GH-51224: [C++] Keep the correct nulls when winsorizing a sliced array

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

@@ -26,6 +26,7 @@
 #include "arrow/compute/function.h"
 #include "arrow/compute/kernel.h"
 #include "arrow/compute/kernels/codegen_internal.h"
+#include "arrow/compute/kernels/util_internal.h"
 #include "arrow/compute/registry.h"
 #include "arrow/compute/registry_internal.h"
 #include "arrow/result.h"
@@ -71,6 +72,7 @@ struct Winsorize {
       // Only nulls and NaNs => return input as-is
       out_data->null_count = data->null_count.load();
       out_data->length = data->length;
+      out_data->offset = data->offset;
       out_data->buffers = data->buffers;
       return Status::OK();
     }
@@ -127,7 +129,9 @@ struct Winsorize {
     DCHECK_EQ(out->buffers.size(), data.buffers.size());
     out->null_count = data.null_count.load();
     out->length = data.length;
-    out->buffers[0] = data.buffers[0];
+    out->offset = 0;
+    ARROW_ASSIGN_OR_RAISE(out->buffers[0],
+                          GetOrCopyNullBitmapBuffer(data, ctx->memory_pool()));
     ARROW_ASSIGN_OR_RAISE(out->buffers[1], ctx->Allocate(out->length * sizeof(CType)));
     // Avoid leaving uninitialized memory under null entries
     std::memset(out->buffers[1]->mutable_data(), 0, out->length * sizeof(CType));

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

@@ -87,6 +87,45 @@ TEST_F(TestWinsorize, FloatingPoint) {
   }
 }
 
+TEST_F(TestWinsorize, SlicedInput) {
+  options_.lower_limit = 0.0;
+  options_.upper_limit = 1.0;
+  auto parent = ArrayFromJSON(float64(), "[1.1, 2.2, null, 4.4, null, 6.6, 7.7, 8.8]");
+  auto expected = ArrayFromJSON(float64(), "[null, 4.4, null, 6.6, 7.7]");
+  CheckWinsorize(parent->Slice(2, 5), expected);
+
+  options_.lower_limit = 0.25;
+  options_.upper_limit = 0.75;
+  auto dense = ArrayFromJSON(float64(), "[1.0, 2.0, 3.0, 44.0, 55.0, 66.0, 77.0]");
+  CheckWinsorize(dense->Slice(1, 5),
+                 ArrayFromJSON(float64(), "[3.0, 3.0, 44.0, 55.0, 55.0]"));
+}
+
+TEST_F(TestWinsorize, SlicedChunkedInput) {
+  options_.lower_limit = 0.0;
+  options_.upper_limit = 1.0;
+  auto parent = ArrayFromJSON(float64(), "[1.1, 2.2, null, 4.4, null, 6.6, 7.7, 8.8]");
+  auto chunked = std::make_shared<ChunkedArray>(
+      ArrayVector{parent->Slice(2, 3), parent->Slice(5, 3)});
+  auto expected = std::make_shared<ChunkedArray>(ArrayVector{
+      ArrayFromJSON(float64(), "[null, 4.4, null]"),
+      ArrayFromJSON(float64(), "[6.6, 7.7, 8.8]"),
+  });
+  CheckWinsorize(chunked, expected);
+}
+
+TEST_F(TestWinsorize, SlicedInputWithoutQuantiles) {
+  for (const auto* json_input :
+       {"[1, 2, null, null, null, 3]", "[1, 2, NaN, null, NaN, 3]",
+        "[1, 2, NaN, NaN, NaN, 3]"}) {
+    auto parent = ArrayFromJSON(float64(), json_input);
+    auto sliced = parent->Slice(2, 3);
+    CheckWinsorize(sliced, sliced);
+    auto chunked = std::make_shared<ChunkedArray>(ArrayVector{sliced});
+    CheckWinsorize(chunked, chunked);
+  }
+}
+
 TEST_F(TestWinsorize, Integral) {
   for (auto type : IntTypes()) {
     options_.lower_limit = 0.25;