// You should have received a copy of the GNU Lesser General Public
// License along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

using System;
using System.Collections.Generic;

namespace OpenNLP.Tools.SentenceDetect
{
	/// <summary> 
	/// Generate event contexts for maxent decisions for sentence detection.
	/// </summary>
    public class SentenceDetectionContextGenerator : SharpEntropy.IContextGenerator<Util.Pair<System.Text.StringBuilder, int>>
	{		
		private System.Text.StringBuilder mBuffer = new System.Text.StringBuilder();
        private List<string> mCollectFeatures = new List<string>();
		private Util.Set<string> mInducedAbbreviations;
		private char[] mEndOfSentenceCharacters;
		
		/// <summary>
		/// Creates a new <code>SentenceDetectionContextGenerator</code> instance with
		/// no induced abbreviations.
		/// </summary>
		public SentenceDetectionContextGenerator(char[] endOfSentenceCharacters) : this(new Util.Set<string>(), endOfSentenceCharacters)
		{
		}
		
		/// <summary> 
		/// Creates a new <code>SentenceDetectionContextGenerator</code> instance which uses
		/// the set of induced abbreviations.
		/// </summary>
		/// <param name="inducedAbbreviations">
		/// a <code>Set</code> of strings
		/// representing induced abbreviations in the training data.
		/// Example: Mr.
		/// </param>
		/// <param name="endOfSentenceCharacters">
		/// Character array of end of sentence characters.
		/// </param>
		public SentenceDetectionContextGenerator(Util.Set<string> inducedAbbreviations, char[] endOfSentenceCharacters)
		{
			mInducedAbbreviations = inducedAbbreviations;
			mEndOfSentenceCharacters = endOfSentenceCharacters;
		}
		
		/// <summary>
		/// Builds up the list of features, anchored around a position within the
		/// StringBuilder. 
		/// </summary>
        public virtual string[] GetContext(Util.Pair<System.Text.StringBuilder, int> pair)
		{
			string prefix;				//string preceeding the eos character in the eos token. 
			string previousToken;	//space delimited token preceding token containing eos character. 
			string suffix;				//string following the eos character in the eos token. 
			string nextToken;				//space delimited token following token containsing eos character. 

            System.Text.StringBuilder buffer = pair.FirstValue;
			int position = pair.SecondValue; //character offset of eos character in 

            //if (first is string[])
            //{
            //    string[] firstList = (string[])first;
            //    previousToken = firstList[0];
            //    string current = firstList[1];
            //    prefix = current.Substring(0, (position) - (0));
            //    suffix = current.Substring(position + 1);
            //    if (suffix.StartsWith(" "))
            //    {
            //        mCollectFeatures.Add("sn");
            //    }
            //    if (prefix.EndsWith(" "))
            //    {
            //        mCollectFeatures.Add("pn");
            //    }
            //    mCollectFeatures.Add("eos=" + current[position]);
            //    nextToken = firstList[2];
            //}
            //else
            //{
            //    //compute previous, next, prefix and suffix strings and space previous, space next features and eos features. 
            //    System.Text.StringBuilder buffer = (System.Text.StringBuilder)((Util.Pair)input).FirstValue;
			int lastIndex = buffer.Length - 1;
		
			// compute space previousToken and space next features.
			if (position > 0 && buffer[position - 1] == ' ')
			{
				mCollectFeatures.Add("sp");
			}
			if (position < lastIndex && buffer[position + 1] == ' ')
			{
				mCollectFeatures.Add("sn");
			}
			mCollectFeatures.Add("eos=" + buffer[position]);
		
			int prefixStart = PreviousSpaceIndex(buffer, position);
			
			int currentPosition = position;
		
			//assign prefix, stop if you run into a period though otherwise stop at space
			while (--currentPosition > prefixStart)
			{
				for (int currentEndOfSentenceCharacter = 0, endOfSentenceCharactersLength = mEndOfSentenceCharacters.Length; currentEndOfSentenceCharacter < endOfSentenceCharactersLength; currentEndOfSentenceCharacter++)
				{
					if (buffer[currentPosition] == mEndOfSentenceCharacters[currentEndOfSentenceCharacter])
					{
						prefixStart = currentPosition;
						currentPosition++; // this gets us out of while loop.
						break;
					}
				}
			}
			
			prefix = buffer.ToString(prefixStart, position - prefixStart).Trim();
	
			int previousStart = PreviousSpaceIndex(buffer, prefixStart);
			previousToken = buffer.ToString(previousStart, prefixStart - previousStart).Trim();
			
			int suffixEnd = NextSpaceIndex(buffer, position, lastIndex);
		
			currentPosition = position;
			while (++currentPosition < suffixEnd)
			{
				for (int currentEndOfSentenceCharacter = 0, endOfSentenceCharactersLength = mEndOfSentenceCharacters.Length; currentEndOfSentenceCharacter < endOfSentenceCharactersLength; currentEndOfSentenceCharacter++)
				{
					if (buffer[currentPosition] == mEndOfSentenceCharacters[currentEndOfSentenceCharacter])
					{
						suffixEnd = currentPosition;
						currentPosition--; // this gets us out of while loop.
						break;
					}
				}
			}
		
			int nextEnd = NextSpaceIndex(buffer, suffixEnd + 1, lastIndex + 1);
			if (position == lastIndex)
			{
				suffix = "";
				nextToken = "";
			}
			else
			{
				suffix = buffer.ToString(position + 1, suffixEnd - (position + 1)).Trim();
				nextToken = buffer.ToString(suffixEnd + 1, nextEnd - (suffixEnd + 1)).Trim();
			}
			
			mBuffer.Append("x=");
			mBuffer.Append(prefix);
			mCollectFeatures.Add(mBuffer.ToString());
			mBuffer.Length = 0;
			if (prefix.Length > 0)
			{
				mCollectFeatures.Add(System.Convert.ToString(prefix.Length, System.Globalization.CultureInfo.InvariantCulture));
				if (IsFirstUpper(prefix))
				{
					mCollectFeatures.Add("xcap");
				}
				if (mInducedAbbreviations.Contains(prefix))
				{
					mCollectFeatures.Add("xabbrev");
				}
			}
			
			mBuffer.Append("v=");
			mBuffer.Append(previousToken);
			mCollectFeatures.Add(mBuffer.ToString());
			mBuffer.Length = 0;
			if (previousToken.Length > 0)
			{
				if (IsFirstUpper(previousToken))
				{
					mCollectFeatures.Add("vcap");
				}
				if (mInducedAbbreviations.Contains(previousToken))
				{
					mCollectFeatures.Add("vabbrev");
				}
			}
			
			mBuffer.Append("s=");
			mBuffer.Append(suffix);
			mCollectFeatures.Add(mBuffer.ToString());
			mBuffer.Length = 0;
			if (suffix.Length > 0)
			{
				if (IsFirstUpper(suffix))
				{
					mCollectFeatures.Add("scap");
				}
				if (mInducedAbbreviations.Contains(suffix))
				{
					mCollectFeatures.Add("sabbrev");
				}
			}
			
			mBuffer.Append("n=");
			mBuffer.Append(nextToken);
			mCollectFeatures.Add(mBuffer.ToString());
			mBuffer.Length = 0;
			if (nextToken.Length > 0)
			{
				if (IsFirstUpper(nextToken))
				{
					mCollectFeatures.Add("ncap");
				}
				if (mInducedAbbreviations.Contains(nextToken))
				{
					mCollectFeatures.Add("nabbrev");
				}
			}
			
			string[] context = mCollectFeatures.ToArray();
			mCollectFeatures.Clear();
			return context;
		}
		
		private static bool IsFirstUpper(string input)
		{
			return char.IsUpper(input[0]);
		}
		
		/// <summary> 
		/// Finds the index of the nearest space before a specified index.
		/// </summary>
		/// <param name="buffer">
		/// The string buffer which contains the text being examined.
		/// </param>
		/// <param name="seek">
		/// The index to begin searching from.
		/// </param>
		/// <returns>
		/// The index which contains the nearest space.
		/// </returns>
		private static int PreviousSpaceIndex(System.Text.StringBuilder buffer, int seek)
		{
			seek--;
			while (seek > 0)
			{
				if (buffer[seek] == ' ')
				{
					while (seek > 0 && buffer[seek - 1] == ' ')
						seek--;
					return seek;
				}
				seek--;
			}
			return 0;
		}
		
		/// <summary>
		///  Finds the index of the nearest space after a specified index.
		/// </summary>
		/// <param name="buffer">
		/// The string buffer which contains the text being examined.
		/// </param>
		/// <param name="seek">
		/// The index to begin searching from.
		/// </param>
		/// <param name="lastIndex">
		/// The highest index of the StringBuffer sb.
		/// </param>
		/// <returns>
		/// The index which contains the nearest space.
		/// </returns>
		private static int NextSpaceIndex(System.Text.StringBuilder buffer, int seek, int lastIndex)
		{
			seek++;
			char currentChar;
			while (seek < lastIndex)
			{
				currentChar = buffer[seek];
				if (currentChar == ' ' || currentChar == '\n')
				{
					while (buffer.Length > seek + 1 && buffer[seek + 1] == ' ')
						seek++;
					return seek;
				}
				seek++;
			}
			return lastIndex;
		}
	}
}

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About the Author

Richard Northedge
Web Developer
United Kingdom United Kingdom
Richard Northedge is a senior developer with a UK Microsoft Gold Partner company. He has a postgraduate degree in English Literature, has been programming professionally since 1998 and has been an MCSD since 2000.

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