[GitHub] [arrow-rs] alamb commented on a diff in pull request #4319: Handle trailing padding when skipping repetition levels (#3911)

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

alamb commented on code in PR #4319:
URL: https://github.com/apache/arrow-rs/pull/4319#discussion_r1213042654


##########
parquet/src/column/reader/decoder.rs:
##########
@@ -473,17 +485,39 @@ mod tests {
     }
 
     #[test]
-    fn test_skip() {
-        let mut rng = thread_rng();
-        let total_len = 10000;
-        let encoded: Vec<i16> = (0..total_len).map(|_| rng.gen_range(0..5)).collect();
-        let mut encoder = RleEncoder::new(3, 1024);
-        for v in &encoded {
-            encoder.put(*v as _)
-        }
+    fn test_skip_padding() {
+        let mut encoder = RleEncoder::new(1, 1024);
+        encoder.put(0);
+        (0..3).for_each(|_| encoder.put(1));
         let data = ByteBufferPtr::new(encoder.consume());
 
+        let mut decoder = ColumnLevelDecoderImpl::new(1);
+        decoder.set_data(Encoding::RLE, data.clone());
+        let (records, levels) = decoder.skip_rep_levels(100, 4).unwrap();
+        assert_eq!(records, 1);
+        assert_eq!(levels, 4);
+
+        // The length of the final bit packed run is ambiguous, so without the correct
+        // levels limit, it will decode zero padding
+        let mut decoder = ColumnLevelDecoderImpl::new(1);
+        decoder.set_data(Encoding::RLE, data);
+        let (records, levels) = decoder.skip_rep_levels(100, 6).unwrap();
+        assert_eq!(records, 3);
+        assert_eq!(levels, 6);
+    }
+
+    #[test]
+    fn test_skip() {
         for _ in 0..10 {
+            let mut rng = thread_rng();

Review Comment:
   it might make sense to add a note to the test giving the rationale for the loop (and maybe name the test 'test_ski-_fuzz` or something



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