[C++] Grouper produce a wrong num_groups when set are_cols_in_encoding_order to false

[ZhangHuiGui]

Describe the bug, including details regarding any error messages, version, and platform.

Bug found from blew comment:
#41036 (comment)

It's bug in grouper when set are_cols_in_encoding_order=false in below codes:

arrow/cpp/src/arrow/compute/row/grouper.cc

Line 582 in 2979d69

/* are_cols_in_encoding_order=*/true);

It will cause the num_group different with are_cols_in_encoding_order=true condition.

The encoder will sort columns by default, when we only set this compare args to false, the CompareColumnsToRows's input impl_ptr->encoder_.batch_all_cols(), impl_ptr->rows_, are all sorted, but use the incorrect column_offset to access compared column:

arrow/cpp/src/arrow/compute/row/compare_internal.cc

Lines 366 to 369 in 2979d69

	uint32_t offset_within_row = rows.metadata().encoded_field_offset(
	are_cols_in_encoding_order
	? static_cast<uint32_t>(icol)
	: rows.metadata().pos_after_encoding(static_cast<uint32_t>(icol)));

Component(s)

C++