[GitHub] [arrow] lidavidm commented on a change in pull request #10317: ARROW-12713 [C++] String reverse kernel

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

lidavidm commented on a change in pull request #10317:
URL: https://github.com/apache/arrow/pull/10317#discussion_r632615943



##########
File path: cpp/src/arrow/compute/kernels/scalar_string.cc
##########
@@ -266,6 +266,52 @@ void EnsureLookupTablesFilled() {}
 
 #endif  // ARROW_WITH_UTF8PROC
 
+template <typename Type>
+struct AsciiReverse : StringTransform<Type, AsciiReverse<Type>> {
+  using Base = StringTransform<Type, AsciiReverse<Type>>;
+  using offset_type = typename Base::offset_type;
+
+  bool Transform(const uint8_t* input, offset_type input_string_ncodeunits,
+                 uint8_t* output, offset_type* output_written) {
+    uint8_t utf8_char_found = 0;
+    for (offset_type i = 0; i < input_string_ncodeunits; i++) {
+      // if a utf8 char is found, report to utf8_char_found
+      utf8_char_found |= input[i] & 0x80;

Review comment:
       IMO if the check is relatively cheap then it's good to keep. If someone is using the ASCII kernel explicitly, presumably either
   1) they want an assurance their data is ASCII
   2) they know data is ASCII and assume it's faster to reverse knowing this




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