Re: [PR] GH-44393: [C++][Compute] Swizzle vector functions [arrow]

["pitrou (via GitHub)" <gi...@apache.org>]

pitrou commented on code in PR #44394:
URL: https://github.com/apache/arrow/pull/44394#discussion_r1834697105


##########
cpp/src/arrow/compute/api_vector.h:
##########
@@ -705,5 +737,52 @@ Result<std::shared_ptr<Array>> PairwiseDiff(const Array& array,
                                             bool check_overflow = false,
                                             ExecContext* ctx = NULLPTR);
 
+/// \brief Return the inverse permutation of the given indices.
+///
+/// For indices[i] = x, inverse_permutation[x] = i. And inverse_permutation[x] = null if x
+/// does not appear in the input indices. For indices[i] = x where x < 0 or x > max_index,
+/// 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 inverse permutation is
+///   [1, 6, 3]                    if max_index = 2,
+///   [1, 6, 3, 2, 4, null, null]  if max_index = 6.
+///
+/// \param[in] indices array-like indices
+/// \param[in] options configures the max index and the output type
+/// \param[in] ctx the function execution context, optional
+/// \return the resulting inverse permutation
+///
+/// \since 19.0.0
+/// \note API not yet finalized
+ARROW_EXPORT
+Result<Datum> InversePermutation(
+    const Datum& indices,
+    const InversePermutationOptions& options = InversePermutationOptions::Defaults(),
+    ExecContext* ctx = NULLPTR);
+
+/// \brief Scatter the values into specified positions according to the indices.
+///
+/// For indices[i] = x, output[x] = values[i]. And output[x] = null if x does not appear
+/// in the input indices. For indices[i] = x where x < 0 or x > max_index, values[i]
+/// is ignored. If multiple indices point to the same value, the last one is used.

Review Comment:
   ```suggestion
   /// in the input indices. For indices[i] = x where x < 0 or x > max_index, values[i]
   /// is ignored. If multiple indices point to the same value, the last one is used.
   ```
   Can you explain the point of the lenient behavior wrt. negative indices and the max_index option?
   Is there a use case that this enables?



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