[GitHub] [lucene-solr] msokolov commented on a change in pull request #892: LUCENE-8972: Add ICUTransformCharFilter, to support pre-tokenizer ICU text transformation

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msokolov commented on a change in pull request #892: LUCENE-8972: Add ICUTransformCharFilter, to support pre-tokenizer ICU text transformation
URL: https://github.com/apache/lucene-solr/pull/892#discussion_r329308264
 
 

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 File path: lucene/analysis/icu/src/java/org/apache/lucene/analysis/icu/ICUTransformCharFilter.java
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+/*
+ * 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.
+ */
+
+package org.apache.lucene.analysis.icu;
+
+import java.io.IOException;
+import java.io.Reader;
+
+import com.ibm.icu.text.ReplaceableString;
+import com.ibm.icu.text.Transliterator;
+import com.ibm.icu.text.Transliterator.Position;
+import com.ibm.icu.text.UTF16;
+
+import org.apache.lucene.analysis.CharFilter;
+import org.apache.lucene.analysis.charfilter.BaseCharFilter;
+import org.apache.lucene.util.ArrayUtil;
+
+/**
+ * A {@link CharFilter} that transforms text with ICU.
+ * <p>
+ * ICU provides text-transformation functionality via its Transliteration API.
+ * Although script conversion is its most common use, a Transliterator can
+ * actually perform a more general class of tasks. In fact, Transliterator
+ * defines a very general API which specifies only that a segment of the input
+ * text is replaced by new text. The particulars of this conversion are
+ * determined entirely by subclasses of Transliterator.
+ * </p>
+ * <p>
+ * Some useful transformations for search are built-in:
+ * <ul>
+ * <li>Conversion from Traditional to Simplified Chinese characters
+ * <li>Conversion from Hiragana to Katakana
+ * <li>Conversion from Fullwidth to Halfwidth forms.
+ * <li>Script conversions, for example Serbian Cyrillic to Latin
+ * </ul>
+ * <p>
+ * Example usage: <blockquote>stream = new ICUTransformCharFilter(reader,
+ * Transliterator.getInstance("Traditional-Simplified"));</blockquote>
+ * <br>
+ * For more details, see the <a
+ * href="http://userguide.icu-project.org/transforms/general">ICU User
+ * Guide</a>.
+ */
+public final class ICUTransformCharFilter extends BaseCharFilter {
+
+  // Transliterator to transform the text
+  private final Transliterator transform;
+
+  // Reusable position object
+  private final Position position = new Position();
+
+  private static final int READ_BUFFER_SIZE = 1024;
+  private final char[] tmpBuffer = new char[READ_BUFFER_SIZE];
+
+  private static final int INITIAL_TRANSLITERATE_BUFFER_SIZE = 1024;
+  private final StringBuffer buffer = new StringBuffer(INITIAL_TRANSLITERATE_BUFFER_SIZE);
+  private final ReplaceableString replaceable = new ReplaceableString(buffer);
+
+  private static final int BUFFER_PRUNE_THRESHOLD = 1024;
+
+  private int outputCursor = 0;
+  private boolean inputFinished = false;
+  private int charCount = 0;
+
+  static final int DEFAULT_MAX_ROLLBACK_BUFFER_CAPACITY = 8192;
+  private final int maxRollbackBufferCapacity;
+
+  private static final int DEFAULT_INITIAL_ROLLBACK_BUFFER_CAPACITY = 4; // must be power of 2
+  private char[] rollbackBuffer;
+  private int rollbackBufferSize = 0;
+
+  ICUTransformCharFilter(Reader in, Transliterator transform) {
+    this(in, transform, DEFAULT_MAX_ROLLBACK_BUFFER_CAPACITY);
+  }
+
+  /**
+   * Construct new {@link ICUTransformCharFilter} with the specified {@link Transliterator}, backed by
+   * the specified {@link Reader}.
+   * @param in input source
+   * @param transform used to perform transliteration
+   * @param maxRollbackBufferCapacityHint used to control the maximum size to which this
+   * {@link ICUTransformCharFilter} will buffer and rollback partial transliteration of input sequences.
+   * The provided hint will be converted to an enforced limit of "the greatest power of 2 (excluding '1')
+   * less than or equal to the specified value". Specifying a negative value allows the rollback buffer to
 
 Review comment:
   Is there any reason to support negative values, rather than having the caller supply MAX_VALUE? I think elsewhere in this codebase we tend to error on negative values in cases like this.

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