[GitHub] [arrow] bkietz commented on a diff in pull request #36334: GH-15281: [C++] Replace bytes_view alias with span

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

bkietz commented on code in PR #36334:
URL: https://github.com/apache/arrow/pull/36334#discussion_r1246999183


##########
cpp/src/arrow/util/span.h:
##########
@@ -0,0 +1,124 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
+#include <type_traits>
+
+namespace arrow::util {
+
+/// std::span polyfill.
+///
+/// Does not support static extents.
+template <typename T>
+class span {
+  static_assert(sizeof(T),
+                R"(
+std::span allows contiguous_iterators instead of just pointers, the enforcement
+of which requires T to be a complete type. arrow::util::span does not support
+contiguous_iterators, but T is still required to be a complete type to prevent
+writing code which would break when it is replaced by std::span.)");
+
+ public:
+  using element_type = T;
+  using value_type = std::remove_cv_t<T>;
+  using iterator = T*;
+  using const_iterator = T const*;
+
+  span() = default;
+  span(const span&) = default;
+  span& operator=(const span&) = default;
+
+  template <typename M, typename = std::enable_if_t<std::is_same_v<T, M const>>>
+  // NOLINTNEXTLINE runtime/explicit
+  constexpr span(span<M> mut) : span{mut.data(), mut.size()} {}
+
+  constexpr span(T* data, size_t count) : data_{data}, size_{count} {}
+
+  constexpr span(T* begin, T* end)
+      : data_{begin}, size_{static_cast<size_t>(end - begin)} {}
+
+  template <
+      typename R,
+      typename DisableUnlessConstructibleFromDataAndSize =
+          decltype(span<T>(std::data(std::declval<R&>()), std::size(std::declval<R&>()))),
+      typename DisableUnlessSimilarTypes = std::enable_if_t<std::is_same_v<
+          std::decay_t<std::remove_pointer_t<decltype(std::data(std::declval<R&>()))>>,
+          std::decay_t<T>>>>
+  // NOLINTNEXTLINE runtime/explicit, non-const reference
+  constexpr span(R& range) : span{std::data(range), std::size(range)} {}
+
+  constexpr T* begin() const { return data_; }
+  constexpr T* end() const { return data_ + size_; }
+  constexpr T* data() const { return data_; }
+
+  constexpr size_t size() const { return size_; }
+  constexpr size_t size_bytes() const { return size_ * sizeof(T); }
+  constexpr bool empty() const { return size_ == 0; }
+
+  constexpr T& operator[](size_t i) { return data_[i]; }
+  constexpr const T& operator[](size_t i) const { return data_[i]; }
+
+  constexpr span subspan(size_t offset) const {
+    if (offset > size_) return {};
+    return {data_ + offset, size_ - offset};
+  }
+
+  constexpr span subspan(size_t offset, size_t count) const {
+    auto out = subspan(offset);
+    if (count < out.size_) {
+      out.size_ = count;
+    }
+    return out;
+  }
+
+  constexpr bool operator==(span const& other) const {
+    if (size_ != other.size_) return false;
+
+    T* ptr = data_;
+    for (T const& e : other) {
+      if (*ptr++ != e) return false;
+    }
+    return true;
+  }

Review Comment:
   I've played with this a bit and couldn't get the compiler to do it automatically. I'll use `if constexpr (std::is_integral_v<T>)` to dispatch to memcmp



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