[GitHub] [arrow] mapleFU commented on a diff in pull request #36183: GH-20505: [C++] Optimize DELTA_BINARY_PACKED encoding

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

mapleFU commented on code in PR #36183:
URL: https://github.com/apache/arrow/pull/36183#discussion_r1236910039


##########
cpp/src/parquet/encoding.cc:
##########
@@ -2189,7 +2193,13 @@ void DeltaBitPackEncoder<DType>::Put(const T* src, int num_values) {
     // making subtraction operations well-defined and correct even in case of overflow.
     // Encoded integers will wrap back around on decoding.
     // See http://en.wikipedia.org/wiki/Modular_arithmetic#Integers_modulo_n
-    deltas_[values_current_block_] = value - current_value_;
+    UT delta = value - current_value_;
+    //update blocks minimum delta
+    if(delta < min_delta_) min_delta_ = delta;
+    //calculate which block a maximum delta belongs to -> whole number division
+    const uint32_t miniblock_index{values_current_block_/values_per_mini_block_};
+    if(delta > max_deltas_[miniblock_index])max_deltas_[miniblock_index] = delta;

Review Comment:
   After that `FlushBlock` is called, should `max_deltas_` being set to 0? Seems that previous block's max will be bring into next block?



##########
cpp/src/parquet/encoding.cc:
##########
@@ -2163,6 +2164,8 @@ class DeltaBitPackEncoder : public EncoderImpl, virtual public TypedEncoder<DTyp
   uint32_t total_value_count_{0};
   UT first_value_{0};
   UT current_value_{0};
+  UT min_delta_;

Review Comment:
   should it default as UT::max ?



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