[GitHub] [arrow] bkietz commented on a change in pull request #7458: ARROW-9122: [C++] Properly handle sliced arrays in ascii_lower, ascii_upper kernels

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

bkietz commented on a change in pull request #7458:
URL: https://github.com/apache/arrow/pull/7458#discussion_r441572451



##########
File path: cpp/src/arrow/compute/kernels/scalar_string.cc
##########
@@ -41,16 +41,50 @@ struct AsciiLength {
 
 using TransformFunc = std::function<void(const uint8_t*, int64_t, uint8_t*)>;
 
+template <typename T>

Review comment:
       ```suggestion
   // Transform a buffer of offsets to one which begins with 0 and has equivalent value lengths
   template <typename T>
   ```

##########
File path: cpp/src/arrow/compute/kernels/scalar_string.cc
##########
@@ -41,16 +41,50 @@ struct AsciiLength {
 
 using TransformFunc = std::function<void(const uint8_t*, int64_t, uint8_t*)>;
 
+template <typename T>
+Status GetShiftedOffsets(KernelContext* ctx, const Buffer& input_buffer, int64_t offset,
+                         int64_t length, std::shared_ptr<Buffer>* out) {
+  ARROW_ASSIGN_OR_RAISE(*out, ctx->Allocate((length + 1) * sizeof(T)));
+  const T* input_offsets = reinterpret_cast<const T*>(input_buffer.data()) + offset;
+  T* out_offsets = reinterpret_cast<T*>((*out)->mutable_data());
+  T first_offset = *input_offsets;
+  for (int64_t i = 0; i < length; ++i) {
+    *out_offsets++ = input_offsets[i] - first_offset;
+  }
+  *out_offsets = input_offsets[length] - first_offset;
+  return Status::OK();
+}
+
+template <typename Type>

Review comment:
       ```suggestion
   // Apply `transform` to input character data- this function cannot change the length
   // of any string value in the input.
   template <typename Type>
   ```




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