Add more methods to Span class

This introduces a rudimentary begin(), end(), operator[], and subspan to Span.

Author: sipa

src/span.h

@@ -7,6 +7,7 @@
 
 #include <type_traits>
 #include <cstddef>
+#include <algorithm>
 
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
@@ -21,9 +22,25 @@ class Span
 public:
     constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
     constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
+    constexpr Span(C* data, C* end) noexcept : m_data(data), m_size(end - data) {}
 
     constexpr C* data() const noexcept { return m_data; }
+    constexpr C* begin() const noexcept { return m_data; }
+    constexpr C* end() const noexcept { return m_data + m_size; }
     constexpr std::ptrdiff_t size() const noexcept { return m_size; }
+    constexpr C& operator[](std::ptrdiff_t pos) const noexcept { return m_data[pos]; }
+
+    constexpr Span<C> subspan(std::ptrdiff_t offset) const noexcept { return Span<C>(m_data + offset, m_size - offset); }
+    constexpr Span<C> subspan(std::ptrdiff_t offset, std::ptrdiff_t count) const noexcept { return Span<C>(m_data + offset, count); }
+    constexpr Span<C> first(std::ptrdiff_t count) const noexcept { return Span<C>(m_data, count); }
+    constexpr Span<C> last(std::ptrdiff_t count) const noexcept { return Span<C>(m_data + m_size - count, count); }
+
+    friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
+    friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
+    friend constexpr bool operator<(const Span& a, const Span& b) noexcept { return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end()); }
+    friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
+    friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
+    friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
 };
 
 /** Create a span to a container exposing data() and size().
@@ -34,6 +51,9 @@ class Span
  *
  * std::span will have a constructor that implements this functionality directly.
  */
+template<typename A, int N>
+constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
+
 template<typename V>
 constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }