felipecrv commented on code in PR #44394:
URL: https://github.com/apache/arrow/pull/44394#discussion_r1823600990
##########
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.
Review Comment:
"reverse index" already means a different thing in database literature.
This operation calculates the "inverse permutation" [2] and there is plenty
of old references to it.
[1] https://en.wikipedia.org/wiki/Reverse_index
[2] https://mathworld.wolfram.com/InversePermutation.html
##########
cpp/src/arrow/compute/api_vector.cc:
##########
@@ -155,6 +155,11 @@ static auto kPairwiseOptionsType =
GetFunctionOptionsType<PairwiseOptions>(
DataMember("periods", &PairwiseOptions::periods));
static auto kListFlattenOptionsType =
GetFunctionOptionsType<ListFlattenOptions>(
DataMember("recursive", &ListFlattenOptions::recursive));
+static auto kReverseIndicesOptionsType =
GetFunctionOptionsType<ReverseIndicesOptions>(
+ DataMember("output_length", &ReverseIndicesOptions::output_length),
+ DataMember("output_type", &ReverseIndicesOptions::output_type));
+static auto kPermuteOptionsType = GetFunctionOptionsType<PermuteOptions>(
+ DataMember("output_length", &PermuteOptions::output_length));
Review Comment:
It's very confusing to call this function permute [1]. It's a scatter [1],
right?
https://en.wikipedia.org/wiki/Permute_instruction (gather and scatter are
different forms of permute)
##########
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.
Review Comment:
I think this explanation is confusing, but we can work on this later.
##########
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:
Is the `ouput_length` parameter that useful? Compared to assuming the same
length as the input?
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