[GitHub] [arrow] milesgranger commented on a diff in pull request #14749: ARROW-18280: [C++][Python] Support slicing to end in list_slice kernel

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

milesgranger commented on code in PR #14749:
URL: https://github.com/apache/arrow/pull/14749#discussion_r1034391074


##########
cpp/src/arrow/compute/kernels/scalar_nested.cc:
##########
@@ -131,13 +124,26 @@ struct ListSlice {
         list_type->id() == arrow::Type::FIXED_SIZE_LIST);
     std::unique_ptr<ArrayBuilder> builder;
 
+    // should have been checked in resolver
+    // if stop not set, then cannot return fixed size list without input being fixed size
+    // list b/c we cannot determine the max list element in type resolving.
+    DCHECK(opts.stop.has_value() ||
+           (!opts.stop.has_value() && (!return_fixed_size_list ||
+                                       list_type->id() == arrow::Type::FIXED_SIZE_LIST)));
+
     // construct array values
     if (return_fixed_size_list) {
-      RETURN_NOT_OK(MakeBuilder(
-          ctx->memory_pool(),
-          fixed_size_list(value_type,
-                          static_cast<int32_t>(opts.stop.value() - opts.start)),
-          &builder));
+      const auto stop = [&]() {
+        if (opts.stop.has_value()) {
+          return static_cast<int32_t>(opts.stop.value());
+        } else {
+          DCHECK_EQ(list_type->id(), arrow::Type::FIXED_SIZE_LIST);
+          return checked_cast<const FixedSizeListType*>(list_type)->list_size();
+        }
+      }();
+      const auto size = std::max(stop - static_cast<int32_t>(opts.start), 0);

Review Comment:
   This covers the case of your suggested case where `stop` is null and the fixed size list stop is then set to the list_size, which could be less than the start.



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