[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_r341510494
 
 

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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;
+  private int offsetDiffAdjust = 0;
+
+  private static final int HARD_MAX_ROLLBACK_BUFFER_CAPACITY = Integer.highestOneBit(Integer.MAX_VALUE);
+  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". It is illegal to specify a negative value. There is no
+   * power of 2 greater than <code>Integer.highestOneBit(Integer.MAX_VALUE))</code>, so to prevent overflow, values
+   * in this range will resolve to an enforced limit of <code>Integer.highestOneBit(Integer.MAX_VALUE))</code>.
+   * Specifying "0" (or "1", in practice) disables rollback. Larger values can in some cases yield more accurate
+   * transliteration, at the cost of performance and resolution/accuracy of offset correction.
+   * This is intended primarily as a failsafe, with a relatively large default value of {@value ICUTransformCharFilter#DEFAULT_MAX_ROLLBACK_BUFFER_CAPACITY}.
+   * See comments "To understand the need for rollback" in private method:
+   * {@link Transliterator#filteredTransliterate(com.ibm.icu.text.Replaceable, Position, boolean, boolean)}
+   */
+  ICUTransformCharFilter(Reader in, Transliterator transform, int maxRollbackBufferCapacityHint) {
+    super(in);
+    this.transform = ICUTransformFilter.optimizeForCommonCase(transform);
+    if (maxRollbackBufferCapacityHint < 0) {
+      throw new IllegalArgumentException("It is illegal to request negative rollback buffer max capacity");
+    } else if (maxRollbackBufferCapacityHint >= HARD_MAX_ROLLBACK_BUFFER_CAPACITY) {
+      // arg is positive, so user wants the largest possible buffer capacity limit
+      // we know what that is (static), protecting for overflow.
+      this.maxRollbackBufferCapacity = HARD_MAX_ROLLBACK_BUFFER_CAPACITY;
+      this.rollbackBuffer = new char[DEFAULT_INITIAL_ROLLBACK_BUFFER_CAPACITY];
+    } else {
+      // greatest power of 2 (excluding "1") less than or equal to the specified hint
+      this.maxRollbackBufferCapacity = Integer.highestOneBit(maxRollbackBufferCapacityHint - 1) << 1;
+      if (this.maxRollbackBufferCapacity == 0) {
+        this.rollbackBuffer = null;
+      } else {
+        this.rollbackBuffer = new char[DEFAULT_INITIAL_ROLLBACK_BUFFER_CAPACITY];
+      }
+    }
+  }
+
+  /**
+   * Reads characters into a portion of an array. This method will block until some input is available, an I/O error
+   * occurs, or the end of the stream is reached.
+   *
+   * @param cbuf
+   *          Destination buffer
+   * @param off
+   *          Offset at which to start storing characters
+   * @param len
+   *          Maximum number of characters to read
+   * @return The number of characters read, or -1 if the end of the stream has been reached
+   * @throws IOException
+   *           If an I/O error occurs
+   */
+  @Override
+  public int read(char[] cbuf, int off, int len) throws IOException {
+    if (off < 0) throw new IndexOutOfBoundsException("specified negative array offset");
+    if (off >= cbuf.length) throw new IndexOutOfBoundsException("specified offset exceeds buffer length");
+    if (len <= 0) throw new IndexOutOfBoundsException("non-positive length specified");
+    if (len > cbuf.length - off) throw new IndexOutOfBoundsException("requested end array index exceeds buffer length");
+
+    // !inputFinished || output remains to be flushed
 
 Review comment:
   Thanks, expanding a little helps a lot! I'm not too concerned about the placement of the comment. I tend to put them inside if the comment is describing the positive condition that is being tested for, though, "if we got here, then this is true"

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