Remove begin() caching based on advice from Tim Song

include/beman/utf_view/to_utf_view.hpp

@@ -127,29 +127,6 @@ class to_utf_view : public std::ranges::view_interface<to_utf_view<V, E, ToType>
 
   V base_ = V(); // @*exposition only*@
 
-/* !PAPER */
-  template <typename V2>
-  struct maybe_cache_begin_impl {
-    constexpr auto begin_impl(to_utf_view& self, auto base) noexcept {
-      return exposition_only_iterator<false>{self, std::move(base)};
-    }
-  };
-
-  template <typename V2>
-  requires (!std::same_as<std::ranges::range_value_t<V2>, char32_t> && std::copy_constructible<V>)
-  struct maybe_cache_begin_impl<V2> {
-    constexpr auto begin_impl(to_utf_view& self, auto base) noexcept {
-      if (!cache_) {
-        cache_.emplace(self, std::move(base));
-      }
-      return *cache_;
-    }
-    std::optional<exposition_only_iterator<false>> cache_;
-  };
-
-  [[no_unique_address]] maybe_cache_begin_impl<V> cache_;
-
-/* PAPER */
 public:
   constexpr to_utf_view()
     requires std::default_initializable<V>
@@ -173,11 +150,7 @@ class to_utf_view : public std::ranges::view_interface<to_utf_view<V, E, ToType>
   /* PAPER:     constexpr @*iterator*@<false> begin(); */
   constexpr exposition_only_iterator<false> begin()
   {
-    if (!std::copy_constructible<V>) {
-      return exposition_only_iterator<false>(*this, std::ranges::begin(base_));
-    } else {
-      return cache_.begin_impl(*this, std::ranges::begin(base_));
-    }
+    return exposition_only_iterator<false>(*this, std::ranges::begin(base_));
   }
   /* PAPER */
   constexpr exposition_only_iterator<true> begin() const

paper/P2728.md

@@ -589,8 +589,6 @@ constexpr @*iterator*@ begin();
 
 _Returns:_ `{*this, std::ranges::begin(base_)}`
 
-_Remarks:_ In order to provide the amortized constant time complexity required by the `range` concept when `to_utf_view` transcodes from UTF-8 or UTF-16, this function caches the result within the `to_utf_view` for use on subsequent calls.
-
 ```cpp
 constexpr auto reserve_hint() requires approximately_sized_range<V>;
 ```
@@ -999,22 +997,21 @@ You can also substitute a different replacement character by changing the
 result of the `else` clause, or add exception-based error handling by throwing
 at that point.
 
-## Caching `begin()`
-
-When we transcode from UTF-8, invoking `begin()` on the transcoding view may read up to
-four elements from the underlying view. Similarly, when transcoding from UTF-16, it may
-read up to two underlying elements. As a result, in order to preserve "amortized O(1)
-complexity," we need to cache `begin()` in these situations.
+## Why We Don't Cache `begin()`
 
-On the other hand, when transcoding from UTF-32, `begin()` reads at most one element from
-the underlying view, so `begin()` is not cached when doing so.
+When we invoke `begin()`, constructing the transcoding iterator may read up to four
+elements from the underlying view if it's transcoding from UTF-8. A previous revision of
+this paper implemented `begin()` caching, based on the idea that iterating the underlying
+range could have unbounded compexity.
 
-We also incorporate [@P3725R1]'s approach, in that when a transcoding view wraps a
-const-iterable `input_range` that is not a `forward_range`, the transcoding view provides
-a `const` overload of `begin()` that is non-caching.
+However, Tim Song pointed to the wording in `[iterator.requirements.general]` stating that
+"All the categories of iterators require only those functions that are realizable for a
+given category in constant time (amortized)." This means that we should be making the
+assumption that the underlying iterator operations used by `begin()` are "amortized
+constant time" (in a hand-wavey sense). Tim also pointed out that transcoding from UTF is
+equivalent to `views::adjacent<4>`, which doesn't cache.
 
-However, we do not provide [@P3725R1]-style `views::input_to_utf8` CPOs; for that use
-case, we expect users to simply spell `views::to_input | views::to_utf8`.
+Based on this reasoning, the transcoding views don't cache `begin()`.
 
 ## Optimizing for Double-Transcoding
 
@@ -1205,6 +1202,7 @@ These concepts are true when the type in question is the iterator/sentinel of a
 - Remove section stating that the concept of a CPO template is a "novelty"
   after it was pointed out in Kona 2025 that it has precedent in
   `views::adjacent_transform<N>` and elsewhere
+- Don't cache `begin()`
 
 ## Changes since R8