From 0a7cd59eb9a87cb1009ea9a2a4828d461f912538 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Fri, 19 Jul 2024 16:41:10 +1000
Subject: [PATCH] ranges: add a (simplistic) implementation of chunk

Taken from the illustrative text at https://en.cppreference.com/w/cpp/ranges/chunk_view
---
 conanfile.txt               |   1 -
 cruft/util/CMakeLists.txt   |   1 +
 cruft/util/ranges/chunk.hpp | 265 ++++++++++++++++++++++++++++++++++++
 test/CMakeLists.txt         |   1 +
 test/ranges/chunk.cpp       |  42 ++++++
 5 files changed, 309 insertions(+), 1 deletion(-)
 create mode 100644 cruft/util/ranges/chunk.hpp
 create mode 100644 test/ranges/chunk.cpp

diff --git a/conanfile.txt b/conanfile.txt
index 7fe23b48..8877d6e5 100644
--- a/conanfile.txt
+++ b/conanfile.txt
@@ -1,3 +1,2 @@
 [requires]
 fmt/10.1.1
-range-v3/0.11.0
diff --git a/cruft/util/CMakeLists.txt b/cruft/util/CMakeLists.txt
index 449f59ff..94c8d0a7 100644
--- a/cruft/util/CMakeLists.txt
+++ b/cruft/util/CMakeLists.txt
@@ -544,6 +544,7 @@ list (
     random.hpp
     range.cpp
     range.hpp
+    ranges/chunk.hpp
     rational.cpp
     rational.hpp
     region.cpp
diff --git a/cruft/util/ranges/chunk.hpp b/cruft/util/ranges/chunk.hpp
new file mode 100644
index 00000000..0dbc90f9
--- /dev/null
+++ b/cruft/util/ranges/chunk.hpp
@@ -0,0 +1,265 @@
+#pragma once
+
+#include <ranges>
+#include <type_traits>
+
+#include <cruft/util/platform.hpp>
+#include <cruft/util/debug/assert.hpp>
+
+
+namespace cruft::ranges {
+    // Largely based on the exposition elements of the specification at
+    // https://en.cppreference.com/w/cpp/ranges/chunk_view
+    //
+    // A lot of the implementation is simplified by assuming we're only working with forward ranges.
+    // But we should remove this code when clang-19 gets released.
+    //
+    // clang#18: remove me when clang gets std::views::chunk
+    template <std::ranges::view V>
+    requires std::ranges::forward_range<V>
+    class chunk_view : public std::ranges::view_interface<chunk_view<V>> {
+    private:
+        V base_;
+        std::ranges::range_difference_t<V> n_;
+
+    public:
+        template <bool Const>
+        class iterator {
+            friend chunk_view;
+
+        private:
+            using Parent = std::conditional_t<Const, const chunk_view, chunk_view>;
+            using Base = std::conditional_t<Const, const V, V>;
+
+            std::ranges::iterator_t<Base> current_;
+            std::ranges::iterator_t<Base> end_;
+            std::ranges::range_difference_t<Base> n_;
+            std::ranges::range_difference_t<Base> missing_;
+
+        public:
+            using iterator_category = std::input_iterator_tag;
+            // HACK: assuming we're modelling only forward iterator
+            using iterator_concept = std::forward_iterator_tag;
+
+            using value_type = decltype(
+                std::views::take(
+                    std::ranges::subrange(current_, end_),
+                    n_
+                )
+            );
+            using difference_type = std::ranges::range_difference_t<Base>;
+
+            iterator() = default;
+
+            constexpr
+            iterator (iterator<!Const> i)
+            requires
+                    Const and
+                    std::convertible_to<std::ranges::iterator_t<V>, std::ranges::iterator_t<Base>> and
+                    std::convertible_to<std::ranges::sentinel_t<V>, std::ranges::sentinel_t<Base>>
+                : current_ (std::move (i.current_))
+                , end_ (std::move (i.end_))
+                , n_ (i.n_)
+                , missing_ (i.missing_)
+            { ; }
+        private:
+            constexpr
+            iterator (
+                    Parent* parent,
+                    std::ranges::iterator_t<Base> current,
+                    std::ranges::range_difference_t<Base> missing = 0
+            )   : current_ (current)
+                , end_ (std::ranges::end (parent->base_))
+                , n_ (parent->n_)
+                , missing_ (missing)
+            {}
+
+        public:
+            constexpr value_type
+            operator*() const
+            {
+                CHECK_NEQ (current_, end_);
+                return std::views::take (
+                    std::ranges::subrange (current_, end_), n_
+                );
+            }
+
+            constexpr iterator&
+            operator++()
+            {
+                missing_ = std::ranges::advance(current_, n_, end_);
+                return *this;
+            }
+
+            constexpr iterator
+            operator++(int)
+            {
+                auto tmp = *this;
+                ++*this;
+                return tmp;
+            }
+
+            constexpr iterator&
+            operator--()
+                requires std::ranges::bidirectional_range<Base>
+            {
+                std::ranges::advance(current_, missing_ - n_);
+                missing_ = 0;
+                return *this;
+            }
+
+            constexpr iterator
+            operator+= (difference_type x)
+                requires std::ranges::random_access_range<Base>
+            {
+                if (x > 0) {
+                    CHECK (std::ranges::distance(current_, end_) > n_ * (x - 1));
+                    std::ranges::advance(current_, n_ * (x - 1));
+                    missing_ = std::ranges::advance(current_, n_, end_);
+                }
+                else if (x < 0) {
+                    std::ranges::advance(current_, n_ * x + missing_);
+                    missing_ = 0;
+                }
+
+                return *this;
+            }
+
+            constexpr iterator&
+            operator-= (difference_type x)
+                requires std::ranges::random_access_range<Base>
+            {
+                return *this += -x;
+            }
+
+            friend constexpr
+            bool
+            operator== (iterator const& x, iterator const &y)
+            {
+                return x.current_ == y.current_;
+            }
+
+            friend constexpr
+            bool
+            operator== (iterator const& x, std::default_sentinel_t)
+            {
+                return x.current_ == x.end_;
+            }
+
+            friend constexpr
+            auto
+            operator<=> (iterator const& x, iterator const& y)
+                requires
+                    std::ranges::random_access_range<Base> &&
+                    std::three_way_comparable<std::ranges::iterator_t<Base>>
+            {
+                return x.current_ <=> y.current_;
+            }
+        };
+
+    public:
+        constexpr explicit
+        chunk_view (V base, std::ranges::range_difference_t<V> n)
+            : base_ (std::move (base))
+            , n_ (n)
+        { ; }
+
+        constexpr V
+        base () const&
+            requires std::copy_constructible<V>
+        { return base_; }
+
+        constexpr V base() &&
+        { return std::move(base_); }
+
+        constexpr auto
+        begin ()
+        {
+            return iterator<false>(this, std::ranges::begin(base_));
+        }
+
+        constexpr auto
+        begin () const
+            requires std::ranges::forward_range<const V>
+        {
+            return iterator<true>(
+                this,
+                std::ranges::begin(base_)
+            );
+        }
+
+        constexpr auto
+        end ()
+        {
+            if constexpr (std::ranges::common_range<V> and std::ranges::sized_range<V>)
+            {
+                auto missing = (n_ - std::ranges::distance (base_) % n_) % n_;
+                return iterator<false>(this, std::ranges::end (base_), missing);
+            }
+            else if constexpr (std::ranges::common_range<V> and !std::ranges::bidirectional_range<V>)
+                return iterator<false>(this, std::ranges::end(base_));
+            else
+                return std::default_sentinel;
+        }
+
+        constexpr auto
+        end () const requires std::ranges::forward_range<const V>
+        {
+            if constexpr (std::ranges::common_range<const V> and std::ranges::sized_range<const V>) {
+                auto missing = (n_ - std::ranges::distance (base_) % n_) % n_;
+                return iterator<true>(this, std::ranges::end (base_), missing);
+            }
+            else if constexpr (std::ranges::common_range<const V> and !std::ranges::bidirectional_range<const V>)
+                return iterator<true>(this, std::ranges::end(base_));
+            else
+                return std::default_sentinel;
+        }
+    };
+
+    template <class R>
+    chunk_view (R&&, std::ranges::range_difference_t<R>) -> chunk_view<std::views::all_t<R>>;
+
+    // HACK: clang#18 does not expose range_adaptor_closure, so we dig into the internals.
+#if defined(COMPILER_GCC)
+#define _CRUFT_RANGE_ADAPTOR_BASE std::ranges::range_adaptor_closure
+#elif defined(COMPILER_CLANG)
+#define _CRUFT_RANGE_ADAPTOR_BASE std::__range_adaptor_closure
+#endif
+
+
+    namespace detail {
+        struct chunk_closure_t : _CRUFT_RANGE_ADAPTOR_BASE<chunk_closure_t> {
+            int n;
+
+            template <typename R>
+            constexpr auto
+            operator() (R &&t) const
+            {
+                return chunk_view (std::forward<R> (t), n);
+            }
+        };
+
+
+        struct chunk_base {
+            template <std::ranges::viewable_range R>
+            constexpr auto
+            operator() ( R&& r, std::ranges::range_difference_t<R> n ) const
+            {
+                return chunk_view (std::views::all (std::forward<R> (r)), n);
+            }
+
+
+            constexpr auto
+            operator() (int n) const
+            {
+                return chunk_closure_t {.n = n};
+            }
+        };
+    }
+
+    constexpr detail::chunk_base chunk;
+}
+
+template< class V >
+constexpr bool std::ranges::enable_borrowed_range<cruft::ranges::chunk_view<V>> =
+    std::ranges::forward_range<V> && std::ranges::enable_borrowed_range<V>;
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index a6681b7f..b50c1d41 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -94,6 +94,7 @@ if (TESTS)
         rand/generator/normal
         random
         range
+        ranges/chunk
         rational
         region
         registrar
diff --git a/test/ranges/chunk.cpp b/test/ranges/chunk.cpp
new file mode 100644
index 00000000..88d5978c
--- /dev/null
+++ b/test/ranges/chunk.cpp
@@ -0,0 +1,42 @@
+#include <cruft/util/tap.hpp>
+
+#include <cruft/util/ranges/chunk.hpp>
+#include <cruft/util/view.hpp>
+
+
+int main ()
+{
+    cruft::TAP::logger tap;
+
+    static constexpr int src[] = {
+        1, 2, 3,
+        4, 5, 6,
+        7, 8,
+    };
+
+    static constexpr int expected[] = {
+        1, 2, 3, -1,
+        4, 5, 6, -2,
+        7, 8,    -3,
+    };
+
+    std::vector<int> dst;
+    dst.reserve (std::size (expected));
+
+    // Copy out the src array in groups of 3, inserting a decrementing 'line number' value.
+    // It's a little easier that manually juggling iterators and such.
+
+    int line = 0;
+    for (auto const chunk: src | cruft::ranges::chunk (3)) {
+        // This could be made more efficient by directly insert the chunk in one go,
+        // but it's better to exercise the single iteration case for increased code coverage.
+        for (auto const &i: chunk)
+            dst.push_back (i);
+
+        dst.push_back (--line);
+    }
+
+    tap.expect (cruft::equal (cruft::view (dst), cruft::view (expected)), "chunk(3)");
+
+    return tap.status ();
+}
\ No newline at end of file