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The Waikato Environment for Knowledge Analysis (WEKA), a machine
learning workbench. This is the stable version. Apart from bugfixes, this version
does not receive any other updates.
/*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program 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 General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/*
* StringKernel.java
*
* Copyright (C) 2006 University of Waikato, Hamilton, New Zealand
*/
package weka.classifiers.functions.supportVector;
import weka.core.Attribute;
import weka.core.Capabilities;
import weka.core.Instance;
import weka.core.Instances;
import weka.core.Option;
import weka.core.RevisionUtils;
import weka.core.SelectedTag;
import weka.core.Tag;
import weka.core.TechnicalInformation;
import weka.core.TechnicalInformationHandler;
import weka.core.Utils;
import weka.core.Capabilities.Capability;
import weka.core.TechnicalInformation.Field;
import weka.core.TechnicalInformation.Type;
import java.util.Enumeration;
import java.util.Vector;
/**
* Implementation of the subsequence kernel (SSK) as described in [1] and of the subsequence kernel with lambda pruning (SSK-LP) as described in [2].
*
* For more information, see
*
* Huma Lodhi, Craig Saunders, John Shawe-Taylor, Nello Cristianini, Christopher J. C. H. Watkins (2002). Text Classification using String Kernels. Journal of Machine Learning Research. 2:419-444.
*
* F. Kleedorfer, A. Seewald (2005). Implementation of a String Kernel for WEKA. Wien, Austria.
*
*
* BibTeX:
*
* @article{Lodhi2002,
* author = {Huma Lodhi and Craig Saunders and John Shawe-Taylor and Nello Cristianini and Christopher J. C. H. Watkins},
* journal = {Journal of Machine Learning Research},
* pages = {419-444},
* title = {Text Classification using String Kernels},
* volume = {2},
* year = {2002},
* HTTP = {http://www.jmlr.org/papers/v2/lodhi02a.html}
* }
*
* @techreport{Kleedorfer2005,
* address = {Wien, Austria},
* author = {F. Kleedorfer and A. Seewald},
* institution = {Oesterreichisches Forschungsinstitut fuer Artificial Intelligence},
* number = {TR-2005-13},
* title = {Implementation of a String Kernel for WEKA},
* year = {2005}
* }
*
*
*
* Valid options are:
*
* -D
* Enables debugging output (if available) to be printed.
* (default: off)
*
* -no-checks
* Turns off all checks - use with caution!
* (default: checks on)
*
* -P <0|1>
* The pruning method to use:
* 0 = No pruning
* 1 = Lambda pruning
* (default: 0)
*
* -C <num>
* The size of the cache (a prime number).
* (default: 250007)
*
* -IC <num>
* The size of the internal cache (a prime number).
* (default: 200003)
*
* -L <num>
* The lambda constant. Penalizes non-continuous subsequence
* matches. Must be in (0,1).
* (default: 0.5)
*
* -ssl <num>
* The length of the subsequence.
* (default: 3)
*
* -ssl-max <num>
* The maximum length of the subsequence.
* (default: 9)
*
* -N
* Use normalization.
* (default: no)
*
*
* Theory
* Overview
* The algorithm computes a measure of similarity between two texts based on
* the number and form of their common subsequences, which need not be
* contiguous. This method can be parametrized by specifying the subsequence
* length k, the penalty factor lambda, which penalizes non-contiguous matches,
* and optional 'lambda pruning', which takes maxLambdaExponent,
* m, as parameter. Lambda pruning causes very 'stretched'
* substring matches not to be counted, thus speeding up the computation. The
* functionality of SSK and SSK-LP is explained in the following using simple
* examples.
*
* Explanation & Examples
* for all of the following examples, we assume these parameter values:
*
*k=2
*lambda=0.5
*m=8 (for SSK-LP examples)
*
*
* SSK
*
* Example 1
*
*
*SSK(2,"ab","axb")=0.5^5 = 0,03125
*
* There is one subsequence of the length of 2 that both strings have in
* common, "ab". The result of SSK is computed by raising lambda to the power
* of L, where L is the length of the subsequence match in the one string plus
* the length of the subsequence match in the other, in our case:
*
* ab axb
*L= 2 + 3 = 5
*
* hence, the kernel yields 0.5^5 = 0,03125
*
* Example 2
*
*SSK(2,"ab","abb")=0.5^5 + 0.5^4 = 0,09375
*
* Here, we also have one subsequence of the length of 2 that both strings have
* in common, "ab". The result of SSK is actually computed by summing over all
* values computed for each occurrence of a common subsequence match. In this
* example, there are two possible cases:
*
*ab abb
*-- -- L=4
*-- - - L=5
*
* we have two matches, one of the length of 2+2=4, one of the length of 2+3=5,
* so we get the result 0.5^5 + 0.5^4 = 0,09375.
*
* SSK-LP
* Without lambda pruning, the string kernel finds *all* common subsequences of
* the given length, whereas with lambda pruning, common subsequence matches
* that are too much stretched in both strings are not taken into account. It
* is argued that the value yielded for such a common subsequence is too low
* (lambda ^(length[match_in_s] + length[match_in_t]) . Tests have
* shown that a tremendous speedup can be achieved using this technique while
* suffering from very little quality loss.
* Lambda pruning is parametrized by the maximum lambda exponent. It is a good
* idea to choose that value to be about 3 or 4 times the subsequence length as
* a rule of thumb. YMMV.
*
* Example 3
* Without lambda pruning, one common subsequence,
* "AB" would be found in the following two strings. (With k=2)
*
*SSK(2,"ab","axb")=0.5^14 = 0,00006103515625
*
* lambda pruning allows for the control of the match length. So, if m
* (the maximum lambda exponent) is e.g. 8, these two strings would
* yield a kernel value of 0:
*
*with lambda pruning: SSK-LP(2,8,"AxxxxxxxxxB","AyB")= 0
*without lambda pruning: SSK(2,"AxxxxxxxxxB","AyB")= 0.5^14 = 0,00006103515625
*
* This is because the exponent for lambda (=the length of the subsequence
* match) would be 14, which is > 8. In Contrast, the next result is
* > 0
*
*m=8
*SSK-LP(2,8,"AxxB","AyyB")=0.5^8 = 0,00390625
*
* because the lambda exponent would be 8, which is just accepted by lambda
* pruning.
*
* Normalization
* When the string kernel is used for its main purpose, as the kernel of a
* support vector machine, it is not normalized. The normalized kernel can be
* switched on by -F (feature space normalization) but is much slower. Like
* most unnormalized kernels, K(x,x) is not a fixed value, see the next
* example.
*
* Example 4
*
*SSK(2,"ab","ab")=0.5^4 = 0.0625
*SSK(2,"AxxxxxxxxxB","AxxxxxxxxxB") = 12.761724710464478
*
* SSK is evaluated twice, each time for two identical strings. A good measure
* of similarity would produce the same value in both cases, which should
* indicate the same level of similarity. The value of the normalized SSK would
* be 1.0 in both cases. So for the purpose of computing string similarity the
* normalized kernel should be used. For SVM the unnormalized kernel is usually
* sufficient.
*
* Complexity of SSK and SSK-LP
* The time complexity of this method (without lambda pruning and with an
* infinitely large cache) is
* O(k*|s|*|t|)
* Lambda Pruning has a complexity (without caching) of
* O(m*binom(m,k)^2*(|s|+n)*|t|)
*
*k... subsequence length (ssl)
*s,t... strings
*|s|... length of string s
*binom(x,y)... binomial coefficient (x!/[(x-y)!y!])
*m... maxLambdaExponent (ssl-max)
*
*
* Keep in mind that execution time can increase fast for long strings
* and big values for k, especially if you don't use lambda pruning.
* With lambda pruning, computation is usually so fast that switching
* on the cache leads to slower computation because of setup costs. Therefore
* caching is switched off for lambda pruning.
*
*
* For details and qualitative experiments about SSK, see [1]
* For details about lambda pruning and performance comparison of SSK
* and SSK-LP (SSK with lambda pruning), see [2]
* Note that the complexity estimation in [2] assumes no caching of
* intermediate results, which has been implemented in the meantime and
* greatly improves the speed of the SSK without lambda pruning.
*
*
*Notes for usage within Weka
* Only instances of the following form can be processed using string kernels:
*
*+----------+-------------+---------------+
*|attribute#| 0 | 1 |
*+----------+-------------+---------------+
*| content | [text data] | [class label] |
*+----------------------------------------+
* ... or ...
*+----------+---------------+-------------+
*|attribute#| 0 | 1 |
*+----------+---------------+-------------+
*| content | [class label] | [text data] |
*+----------------------------------------+
*
*
* @author Florian Kleedorfer ([email protected])
* @author Alexander K. Seewald ([email protected])
* @version $Revision: 5518 $
*/
public class StringKernel
extends Kernel
implements TechnicalInformationHandler {
/** for serialization */
private static final long serialVersionUID = -4902954211202690123L;
/** The size of the cache (a prime number) */
private int m_cacheSize = 250007;
/** The size of the internal cache for intermediate results (a prime number) */
private int m_internalCacheSize = 200003;
/** The attribute number of the string attribute */
private int m_strAttr;
/** Kernel cache (i.e., cache for kernel evaluations) */
private double[] m_storage;
private long[] m_keys;
/** Counts the number of kernel evaluations. */
private int m_kernelEvals;
/** The number of instance in the dataset */
private int m_numInsts;
/** Pruning method: No Pruning */
public final static int PRUNING_NONE = 0;
/** Pruning method: Lambda See [2] for details. */
public final static int PRUNING_LAMBDA = 1;
/** Pruning methods */
public static final Tag [] TAGS_PRUNING = {
new Tag(PRUNING_NONE, "No pruning"),
new Tag(PRUNING_LAMBDA, "Lambda pruning"),
};
/** the pruning method */
protected int m_PruningMethod = PRUNING_NONE;
/** the decay factor that penalizes non-continuous substring matches. See [1]
* for details. */
protected double m_lambda = 0.5;
/** The substring length */
private int m_subsequenceLength = 3;
/** The maximum substring length for lambda pruning */
private int m_maxSubsequenceLength = 9;
/** powers of lambda are prepared prior to kernel evaluations.
* all powers between 0 and this value are precalculated */
protected static final int MAX_POWER_OF_LAMBDA = 10000;
/** the precalculated powers of lambda */
protected double[] m_powersOflambda = null;
/** flag for switching normalization on or off. This defaults to false and
* can be turned on by the switch for feature space normalization in SMO
*/
private boolean m_normalize = false;
/** private cache for intermediate results */
private int maxCache; // is set in unnormalizedKernel(s1,s2)
private double[] cachekh;
private int[] cachekhK;
private double[] cachekh2;
private int[] cachekh2K;
/** cached indexes for private cache */
private int m_multX;
private int m_multY;
private int m_multZ;
private int m_multZZ;
private boolean m_useRecursionCache = true;
/**
* default constructor
*/
public StringKernel() {
super();
}
/**
* creates a new StringKernel object. Initializes the kernel cache and the
* 'lambda cache', i.e. the precalculated powers of lambda from lambda^2 to
* lambda^MAX_POWER_OF_LAMBDA
*
* @param data the dataset to use
* @param cacheSize the size of the cache
* @param subsequenceLength the subsequence length
* @param lambda the lambda value
* @param debug whether to output debug information
* @throws Exception if something goes wrong
*/
public StringKernel(Instances data, int cacheSize, int subsequenceLength,
double lambda, boolean debug) throws Exception {
setDebug(debug);
setCacheSize(cacheSize);
setInternalCacheSize(200003);
setSubsequenceLength(subsequenceLength);
setMaxSubsequenceLength(-1);
setLambda(lambda);
buildKernel(data);
}
/**
* Returns a string describing the kernel
*
* @return a description suitable for displaying in the
* explorer/experimenter gui
*/
public String globalInfo() {
return
"Implementation of the subsequence kernel (SSK) as described in [1] "
+ "and of the subsequence kernel with lambda pruning (SSK-LP) as "
+ "described in [2].\n\n"
+ "For more information, see\n\n"
+ getTechnicalInformation().toString();
}
/**
* Returns an instance of a TechnicalInformation object, containing
* detailed information about the technical background of this class,
* e.g., paper reference or book this class is based on.
*
* @return the technical information about this class
*/
public TechnicalInformation getTechnicalInformation() {
TechnicalInformation result;
TechnicalInformation additional;
result = new TechnicalInformation(Type.ARTICLE);
result.setValue(Field.AUTHOR, "Huma Lodhi and Craig Saunders and John Shawe-Taylor and Nello Cristianini and Christopher J. C. H. Watkins");
result.setValue(Field.YEAR, "2002");
result.setValue(Field.TITLE, "Text Classification using String Kernels");
result.setValue(Field.JOURNAL, "Journal of Machine Learning Research");
result.setValue(Field.VOLUME, "2");
result.setValue(Field.PAGES, "419-444");
result.setValue(Field.HTTP, "http://www.jmlr.org/papers/v2/lodhi02a.html");
additional = result.add(Type.TECHREPORT);
additional.setValue(Field.AUTHOR, "F. Kleedorfer and A. Seewald");
additional.setValue(Field.YEAR, "2005");
additional.setValue(Field.TITLE, "Implementation of a String Kernel for WEKA");
additional.setValue(Field.INSTITUTION, "Oesterreichisches Forschungsinstitut fuer Artificial Intelligence");
additional.setValue(Field.ADDRESS, "Wien, Austria");
additional.setValue(Field.NUMBER, "TR-2005-13");
return result;
}
/**
* Returns an enumeration describing the available options.
*
* @return an enumeration of all the available options.
*/
public Enumeration listOptions() {
Vector result;
Enumeration en;
String desc;
String param;
int i;
SelectedTag tag;
result = new Vector();
en = super.listOptions();
while (en.hasMoreElements())
result.addElement(en.nextElement());
desc = "";
param = "";
for (i = 0; i < TAGS_PRUNING.length; i++) {
if (i > 0)
param += "|";
tag = new SelectedTag(TAGS_PRUNING[i].getID(), TAGS_PRUNING);
param += "" + tag.getSelectedTag().getID();
desc += "\t" + tag.getSelectedTag().getID()
+ " = " + tag.getSelectedTag().getReadable()
+ "\n";
}
result.addElement(new Option(
"\tThe pruning method to use:\n"
+ desc
+ "\t(default: " + PRUNING_NONE + ")",
"P", 1, "-P <" + param + ">"));
result.addElement(new Option(
"\tThe size of the cache (a prime number).\n"
+ "\t(default: 250007)",
"C", 1, "-C "));
result.addElement(new Option(
"\tThe size of the internal cache (a prime number).\n"
+ "\t(default: 200003)",
"IC", 1, "-IC "));
result.addElement(new Option(
"\tThe lambda constant. Penalizes non-continuous subsequence\n"
+ "\tmatches. Must be in (0,1).\n"
+ "\t(default: 0.5)",
"L", 1, "-L "));
result.addElement(new Option(
"\tThe length of the subsequence.\n"
+ "\t(default: 3)",
"ssl", 1, "-ssl "));
result.addElement(new Option(
"\tThe maximum length of the subsequence.\n"
+ "\t(default: 9)",
"ssl-max", 1, "-ssl-max "));
result.addElement(new Option(
"\tUse normalization.\n"
+ "\t(default: no)",
"N", 0, "-N"));
return result.elements();
}
/**
* Parses a given list of options.
*
* Valid options are:
*
* -D
* Enables debugging output (if available) to be printed.
* (default: off)
*
* -no-checks
* Turns off all checks - use with caution!
* (default: checks on)
*
* -P <0|1>
* The pruning method to use:
* 0 = No pruning
* 1 = Lambda pruning
* (default: 0)
*
* -C <num>
* The size of the cache (a prime number).
* (default: 250007)
*
* -IC <num>
* The size of the internal cache (a prime number).
* (default: 200003)
*
* -L <num>
* The lambda constant. Penalizes non-continuous subsequence
* matches. Must be in (0,1).
* (default: 0.5)
*
* -ssl <num>
* The length of the subsequence.
* (default: 3)
*
* -ssl-max <num>
* The maximum length of the subsequence.
* (default: 9)
*
* -N
* Use normalization.
* (default: no)
*
*
* @param options the list of options as an array of strings
* @throws Exception if an option is not supported
*/
public void setOptions(String[] options) throws Exception {
String tmpStr;
tmpStr = Utils.getOption('P', options);
if (tmpStr.length() != 0)
setPruningMethod(
new SelectedTag(Integer.parseInt(tmpStr), TAGS_PRUNING));
else
setPruningMethod(
new SelectedTag(PRUNING_NONE, TAGS_PRUNING));
tmpStr = Utils.getOption('C', options);
if (tmpStr.length() != 0)
setCacheSize(Integer.parseInt(tmpStr));
else
setCacheSize(250007);
tmpStr = Utils.getOption("IC", options);
if (tmpStr.length() != 0)
setInternalCacheSize(Integer.parseInt(tmpStr));
else
setInternalCacheSize(200003);
tmpStr = Utils.getOption('L', options);
if (tmpStr.length() != 0)
setLambda(Double.parseDouble(tmpStr));
else
setLambda(0.5);
tmpStr = Utils.getOption("ssl", options);
if (tmpStr.length() != 0)
setSubsequenceLength(Integer.parseInt(tmpStr));
else
setSubsequenceLength(3);
tmpStr = Utils.getOption("ssl-max", options);
if (tmpStr.length() != 0)
setMaxSubsequenceLength(Integer.parseInt(tmpStr));
else
setMaxSubsequenceLength(9);
setUseNormalization(Utils.getFlag('N', options));
if (getMaxSubsequenceLength()<2*getSubsequenceLength()) {
throw new IllegalArgumentException("Lambda Pruning forbids even contiguous substring matches! " +
"Use a bigger value for ssl-max (at least 2*ssl).");
}
super.setOptions(options);
}
/**
* Gets the current settings of the Kernel.
*
* @return an array of strings suitable for passing to setOptions
*/
public String[] getOptions() {
int i;
Vector result;
String[] options;
result = new Vector();
options = super.getOptions();
for (i = 0; i < options.length; i++)
result.add(options[i]);
result.add("-P");
result.add("" + m_PruningMethod);
result.add("-C");
result.add("" + getCacheSize());
result.add("-IC");
result.add("" + getInternalCacheSize());
result.add("-L");
result.add("" + getLambda());
result.add("-ssl");
result.add("" + getSubsequenceLength());
result.add("-ssl-max");
result.add("" + getMaxSubsequenceLength());
if (getUseNormalization())
result.add("-L");
return (String[]) result.toArray(new String[result.size()]);
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String pruningMethodTipText() {
return "The pruning method.";
}
/**
* Sets the method used to for pruning.
*
* @param value the pruning method to use.
*/
public void setPruningMethod(SelectedTag value) {
if (value.getTags() == TAGS_PRUNING)
m_PruningMethod = value.getSelectedTag().getID();
}
/**
* Gets the method used for pruning.
*
* @return the pruning method to use.
*/
public SelectedTag getPruningMethod() {
return new SelectedTag(m_PruningMethod, TAGS_PRUNING);
}
/**
* Sets the size of the cache to use (a prime number)
*
* @param value the size of the cache
*/
public void setCacheSize(int value) {
if (value >= 0) {
m_cacheSize = value;
clean();
}
else {
System.out.println(
"Cache size cannot be smaller than 0 (provided: " + value + ")!");
}
}
/**
* Gets the size of the cache
*
* @return the cache size
*/
public int getCacheSize() {
return m_cacheSize;
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String cacheSizeTipText() {
return "The size of the cache (a prime number).";
}
/**
* sets the size of the internal cache for intermediate results. Memory
* consumption is about 16x this amount in bytes. Only use when lambda
* pruning is switched off.
*
* @param value the size of the internal cache
*/
public void setInternalCacheSize(int value) {
if (value >= 0) {
m_internalCacheSize = value;
clean();
} else {
System.out.println(
"Cache size cannot be smaller than 0 (provided: " + value + ")!");
}
}
/**
* Gets the size of the internal cache
*
* @return the cache size
*/
public int getInternalCacheSize() {
return m_internalCacheSize;
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String internalCacheSizeTipText() {
return "The size of the internal cache (a prime number).";
}
/**
* Sets the length of the subsequence.
*
* @param value the length
*/
public void setSubsequenceLength(int value) {
m_subsequenceLength = value;
}
/**
* Returns the length of the subsequence
*
* @return the length
*/
public int getSubsequenceLength() {
return m_subsequenceLength;
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String subsequenceLengthTipText() {
return "The subsequence length.";
}
/**
* Sets the maximum length of the subsequence.
*
* @param value the maximum length
*/
public void setMaxSubsequenceLength(int value) {
m_maxSubsequenceLength = value;
}
/**
* Returns the maximum length of the subsequence
*
* @return the maximum length
*/
public int getMaxSubsequenceLength() {
return m_maxSubsequenceLength;
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String maxSubsequenceLengthTipText() {
return "The maximum subsequence length (theta in the paper)";
}
/**
* Sets the lambda constant used in the string kernel
*
* @param value the lambda value to use
*/
public void setLambda(double value) {
m_lambda = value;
}
/**
* Gets the lambda constant used in the string kernel
*
* @return the current lambda constant
*/
public double getLambda() {
return m_lambda;
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String lambdaTipText(){
return "Penalizes non-continuous subsequence matches, from (0,1)";
}
/**
* Sets whether to use normalization.
* Each time this value is changed, the kernel cache is cleared.
*
* @param value whether to use normalization
*/
public void setUseNormalization(boolean value) {
if (value != m_normalize)
clean();
m_normalize = value;
}
/**
* Returns whether normalization is used.
*
* @return true if normalization is used
*/
public boolean getUseNormalization() {
return m_normalize;
}
/**
* Returns the tip text for this property
*
* @return tip text for this property suitable for
* displaying in the explorer/experimenter gui
*/
public String useNormalizationTipText(){
return "Whether to use normalization.";
}
/**
* Computes the result of the kernel function for two instances.
* If id1 == -1, eval use inst1 instead of an instance in the dataset.
*
* @param id1 the index of the first instance in the dataset
* @param id2 the index of the second instance in the dataset
* @param inst1 the instance corresponding to id1 (used if id1 == -1)
* @return the result of the kernel function
* @throws Exception if something goes wrong
*/
public double eval(int id1, int id2, Instance inst1) throws Exception {
if (m_Debug && id1>-1 && id2>-1) {
System.err.println("\nEvaluation of string kernel for");
System.err.println(m_data.instance(id1).stringValue(m_strAttr));
System.err.println("and");
System.err.println(m_data.instance(id2).stringValue(m_strAttr));
}
//the normalized kernel returns 1 for comparison of
//two identical strings
if (id1 == id2 && m_normalize)
return 1.0;
double result = 0;
long key = -1;
int location = -1;
// we can only cache if we know the indexes
if ((id1 >= 0) && (m_keys != null)) {
if (id1 > id2) {
key = (long)id1 * m_numInsts + id2;
} else {
key = (long)id2 * m_numInsts + id1;
}
if (key < 0) {
throw new Exception("Cache overflow detected!");
}
location = (int)(key % m_keys.length);
if (m_keys[location] == (key + 1)) {
if (m_Debug)
System.err.println("result (cached): " + m_storage[location]);
return m_storage[location];
}
}
m_kernelEvals++;
long start = System.currentTimeMillis();
Instance inst2 = m_data.instance(id2);
char[] s1 = inst1.stringValue(m_strAttr).toCharArray();
char[] s2 = inst2.stringValue(m_strAttr).toCharArray();
// prevent the kernel from returning NaN
if (s1.length == 0 || s2.length == 0) return 0;
if (m_normalize) {
result = normalizedKernel(s1,s2);
} else {
result = unnormalizedKernel(s1, s2);
}
if (m_Debug) {
long duration = System.currentTimeMillis() - start;
System.err.println("result: " + result);
System.err.println("evaluation time:" + duration +"\n");
}
// store result in cache
if (key != -1){
m_storage[location] = result;
m_keys[location] = (key + 1);
}
return result;
}
/**
* Frees the memory used by the kernel.
* (Useful with kernels which use cache.)
* This function is called when the training is done.
* i.e. after that, eval will be called with id1 == -1.
*/
public void clean() {
m_storage = null;
m_keys = null;
}
/**
* Returns the number of kernel evaluation performed.
*
* @return the number of kernel evaluation performed.
*/
public int numEvals() {
return m_kernelEvals;
}
/**
* Returns the number of dot product cache hits.
*
* @return the number of dot product cache hits, or -1 if not supported by
* this kernel.
*/
public int numCacheHits() {
// TODO: implement!
return -1;
}
/**
* evaluates the normalized kernel between s and t. See [1] for details about
* the normalized SSK.
*
* @param s first input string
* @param t second input string
* @return a double indicating their distance, or similarity
*/
public double normalizedKernel(char[] s, char[] t){
double k1 = unnormalizedKernel(s, s);
double k2 = unnormalizedKernel(t, t);
double normTerm = Math.sqrt( k1*k2 );
return unnormalizedKernel(s, t) / normTerm;
}
/**
* evaluates the unnormalized kernel between s and t. See [1] for details
* about the unnormalized SSK.
*
* @param s first input string
* @param t second input string
* @return a double indicating their distance, or similarity
*/
public double unnormalizedKernel(char[] s, char[] t){
if (t.length > s.length) {
//swap because the algorithm is faster if s is
//the longer string
char[] buf = s;
s = t;
t = buf;
}
if (m_PruningMethod == PRUNING_NONE) {
m_multX=(s.length+1)*(t.length+1);
m_multY=(t.length+1);
m_multZ=1;
maxCache = m_internalCacheSize;
if (maxCache==0) {
maxCache=(m_subsequenceLength+1)*m_multX;
}
else if ((m_subsequenceLength+1)*m_multX See [1] for details
* but note that this code is optimized and may be hard to recognize.
*
* @param n the current length of the matching subsequence
* @param s first string, as a char array
* @param t second string, as a char array
* @param endIndexS the portion of s currently regarded is s[1:endIndexS]
* @param endIndexT the portion of t currently regarded is t[1:endIndexT]
* @return a double indicating the distance or similarity between s and t,
* according to and depending on the initial value for n.
*/
protected double kernel(int n, char[] s,int endIndexS, char[] t,
int endIndexT) {
//normal recursion ending case
if (Math.min(endIndexS+1,endIndexT+1) < n) return getReturnValue(n);
//accumulate all recursion results in one:
double result = 0;
//the tail-recursive function defined in [1] is turned into a
//loop here, preventing stack overflows.
//skim s from back to front
for (int iS=endIndexS; iS > n-2; iS--) {
double buf = 0;
//let the current character in s be x
char x = s[iS];
// iterate over all occurrences of x in t
for (int j=0; j <= endIndexT; j++) {
if (t[j] == x){
//this is a match for the current character, hence
//1. use previous chars in both strings (iS-1, j-1)
//2. decrement the remainingMatchLength (n-1)
//and start a recursion branch for these parameters
buf += kernelHelper(n-1,s,iS-1, t, j-1);
}
}
//ok, all occurrences of x in t have been found
//multiply the result with lambda^2
// (one lambda for x, and the other for all matches of x in t)
result += buf * m_powersOflambda[2];
}
return result;
}
/**
* The kernel helper function, called K' in [1] and [2].
*
* @param n the current length of the matching subsequence
* @param s first string, as a char array
* @param t second string, as a char array
* @param endIndexS the portion of s currently regarded is s[1:endIndexS]
* @param endIndexT the portion of t currently regarded is t[1:endIndexT]
* @return a partial result for K
*/
protected double kernelHelper (int n, char[] s,int endIndexS, char[] t,
int endIndexT) {
//recursion ends if the current subsequence has maximal length,
//which is the case here
if (n <= 0 ) {
return getReturnValue(n);
}
//recursion ends, too, if the current subsequence is shorter than
//maximal length, but there is no chance that it will reach maximal length.
//in this case, normally 0 is returned, but the EXPERIMENTAL
//minSubsequenceLength feature allows shorter subsequence matches
//also to contribute
if (Math.min(endIndexS+1,endIndexT+1) < n) {
return getReturnValue(n);
}
int adr = 0;
if (m_useRecursionCache) {
adr=m_multX*n+m_multY*endIndexS+m_multZ*endIndexT;
if ( cachekhK[adr % maxCache] == adr+1) return cachekh[adr % maxCache];
}
//the tail-recursive function defined in [1] is turned into a
//loop here, preventing stack overflows.
//loop over s, nearly from the start (skip the first n-1 characters)
//and only up until endIndexS, and recursively apply K''. Thus, every
//character between n-1 and endIndexS in s is counted once as
//being part of the subsequence match and once just as a gap.
//In both cases lambda is multiplied with the result.
double result = 0;
/*
for (int iS = n-1; iS <= endIndexS;iS++) {
result *= m_lambda;
result += kernelHelper2(n,s,iS, t, endIndexT);
}
if (m_useRecursionCache) {
cachekhK[adr % maxCache]=adr+1; cachekh[adr % maxCache]=result;
}
return result;
*/
/* ^^^ again, above code segment does not store some intermediate results... */
result = m_lambda*kernelHelper(n,s,endIndexS-1,t,endIndexT)
+ kernelHelper2(n,s,endIndexS,t,endIndexT);
if (m_useRecursionCache) {
cachekhK[adr % maxCache]=adr+1; cachekh[adr % maxCache]=result;
}
return result;
}
/**
* helper function for the evaluation of the kernel K'' see section
* 'Efficient Computation of SSK' in [1]
*
* @param n the current length of the matching subsequence
* @param s first string, as a char array
* @param t second string, as a char array
* @param endIndexS the portion of s currently regarded is s[1:endIndexS]
* @param endIndexT the portion of t currently regarded is t[1:endIndexT]
* @return a partial result for K'
*/
protected double kernelHelper2(int n, char[] s, int endIndexS, char[] t,
int endIndexT) {
//recursion ends if one of the indices in both strings is <0
if (endIndexS <0 || endIndexT <0) {
return getReturnValue(n);
}
int adr = 0;
if (m_useRecursionCache) {
adr=m_multX*n+m_multY*endIndexS+m_multZ*endIndexT;
if ( cachekh2K[adr % maxCache] == adr+1) return cachekh2[adr % maxCache];
}
//spot the last character in s, we'll need it
char x = s[endIndexS];
//recurse if the last characters of s and t, x (and y) are identical.
//which is an easy case: just add up two recursions,
// 1. one that counts x and y as a part of the subsequence match
// -> n, endIndexS and endIndexT are decremented for next recursion level
// -> lambda^2 is multiplied with the result to account for the length
// of 2 that has been added to the length of the subsequence match
// by accepting x and y.
// 2. one that counts y as a gap in the match
// -> only endIndexT is decremented for next recursion level
// -> lambda is multiplied with the result to account for the length
// of 1 that has been added to the length of the subsequence match
// by omitting y.
if (x == t[endIndexT]) {
double ret = m_lambda * (kernelHelper2(n,s,endIndexS, t, endIndexT-1)
+ m_lambda * kernelHelper(n-1,s,endIndexS-1, t, endIndexT-1));
if (m_useRecursionCache) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret;
}
return ret;
} else {
double ret = m_lambda*kernelHelper2(n,s,endIndexS,t,endIndexT-1);
if (m_useRecursionCache) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret;
}
return ret;
}
//look for x in t from back to front.
//this is actually an optimization from [1] that spares unneccessary
//recursions iff
//x is actually found in t, but not at the last position.
/*
int i;
int threshold = n>0?n-1:0;
for (i=endIndexT-1; i >= threshold;i--) {
if (x == t[i]) {
double ret=getPowerOfLambda(endIndexT-i) * kernelHelper2(n,s,endIndexS, t, i);
if (m_useRecursionCache) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret;
}
return ret;
}
}
*/
//end the recursion if x is not found in t.
/* double ret = getReturnValue(n);
if (m_useRecursionCache) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret;
}
return ret;*/
}
/**
* the kernel function K explained in [1] using lambda pruning, explained in
* [2]. An additional parameter is introduced, which denotes the maximum
* length of a subsequence match. This allows for the control of how relaxed
* the subsequence matches are.
*
* @param n the current length of the matching subsequence
* @param s first string, as a char array
* @param t second string, as a char array
* @param endIndexS the portion of s currently regarded is s[1:endIndexS]
* @param endIndexT the portion of t currently regarded is t[1:endIndexT]
* @param remainingMatchLength actually the initial value for
* maxLambdaExponent
* @return a double indicating the distance or similarity between s and t,
* according to and depending on the initial value for n.
*/
protected double kernelLP(int n, char[] s, int endIndexS,char[] t,
int endIndexT,int remainingMatchLength) {
//see code docs in kernel()
if (Math.min(endIndexS+1,endIndexT +1) < n) {
return getReturnValue(n);
}
//lambda pruning check
//stops recursion if the match is so long that the resulting
//power of lambda is smaller than minLambda
//if lambda pruning is not used, the remainingMatchLength is < 0
//and this check never stops the recursion
if (remainingMatchLength == 0) return getReturnValue(n);
double result = 0;
//see code docs in kernel()
for (int iS =endIndexS; iS > n-2; iS--) {
double buf = 0;
char x = s[iS];
for (int j=0; j <= endIndexT; j++) {
if (t[j] == x){
//both t[j] and x are considered part of the subsequence match, hence
//subtract 2 from the remainingMatchLength
buf += kernelHelperLP(n-1,s,iS-1,t,j-1,remainingMatchLength-2);
}
}
result += buf * m_powersOflambda[2];
}
return result;
}
/**
* helper function for the evaluation of the kernel (K'n) using lambda pruning
*
* @param n the current length of the matching subsequence
* @param s first string, as a char array
* @param t second string, as a char array
* @param endIndexS the portion of s currently regarded is s[1:endIndexS]
* @param endIndexT the portion of t currently regarded is t[1:endIndexT]
* @param remainingMatchLength the number of characters that may still be
* used
* for matching (i.e. gaps + matches in both strings)
* @return a partial result for K
*/
protected double kernelHelperLP (int n, char[] s, int endIndexS,char[] t,
int endIndexT,int remainingMatchLength) {
//see code docs in kernelHelper()
if (n == 0) {
return getReturnValue(n);
}
//see code docs in kernelHelper()
if (Math.min(endIndexS+1,endIndexT +1) < n) {;
return getReturnValue(n);
}
//lambda pruning check
//stops recursion if the match is so long that the resulting
//power of lambda is smaller than minLambda
//if lambda pruning is not used, the remainingMatchLength is < 0
//and this check never stops the recursion
if (remainingMatchLength < 2*n) {
return getReturnValue(n);
}
int adr=0;
if (m_useRecursionCache) {
adr = m_multX*n+m_multY*endIndexS+m_multZ*endIndexT
+ m_multZZ * remainingMatchLength;
if (cachekh2K[adr % maxCache]==adr+1) {
return cachekh2[adr % maxCache];
}
}
int rml = 0; //counts the remaining match length
double result = 0;
//see code docs in kernelHelper()
//difference to implementation in kernelHelper:
//*)choose different starting point, which is found counting
//the maximal remaining match length from endIndexS.
//*)keep track of the remaining match length, rml, which is
// incremented each loop
for (int iS = (endIndexS-remainingMatchLength); iS <= endIndexS;iS++) {
result *= m_lambda;
result += kernelHelper2LP(n,s,iS, t, endIndexT,rml++);
}
if (m_useRecursionCache && endIndexS >= 0 && endIndexT >= 0 && n >= 0) {
cachekhK[adr % maxCache]=adr+1; cachekh[adr % maxCache]=result;
}
return result;
}
/**
* helper function for the evaluation of the kernel (K''n) using lambda
* pruning
*
* @param n the current length of the matching subsequence
* @param s first string, as a char array
* @param t second string, as a char array
* @param endIndexS the portion of s currently regarded is s[1:endIndexS]
* @param endIndexT the portion of t currently regarded is t[1:endIndexT]
* @param remainingMatchLength the number of characters that may still be
* used
* for matching (i.e. gaps + matches in both strings)
* @return a partial result for K'
*/
protected double kernelHelper2LP(int n, char[] s, int endIndexS,char[] t,
int endIndexT,int remainingMatchLength) {
//lambda pruning check
//stops recursion if the match is so long that the resulting
//power of lambda is smaller than minLambda
//if lambda pruning is not used, the remainingMatchLength is < 0
//and this check never stops the recursion
//if (remainingMatchLength <= 0) return 0;
if (remainingMatchLength < 2*n) return getReturnValue(n);
//see code docs in kernelHelper2()
if (endIndexS <0 || endIndexT <0) return getReturnValue(n);
int adr=0;
if (m_useRecursionCache){
adr = m_multX*n+m_multY*endIndexS+m_multZ*endIndexT
+ m_multZZ * remainingMatchLength;
if (cachekh2K[adr % maxCache]==adr+1) {
return cachekh2[adr % maxCache];
}
}
char x = s[endIndexS];
if (x == t[endIndexT]) {
double ret =
m_lambda
* (kernelHelper2LP(n,s,endIndexS,t,endIndexT-1,remainingMatchLength-1)
+ m_lambda
* kernelHelperLP(n-1,s,endIndexS-1,t,endIndexT-1,remainingMatchLength-2));
if (m_useRecursionCache && endIndexS >= 0 && endIndexT >= 0 && n >= 0) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret; }
return ret;
}
//see code docs in kernelHelper()
//differences to implementation in kernelHelper():
//*) choose a different ending point for the loop
// based on the remaining match length
int i;
int minIndex = endIndexT - remainingMatchLength;
if (minIndex < 0) minIndex = 0;
for (i=endIndexT; i >= minIndex;i--) {
if (x == t[i]) {
int skipLength = endIndexT -i;
double ret = getPowerOfLambda(skipLength) *
kernelHelper2LP(n,s,endIndexS,t,i,remainingMatchLength-skipLength);
if (m_useRecursionCache && endIndexS >= 0 && endIndexT >= 0 && n >= 0) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret;
}
return ret;
}
}
double ret = getReturnValue(n);
if (m_useRecursionCache && endIndexS >= 0 && endIndexT >= 0 && n >= 0) {
cachekh2K[adr % maxCache]=adr+1; cachekh2[adr % maxCache]=ret;
}
return ret;
}
/**
* precalculates small powers of lambda to speed up the kernel evaluation
*
* @return the powers
*/
private double[] calculatePowersOfLambda(){
double[] powers = new double[MAX_POWER_OF_LAMBDA+1];
powers[0] = 1.0;
double val = 1.0;
for (int i = 1; i<=MAX_POWER_OF_LAMBDA;i++) {
val *= m_lambda;
powers[i] = val;
}
return powers;
}
/**
* retrieves a power of lambda from the lambda cache or calculates it
* directly
*
* @param exponent the exponent to calculate
* @return the exponent-th power of lambda
*/
private double getPowerOfLambda(int exponent){
if (exponent > MAX_POWER_OF_LAMBDA)
return Math.pow(m_lambda,exponent);
if (exponent < 0)
throw new IllegalArgumentException(
"only positive powers of lambda may be computed");
return m_powersOflambda[exponent];
}
/**
* initializes variables etc.
*
* @param data the data to use
*/
protected void initVars(Instances data) {
super.initVars(data);
m_kernelEvals = 0;
// take the first string attribute
m_strAttr = -1;
for (int i = 0; i < data.numAttributes(); i++) {
if (i == data.classIndex())
continue;
if (data.attribute(i).type() == Attribute.STRING) {
m_strAttr = i;
break;
}
}
m_numInsts = m_data.numInstances();
m_storage = new double[m_cacheSize];
m_keys = new long[m_cacheSize];
m_powersOflambda = calculatePowersOfLambda();
}
/**
* Returns the Capabilities of this kernel.
*
* @return the capabilities of this object
* @see Capabilities
*/
public Capabilities getCapabilities() {
Capabilities result = super.getCapabilities();
result.disableAll();
result.enable(Capability.STRING_ATTRIBUTES);
result.enableAllClasses();
result.enable(Capability.MISSING_CLASS_VALUES);
return result;
}
/**
* builds the kernel with the given data.
*
* @param data the data to base the kernel on
* @throws Exception if something goes wrong, e.g., the data does not
* consist of one string attribute and the class
*/
public void buildKernel(Instances data) throws Exception {
super.buildKernel(data);
}
/**
* Returns the revision string.
*
* @return the revision
*/
public String getRevision() {
return RevisionUtils.extract("$Revision: 5518 $");
}
}
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