lottie/vector: add a local arena allocator.

Change-Id: I7f76ab73d457ee9f4e9b5e35c1303c63c14efcea

src/vector/vstackallocator.h

@@ -0,0 +1,134 @@
+#ifndef VSTACK_ALLOCATOR_H
+#define VSTACK_ALLOCATOR_H
+
+#include <cstddef>
+#include <cassert>
+
+template <std::size_t N, std::size_t alignment = alignof(std::max_align_t)>
+class arena
+{
+    alignas(alignment) char buf_[N];
+    char* ptr_;
+
+public:
+    ~arena() {ptr_ = nullptr;}
+    arena() noexcept : ptr_(buf_) {}
+    arena(const arena&) = delete;
+    arena& operator=(const arena&) = delete;
+
+    template <std::size_t ReqAlign> char* allocate(std::size_t n);
+    void deallocate(char* p, std::size_t n) noexcept;
+
+    static constexpr std::size_t size() noexcept {return N;}
+    std::size_t used() const noexcept {return static_cast<std::size_t>(ptr_ - buf_);}
+    void reset() noexcept {ptr_ = buf_;}
+
+private:
+    static
+    std::size_t
+    align_up(std::size_t n) noexcept
+        {return (n + (alignment-1)) & ~(alignment-1);}
+
+    bool
+    pointer_in_buffer(char* p) noexcept
+        {return buf_ <= p && p <= buf_ + N;}
+};
+
+template <std::size_t N, std::size_t alignment>
+template <std::size_t ReqAlign>
+char*
+arena<N, alignment>::allocate(std::size_t n)
+{
+    static_assert(ReqAlign <= alignment, "alignment is too small for this arena");
+    assert(pointer_in_buffer(ptr_) && "stack_alloc has outlived arena");
+    auto const aligned_n = align_up(n);
+    if (static_cast<decltype(aligned_n)>(buf_ + N - ptr_) >= aligned_n)
+    {
+        char* r = ptr_;
+        ptr_ += aligned_n;
+        return r;
+    }
+
+    static_assert(alignment <= alignof(std::max_align_t), "you've chosen an "
+                  "alignment that is larger than alignof(std::max_align_t), and "
+                  "cannot be guaranteed by normal operator new");
+    return static_cast<char*>(::operator new(n));
+}
+
+template <std::size_t N, std::size_t alignment>
+void
+arena<N, alignment>::deallocate(char* p, std::size_t n) noexcept
+{
+    assert(pointer_in_buffer(ptr_) && "stack_alloc has outlived arena");
+    if (pointer_in_buffer(p))
+    {
+        n = align_up(n);
+        if (p + n == ptr_)
+            ptr_ = p;
+    }
+    else
+        ::operator delete(p);
+}
+
+template <class T, std::size_t N, std::size_t Align = alignof(std::max_align_t)>
+class stack_alloc
+{
+public:
+    using value_type = T;
+    static auto constexpr alignment = Align;
+    static auto constexpr size = N;
+    using arena_type = arena<size, alignment>;
+
+private:
+    arena_type& a_;
+
+public:
+    stack_alloc(const stack_alloc&) = default;
+    stack_alloc& operator=(const stack_alloc&) = delete;
+
+    stack_alloc(arena_type& a) noexcept : a_(a)
+    {
+        static_assert(size % alignment == 0,
+                      "size N needs to be a multiple of alignment Align");
+    }
+    template <class U>
+        stack_alloc(const stack_alloc<U, N, alignment>& a) noexcept
+            : a_(a.a_) {}
+
+    template <class _Up> struct rebind {using other = stack_alloc<_Up, N, alignment>;};
+
+    T* allocate(std::size_t n)
+    {
+        return reinterpret_cast<T*>(a_.template allocate<alignof(T)>(n*sizeof(T)));
+    }
+    void deallocate(T* p, std::size_t n) noexcept
+    {
+        a_.deallocate(reinterpret_cast<char*>(p), n*sizeof(T));
+    }
+
+    template <class T1, std::size_t N1, std::size_t A1,
+              class U, std::size_t M, std::size_t A2>
+    friend
+    bool
+    operator==(const stack_alloc<T1, N1, A1>& x, const stack_alloc<U, M, A2>& y) noexcept;
+
+    template <class U, std::size_t M, std::size_t A> friend class stack_alloc;
+};
+
+template <class T, std::size_t N, std::size_t A1, class U, std::size_t M, std::size_t A2>
+inline
+bool
+operator==(const stack_alloc<T, N, A1>& x, const stack_alloc<U, M, A2>& y) noexcept
+{
+    return N == M && A1 == A2 && &x.a_ == &y.a_;
+}
+
+template <class T, std::size_t N, std::size_t A1, class U, std::size_t M, std::size_t A2>
+inline
+bool
+operator!=(const stack_alloc<T, N, A1>& x, const stack_alloc<U, M, A2>& y) noexcept
+{
+    return !(x == y);
+}
+
+#endif  // VSTACK_ALLOCATOR_H