[GitHub] [arrow] pitrou commented on a diff in pull request #15124: GH-15052: [C++][Parquet] Fixing DELTA_BINARY_PACKED when reading only 1

[GitBox <gi...@apache.org>]

pitrou commented on code in PR #15124:
URL: https://github.com/apache/arrow/pull/15124#discussion_r1060500345


##########
cpp/src/parquet/encoding.cc:
##########
@@ -2479,7 +2481,18 @@ class DeltaBitPackDecoder : public DecoderImpl, virtual public TypedDecoder<DTyp
       if (ARROW_PREDICT_FALSE(values_current_mini_block_ == 0)) {
         if (ARROW_PREDICT_FALSE(!block_initialized_)) {
           buffer[i++] = last_value_;
-          if (ARROW_PREDICT_FALSE(i == max_values)) break;
+          if (ARROW_PREDICT_FALSE(i == max_values)) {
+            // When block is uninitialized and i reaches max_values we have two
+            // different possibilities:
+            // 1. i == total_value_count_, which means that the page may have only one
+            // value and we should not initialize any block.
+            // 2. i != total_value_count_ which means that user just read the first value
+            // in the page, so we should initialize the incoming block.
+            if (i != static_cast<int>(total_value_count_)) {
+              InitBlock();
+            }

Review Comment:
   Wouldn't it be clearer to write this as:
   ```suggestion
               if (total_value_count_ != 1) {
                 InitBlock();
               }
   ```



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