[GitHub] [arrow] pitrou commented on a change in pull request #8621: ARROW-9128: [C++] Implement string space trimming kernels: trim, ltrim, and rtrim

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

pitrou commented on a change in pull request #8621:
URL: https://github.com/apache/arrow/pull/8621#discussion_r551437108



##########
File path: cpp/src/arrow/util/utf8.h
##########
@@ -456,6 +456,67 @@ static inline bool UTF8Transform(const uint8_t* first, const uint8_t* last,
   return true;
 }
 
+template <class Predicate>
+static inline bool UTF8FindIf(const uint8_t* first, const uint8_t* last,
+                              Predicate&& predicate, const uint8_t** position) {
+  const uint8_t* i = first;
+  while (i < last) {
+    uint32_t codepoint = 0;
+    const uint8_t* current = i;
+    if (ARROW_PREDICT_FALSE(!UTF8Decode(&i, &codepoint))) {
+      return false;
+    }
+    if (predicate(codepoint)) {
+      *position = current;
+      return true;
+    }
+  }
+  *position = last;
+  return true;
+}
+
+// same semantics as std::find_if using reverse iterators when the return value
+// having the same semantics as std::reverse_iterator<..>.base()
+template <class Predicate>
+static inline bool UTF8FindIfReverse(const uint8_t* first, const uint8_t* last,
+                                     Predicate&& predicate, const uint8_t** position) {
+  const uint8_t* i = last - 1;
+  while (i >= first) {
+    uint32_t codepoint = 0;
+    const uint8_t* current = i;
+    if (ARROW_PREDICT_FALSE(!UTF8DecodeReverse(&i, &codepoint))) {
+      return false;
+    }
+    if (predicate(codepoint)) {
+      *position = current + 1;

Review comment:
       This is a bit weird. It returns the position to the next codepoint? The docstring should be a bit clearer about that (the current spelling is cryptic to me).

##########
File path: cpp/src/arrow/compute/kernels/scalar_string.cc
##########
@@ -186,6 +172,40 @@ struct UTF8Transform {
   }
 };
 
+#ifdef ARROW_WITH_UTF8PROC
+
+template <typename Type, typename Derived>
+struct UTF8Transform : StringTransform<Type, Derived> {

Review comment:
       I don't exactly understand this refactor. There's a `UTF8Transform` with a `Transform` method for utf8 kernels but no corresponding class with a `Transform` method for ascii kernels, is that right?




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