From aad49d666a68c893df2fe01d084b2f021ad402aa Mon Sep 17 00:00:00 2001
From: Chris Robinson <chris.kcat@gmail.com>
Date: Mon, 27 May 2019 15:36:10 -0700
Subject: Implement static-sized spans, and handle overload requirements

Note that span is specialized such that a static-sized span only has a single
data member, making it a suitable replacement for  Type (&arg)[Size]  style
variables/parameters.
---
 common/alspan.h | 197 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 191 insertions(+), 6 deletions(-)

diff --git a/common/alspan.h b/common/alspan.h
index 12254812..e5a91686 100644
--- a/common/alspan.h
+++ b/common/alspan.h
@@ -43,8 +43,45 @@ constexpr const T* data(std::initializer_list<T> list) noexcept
 { return list.begin(); }
 
 
-template<typename T>
+template<typename T, size_t E=static_cast<size_t>(-1)>
+class span;
+
+namespace detail_ {
+    template<typename... Ts>
+    struct make_void { using type = void; };
+    template<typename... Ts>
+    using void_t = typename make_void<Ts...>::type;
+
+    template<typename T>
+    struct is_span : std::false_type { };
+    template<typename T, size_t E>
+    struct is_span<span<T,E>> : std::true_type { };
+
+    template<typename T>
+    struct is_std_array : std::false_type { };
+    template<typename T, size_t N>
+    struct is_std_array<std::array<T,N>> : std::true_type { };
+
+    template<typename T, typename = void>
+    struct has_size_and_data : std::false_type { };
+    template<typename T>
+    struct has_size_and_data<T,
+        void_t<decltype(al::size(std::declval<T>())), decltype(al::data(std::declval<T>()))>>
+        : std::true_type { };
+} // namespace detail_
+
+#define REQUIRES(...) typename std::enable_if<(__VA_ARGS__),int>::type = 0
+#define IS_VALID_CONTAINER(C)                                                 \
+    !detail_::is_span<typename std::remove_cv<C>::type>::value &&             \
+    !detail_::is_std_array<typename std::remove_cv<C>::type>::value &&        \
+    !std::is_array<typename std::remove_cv<C>::type>::value &&                \
+    detail_::has_size_and_data<typename std::remove_cv<C>::type>::value &&    \
+    std::is_convertible<typename std::remove_pointer<decltype(al::data(std::declval<C>()))>::type(*)[],element_type(*)[]>::value
+
+template<typename T, size_t E>
 class span {
+    static constexpr size_t dynamic_extent{static_cast<size_t>(-1)};
+
 public:
     using element_type = T;
     using value_type = typename std::remove_cv<T>::type;
@@ -61,22 +98,131 @@ public:
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
+    static constexpr size_t extent{E};
+
+    template<REQUIRES(extent == 0)>
+    constexpr span() noexcept { }
+    constexpr span(pointer ptr, index_type /*count*/) : mData{ptr} { }
+    constexpr span(pointer first, pointer /*last*/) : mData{first} { }
+    template<size_t N, REQUIRES(extent == N)>
+    constexpr span(element_type (&arr)[N]) noexcept : span{al::data(arr), al::size(arr)} { }
+    template<size_t N, REQUIRES(extent == N && std::is_convertible<value_type(*)[],element_type(*)[]>::value)>
+    constexpr span(std::array<value_type,N> &arr) noexcept : span{al::data(arr), al::size(arr)} { }
+    template<size_t N, REQUIRES(extent == N && std::is_convertible<value_type(*)[],element_type(*)[]>::value)>
+    constexpr span(const std::array<value_type,N> &arr) noexcept : span{al::data(arr), al::size(arr)} { }
+    template<typename U, REQUIRES(IS_VALID_CONTAINER(U))>
+    constexpr span(U &cont) : span{al::data(cont), al::size(cont)} { }
+    template<typename U, REQUIRES(IS_VALID_CONTAINER(U))>
+    constexpr span(const U &cont) : span{al::data(cont), al::size(cont)} { }
+    template<typename U, size_t N, REQUIRES(extent == N && std::is_convertible<U(*)[],element_type(*)[]>::value)>
+    constexpr span(const span<U,N> &span_) noexcept : span{al::data(span_), al::size(span_)} { }
+    constexpr span(const span&) noexcept = default;
+
+    span& operator=(const span &rhs) noexcept = default;
+    template<typename U, size_t N, REQUIRES(extent == N && std::is_convertible<U(*)[],element_type(*)[]>::value)>
+    span& operator=(const span<U,N> &rhs) noexcept
+    { mData = rhs.data(); return *this; }
+
+    constexpr reference front() const { return *mData; }
+    constexpr reference back() const { return *(mData+E-1); }
+    constexpr reference operator[](index_type idx) const { return mData[idx]; }
+    constexpr pointer data() const noexcept { return mData; }
+
+    constexpr index_type size() const noexcept { return E; }
+    constexpr index_type size_bytes() const noexcept { return E * sizeof(value_type); }
+    constexpr bool empty() const noexcept { return E == 0; }
+
+    constexpr iterator begin() const noexcept { return mData; }
+    constexpr iterator end() const noexcept { return mData+E; }
+    constexpr const_iterator cbegin() const noexcept { return mData; }
+    constexpr const_iterator cend() const noexcept { return mData+E; }
+
+    constexpr reverse_iterator rbegin() const noexcept { return end(); }
+    constexpr reverse_iterator rend() const noexcept { return begin(); }
+    constexpr const_reverse_iterator crbegin() const noexcept { return cend(); }
+    constexpr const_reverse_iterator crend() const noexcept { return cbegin(); }
+
+    template<size_t C>
+    constexpr span<element_type,C> first() const
+    {
+        static_assert(E >= C, "New size exceeds original capacity");
+        return span<element_type,C>{mData, C};
+    }
+
+    template<size_t C>
+    constexpr span<element_type,C> last() const
+    {
+        static_assert(E >= C, "New size exceeds original capacity");
+        return span<element_type,C>{mData+(E-C), C};
+    }
+
+    template<size_t O, size_t C, size_t RealC=((C==dynamic_extent) ? E-O : C)>
+    constexpr span<element_type,RealC> subspan() const
+    {
+        static_assert(E >= O, "Offset exceeds extent");
+        static_assert(E-O >= RealC, "New size exceeds original capacity");
+        return span<element_type,RealC>{mData+O, RealC};
+    }
+
+    /* NOTE: Can't declare objects of a specialized template class prior to
+     * defining the specialization. As a result, these methods need to be
+     * defined later.
+     */
+    constexpr span<element_type,dynamic_extent> first(size_t count) const;
+    constexpr span<element_type,dynamic_extent> last(size_t count) const;
+    constexpr span<element_type,dynamic_extent> subspan(size_t offset, size_t count=dynamic_extent) const;
+
+private:
+    pointer mData{nullptr};
+};
+
+template<typename T>
+class span<T,static_cast<size_t>(-1)> {
+    static constexpr size_t dynamic_extent{static_cast<size_t>(-1)};
+
+public:
+    using element_type = T;
+    using value_type = typename std::remove_cv<T>::type;
+    using index_type = size_t;
+    using difference_type = ptrdiff_t;
+
+    using pointer = T*;
+    using const_pointer = const T*;
+    using reference = T&;
+    using const_reference = const T&;
+
+    using iterator = pointer;
+    using const_iterator = const_pointer;
+    using reverse_iterator = std::reverse_iterator<iterator>;
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
+    static constexpr size_t extent{static_cast<size_t>(-1)};
+
     constexpr span() noexcept = default;
     constexpr span(pointer ptr, index_type count) : mData{ptr}, mDataEnd{ptr+count} { }
     constexpr span(pointer first, pointer last) : mData{first}, mDataEnd{last} { }
     template<size_t N>
     constexpr span(element_type (&arr)[N]) noexcept : span{al::data(arr), al::size(arr)} { }
-    template<size_t N>
+    template<size_t N, REQUIRES(std::is_convertible<value_type(*)[],element_type(*)[]>::value)>
     constexpr span(std::array<value_type,N> &arr) noexcept : span{al::data(arr), al::size(arr)} { }
-    template<size_t N>
+    template<size_t N, REQUIRES(std::is_convertible<value_type(*)[],element_type(*)[]>::value)>
     constexpr span(const std::array<value_type,N> &arr) noexcept : span{al::data(arr), al::size(arr)} { }
-    template<typename U>
+    template<typename U, REQUIRES(IS_VALID_CONTAINER(U))>
     constexpr span(U &cont) : span{al::data(cont), al::size(cont)} { }
-    template<typename U>
+    template<typename U, REQUIRES(IS_VALID_CONTAINER(U))>
     constexpr span(const U &cont) : span{al::data(cont), al::size(cont)} { }
+    template<typename U, size_t N, REQUIRES(std::is_convertible<U(*)[],element_type(*)[]>::value)>
+    constexpr span(const span<U,N> &span_) noexcept : span{al::data(span_), al::size(span_)} { }
     constexpr span(const span&) noexcept = default;
 
     span& operator=(const span &rhs) noexcept = default;
+    template<typename U, size_t N, REQUIRES(std::is_convertible<U(*)[],element_type(*)[]>::value)>
+    span& operator=(const span<U,N> &rhs) noexcept
+    {
+        mData = rhs.data();
+        mDataEnd = mData + rhs.size();
+        return *this;
+    }
 
     constexpr reference front() const { return *mData; }
     constexpr reference back() const { return *(mDataEnd-1); }
@@ -97,11 +243,25 @@ public:
     constexpr const_reverse_iterator crbegin() const noexcept { return cend(); }
     constexpr const_reverse_iterator crend() const noexcept { return cbegin(); }
 
+    template<size_t C>
+    constexpr span<element_type,C> first() const
+    { return span<element_type,C>{mData, C}; }
+
     constexpr span first(size_t count) const
     { return (count >= size()) ? *this : span{mData, mData+count}; }
+
+    template<size_t C>
+    constexpr span<element_type,C> last() const
+    { return span<element_type,C>{mDataEnd-C, C}; }
+
     constexpr span last(size_t count) const
     { return (count >= size()) ? *this : span{mDataEnd-count, mDataEnd}; }
-    constexpr span subspan(size_t offset, size_t count=static_cast<size_t>(-1)) const
+
+    template<size_t O, size_t C>
+    constexpr span<element_type,C> subspan() const
+    { return span<element_type,C>{mData+O, C}; }
+
+    constexpr span subspan(size_t offset, size_t count=dynamic_extent) const
     {
         return (offset >= size()) ? span{} :
             (count >= size()-offset) ? span{mData+offset, mDataEnd} :
@@ -113,6 +273,31 @@ private:
     pointer mDataEnd{nullptr};
 };
 
+template<typename T, size_t E>
+constexpr inline auto span<T,E>::first(size_t count) const -> span<element_type,dynamic_extent>
+{
+    return (count >= size()) ? span<element_type>{mData, extent} :
+        span<element_type>{mData, count};
+}
+
+template<typename T, size_t E>
+constexpr inline auto span<T,E>::last(size_t count) const -> span<element_type,dynamic_extent>
+{
+    return (count >= size()) ? span<element_type>{mData, extent} :
+        span<element_type>{mData+extent-count, count};
+}
+
+template<typename T, size_t E>
+constexpr inline auto span<T,E>::subspan(size_t offset, size_t count) const -> span<element_type,dynamic_extent>
+{
+    return (offset >= size()) ? span<element_type>{} :
+        (count >= size()-offset) ? span<element_type>{mData+offset, mData+extent} :
+        span<element_type>{mData+offset, mData+offset+count};
+}
+
+#undef IS_VALID_CONTAINER
+#undef REQUIRES
+
 } // namespace al
 
 #endif /* AL_SPAN_H */
-- 
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