apacheGH-34335: [C++][Parquet] Optimize Decoding DELTA_LENGTH_BYTE_AR…

…RAY (apache#34955)

### Rationale for this change

According to apache#34323 . DELTA_LENGTH_BYTE_ARRAY is much more slower. So do some optimizations.

### What changes are included in this PR?

Some tiny changes

### Are these changes tested?

### Are there any user-facing changes?

* Closes: apache#34335

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>

cpp/src/parquet/encoding.cc

@@ -2718,21 +2718,14 @@ class DeltaLengthByteArrayDecoder : public DecoderImpl,
                                        MemoryPool* pool = ::arrow::default_memory_pool())
       : DecoderImpl(descr, Encoding::DELTA_LENGTH_BYTE_ARRAY),
         len_decoder_(nullptr, pool),
-        buffered_length_(AllocateBuffer(pool, 0)),
-        buffered_data_(AllocateBuffer(pool, 0)) {}
+        buffered_length_(AllocateBuffer(pool, 0)) {}
 
   void SetData(int num_values, const uint8_t* data, int len) override {
-    num_values_ = num_values;
+    DecoderImpl::SetData(num_values, data, len);
     decoder_ = std::make_shared<::arrow::bit_util::BitReader>(data, len);
     DecodeLengths();
   }
 
-  void SetDecoder(int num_values, std::shared_ptr<::arrow::bit_util::BitReader> decoder) {
-    num_values_ = num_values;
-    decoder_ = decoder;
-    DecodeLengths();
-  }
-
   int Decode(ByteArray* buffer, int max_values) override {
     // Decode up to `max_values` strings into an internal buffer
     // and reference them into `buffer`.
@@ -2745,6 +2738,7 @@ class DeltaLengthByteArrayDecoder : public DecoderImpl,
     int32_t data_size = 0;
     const int32_t* length_ptr =
         reinterpret_cast<const int32_t*>(buffered_length_->data()) + length_idx_;
+    int bytes_offset = len_ - decoder_->bytes_left();
     for (int i = 0; i < max_values; ++i) {
       int32_t len = length_ptr[i];
       if (ARROW_PREDICT_FALSE(len < 0)) {
@@ -2756,13 +2750,10 @@ class DeltaLengthByteArrayDecoder : public DecoderImpl,
       }
     }
     length_idx_ += max_values;
-
-    PARQUET_THROW_NOT_OK(buffered_data_->Resize(data_size));
-    if (decoder_->GetBatch(8, buffered_data_->mutable_data(), data_size) != data_size) {
+    if (ARROW_PREDICT_FALSE(!decoder_->Advance(8 * static_cast<int64_t>(data_size)))) {
       ParquetException::EofException();
     }
-    const uint8_t* data_ptr = buffered_data_->data();
-
+    const uint8_t* data_ptr = data_ + bytes_offset;
     for (int i = 0; i < max_values; ++i) {
       buffer[i].ptr = data_ptr;
       data_ptr += buffer[i].len;
@@ -2850,7 +2841,6 @@ class DeltaLengthByteArrayDecoder : public DecoderImpl,
   int num_valid_values_;
   uint32_t length_idx_;
   std::shared_ptr<ResizableBuffer> buffered_length_;
-  std::shared_ptr<ResizableBuffer> buffered_data_;
 };
 
 // ----------------------------------------------------------------------
@@ -3071,8 +3061,12 @@ class DeltaByteArrayDecoder : public DecoderImpl,
     prefix_len_offset_ = 0;
     num_valid_values_ = num_prefix;
 
+    int bytes_left = decoder_->bytes_left();
+    // If len < bytes_left, prefix_len_decoder.Decode will throw exception.
+    DCHECK_GE(len, bytes_left);
+    int suffix_begins = len - bytes_left;
     // at this time, the decoder_ will be at the start of the encoded suffix data.
-    suffix_decoder_.SetDecoder(num_values, decoder_);
+    suffix_decoder_.SetData(num_values, data + suffix_begins, bytes_left);
 
     // TODO: read corrupted files written with bug(PARQUET-246). last_value_ should be set
     // to last_value_in_previous_page_ when decoding a new page(except the first page)