GH-32884: [C++] Add ordered aggregation

cpp/src/arrow/compute/exec.cc

@@ -147,6 +147,18 @@ ExecBatch ExecBatch::Slice(int64_t offset, int64_t length) const {
   return out;
 }
 
+Result<ExecBatch> ExecBatch::SelectValues(const std::vector<int>& ids) const {
+  std::vector<Datum> selected_values;
+  selected_values.reserve(ids.size());
+  for (int id : ids) {
+    if (id < 0 || static_cast<size_t>(id) >= values.size()) {
+      return Status::Invalid("ExecBatch invalid value selection: ", id);
+    }
+    selected_values.push_back(values[id]);
+  }
+  return ExecBatch(std::move(selected_values), length);
+}
+
 namespace {
 
 enum LengthInferenceError {

cpp/src/arrow/compute/exec.h

@@ -181,6 +181,12 @@ struct ARROW_EXPORT ExecBatch {
   /// \brief Infer the ExecBatch length from values.
   static Result<int64_t> InferLength(const std::vector<Datum>& values);
 
+  /// Creates an ExecBatch with length-validation.
+  ///
+  /// If any value is given, then all values must have a common length. If the given
+  /// length is negative, then the length of the ExecBatch is set to this common length,
+  /// or to 1 if no values are given. Otherwise, the given length must equal the common
+  /// length, if any value is given.
   static Result<ExecBatch> Make(std::vector<Datum> values, int64_t length = -1);
 
   Result<std::shared_ptr<RecordBatch>> ToRecordBatch(
@@ -240,6 +246,8 @@ struct ARROW_EXPORT ExecBatch {
 
   ExecBatch Slice(int64_t offset, int64_t length) const;
 
+  Result<ExecBatch> SelectValues(const std::vector<int>& ids) const;
+
   /// \brief A convenience for returning the types from the batch.
   std::vector<TypeHolder> GetTypes() const {
     std::vector<TypeHolder> result;