[GitHub] [arrow] mapleFU commented on a diff in pull request #37471: GH-37466: [C++][Parquet] Fix Valgrind failure in DELTA_BYTE_ARRAY decoder

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

mapleFU commented on code in PR #37471:
URL: https://github.com/apache/arrow/pull/37471#discussion_r1310490219


##########
cpp/src/parquet/encoding.cc:
##########
@@ -2522,32 +2522,37 @@ class DeltaBitPackDecoder : public DecoderImpl, virtual public TypedDecoder<DTyp
     }
 
     int i = 0;
+
+    if (ARROW_PREDICT_FALSE(!first_block_initialized_)) {
+      // This is the first time we decode this data page, first output the
+      // last value and initialize the first block.
+      buffer[i++] = last_value_;
+      if (ARROW_PREDICT_FALSE(i == max_values)) {
+        // When i reaches max_values here we have two different possibilities:
+        // 1. total_value_count_ == 1, which means that the page may have only
+        //    one value (encoded in the header), and we should not initialize
+        //    any block, nor should we skip any padding bits below.
+        // 2. total_value_count_ != 1, which means we should initialize the
+        //    incoming block for subsequent reads.
+        if (total_value_count_ != 1) {
+          InitBlock();
+        }
+        total_values_remaining_ -= max_values;
+        this->num_values_ -= max_values;
+        return max_values;
+      }
+      InitBlock();
+    }
+
+    DCHECK(first_block_initialized_);
     while (i < max_values) {
+      // Ensure we have an initialized mini-block

Review Comment:
   comment should before 2547?
   
   By the way, is this related to the valgrind error?



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