etseidl commented on code in PR #12978:
URL: https://github.com/apache/datafusion/pull/12978#discussion_r1882425151


##########
datafusion/core/src/physical_optimizer/pruning.rs:
##########
@@ -3443,6 +3583,425 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_increment_utf8() {
+        // Basic ASCII
+        assert_eq!(increment_utf8("abc").unwrap(), "abd");
+        assert_eq!(increment_utf8("abz").unwrap(), "ab{");
+
+        // Test around ASCII 127 (DEL)
+        assert_eq!(increment_utf8("~").unwrap(), "\u{7f}"); // 126 -> 127
+        assert_eq!(increment_utf8("\u{7f}").unwrap(), "\u{80}"); // 127 -> 128
+
+        // Test 2-byte UTF-8 sequences
+        assert_eq!(increment_utf8("ß").unwrap(), "à"); // U+00DF -> U+00E0
+
+        // Test 3-byte UTF-8 sequences
+        assert_eq!(increment_utf8("℣").unwrap(), "ℤ"); // U+2123 -> U+2124
+
+        // Test at UTF-8 boundaries
+        assert_eq!(increment_utf8("\u{7FF}").unwrap(), "\u{800}"); // 2-byte 
to 3-byte boundary
+        assert_eq!(increment_utf8("\u{FFFF}").unwrap(), "\u{10000}"); // 
3-byte to 4-byte boundary
+
+        // Test that if we can't increment we return None
+        assert!(increment_utf8("").is_none());
+        assert!(increment_utf8("\u{10FFFF}").is_none()); // U+10FFFF is the 
max code point
+
+        // Test that if we can't increment the last character we do the 
previous one and truncate
+        assert_eq!(increment_utf8("a\u{10FFFF}").unwrap(), "b");
+
+        // Test surrogate pair range (0xD800..=0xDFFF)
+        assert_eq!(increment_utf8("a\u{D7FF}").unwrap(), "b");
+        assert!(increment_utf8("\u{D7FF}").is_none());

Review Comment:
   > why so? 0xD7FF + 1 would be 0xD800, not 0xE800
   
   Because 0xD800-0xDFFF are not valid code points. The next higher valid code 
point is 0xE000. It's a happy coincidence that this happens.



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