felipecrv commented on code in PR #44394:
URL: https://github.com/apache/arrow/pull/44394#discussion_r1824544446
##########
cpp/src/arrow/compute/api_vector.h:
##########
@@ -705,5 +736,56 @@ Result<std::shared_ptr<Array>> PairwiseDiff(const Array&
array,
bool check_overflow = false,
ExecContext* ctx = NULLPTR);
+/// \brief Return the reverse indices of the given indices.
+///
+/// For indices[i] = x, reverse_indices[x] = i. And reverse_indices[x] = null
if x does
+/// not appear in the input indices. For indices[i] = x where x < 0 or x >=
output_length,
+/// it is ignored. If multiple indices point to the same value, the last one
is used.
+///
+/// For example, with indices = [null, 0, 3, 2, 4, 1, 1], the reverse indices
is
+/// [1, 6, 3] if output_length = 3,
+/// [1, 6, 3, 2, 4, null, null] if output_length = 7.
Review Comment:
Right, so could you call it `n` or `max_index` (i.e. `n - 1`) instead. The
output length becomes a consequence of how far the loop goes. So at least to me
it was easier to understand this parameter as a constraint on the loop and the
output length being just a consequence of that.
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