ARROW-15643: [C++] Allow selecting subset of fields of a StructArray via cast

cpp/src/arrow/compute/kernels/scalar_cast_nested.cc

@@ -153,27 +153,32 @@ void AddListCast(CastFunction* func) {
 struct CastStruct {
   static Status Exec(KernelContext* ctx, const ExecBatch& batch, Datum* out) {
     const CastOptions& options = CastState::Get(ctx);
-    const StructType& in_type = checked_cast<const StructType&>(*batch[0].type());
-    const StructType& out_type = checked_cast<const StructType&>(*out->type());
-    const auto in_field_count = in_type.num_fields();
-
-    if (in_field_count != out_type.num_fields()) {
-      return Status::TypeError("struct field sizes do not match: ", in_type.ToString(),
-                               " ", out_type.ToString());
-    }
-
-    for (int i = 0; i < in_field_count; ++i) {
-      const auto in_field = in_type.field(i);
-      const auto out_field = out_type.field(i);
-      if (in_field->name() != out_field->name()) {
-        return Status::TypeError("struct field names do not match: ", in_type.ToString(),
-                                 " ", out_type.ToString());
+    const auto& in_type = checked_cast<const StructType&>(*batch[0].type());
+    const auto& out_type = checked_cast<const StructType&>(*out->type());
+    const int in_field_count = in_type.num_fields();
+    const int out_field_count = out_type.num_fields();
+
+    std::vector<int> fields_to_select(out_field_count, -1);
+
+    int out_field_index = 0;
+    for (int in_field_index = 0;
+         in_field_index < in_field_count && out_field_index < out_field_count;
+         ++in_field_index) {
+      const auto& in_field = in_type.field(in_field_index);
+      const auto& out_field = out_type.field(out_field_index);
+      if (in_field->name() == out_field->name()) {
+        if (in_field->nullable() && !out_field->nullable()) {
+          return Status::TypeError("cannot cast nullable field to non-nullable field: ",
+                                   in_type.ToString(), " ", out_type.ToString());
+        }
+        fields_to_select[out_field_index++] = in_field_index;
       }
+    }
 
-      if (in_field->nullable() && !out_field->nullable()) {
-        return Status::TypeError("cannot cast nullable struct to non-nullable struct: ",
-                                 in_type.ToString(), " ", out_type.ToString());
-      }
+    if (out_field_index < out_field_count) {
+      return Status::TypeError(
+          "struct fields don't match or are in the wrong order: Input fields: ",
+          in_type.ToString(), " output fields: ", out_type.ToString());
     }
 
     if (out->kind() == Datum::SCALAR) {
@@ -182,9 +187,10 @@ struct CastStruct {
 
       DCHECK(!out_scalar->is_valid);
       if (in_scalar.is_valid) {
-        for (int i = 0; i < in_field_count; i++) {
-          auto values = in_scalar.value[i];
-          auto target_type = out->type()->field(i)->type();
+        out_field_index = 0;
+        for (int field_index : fields_to_select) {
+          const auto& values = in_scalar.value[field_index];
+          const auto& target_type = out->type()->field(out_field_index++)->type();
           ARROW_ASSIGN_OR_RAISE(Datum cast_values,
                                 Cast(values, target_type, options, ctx->exec_context()));
           DCHECK_EQ(Datum::SCALAR, cast_values.kind());
@@ -204,9 +210,11 @@ struct CastStruct {
                                        in_array.offset, in_array.length));
     }
 
-    for (int i = 0; i < in_field_count; ++i) {
-      auto values = in_array.child_data[i]->Slice(in_array.offset, in_array.length);
-      auto target_type = out->type()->field(i)->type();
+    out_field_index = 0;
+    for (int field_index : fields_to_select) {
+      const auto& values =
+          in_array.child_data[field_index]->Slice(in_array.offset, in_array.length);
+      const auto& target_type = out->type()->field(out_field_index++)->type();
 
       ARROW_ASSIGN_OR_RAISE(Datum cast_values,
                             Cast(values, target_type, options, ctx->exec_context()));