Re: [PR] [SPARK-46833][SQL] Collations - Introducing CollationFactory which provides comparison and hashing rules for supported collations [spark]

["dbatomic (via GitHub)" <gi...@apache.org>]

dbatomic commented on code in PR #44968:
URL: https://github.com/apache/spark/pull/44968#discussion_r1472598191


##########
common/unsafe/src/main/java/org/apache/spark/sql/catalyst/util/CollationFactory.java:
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@@ -0,0 +1,171 @@
+/*
+ * 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.spark.sql.catalyst.util;
+
+import java.util.*;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
+import com.ibm.icu.util.ULocale;
+import com.ibm.icu.text.Collator;
+
+import org.apache.spark.SparkIllegalArgumentException;
+import org.apache.spark.unsafe.types.UTF8String;
+
+/**
+ * Static entry point for collation aware string functions.
+ * Provides functionality to the UTF8String object which respects defined collation settings.
+ */
+public final class CollationFactory {
+  /**
+   * Entry encapsulating all information about a collation.
+   */
+  public static class CollationInfo {
+    public final String collationName;
+    public final Collator collator;
+    public final Comparator<UTF8String> comparator;
+
+    /**
+     * Version of the collation. This is the version of the ICU library used to create the collator.
+     * For non-ICU collations (e.g. UTF8 Binary) the version is set to "1.0".
+     */
+    public final String version;
+
+    /**
+     * Collation sensitive hash function. Output for two UTF8Strings will be the same if they are
+     * equal according to the collation.
+     */
+    public final Function<UTF8String, Integer> hashFunction;
+
+    /**
+     * Potentially faster way than using comparator to compare two UTF8Strings for equality.
+     * Falls back to binary comparison if the collation is binary.
+     */
+    public final BiFunction<UTF8String, UTF8String, Boolean> equalsFunction;
+
+    /**
+     * Binary collation implies that UTF8Strings are considered equal only if they are
+     * byte for byte equal. All accent or case-insensitive collations are considered non-binary.
+     */
+    public final boolean isBinaryCollation;
+
+    public CollationInfo(
+      String collationName,
+      Collator collator,
+      Comparator<UTF8String> comparator,
+      String version,
+      Function<UTF8String, Integer> hashFunction,
+      boolean isBinaryCollation) {
+      this.collationName = collationName;
+      this.collator = collator;
+      this.comparator = comparator;
+      this.version = version;
+      this.hashFunction = hashFunction;
+      this.isBinaryCollation = isBinaryCollation;
+
+      if (isBinaryCollation) {
+        this.equalsFunction = UTF8String::binaryEquals;
+      } else {
+        this.equalsFunction = (s1, s2) -> this.comparator.compare(s1, s2) == 0;
+      }
+    }
+
+    /**
+     * Constructor with comparators that are inherited from the given collator.
+     */
+    public CollationInfo(
+      String collationName, Collator collator, String version, boolean isBinaryCollation) {
+      this(
+        collationName,
+        collator,
+        (s1, s2) -> collator.compare(s1.toString(), s2.toString()),
+        version,
+        s -> collator.getCollationKey(s.toString()).hashCode(),
+        isBinaryCollation);
+    }
+  }
+
+  private final CollationInfo[] collatorTable;
+  private final Hashtable<String, Integer> collationNameToIdMap = new Hashtable<>();
+
+  private CollationFactory() {
+    collatorTable = new CollationInfo[4];
+
+    // Binary comparison. This is the default collation.
+    // No custom comparators will be used for this collation.
+    // Instead, we rely on byte for byte comparison.
+    collatorTable[0] = new CollationInfo(
+      "UCS_BASIC",
+      null,
+      UTF8String::binaryCompare,
+      "1.0",
+      UTF8String::binaryHash,
+      true);
+
+    // Case-insensitive UTF8 binary collation.
+    // TODO: Do in place comparisons instead of creating new strings.
+    collatorTable[1] = new CollationInfo(
+      "UCS_BASIC_LCASE",
+      null,
+      (s1, s2) -> s1.toLowerCase().binaryCompare(s2.toLowerCase()), "1.0",
+      (s) -> s.toLowerCase().binaryHash(),
+      false);
+
+    // UNICODE case sensitive comparison (ROOT locale, in ICU).
+    collatorTable[2] = new CollationInfo(
+      "UNICODE", Collator.getInstance(ULocale.ROOT), "153.120.0.0", true);
+    collatorTable[2].collator.setStrength(Collator.TERTIARY);
+
+
+    // UNICODE case-insensitive comparison (ROOT locale, in ICU + Secondary strength).
+    collatorTable[3] = new CollationInfo(
+            "UNICODE_CI", Collator.getInstance(ULocale.ROOT), "153.120.0.0", false);
+    collatorTable[3].collator.setStrength(Collator.SECONDARY);
+
+    for (int i = 0; i < collatorTable.length; i++) {
+      this.collationNameToIdMap.put(collatorTable[i].collationName, i);
+    }
+  }
+
+  private static final CollationFactory instance = new CollationFactory();
+
+  /**
+   * Returns the collation id for the given collation name.
+   */
+  public int collationNameToId(String collationName) {
+    String normalizedName = collationName.toUpperCase();
+    if (collationNameToIdMap.containsKey(normalizedName)) {
+      return collationNameToIdMap.get(normalizedName);
+    } else {
+      throw SparkIllegalArgumentException.apply(
+        "COLLATION_INVALID_NAME", Collections.singletonMap("collationName", collationName));
+    }
+  }
+
+  public CollationInfo fetchCollationInfo(int collationId) {
+    return collatorTable[collationId];
+  }
+
+  public CollationInfo fetchCollationInfo(String collationName) {
+    int collationId = collationNameToId(collationName);
+    return collatorTable[collationId];
+  }
+
+  public static CollationFactory getInstance() {

Review Comment:
   @cloud-fan and @MaxGekk - looking for feedback here on proper design. `CollationFactory` is supposed to be readonly per JVM object. Let me know if singleton pattern is the proper approach here or this object should be nested into some other global object (e.g. `SparkEnv`?)



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