[GitHub] [arrow] edponce commented on a diff in pull request #13009: ARROW-602: [C++] Provide iterator access to primitive elements inside an Array

[GitBox <gi...@apache.org>]

edponce commented on code in PR #13009:
URL: https://github.com/apache/arrow/pull/13009#discussion_r859921831


##########
cpp/src/arrow/stl_iterator.h:
##########
@@ -128,6 +131,215 @@ class ArrayIterator {
   int64_t index_;
 };
 
+template <typename ArrayType,
+          typename ValueAccessor = detail::DefaultValueAccessor<ArrayType>>
+class ChunkedArrayIterator {
+ public:
+  using value_type = arrow::util::optional<typename ValueAccessor::ValueType>;
+  using difference_type = int64_t;
+  using pointer = value_type*;
+  using reference = value_type&;
+  using iterator_category = std::random_access_iterator_tag;
+
+  // Some algorithms need to default-construct an iterator
+  ChunkedArrayIterator() : chunked_array_(NULLPTR), index_(0), current_chunk_index_(0) {}
+
+  explicit ChunkedArrayIterator(const ChunkedArray& chunked_array, int64_t index = 0)
+      : chunked_array_(&chunked_array), index_(index) {
+    InitializeComponents(*this);
+  }
+
+  // Value access
+  value_type operator*() const { return *iterators_list_[current_chunk_index_]; }
+
+  value_type operator[](difference_type n) const {
+    int64_t chunk_index = GetChunkIndex(index_ + n);
+    int64_t index_in_chunk =
+        chunk_index ? index_ + n - chunks_lengths_[chunk_index - 1] : index_ + n;
+    return iterators_list_[chunk_index]
+                          [index_in_chunk - iterators_list_[chunk_index].index()];

Review Comment:
   [`ChunkedArray`](https://github.com/apache/arrow/blob/master/cpp/src/arrow/chunked_array.h#L145) already has `GetScalar(index)` which is efficient for accessing sequential scalars at particular logical indices. I would expect this iterator to make use of it.



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