From b59d0a5cd5219375cf77850a449639f5a3e35a9f Mon Sep 17 00:00:00 2001
From: Vinnie Falco <vinnie.falco@gmail.com>
Date: Sun, 26 Nov 2017 09:55:31 -0800
Subject: [PATCH] Use variant in buffers_cat_view

---
 CHANGELOG.md                                  |   1 +
 include/boost/beast/core/detail/variant.hpp   | 141 +++++--
 include/boost/beast/core/impl/buffers_cat.ipp | 383 ++++++------------
 3 files changed, 237 insertions(+), 288 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a12b0719..c59fd08a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ Version 146:
 * Documentation tidying
 * Fix typo in examples documentation
 * Add detail::aligned_union and tidy up
+* Use variant in buffers_cat_view
 
 API Changes:
 
diff --git a/include/boost/beast/core/detail/variant.hpp b/include/boost/beast/core/detail/variant.hpp
index 064ebf1c..536cb759 100644
--- a/include/boost/beast/core/detail/variant.hpp
+++ b/include/boost/beast/core/detail/variant.hpp
@@ -36,7 +36,7 @@ class variant
 
     template<std::size_t I>
     using type = typename std::tuple_element<
-        I , std::tuple<TN...>>::type;
+        I, std::tuple<TN...>>::type;
 
     template<std::size_t I>
     using C = std::integral_constant<std::size_t, I>;
@@ -46,8 +46,15 @@ public:
 
     ~variant()
     {
-        if(i_)
-            destroy(C<0>{});
+        destroy(C<0>{});
+    }
+
+    bool
+    operator==(variant const& other) const
+    {
+        if(i_ != other.i_)
+            return false;
+        return equal(other, C<0>{});
     }
 
     // 0 = empty
@@ -61,6 +68,7 @@ public:
     variant(variant&& other)
     {
         i_ = other.move(&buf_, C<0>{});
+        other.i_ = 0;
     }
 
     variant(variant const& other)
@@ -71,17 +79,22 @@ public:
     // moved-from object becomes empty
     variant& operator=(variant&& other)
     {
-        if(i_ != 0)
+        if(this != &other)
+        {
             destroy(C<0>{});
-        i_ = other.move(&buf_, C<0>{});
+            i_ = other.move(&buf_, C<0>{});
+            other.i_ = 0;
+        }
         return *this;
     }
 
     variant& operator=(variant const& other)
     {
-        if(i_ != 0)
+        if(this != &other)
+        {
             destroy(C<0>{});
-        i_ = other.copy(&buf_, C<0>{});
+            i_ = other.copy(&buf_, C<0>{});
+        }
         return *this;
     }
 
@@ -89,8 +102,7 @@ public:
     void
     emplace(Args&&... args)
     {
-        if(i_ != 0)
-            destroy(C<0>{});
+        destroy(C<0>{});
         new(&buf_) type<I-1>(
             std::forward<Args>(args)...);
         i_ = I;
@@ -117,72 +129,135 @@ public:
     void
     reset()
     {
-        if(i_ == 0)
-            return;
         destroy(C<0>{});
     }
 
 private:
     void
-    destroy(C<sizeof...(TN)>)
+    destroy(C<0>)
     {
-        return;
+        auto const I = 0;
+        if(i_ == I)
+            return;
+        destroy(C<I+1>{});
+        i_ = 0;
     }
 
     template<std::size_t I>
     void
     destroy(C<I>)
     {
-        if(i_ == I+1)
+        if(i_ == I)
         {
-            using T = type<I>;
-            get<I+1>().~T();
-            i_ = 0;
+            using T = type<I-1>;
+            get<I>().~T();
             return;
         }
         destroy(C<I+1>{});
     }
 
-    unsigned char
-    move(void*, C<sizeof...(TN)>)
+    void
+    destroy(C<sizeof...(TN)>)
     {
-        return 0;
+        auto const I = sizeof...(TN);
+        BOOST_ASSERT(i_ == I);
+        using T = type<I-1>;
+        get<I>().~T();
+    }
+
+    unsigned char
+    move(void* dest, C<0>)
+    {
+        auto const I = 0;
+        if(i_ == I)
+            return I;
+        return move(dest, C<I+1>{});
     }
 
     template<std::size_t I>
     unsigned char
     move(void* dest, C<I>)
     {
-        if(i_ == I+1)
+        if(i_ == I)
         {
-            using T = type<I>;
-            new(dest) T{std::move(get<I+1>())};
-            get<I+1>().~T();
-            i_ = 0;
-            return I+1;
+            using T = type<I-1>;
+            new(dest) T(std::move(get<I>()));
+            get<I>().~T();
+            return I;
         }
         return move(dest, C<I+1>{});
     }
 
     unsigned char
-    copy(void*, C<sizeof...(TN)>) const
+    move(void* dest, C<sizeof...(TN)>)
     {
-        return 0;
+        auto const I = sizeof...(TN);
+        BOOST_ASSERT(i_ == I);
+        using T = type<I-1>;
+        new(dest) T(std::move(get<I>()));
+        get<I>().~T();
+        return I;
+    }
+
+    unsigned char
+    copy(void* dest, C<0>) const
+    {
+        auto const I = 0;
+        if(i_ == I)
+            return I;
+        return copy(dest, C<I+1>{});
     }
 
     template<std::size_t I>
     unsigned char
     copy(void* dest, C<I>) const
     {
-        if(i_ == I+1)
+        if(i_ == I)
         {
-            using T = type<I>;
-            auto const& t = get<I+1>();
-            new(dest) T{t};
-            return I+1;
+            using T = type<I-1>;
+            auto const& t = get<I>();
+            new(dest) T(t);
+            return I;
         }
         return copy(dest, C<I+1>{});
     }
+
+    unsigned char
+    copy(void* dest, C<sizeof...(TN)>) const
+    {
+        auto const I = sizeof...(TN);
+        BOOST_ASSERT(i_ == I);
+        using T = type<I-1>;
+        auto const& t = get<I>();
+        new(dest) T(t);
+        return I;
+    }
+
+    bool
+    equal(variant const& other, C<0>) const
+    {
+        auto constexpr I = 0;
+        if(i_ == I)
+            return true;
+        return equal(other, C<I+1>{});
+    }
+
+    template<std::size_t I>
+    bool
+    equal(variant const& other, C<I>) const
+    {
+        if(i_ == I)
+            return get<I>() == other.get<I>();
+        return equal(other, C<I+1>{});
+    }
+
+    bool
+    equal(variant const& other, C<sizeof...(TN)>) const
+    {
+        auto constexpr I = sizeof...(TN);
+        BOOST_ASSERT(i_ == I);
+        return get<I>() == other.get<I>();
+    }
 };
 
 } // detail
diff --git a/include/boost/beast/core/impl/buffers_cat.ipp b/include/boost/beast/core/impl/buffers_cat.ipp
index d92d5523..2e82e887 100644
--- a/include/boost/beast/core/impl/buffers_cat.ipp
+++ b/include/boost/beast/core/impl/buffers_cat.ipp
@@ -11,9 +11,9 @@
 #define BOOST_BEAST_IMPL_BUFFERS_CAT_IPP
 
 #include <boost/beast/core/detail/type_traits.hpp>
+#include <boost/beast/core/detail/variant.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/throw_exception.hpp>
-#include <array>
 #include <cstdint>
 #include <iterator>
 #include <new>
@@ -27,39 +27,29 @@ namespace beast {
 template<class... Bn>
 class buffers_cat_view<Bn...>::const_iterator
 {
-    std::size_t n_;
-    std::tuple<Bn...> const* bn_;
-    std::array<char, detail::max_sizeof<
-        typename detail::buffer_sequence_iterator<Bn>::type...>()> buf_;
+    // VFALCO The logic to skip empty sequences fails
+    //        if there is just one buffer in the list.
+    static_assert(sizeof...(Bn) >= 2,
+        "A minimum of two sequences are required");
+
+    struct past_end
+    {
+        operator bool() const noexcept
+        {
+            return true;
+        }
+    };
+
+    std::tuple<Bn...> const* bn_ = nullptr;
+    detail::variant<typename
+        detail::buffer_sequence_iterator<Bn>::type...,
+            past_end> it_;
 
     friend class buffers_cat_view<Bn...>;
 
     template<std::size_t I>
     using C = std::integral_constant<std::size_t, I>;
 
-    template<std::size_t I>
-    using iter_t = typename detail::buffer_sequence_iterator<
-        typename std::tuple_element<I, std::tuple<Bn...>>::type>::type;
-
-    template<std::size_t I>
-    iter_t<I>&
-    iter()
-    {
-        // type-pun
-        return *reinterpret_cast<
-            iter_t<I>*>(static_cast<void*>(buf_.data()));
-    }
-
-    template<std::size_t I>
-    iter_t<I> const&
-    iter() const
-    {
-        // type-pun
-        return *reinterpret_cast<
-            iter_t<I> const*>(static_cast<
-                void const*>(buf_.data()));
-    }
-
 public:
     using value_type = typename
         detail::common_buffers_type<Bn...>::type;
@@ -69,12 +59,11 @@ public:
     using iterator_category =
         std::bidirectional_iterator_tag;
 
-    ~const_iterator();
-    const_iterator();
-    const_iterator(const_iterator&& other);
-    const_iterator(const_iterator const& other);
-    const_iterator& operator=(const_iterator&& other);
-    const_iterator& operator=(const_iterator const& other);
+    const_iterator() = default;
+    const_iterator(const_iterator&& other) = default;
+    const_iterator(const_iterator const& other) = default;
+    const_iterator& operator=(const_iterator&& other) = default;
+    const_iterator& operator=(const_iterator const& other) = default;
 
     bool
     operator==(const_iterator const& other) const;
@@ -97,9 +86,11 @@ public:
     const_iterator
     operator++(int);
 
+    // deprecated
     const_iterator&
     operator--();
 
+    // deprecated
     const_iterator
     operator--(int);
 
@@ -107,13 +98,6 @@ private:
     const_iterator(
         std::tuple<Bn...> const& bn, bool at_end);
 
-    void
-    construct(C<sizeof...(Bn)> const&)
-    {
-        auto constexpr I = sizeof...(Bn);
-        n_ = I;
-    }
-
     template<std::size_t I>
     void
     construct(C<I> const&)
@@ -121,122 +105,85 @@ private:
         if(boost::asio::buffer_size(
             std::get<I>(*bn_)) != 0)
         {
-            n_ = I;
-            new(&buf_[0]) iter_t<I>{
+            it_.template emplace<I+1>(
                 boost::asio::buffer_sequence_begin(
-                std::get<I>(*bn_))};
+                    std::get<I>(*bn_)));
             return;
         }
         construct(C<I+1>{});
     }
 
     void
-    rconstruct(C<0> const&)
+    construct(C<sizeof...(Bn)-1> const&)
+    {
+        auto constexpr I = sizeof...(Bn)-1;
+        it_.template emplace<I+1>(
+            boost::asio::buffer_sequence_begin(
+                std::get<I>(*bn_)));
+    }
+
+    void
+    construct(C<sizeof...(Bn)> const&)
+    {
+        // end
+        auto constexpr I = sizeof...(Bn);
+        it_.template emplace<I+1>();
+    }
+
+    template<std::size_t I>
+    void
+    next(C<I> const&)
+    {
+        if(boost::asio::buffer_size(
+            std::get<I>(*bn_)) != 0)
+        {
+            it_.template emplace<I+1>(
+                boost::asio::buffer_sequence_begin(
+                    std::get<I>(*bn_)));
+            return;
+        }
+        next(C<I+1>{});
+    }
+
+    void
+    next(C<sizeof...(Bn)> const&)
+    {
+        // end
+        auto constexpr I = sizeof...(Bn);
+        it_.template emplace<I+1>();
+    }
+
+    template<std::size_t I>
+    void
+    prev(C<I> const&)
+    {
+        if(boost::asio::buffer_size(
+            std::get<I>(*bn_)) != 0)
+        {
+            it_.template emplace<I+1>(
+                boost::asio::buffer_sequence_end(
+                    std::get<I>(*bn_)));
+            return;
+        }
+        prev(C<I-1>{});
+    }
+
+    void
+    prev(C<0> const&)
     {
         auto constexpr I = 0;
-        if(boost::asio::buffer_size(
-            std::get<I>(*bn_)) != 0)
-        {
-            n_ = I;
-            new(&buf_[0]) iter_t<I>{
-                boost::asio::buffer_sequence_end(
-                std::get<I>(*bn_))};
-            return;
-        }
-        BOOST_THROW_EXCEPTION(std::logic_error{
-            "invalid iterator"});
+        it_.template emplace<I+1>(
+            boost::asio::buffer_sequence_end(
+                std::get<I>(*bn_)));
     }
 
     template<std::size_t I>
-    void
-    rconstruct(C<I> const&)
+    reference
+    dereference(C<I> const&) const
     {
-        if(boost::asio::buffer_size(
-            std::get<I>(*bn_)) != 0)
-        {
-            n_ = I;
-            new(&buf_[0]) iter_t<I>{
-                boost::asio::buffer_sequence_end(
-                    std::get<I>(*bn_))};
-            return;
-        }
-        rconstruct(C<I-1>{});
-    }
-
-    void
-    destroy(C<sizeof...(Bn)> const&)
-    {
-        return;
-    }
-
-    template<std::size_t I>
-    void
-    destroy(C<I> const&)
-    {
-        if(n_ == I)
-        {
-            using Iter = iter_t<I>;
-            iter<I>().~Iter();
-            return;
-        }
-        destroy(C<I+1>{});
-    }
-
-    void
-    move(const_iterator&&,
-        C<sizeof...(Bn)> const&)
-    {
-    }
-
-    template<std::size_t I>
-    void
-    move(const_iterator&& other,
-        C<I> const&)
-    {
-        if(n_ == I)
-        {
-            new(&buf_[0]) iter_t<I>{
-                std::move(other.iter<I>())};
-            return;
-        }
-        move(std::move(other), C<I+1>{});
-    }
-
-    void
-    copy(const_iterator const&,
-        C<sizeof...(Bn)> const&)
-    {
-    }
-
-    template<std::size_t I>
-    void
-    copy(const_iterator const& other,
-        C<I> const&)
-    {
-        if(n_ == I)
-        {
-            new(&buf_[0]) iter_t<I>{
-                other.iter<I>()};
-            return;
-        }
-        copy(other, C<I+1>{});
-    }
-
-    bool
-    equal(const_iterator const&,
-        C<sizeof...(Bn)> const&) const
-    {
-        return true;
-    }
-
-    template<std::size_t I>
-    bool
-    equal(const_iterator const& other,
-        C<I> const&) const
-    {
-        if(n_ == I)
-            return iter<I>() == other.iter<I>();
-        return equal(other, C<I+1>{});
+        if(it_.index() == I+1)
+            return *it_.template get<I+1>();
+        return dereference(C<I+1>{});
     }
 
     [[noreturn]]
@@ -248,12 +195,18 @@ private:
     }
 
     template<std::size_t I>
-    reference
-    dereference(C<I> const&) const
+    void
+    increment(C<I> const&)
     {
-        if(n_ == I)
-            return *iter<I>();
-        return dereference(C<I+1>{});
+        if(it_.index() == I+1)
+        {
+            if(++it_.template get<I+1>() !=
+                boost::asio::buffer_sequence_end(
+                    std::get<I>(*bn_)))
+                return;
+            return next(C<I+1>{});
+        }
+        increment(C<I+1>{});
     }
 
     [[noreturn]]
@@ -264,29 +217,12 @@ private:
             "invalid iterator"});
     }
 
-    template<std::size_t I>
-    void
-    increment(C<I> const&)
-    {
-        if(n_ == I)
-        {
-            if(++iter<I>() !=
-                boost::asio::buffer_sequence_end(
-                    std::get<I>(*bn_)))
-                return;
-            using Iter = iter_t<I>;
-            iter<I>().~Iter();
-            return construct(C<I+1>{});
-        }
-        increment(C<I+1>{});
-    }
-
     void
     decrement(C<sizeof...(Bn)> const&)
     {
         auto constexpr I = sizeof...(Bn);
-        if(n_ == I)
-            rconstruct(C<I-1>{});
+        if(it_.index() == I+1)
+            prev(C<I-1>{});
         decrement(C<I-1>{});
     }
 
@@ -294,19 +230,16 @@ private:
     void
     decrement(C<I> const&)
     {
-        if(n_ == I)
+        if(it_.index() == I+1)
         {
-            if(iter<I>() !=
+            if(it_.template get<I+1>() !=
                 boost::asio::buffer_sequence_begin(
                     std::get<I>(*bn_)))
             {
-                --iter<I>();
+                --it_.template get<I+1>();
                 return;
             }
-            --n_;
-            using Iter = iter_t<I>;
-            iter<I>().~Iter();
-            rconstruct(C<I-1>{});
+            prev(C<I-1>{});
         }
         decrement(C<I-1>{});
     }
@@ -315,11 +248,11 @@ private:
     decrement(C<0> const&)
     {
         auto constexpr I = 0;
-        if(iter<I>() !=
+        if(it_.template get<I+1>() !=
             boost::asio::buffer_sequence_begin(
                 std::get<I>(*bn_)))
         {
-            --iter<I>();
+            --it_.template get<I+1>();
             return;
         }
         BOOST_THROW_EXCEPTION(std::logic_error{
@@ -331,97 +264,33 @@ private:
 
 template<class... Bn>
 buffers_cat_view<Bn...>::
-const_iterator::~const_iterator()
-{
-    destroy(C<0>{});
-}
-
-template<class... Bn>
-buffers_cat_view<Bn...>::
-const_iterator::const_iterator()
-    : n_(sizeof...(Bn))
-    , bn_(nullptr)
-{
-}
-
-template<class... Bn>
-buffers_cat_view<Bn...>::
-const_iterator::const_iterator(
+const_iterator::
+const_iterator(
     std::tuple<Bn...> const& bn, bool at_end)
     : bn_(&bn)
 {
-    if(at_end)
-        n_ = sizeof...(Bn);
-    else
+    if(! at_end)
         construct(C<0>{});
-}
-
-template<class... Bn>
-buffers_cat_view<Bn...>::
-const_iterator::const_iterator(const_iterator&& other)
-    : n_(other.n_)
-    , bn_(other.bn_)
-{
-    move(std::move(other), C<0>{});
-}
-
-template<class... Bn>
-buffers_cat_view<Bn...>::
-const_iterator::const_iterator(const_iterator const& other)
-    : n_(other.n_)
-    , bn_(other.bn_)
-{
-    copy(other, C<0>{});
-}
-
-template<class... Bn>
-auto
-buffers_cat_view<Bn...>::
-const_iterator::operator=(const_iterator&& other) ->
-    const_iterator&
-{
-    if(&other == this)
-        return *this;
-    destroy(C<0>{});
-    n_ = other.n_;
-    bn_ = other.bn_;
-    // VFALCO What about exceptions?
-    move(std::move(other), C<0>{});
-    return *this;
-}
-
-template<class... Bn>
-auto
-buffers_cat_view<Bn...>::
-const_iterator::operator=(const_iterator const& other) ->
-const_iterator&
-{
-    if(&other == this)
-        return *this;
-    destroy(C<0>{});
-    n_ = other.n_;
-    bn_ = other.bn_;
-    // VFALCO What about exceptions?
-    copy(other, C<0>{});
-    return *this;
+    else
+        construct(C<sizeof...(Bn)>{});
 }
 
 template<class... Bn>
 bool
 buffers_cat_view<Bn...>::
-const_iterator::operator==(const_iterator const& other) const
+const_iterator::
+operator==(const_iterator const& other) const
 {
     if(bn_ != other.bn_)
         return false;
-    if(n_ != other.n_)
-        return false;
-    return equal(other, C<0>{});
+    return it_ == other.it_;
 }
 
 template<class... Bn>
 auto
 buffers_cat_view<Bn...>::
-const_iterator::operator*() const ->
+const_iterator::
+operator*() const ->
     reference
 {
     return dereference(C<0>{});
@@ -430,7 +299,8 @@ const_iterator::operator*() const ->
 template<class... Bn>
 auto
 buffers_cat_view<Bn...>::
-const_iterator::operator++() ->
+const_iterator::
+operator++() ->
     const_iterator&
 {
     increment(C<0>{});
@@ -440,7 +310,8 @@ const_iterator::operator++() ->
 template<class... Bn>
 auto
 buffers_cat_view<Bn...>::
-const_iterator::operator++(int) ->
+const_iterator::
+operator++(int) ->
     const_iterator
 {
     auto temp = *this;
@@ -451,7 +322,8 @@ const_iterator::operator++(int) ->
 template<class... Bn>
 auto
 buffers_cat_view<Bn...>::
-const_iterator::operator--() ->
+const_iterator::
+operator--() ->
     const_iterator&
 {
     decrement(C<sizeof...(Bn)>{});
@@ -461,7 +333,8 @@ const_iterator::operator--() ->
 template<class... Bn>
 auto
 buffers_cat_view<Bn...>::
-const_iterator::operator--(int) ->
+const_iterator::
+operator--(int) ->
     const_iterator
 {
     auto temp = *this;