From 8e6e99093f493bbe517cdec102d2dca01d2fcac5 Mon Sep 17 00:00:00 2001
From: Rok Mihevc <rok@mihevc.org>
Date: Fri, 29 Mar 2024 23:56:03 +0100
Subject: [PATCH] refactor ComputeStrides

---
 .../extension/tensor_extension_array_test.cc  |  8 ++--
 cpp/src/arrow/extension/tensor_internal.cc    | 39 ++++---------------
 2 files changed, 11 insertions(+), 36 deletions(-)

diff --git a/cpp/src/arrow/extension/tensor_extension_array_test.cc b/cpp/src/arrow/extension/tensor_extension_array_test.cc
index 83c3760888713..530ae27c85a14 100644
--- a/cpp/src/arrow/extension/tensor_extension_array_test.cc
+++ b/cpp/src/arrow/extension/tensor_extension_array_test.cc
@@ -514,16 +514,16 @@ TEST_F(TestFixedShapeTensorType, ComputeStrides) {
 
   auto ext_type_5 = internal::checked_pointer_cast<FixedShapeTensorType>(
       fixed_shape_tensor(int64(), {3, 4, 7}, {1, 0, 2}));
-  ASSERT_EQ(ext_type_5->strides(), (std::vector<int64_t>{56, 224, 8}));
+  ASSERT_EQ(ext_type_5->strides(), (std::vector<int64_t>{56, 168, 8}));
   ASSERT_EQ(ext_type_5->Serialize(), R"({"shape":[3,4,7],"permutation":[1,0,2]})");
 
   auto ext_type_6 = internal::checked_pointer_cast<FixedShapeTensorType>(
       fixed_shape_tensor(int64(), {3, 4, 7}, {1, 2, 0}, {}));
-  ASSERT_EQ(ext_type_6->strides(), (std::vector<int64_t>{56, 8, 224}));
+  ASSERT_EQ(ext_type_6->strides(), (std::vector<int64_t>{32, 8, 96}));
   ASSERT_EQ(ext_type_6->Serialize(), R"({"shape":[3,4,7],"permutation":[1,2,0]})");
   auto ext_type_7 = internal::checked_pointer_cast<FixedShapeTensorType>(
       fixed_shape_tensor(int32(), {3, 4, 7}, {2, 0, 1}, {}));
-  ASSERT_EQ(ext_type_7->strides(), (std::vector<int64_t>{4, 112, 16}));
+  ASSERT_EQ(ext_type_7->strides(), (std::vector<int64_t>{4, 84, 12}));
   ASSERT_EQ(ext_type_7->Serialize(), R"({"shape":[3,4,7],"permutation":[2,0,1]})");
 }
 
@@ -594,7 +594,7 @@ TEST_F(TestFixedShapeTensorType, GetTensor) {
     // Get tensor from extension array with non-trivial permutation
     ASSERT_OK_AND_ASSIGN(auto expected_permuted_tensor,
                          Tensor::Make(value_type_, Buffer::Wrap(element_values[i]),
-                                      {4, 3}, {8, 24}, {"y", "x"}));
+                                      {4, 3}, {8, 32}, {"y", "x"}));
     ASSERT_OK_AND_ASSIGN(scalar, permuted_array->GetScalar(i));
     ASSERT_OK_AND_ASSIGN(auto actual_permuted_tensor,
                          exact_permuted_ext_type->MakeTensor(
diff --git a/cpp/src/arrow/extension/tensor_internal.cc b/cpp/src/arrow/extension/tensor_internal.cc
index d021553702efb..b3d3f626441ae 100644
--- a/cpp/src/arrow/extension/tensor_internal.cc
+++ b/cpp/src/arrow/extension/tensor_internal.cc
@@ -47,43 +47,18 @@ Status IsPermutationValid(const std::vector<int64_t>& permutation) {
 Result<std::vector<int64_t>> ComputeStrides(const std::shared_ptr<DataType>& value_type,
                                             const std::vector<int64_t>& shape,
                                             const std::vector<int64_t>& permutation) {
-  const auto fixed_width_type =
-      internal::checked_pointer_cast<FixedWidthType>(value_type);
-
+  const auto& fw_type = checked_cast<const FixedWidthType&>(*value_type);
   std::vector<int64_t> strides;
-  if (permutation.empty()) {
-    ARROW_DCHECK_OK(
-        internal::ComputeRowMajorStrides(*fixed_width_type.get(), shape, &strides));
-    return strides;
-  }
-  const int byte_width = value_type->byte_width();
-
-  int64_t remaining = 0;
-  if (!shape.empty() && shape.front() > 0) {
-    remaining = byte_width;
-    for (auto i : permutation) {
-      if (i > 0) {
-        if (internal::MultiplyWithOverflow(remaining, shape[i], &remaining)) {
-          return Status::Invalid(
-              "Strides computed from shape would not fit in 64-bit integer");
-        }
-      }
-    }
-  }
 
-  if (remaining == 0) {
-    strides.assign(shape.size(), byte_width);
+  if (permutation.empty()) {
+    ARROW_DCHECK_OK(internal::ComputeRowMajorStrides(fw_type, shape, &strides));
     return strides;
   }
 
-  strides.push_back(remaining);
-  for (auto i : permutation) {
-    if (i > 0) {
-      remaining /= shape[i];
-      strides.push_back(remaining);
-    }
-  }
-  DCHECK_EQ(strides.back(), byte_width);
+  auto permuted_shape = std::move(shape);
+  auto reverse_permutation = internal::ArgSort(permutation, std::less<>());
+  Permute(reverse_permutation, &permuted_shape);
+  ARROW_DCHECK_OK(internal::ComputeRowMajorStrides(fw_type, permuted_shape, &strides));
   Permute(permutation, &strides);
 
   return strides;