From ba98a234f254a850b28b9773e07d9862d8aef162 Mon Sep 17 00:00:00 2001
From: Wes McKinney <wesm+git@apache.org>
Date: Mon, 19 Aug 2019 19:29:16 -0500
Subject: [PATCH] Support view from one dictionary type to another in
 Array::View

---
 cpp/src/arrow/array.cc           | 28 +++++++++++++++-------------
 cpp/src/arrow/array_view_test.cc | 31 +++++++++++++++++++++++++++----
 2 files changed, 42 insertions(+), 17 deletions(-)

diff --git a/cpp/src/arrow/array.cc b/cpp/src/arrow/array.cc
index 72997030e48..cfcc65a6743 100644
--- a/cpp/src/arrow/array.cc
+++ b/cpp/src/arrow/array.cc
@@ -1044,15 +1044,6 @@ struct ViewDataImpl {
     return Status::OK();
   }
 
-  Status CheckInputHasNoDictionaries() {
-    for (const auto& layout : in_layouts) {
-      if (layout.has_dictionary) {
-        return InvalidView("input has dictionary");
-      }
-    }
-    return Status::OK();
-  }
-
   Status CheckInputAtZeroOffset() {
     for (const auto& data : in_data) {
       if (data->offset != 0) {
@@ -1062,18 +1053,28 @@ struct ViewDataImpl {
     return Status::OK();
   }
 
+  Status GetDictionaryView(const DataType& out_type, std::shared_ptr<Array>* out) {
+    if (in_data[in_layout_idx]->type->id() != Type::DICTIONARY) {
+      return InvalidView("Cannot get view as dictionary type");
+    }
+    const auto& dict_out_type = static_cast<const DictionaryType&>(out_type);
+    return in_data[in_layout_idx]->dictionary->View(dict_out_type.value_type(), out);
+  }
+
   Status MakeDataView(const std::shared_ptr<Field>& out_field,
                       std::shared_ptr<ArrayData>* out) {
     const auto out_type = out_field->type();
     const auto out_layout = out_type->layout();
-    if (out_layout.has_dictionary) {
-      return InvalidView("view type requires dictionary");
-    }
 
     AdjustInputPointer();
     int64_t out_length = in_data_length;
     int64_t out_null_count;
 
+    std::shared_ptr<Array> dictionary;
+    if (out_type->id() == Type::DICTIONARY) {
+      RETURN_NOT_OK(GetDictionaryView(*out_type, &dictionary));
+    }
+
     // No type has a purely empty layout
     DCHECK_GT(out_layout.bit_widths.size(), 0);
 
@@ -1143,6 +1144,8 @@ struct ViewDataImpl {
 
     std::shared_ptr<ArrayData> out_data =
         ArrayData::Make(out_type, out_length, std::move(out_buffers), out_null_count);
+    out_data->dictionary = dictionary;
+
     // Process children recursively, depth-first
     for (const auto& child_field : out_type->children()) {
       std::shared_ptr<ArrayData> child_data;
@@ -1166,7 +1169,6 @@ Status Array::View(const std::shared_ptr<DataType>& out_type,
   impl.in_data_length = data_->length;
 
   std::shared_ptr<ArrayData> out_data;
-  RETURN_NOT_OK(impl.CheckInputHasNoDictionaries());
   RETURN_NOT_OK(impl.CheckInputAtZeroOffset());
   // Dummy field for output type
   auto out_field = field("", out_type);
diff --git a/cpp/src/arrow/array_view_test.cc b/cpp/src/arrow/array_view_test.cc
index 8d94242d8f3..4d4c5704b0b 100644
--- a/cpp/src/arrow/array_view_test.cc
+++ b/cpp/src/arrow/array_view_test.cc
@@ -355,14 +355,37 @@ TEST(TestArrayView, DecimalRoundTrip) {
 }
 
 TEST(TestArrayView, Dictionaries) {
-  // Can't view dictionaries
+  // ARROW-6049
   auto ty1 = dictionary(int8(), float32());
+  auto ty2 = dictionary(int8(), int32());
+
   auto indices = ArrayFromJSON(int8(), "[0, 2, null, 1]");
   auto values = ArrayFromJSON(float32(), "[0.0, 1.5, -2.5]");
-  std::shared_ptr<Array> arr;
+
+  std::shared_ptr<Array> arr, expected, expected_dict;
+  ASSERT_OK(values->View(int32(), &expected_dict));
   ASSERT_OK(DictionaryArray::FromArrays(ty1, indices, values, &arr));
-  CheckViewFails(arr, int8());
-  CheckViewFails(indices, ty1);
+  ASSERT_OK(DictionaryArray::FromArrays(ty2, indices, expected_dict, &expected));
+
+  CheckView(arr, expected);
+  CheckView(expected, arr);
+
+  // Incompatible index type
+  auto ty3 = dictionary(int16(), int32());
+  CheckViewFails(arr, ty3);
+
+  // Incompatible dictionary type
+  auto ty4 = dictionary(int16(), float64());
+  CheckViewFails(arr, ty4);
+
+  // Check dictionary-encoded child
+  auto offsets = ArrayFromJSON(int32(), "[0, 2, 2, 4]");
+  std::shared_ptr<Array> list_arr, expected_list_arr;
+  ASSERT_OK(ListArray::FromArrays(*offsets, *arr, default_memory_pool(), &list_arr));
+  ASSERT_OK(ListArray::FromArrays(*offsets, *expected, default_memory_pool(),
+                                  &expected_list_arr));
+  CheckView(list_arr, expected_list_arr);
+  CheckView(expected_list_arr, list_arr);
 }
 
 TEST(TestArrayView, ExtensionType) {