FeatureHasher should have an option to not hash the values.

Data/src/main/java/org/tribuo/data/text/FeatureTransformer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2022, Oracle and/or its affiliates. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,8 +24,11 @@
 import java.util.List;
 
 /**
- * A feature transformer maps a list of features to a new list of features
- * Useful for example to apply the hashing trick to a set of features
+ * A feature transformer maps a list of features to a new list of features.
+ * Useful for example to apply the hashing trick to a set of features.
+ * <p>
+ * Note a list of features returned by a {@code FeatureTransformer} may contain
+ * duplicate features, and should be reduced to ensure that each feature is unique.
  */
 public interface FeatureTransformer extends Configurable, Provenancable<ConfiguredObjectProvenance> {
 

Data/src/main/java/org/tribuo/data/text/impl/FeatureHasher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2022, Oracle and/or its affiliates. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,15 +36,62 @@ public class FeatureHasher implements FeatureTransformer {
 
     private static final Logger logger = Logger.getLogger(FeatureHasher.class.getName());
 
+    /**
+     * Default value for the hash function seed.
+     */
+    public static final int DEFAULT_HASH_SEED = 38495;
+
+    /**
+     * Default value for the value hash function seed.
+     */
+    public static final int DEFAULT_HASH_VALUE_SEED = 77777;
+
     @Config(mandatory = true,description="Dimension to map the hash into.")
     private int dimension;
 
+    @Config(description = "Seed used in the hash function.")
+    private int hashSeed = DEFAULT_HASH_SEED;
+
+    @Config(description = "Seed used for value hash function.")
+    private int hashValueSeed = DEFAULT_HASH_VALUE_SEED;
+
+    @Config(description = "Preserve input feature value.")
+    private boolean preserveValue = false;
+
     /**
      * Constructs a feature hasher using the supplied hash dimension.
+     * <p>
+     * Note the hasher also hashes the feature value into {-1, 1}.
      * @param dimension The dimension to reduce the hashed features into.
      */
     public FeatureHasher(int dimension) {
+        this(dimension, DEFAULT_HASH_SEED, DEFAULT_HASH_VALUE_SEED, false);
+    }
+
+    /**
+     * Constructs a feature hasher using the supplied hash dimension.
+     * @param dimension The dimension to reduce the hashed features into.
+     * @param preserveValue If true the feature value is used unaltered in the new features,
+     *                      if false it is hashed into the values {-1, 1}.
+     */
+    public FeatureHasher(int dimension, boolean preserveValue) {
+        this(dimension, DEFAULT_HASH_SEED, DEFAULT_HASH_VALUE_SEED, preserveValue);
+    }
+
+    /**
+     * Constructs a feature hasher using the supplied hash dimension and seed values.
+     * @param dimension The dimension to reduce the hashed features into.
+     * @param hashSeed The seed used in the murmurhash computation.
+     * @param hashValueSeed The seed used in the murmurhash computation for the feature value,
+     *                      unused if {@code preserveValue} is true.
+     * @param preserveValue If true the feature value is used unaltered in the new features,
+     *                      if false it is hashed into the values {-1, 1}.
+     */
+    public FeatureHasher(int dimension, int hashSeed, int hashValueSeed, boolean preserveValue) {
         this.dimension = dimension;
+        this.hashSeed = hashSeed;
+        this.hashValueSeed = hashValueSeed;
+        this.preserveValue = preserveValue;
     }
 
     /**
@@ -58,20 +105,23 @@ public List<Feature> map(String tag, List<Feature> features) {
         List<Feature> hashedFeatures = new ArrayList<>();
 
         for (Feature feature : features) {
-            int hash = MurmurHash3.murmurhash3_x86_32(feature.getName(), 0, feature.getName().length(), 38495);
-            //int bit = hash & 1;
-            int bit = MurmurHash3.murmurhash3_x86_32(feature.getName(), 0, feature.getName().length(), 77777) & 1; 
+            int hash = MurmurHash3.murmurhash3_x86_32(feature.getName(), 0, feature.getName().length(), hashSeed);
             hash = hash >>> 1;
             int code = hash % dimension;
-
-            int change = bit == 1 ? 1 : -1;
 
-            Feature newFeature = new Feature(tag + "-hash="+code,change);
+            double value;
+            if (preserveValue) {
+                value = feature.getValue();
+            } else {
+                int bit = MurmurHash3.murmurhash3_x86_32(feature.getName(), 0, feature.getName().length(), hashValueSeed) & 1;
+                value = bit == 1 ? 1 : -1;
+            }
+
+            Feature newFeature = new Feature(tag + "-hash="+code, value);
             hashedFeatures.add(newFeature);
         }
 
         return hashedFeatures;
-
     }
 
     @Override

Data/src/main/java/org/tribuo/data/text/impl/TokenPipeline.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2022, Oracle and/or its affiliates. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,16 +43,19 @@ public class TokenPipeline implements TextPipeline {
     private List<FeatureTransformer> transformers = new ArrayList<>();
     private FeatureAggregator aggregator;
 
-    @Config(mandatory = true,description="Use term counting, otherwise emit binary features.")
+    @Config(mandatory = true, description = "Use term counting, otherwise emit binary features.")
     private boolean termCounting;
 
-    @Config(description="Dimension to map the hash into.")
+    @Config(description = "Dimension to map the hash into.")
     private int hashDim = -1;
 
-    @Config(mandatory = true,description="Tokenizer to use.")
+    @Config(description = "Should feature hashing preserve the value?")
+    private boolean hashPreserveValue = true;
+
+    @Config(mandatory = true, description = "Tokenizer to use.")
     private Tokenizer tokenizer;
 
-    @Config(description="n in the n-gram to emit.")
+    @Config(description = "n in the n-gram to emit.")
     private int ngram = 2;
 
     /**
@@ -73,7 +76,31 @@ public class TokenPipeline implements TextPipeline {
     public TokenPipeline(Tokenizer tokenizer, int ngram, boolean termCounting) {
         this(tokenizer, ngram, termCounting, -1);
     }
-
+
+    /**
+     * Creates a new token pipeline.
+     *
+     * @param tokenizer    The tokenizer to use to split up the text into words
+     *                     (i.e., features.)
+     * @param ngram        The maximum size of ngram features to add to the features
+     *                     generated by the pipeline. A value of {@code n} means that ngram
+     *                     features of size 1-n will be generated. A good standard value to use is
+     *                     2, which means that unigram and bigram features will be generated. You
+     *                     will very likely see diminishing returns for larger values of
+     *                     {@code n} but there will be times when they will be necessary.
+     * @param termCounting If {@code true}, multiple occurrences of terms
+     *                     in the document will be counted and the count will be used as the value
+     *                     of the features that are produced.
+     * @param dimension    The maximum dimension for the feature space. If this value
+     *                     is greater than 0, then at most {@code dimension} features will be
+     *                     through the use of a hashing function that will collapse the feature
+     *                     space. This {@code TokenPipeline} will preserve the feature values when hashing,
+     *                     w.
+     */
+    public TokenPipeline(Tokenizer tokenizer, int ngram, boolean termCounting, int dimension) {
+        this(tokenizer, ngram, termCounting, dimension, true);
+    }
+
     /**
      * Creates a new token pipeline.
      *
@@ -88,16 +115,19 @@ public TokenPipeline(Tokenizer tokenizer, int ngram, boolean termCounting) {
      * @param termCounting If {@code true}, multiple occurrences of terms
      * in the document will be counted and the count will be used as the value
      * of the features that are produced.
-     * @param dimension The maximum dimension for the feature space. If this value 
+     * @param dimension The maximum dimension for the feature space. If this value
      * is greater than 0, then at most {@code dimension} features will be
-     * through the use of a hashing function that will collapse the feature 
+     * through the use of a hashing function that will collapse the feature
      * space.
+     * @param hashPreserveValue If true, the hash function preserves the feature value, if false
+     *                          it hashes it into the values {-1, 1}.
      */
-    public TokenPipeline(Tokenizer tokenizer, int ngram, boolean termCounting, int dimension) {
+    public TokenPipeline(Tokenizer tokenizer, int ngram, boolean termCounting, int dimension, boolean hashPreserveValue) {
         this.tokenizer = tokenizer;
         this.ngram = ngram;
         this.hashDim = dimension;
         this.termCounting = termCounting;
+        this.hashPreserveValue = hashPreserveValue;
         postConfig();
     }
 
@@ -115,7 +145,7 @@ public void postConfig() {
             processors.add(new NgramProcessor(tokenizer,i,1));
         }
         if (hashDim > 0) {
-            transformers.add(new FeatureHasher(hashDim));
+            transformers.add(new FeatureHasher(hashDim, hashPreserveValue));
         }
         if (termCounting) {
             aggregator = new SumAggregator();

Data/src/test/java/org/tribuo/data/text/TextPipelineTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2022, Oracle and/or its affiliates. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -101,6 +101,34 @@ public void testTokenPipeline() {
         assertTrue(featureList.contains(new Feature("2-N=input/text",1.0)));
     }
 
+    @Test
+    public void testHashingTokenPipeline() {
+        String input = "This is some input text.";
+
+        TokenPipeline pipeline = new TokenPipeline(new BreakIteratorTokenizer(Locale.US),2,true, 10);
+
+        List<Feature> featureList = pipeline.process("test",input);
+
+        assertTrue(featureList.contains(new Feature("test-hash=1",1.0)));
+        assertTrue(featureList.contains(new Feature("test-hash=2",2.0)));
+        assertTrue(featureList.contains(new Feature("test-hash=3",5.0)));
+        assertTrue(featureList.contains(new Feature("test-hash=5",1.0)));
+        assertTrue(featureList.contains(new Feature("test-hash=6",1.0)));
+        assertTrue(featureList.contains(new Feature("test-hash=7",1.0)));
+
+        TokenPipeline hashedValuePipeline = new TokenPipeline(new BreakIteratorTokenizer(Locale.US),2,true, 10, false);
+
+        List<Feature> hashedValueFeatureList = hashedValuePipeline.process("test",input);
+
+        assertTrue(hashedValueFeatureList.contains(new Feature("test-hash=1",1.0)));
+        assertTrue(hashedValueFeatureList.contains(new Feature("test-hash=2",0.0)));
+        assertTrue(hashedValueFeatureList.contains(new Feature("test-hash=3",-1.0)));
+        assertTrue(hashedValueFeatureList.contains(new Feature("test-hash=5",-1.0)));
+        assertTrue(hashedValueFeatureList.contains(new Feature("test-hash=6",1.0)));
+        assertTrue(hashedValueFeatureList.contains(new Feature("test-hash=7",-1.0)));
+    }
+
+
     @Test
     public void testTokenPipelineTagging() {
         String input = "This is some input text.";

Data/src/test/java/org/tribuo/data/text/impl/FeatureHasherTest.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ *
+ * 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 implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.tribuo.data.text.impl;
+
+import org.junit.jupiter.api.Test;
+import org.tribuo.Feature;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class FeatureHasherTest {
+
+    @Test
+    public void negativeValuesTest() {
+        List<Feature> input = new ArrayList<>();
+        Feature posValue = new Feature("Testing", 2.0);
+        input.add(posValue);
+        Feature negValue = new Feature("Test",2.0);
+        input.add(negValue);
+
+        FeatureHasher preserving = new FeatureHasher(10, true);
+        FeatureHasher notPreserving = new FeatureHasher(10, false);
+
+        List<Feature> preservingOutput = preserving.map("test", input);
+        List<Feature> notPreservingOutput = notPreserving.map("test", input);
+
+        assertEquals(2.0, preservingOutput.get(0).getValue());
+        assertEquals(2.0, preservingOutput.get(1).getValue());
+
+        assertEquals(1.0, notPreservingOutput.get(0).getValue());
+        assertEquals(-1.0, notPreservingOutput.get(1).getValue());
+    }
+
+}