apacheGH-39976: [C++] Fix out-of-line data size calculation in Binary…

…ViewBuilder::AppendArraySlice (apache#39994)

### Rationale for this change

Fix the bug in `BinaryViewBuilder::AppendArraySlice` that, when calculating out-of-line data size, the array is wrongly iterated.

### What changes are included in this PR?

Fix and UT.

### Are these changes tested?

UT included.

### Are there any user-facing changes?

No.

* Closes: apache#39976

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/array/array_test.cc

@@ -905,6 +905,29 @@ TEST_F(TestArray, TestAppendArraySlice) {
   }
 }
 
+// GH-39976: Test out-of-line data size calculation in
+// BinaryViewBuilder::AppendArraySlice.
+TEST_F(TestArray, TestBinaryViewAppendArraySlice) {
+  BinaryViewBuilder src_builder(pool_);
+  ASSERT_OK(src_builder.AppendNull());
+  ASSERT_OK(src_builder.Append("long string; not inlined"));
+  ASSERT_EQ(2, src_builder.length());
+  ASSERT_OK_AND_ASSIGN(auto src, src_builder.Finish());
+  ASSERT_OK(src->ValidateFull());
+
+  ArraySpan span;
+  span.SetMembers(*src->data());
+  BinaryViewBuilder dst_builder(pool_);
+  ASSERT_OK(dst_builder.AppendArraySlice(span, 0, 1));
+  ASSERT_EQ(1, dst_builder.length());
+  ASSERT_OK(dst_builder.AppendArraySlice(span, 1, 1));
+  ASSERT_EQ(2, dst_builder.length());
+  ASSERT_OK_AND_ASSIGN(auto dst, dst_builder.Finish());
+  ASSERT_OK(dst->ValidateFull());
+
+  AssertArraysEqual(*src, *dst);
+}
+
 TEST_F(TestArray, ValidateBuffersPrimitive) {
   auto empty_buffer = std::make_shared<Buffer>("");
   auto null_buffer = Buffer::FromString("\xff");

cpp/src/arrow/array/builder_binary.cc

@@ -54,7 +54,7 @@ Status BinaryViewBuilder::AppendArraySlice(const ArraySpan& array, int64_t offse
 
   int64_t out_of_line_total = 0, i = 0;
   VisitNullBitmapInline(
-      array.buffers[0].data, array.offset, array.length, array.null_count,
+      array.buffers[0].data, array.offset + offset, length, array.null_count,
       [&] {
         if (!values[i].is_inline()) {
           out_of_line_total += static_cast<int64_t>(values[i].size());