From 8eeb35cf365aa60ed538a47e8f6dd5bb30837aee Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Mon, 11 Nov 2024 11:19:33 +1000
Subject: [PATCH] ranges: add a naive implemenation of "adjacent"

This works around a diffiency in the clang-19 ranges library.
---
 cruft/util/CMakeLists.txt      |   1 +
 cruft/util/ranges/adjacent.hpp | 177 +++++++++++++++++++++++++++++++++
 test/CMakeLists.txt            |   1 +
 test/ranges/adjacent.cpp       |  29 ++++++
 4 files changed, 208 insertions(+)
 create mode 100644 cruft/util/ranges/adjacent.hpp
 create mode 100644 test/ranges/adjacent.cpp

diff --git a/cruft/util/CMakeLists.txt b/cruft/util/CMakeLists.txt
index 815a920c..ce7fd682 100644
--- a/cruft/util/CMakeLists.txt
+++ b/cruft/util/CMakeLists.txt
@@ -544,6 +544,7 @@ list (
     random.hpp
     range.cpp
     range.hpp
+    ranges/adjacent.hpp
     ranges/chunk.hpp
     ranges/enumerate.hpp
     ranges/izip.hpp
diff --git a/cruft/util/ranges/adjacent.hpp b/cruft/util/ranges/adjacent.hpp
new file mode 100644
index 00000000..5848940a
--- /dev/null
+++ b/cruft/util/ranges/adjacent.hpp
@@ -0,0 +1,177 @@
+#pragma once
+
+#include <array>
+#include <ranges>
+
+
+namespace cruft::ranges {
+    /// A very naive implementation of std::views::adjacent provided because clang-19 lacks it
+    ///
+    /// clang#19: check me again in clang-20
+    ///
+    /// Models the main iterator as an array of iterators, and the sentinel as a thin wrapper
+    // around the underlying range's sentinel.
+    template <std::ranges::forward_range V, std::size_t N>
+    requires std::ranges::view<V> and (N > 0)
+    class adjacent_view
+        : public std::ranges::view_interface<adjacent_view<V, N>>
+    {
+    private:
+        V base_;
+
+    public:
+        class sentinel;
+
+        class iterator {
+        public:
+            using iterator_category = std::forward_iterator_tag;
+            using iterator_concept  = std::forward_iterator_tag;
+            using value_type = std::array<std::ranges::range_value_t<V>, N>;
+            using difference_type = std::ranges::range_difference_t<V>;
+
+        private:
+            // Uses an index sequence in the hope to avoid default initialisation and copying for
+            // problematic value_type.
+            template <std::size_t ...I>
+            value_type
+            dereference (std::index_sequence<I...>) const
+            {
+                return { *m_cursors[I]..., };
+            }
+
+            std::array<std::ranges::iterator_t<V>, N> m_cursors;
+
+        public:
+            iterator ();
+
+            iterator (
+                std::ranges::iterator_t<V> first,
+                std::ranges::sentinel_t<V> last_
+            ) {
+                for (std::size_t i = 0; i < N; ++i) {
+                    m_cursors[i] = first;
+                    if (first != last_)
+                        ++first;
+                }
+            }
+
+            iterator (iterator const&) = default;
+            iterator (iterator &&) noexcept = default;
+
+            iterator& operator= (iterator const&) = default;
+            iterator& operator= (iterator &&) noexcept = default;
+
+            iterator&
+            operator++ ()
+            {
+                for (auto &i: m_cursors)
+                    ++i;
+                return *this;
+            }
+
+            iterator operator++ (int) const;
+
+            value_type
+            operator* () const
+            {
+                return dereference (std::make_index_sequence<N> ());
+            }
+
+            bool
+            operator==(iterator const &rhs) const
+            {
+                return m_cursors.back () == rhs.m_cursors.back ();
+            }
+
+            bool
+            operator== (sentinel const &rhs) const
+            {
+                return m_cursors.back () == rhs.inner ();
+            }
+        };
+
+        class sentinel {
+        private:
+            std::ranges::sentinel_t<V> m_end;
+
+        public:
+            sentinel (std::ranges::sentinel_t<V> end_)
+                : m_end (std::move (end_))
+            { ; }
+
+            sentinel () = default;
+
+            sentinel (sentinel const&) = default;
+            sentinel (sentinel&&) = default;
+
+            sentinel& operator= (sentinel const&) = default;
+            sentinel& operator= (sentinel &&) noexcept = default;
+
+            auto const&
+            inner (void) const&
+            { return m_end; }
+        };
+
+        adjacent_view (V base)
+            : base_ (std::move (base))
+        { ; }
+
+        iterator
+        begin (void) const
+        {
+            return iterator (
+                std::ranges::begin (base_),
+                std::ranges::end   (base_)
+            );
+        }
+
+        iterator
+        begin (void)
+        {
+            return iterator (
+                std::ranges::begin (base_),
+                std::ranges::end   (base_)
+            );
+        }
+
+        sentinel
+        end (void)
+        {
+            return sentinel { std::ranges::end (base_) };
+        }
+
+        sentinel
+        end (void) const
+        {
+            return sentinel { std::ranges::end (base_) };
+        }
+    };
+
+
+    namespace detail {
+        // Subclasses `closure` so we get the pipe operator and such.
+        template <std::size_t N>
+        struct adjacent_adapter
+            : std::ranges::range_adaptor_closure<adjacent_adapter<N>>
+        {
+            template <std::ranges::forward_range V>
+            decltype (auto)
+            operator () (V &&v) const
+            {
+                return adjacent_view<V, N> (
+                    std::forward<V> (v)
+                );
+            }
+        };
+    }
+
+    template <std::size_t N>
+    constexpr detail::adjacent_adapter<N> adjacent;
+
+    constexpr detail::adjacent_adapter<2> pairwise;
+}
+
+
+template <class V, std::size_t N>
+constexpr bool std::ranges::enable_borrowed_range<cruft::ranges::adjacent_view<V, N>> =
+    std::ranges::forward_range<V> && std::ranges::enable_borrowed_range<V>;
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index b50c1d41..87029a9a 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -94,6 +94,7 @@ if (TESTS)
         rand/generator/normal
         random
         range
+        ranges/adjacent
         ranges/chunk
         rational
         region
diff --git a/test/ranges/adjacent.cpp b/test/ranges/adjacent.cpp
new file mode 100644
index 00000000..97cae2d6
--- /dev/null
+++ b/test/ranges/adjacent.cpp
@@ -0,0 +1,29 @@
+#include <cruft/util/ranges/adjacent.hpp>
+
+#include <cruft/util/tap.hpp>
+
+#include <algorithm>
+
+int main()
+{
+    static int constexpr INPUT[] = { 1, 1, 2, 3, 5, 8 };
+    static int constexpr EXPECTED[][2] = {
+        { 1, 1 },
+        { 1, 2 },
+        { 2, 3 },
+        { 3, 5 },
+        { 5, 8 },
+    };
+
+    cruft::TAP::logger tap;
+
+    const bool same = std::ranges::equal (
+        std::views::all (INPUT) | cruft::ranges::pairwise,
+        EXPECTED,
+        [] (auto const &a, auto const &b) { return std::ranges::equal (a, b); }
+    );
+
+    tap.expect (same, "ranges::pairwise fibonacci");
+
+    return tap.status ();
+}
\ No newline at end of file