ARROW-1699: [C++] forward, backward fill kernel functions

cpp/src/arrow/compute/api_vector.cc

@@ -220,6 +220,14 @@ Result<Datum> ReplaceWithMask(const Datum& values, const Datum& mask,
   return CallFunction("replace_with_mask", {values, mask, replacements}, ctx);
 }
 
+Result<Datum> FillNullForward(const Datum& values, ExecContext* ctx) {
+  return CallFunction("fill_null_forward", {values}, ctx);
+}
+
+Result<Datum> FillNullBackward(const Datum& values, ExecContext* ctx) {
+  return CallFunction("fill_null_backward", {values}, ctx);
+}
+
 Result<std::shared_ptr<Array>> SortIndices(const Array& values,
                                            const ArraySortOptions& options,
                                            ExecContext* ctx) {

cpp/src/arrow/compute/api_vector.h

@@ -246,6 +246,34 @@ ARROW_EXPORT
 Result<Datum> ReplaceWithMask(const Datum& values, const Datum& mask,
                               const Datum& replacements, ExecContext* ctx = NULLPTR);
 
+/// \brief FillNullForward fill null values in forward direction
+///
+/// The output array will be of the same type as the input values
+/// array, with replaced null values in forward direction.
+///
+/// For example given values = ["a", "b", "c", null, null, "f"],
+/// the output will be = ["a", "b", "c", "c", "c", "f"]
+///
+/// \param[in] values datum from which to take
+/// \param[in] ctx the function execution context, optional
+/// \return the resulting datum
+ARROW_EXPORT
+Result<Datum> FillNullForward(const Datum& values, ExecContext* ctx = NULLPTR);
+
+/// \brief FillNullBackward fill null values in backward direction
+///
+/// The output array will be of the same type as the input values
+/// array, with replaced null values in backward direction.
+///
+/// For example given values = ["a", "b", "c", null, null, "f"],
+/// the output will be = ["a", "b", "c", "f", "f", "f"]
+///
+/// \param[in] values datum from which to take
+/// \param[in] ctx the function execution context, optional
+/// \return the resulting datum
+ARROW_EXPORT
+Result<Datum> FillNullBackward(const Datum& values, ExecContext* ctx = NULLPTR);
+
 /// \brief Take from an array of values at indices in another array
 ///
 /// The output array will be of the same type as the input values