From 2721134715b7dedfa2715bcf47548728ff702d5a Mon Sep 17 00:00:00 2001
From: mwish <maplewish117@gmail.com>
Date: Thu, 1 Feb 2024 21:24:42 +0800
Subject: [PATCH] GH-39845: [C++][Parquet] Minor: avoid creating a new Reader
 object in Decoder::SetData (#39847)

### Rationale for this change

avoid creating a new Reader object in Decoder::SetData

### What changes are included in this PR?

avoid creating a new Reader object in Decoder::SetData

### Are these changes tested?

Already

### Are there any user-facing changes?

no

* Closes: #39845

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: mwish <maplewish117@gmail.com>
---
 cpp/src/parquet/encoding.cc | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/cpp/src/parquet/encoding.cc b/cpp/src/parquet/encoding.cc
index b801b5ab11bb9..5573f5b9aed4c 100644
--- a/cpp/src/parquet/encoding.cc
+++ b/cpp/src/parquet/encoding.cc
@@ -2411,7 +2411,11 @@ class DeltaBitPackDecoder : public DecoderImpl, virtual public TypedDecoder<DTyp
   void SetData(int num_values, const uint8_t* data, int len) override {
     // num_values is equal to page's num_values, including null values in this page
     this->num_values_ = num_values;
-    decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
+    if (decoder_ == nullptr) {
+      decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
+    } else {
+      decoder_->Reset(data, len);
+    }
     InitHeader();
   }
 
@@ -2769,7 +2773,11 @@ class DeltaLengthByteArrayDecoder : public DecoderImpl,
 
   void SetData(int num_values, const uint8_t* data, int len) override {
     DecoderImpl::SetData(num_values, data, len);
-    decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
+    if (decoder_ == nullptr) {
+      decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
+    } else {
+      decoder_->Reset(data, len);
+    }
     DecodeLengths();
   }