paleolimbot commented on code in PR #13397:
URL: https://github.com/apache/arrow/pull/13397#discussion_r918234089
##########
r/src/compute.cpp:
##########
@@ -574,3 +576,171 @@ SEXP compute__CallFunction(std::string func_name,
cpp11::list args, cpp11::list
std::vector<std::string> compute__GetFunctionNames() {
return arrow::compute::GetFunctionRegistry()->GetFunctionNames();
}
+
+class RScalarUDFKernelState : public arrow::compute::KernelState {
+ public:
+ RScalarUDFKernelState(cpp11::sexp exec_func, cpp11::sexp resolver)
+ : exec_func_(exec_func), resolver_(resolver) {}
+
+ cpp11::function exec_func_;
+ cpp11::function resolver_;
+};
+
+arrow::Result<arrow::TypeHolder> ResolveScalarUDFOutputType(
+ arrow::compute::KernelContext* context,
+ const std::vector<arrow::TypeHolder>& input_types) {
+ return SafeCallIntoR<arrow::TypeHolder>(
+ [&]() -> arrow::TypeHolder {
+ auto kernel =
+ reinterpret_cast<const
arrow::compute::ScalarKernel*>(context->kernel());
+ auto state =
std::dynamic_pointer_cast<RScalarUDFKernelState>(kernel->data);
+
+ cpp11::writable::list input_types_sexp(input_types.size());
+ for (size_t i = 0; i < input_types.size(); i++) {
+ input_types_sexp[i] =
+ cpp11::to_r6<arrow::DataType>(input_types[i].GetSharedPtr());
+ }
+
+ cpp11::sexp output_type_sexp = state->resolver_(input_types_sexp);
+ if (!Rf_inherits(output_type_sexp, "DataType")) {
+ cpp11::stop("arrow_scalar_function resolver must return a DataType");
+ }
+
+ return arrow::TypeHolder(
+ cpp11::as_cpp<std::shared_ptr<arrow::DataType>>(output_type_sexp));
+ },
+ "resolve scalar user-defined function output data type");
+}
+
+arrow::Status CallRScalarUDF(arrow::compute::KernelContext* context,
+ const arrow::compute::ExecSpan& span,
+ arrow::compute::ExecResult* result) {
+ if (result->is_array_span()) {
+ return arrow::Status::NotImplemented("ArraySpan result from R scalar UDF");
+ }
+
+ return SafeCallIntoRVoid(
+ [&]() {
+ auto kernel =
+ reinterpret_cast<const
arrow::compute::ScalarKernel*>(context->kernel());
+ auto state =
std::dynamic_pointer_cast<RScalarUDFKernelState>(kernel->data);
+
+ cpp11::writable::list args_sexp(span.num_values());
+
+ for (int i = 0; i < span.num_values(); i++) {
+ const arrow::compute::ExecValue& exec_val = span[i];
+ if (exec_val.is_array()) {
+ std::shared_ptr<arrow::Array> array = exec_val.array.ToArray();
+ args_sexp[i] = cpp11::to_r6<arrow::Array>(array);
+ } else if (exec_val.is_scalar()) {
+ std::shared_ptr<arrow::Scalar> scalar =
exec_val.scalar->GetSharedPtr();
+ args_sexp[i] = cpp11::to_r6<arrow::Scalar>(scalar);
+ }
+ }
+
+ cpp11::sexp batch_length_sexp = cpp11::as_sexp(span.length);
+
+ std::shared_ptr<arrow::DataType> output_type =
result->type()->GetSharedPtr();
+ cpp11::sexp output_type_sexp =
cpp11::to_r6<arrow::DataType>(output_type);
+ cpp11::writable::list udf_context = {batch_length_sexp,
output_type_sexp};
+ udf_context.names() = {"batch_length", "output_type"};
+
+ cpp11::sexp func_result_sexp = state->exec_func_(udf_context,
args_sexp);
+
+ if (Rf_inherits(func_result_sexp, "Array")) {
+ auto array =
cpp11::as_cpp<std::shared_ptr<arrow::Array>>(func_result_sexp);
+
+ // handle an Array result of the wrong type
+ if (!result->type()->Equals(array->type())) {
+ arrow::Datum out = ValueOrStop(arrow::compute::Cast(array,
result->type()));
+ std::shared_ptr<arrow::Array> out_array = out.make_array();
+ array.swap(out_array);
+ }
+
+ result->value = std::move(array->data());
+ } else if (Rf_inherits(func_result_sexp, "Scalar")) {
+ auto scalar =
cpp11::as_cpp<std::shared_ptr<arrow::Scalar>>(func_result_sexp);
+
+ // handle a Scalar result of the wrong type
+ if (!result->type()->Equals(scalar->type)) {
+ arrow::Datum out = ValueOrStop(arrow::compute::Cast(scalar,
result->type()));
+ std::shared_ptr<arrow::Scalar> out_scalar = out.scalar();
+ scalar.swap(out_scalar);
+ }
+
+ auto array = ValueOrStop(
+ arrow::MakeArrayFromScalar(*scalar, span.length,
context->memory_pool()));
+ result->value = std::move(array->data());
+ } else {
+ cpp11::stop("arrow_scalar_function must return an Array or Scalar");
+ }
+ },
+ "execute scalar user-defined function");
+}
+
+// [[arrow::export]]
+void RegisterScalarUDF(std::string name, cpp11::sexp func_sexp) {
+ cpp11::list in_type_r(func_sexp.attr("in_type"));
+ cpp11::list out_type_r(func_sexp.attr("out_type"));
+ R_xlen_t n_kernels = in_type_r.size();
+
+ if (n_kernels == 0) {
+ cpp11::stop("Can't register user-defined function with zero kernels");
+ }
+
+ // compute the Arity from the list of input kernels
+ std::vector<int64_t> n_args(n_kernels);
+ for (R_xlen_t i = 0; i < n_kernels; i++) {
+ auto in_types =
cpp11::as_cpp<std::shared_ptr<arrow::Schema>>(in_type_r[i]);
+ n_args[i] = in_types->num_fields();
+ }
+
+ const int64_t min_args = *std::min_element(n_args.begin(), n_args.end());
+ const int64_t max_args = *std::max_element(n_args.begin(), n_args.end());
+
+ // We can't currently handle variable numbers of arguments in a user-defined
+ // function and we don't have a mechanism for the user to specify a variable
+ // number of arguments at the end of a signature.
+ if (min_args != max_args) {
+ cpp11::stop(
+ "User-defined function with a variable number of arguments is not
supported");
+ }
+
+ arrow::compute::Arity arity(min_args, false);
+
+ // The function documentation isn't currently accessible from R but is
required
+ // for the C++ function constructor.
Review Comment:
Oh that's awesome! In R the pattern would be to provide a function in a
package (e.g., `sf::st_length()`) that already has documentation and match its
behaviour using an `arrow_scalar_function()` which would be called as part of a
dplyr pipeline (e.g., `mutate()` or `filter()`). Unless somebody asks for this
functionality I'm not keen to spend time investing in an API to specify it
(although I certainly could if this really is important).
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