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

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

bkietz commented on code in PR #13009:
URL: https://github.com/apache/arrow/pull/13009#discussion_r870385124


##########
cpp/src/arrow/stl_iterator.h:
##########
@@ -128,6 +131,162 @@ 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) {}
+
+  explicit ChunkedArrayIterator(const ChunkedArray& chunked_array, int64_t index = 0)
+      : chunked_array_(&chunked_array), index_(index) {
+    if (index_ != chunked_array.length()) {
+      auto chunk_location = GetChunkLocation(this->index_);
+      current_array_iterator_ = ArrayIterator<ArrayType>(
+          arrow::internal::checked_cast<const ArrayType&>(
+              *chunked_array_->chunk(static_cast<int>(chunk_location.chunk_index))),
+          chunk_location.index_in_chunk);
+    } else {
+      current_array_iterator_ = {};
+    }
+  }
+
+  // Value access
+  value_type operator*() const { return *current_array_iterator_; }
+
+  value_type operator[](difference_type n) const {
+    auto chunk_location = GetChunkLocation(index_ + n);
+    ArrayIterator<ArrayType> target_iterator{
+        arrow::internal::checked_cast<const ArrayType&>(
+            *chunked_array_->chunk(static_cast<int>(chunk_location.chunk_index)))};
+    return target_iterator[chunk_location.index_in_chunk];
+  }
+
+  int64_t index() const { return index_; }
+
+  // Forward / backward
+  ChunkedArrayIterator& operator++() {
+    (*this) += 1;
+    return *this;
+  }
+  ChunkedArrayIterator& operator--() {
+    (*this) -= 1;
+    return *this;
+  }
+
+  ChunkedArrayIterator operator++(int) {
+    ChunkedArrayIterator tmp(*this);
+    ++*this;
+    return tmp;
+  }
+  ChunkedArrayIterator operator--(int) {
+    ChunkedArrayIterator tmp(*this);
+    --*this;
+    return tmp;
+  }
+
+  // Arithmetic
+  difference_type operator-(const ChunkedArrayIterator& other) const {
+    return index_ - other.index_;
+  }
+  ChunkedArrayIterator operator+(difference_type n) const {
+    return ChunkedArrayIterator(*chunked_array_, index_ + n);
+  }
+  ChunkedArrayIterator operator-(difference_type n) const {
+    return ChunkedArrayIterator(*chunked_array_, index_ - n);
+  }
+  friend inline ChunkedArrayIterator operator+(difference_type diff,
+                                               const ChunkedArrayIterator& other) {
+    return ChunkedArrayIterator(*other.chunked_array_, diff + other.index_);
+  }
+  friend inline ChunkedArrayIterator operator-(difference_type diff,
+                                               const ChunkedArrayIterator& other) {
+    return ChunkedArrayIterator(*other.chunked_array_, diff - other.index_);
+  }
+  ChunkedArrayIterator& operator+=(difference_type n) {
+    index_ += n;
+    if (index_ != chunked_array_->length()) {

Review Comment:
   Personally, my preference is to maximize the similarity of any random access iterators to indices/non-owning pointers. In this case I'd try to avoid accessing elements at all when incrementing the iterator, deferring the resolution of `current_array_iterator_` until something is actually dereferenced. This would also move out-of-bounds/invalid current array checking into the dereferencing member functions where correct behavior is far more obvious.
   
   The specific question of how to validate out-of-bounds iterators becomes easier to answer too: We can construct an iterator with whatever index and compare/increment/etc without worry since that operates only on the index.



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