[GitHub] [arrow] lidavidm commented on a diff in pull request #14395: ARROW-17960: [C++][Python] Implement list_slice kernel

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

lidavidm commented on code in PR #14395:
URL: https://github.com/apache/arrow/pull/14395#discussion_r1010336367


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cpp/src/arrow/compute/kernels/scalar_nested.cc:
##########
@@ -87,6 +89,206 @@ Status GetListElementIndex(const ExecValue& value, T* out) {
   return Status::OK();
 }
 
+template <typename Type, typename IndexType>
+struct ListSlice {
+  using offset_type = typename Type::offset_type;
+
+  static Status Exec(KernelContext* ctx, const ExecSpan& batch, ExecResult* out) {
+    const auto opts = OptionsWrapper<ListSliceOptions>::Get(ctx);
+
+    // Invariants
+    if (opts.start < 0 || (opts.start >= opts.stop && opts.stop != -1)) {
+      // TODO: support start == stop which should give empty lists
+      return Status::Invalid("`start`(", opts.start,
+                             ") should be greater than 0 and smaller than `stop`(",
+                             opts.stop, ")");
+    }
+    if (opts.step != 1) {
+      // TODO: support step in slicing
+      return Status::NotImplemented(
+          "Setting `step` to anything other than 1 is not supported; got step=",
+          opts.step);
+    }
+    if (opts.stop == -1) {
+      // TODO: Support slicing to arbitrary end

Review Comment:
   You'll need to add overloads of GenericToString, GenericFromScalar, and GenericToScalar here: https://github.com/apache/arrow/blob/master/cpp/src/arrow/compute/function_internal.h
   
   (Or, avoid the generic stuff and implement the FunctionOptionsType manually)



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