uros-db commented on code in PR #47771:
URL: https://github.com/apache/spark/pull/47771#discussion_r1718712620


##########
common/unsafe/src/main/java/org/apache/spark/sql/catalyst/util/CollationAwareUTF8String.java:
##########
@@ -1267,6 +1268,128 @@ public static UTF8String[] icuSplitSQL(final UTF8String 
string, final UTF8String
     return strings.toArray(new UTF8String[0]);
   }
 
+  /**
+   * Title casing a string according to a new behaviour. Iterates over the 
string and title cases
+   * the first character in each word, and lowercases every other character. 
Handles lowercasing
+   * capital Greek letter sigma ('Σ') separately, taking into account if it 
should be a small final
+   * Greek sigma ('ς') or small non-final Greek sigma ('σ'). Words are 
separated by ASCII
+   * space(\u0020).
+   *
+   * @param target UTF8String to be title cased
+   * @return title cased target
+   */
+  public static UTF8String toTitleCaseICU(UTF8String target) {
+
+    Iterator<Integer> codepointIterator = target.codePointIterator(
+            CodePointIteratorType.CODE_POINT_ITERATOR_MAKE_VALID);
+
+    // Building the title cased target with 'sb'.
+    StringBuilder sb = new StringBuilder();
+    // 'newWord' is true if the current character is the beginning of a word, 
false otherwise.
+    boolean newWord = true;
+    // We are maintaining if the current character is preceded by a cased 
letter.
+    // This is used when lowercasing capital Greek letter sigma ('Σ'), to 
figure out if it should be
+    // lowercased into σ or ς.
+    boolean precededByCasedLetter = false;
+
+    // 'offset' is a byte offset in target's byte array pointing to the 
beginning of the character
+    // that we need to process next(this is only actually used in 
appendLowerCasedGreekCapitalSigma)
+    int offset = 0;
+
+    while(codepointIterator.hasNext()) {
+      int codepoint = codepointIterator.next();
+      // Appending the correctly cased character onto 'sb'.
+      appendTitleCasedCodepoint(sb, codepoint, newWord, precededByCasedLetter, 
target, offset);
+      // Updating 'newWord', 'precededByCasedLetter' and 'offset' to be ready 
for the next character
+      // that we will process.
+      newWord = (codepoint == ASCII_SPACE_CODEPOINT);
+      if(!UCharacter.hasBinaryProperty(codepoint, UProperty.CASE_IGNORABLE)) {
+        precededByCasedLetter = UCharacter.hasBinaryProperty(codepoint, 
UProperty.CASED);
+      }
+      offset += UTF8String.numBytesForFirstByte(target.getByte(offset));
+    }
+    return UTF8String.fromString(sb.toString());
+  }
+
+  private static void appendTitleCasedCodepoint(
+      StringBuilder sb,
+      int codepoint,
+      boolean isAfterAsciiSpace,
+      boolean precededByCasedLetter,
+      UTF8String target,
+      int offset) {
+    if(isAfterAsciiSpace) {
+      // Titlecasing a character if it is in the beginning of a new word.
+      sb.append(codepointToTitleString(codepoint));
+      return;
+    }
+    if(codepoint == CAPITAL_SIGMA) {
+      // Handling capital Greek letter sigma ('Σ').
+      appendLowerCasedGreekCapitalSigma(sb, precededByCasedLetter, target, 
offset);
+      return;
+    }
+    // If it's not the beginning of a word, or a capital Greek letter sigma 
('Σ'), we lowercase the
+    // character.
+    sb.append(
+      toLowerCase(UTF8String.fromString(new 
String(Character.toChars(codepoint)))).toString());
+  }
+
+  private static void appendLowerCasedGreekCapitalSigma(
+      StringBuilder sb,
+      boolean precededByCasedLetter,
+      UTF8String target,
+      int offset) {
+    int codepoint;
+    if (!followedByCasedLetter(target,offset) && precededByCasedLetter) {
+      codepoint = SMALL_FINAL_SIGMA;
+    }
+    else {
+      codepoint = SMALL_NON_FINAL_SIGMA;
+    }
+    sb.appendCodePoint(codepoint);
+  }
+
+  /**
+   * Checks if the character beginning at 'offset'(in 'targets' byte array) is 
followed by a cased
+   * letter.
+   *
+   * @param target
+   * @param offset
+   * @return
+   */

Review Comment:
   this does not look very helpful - either describe the parameters, or just 
feel free to remove these lines (imo, the header of the doc comment is helpful 
enough)



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