premiere-libtorrent/include/libtorrent/heterogeneous_queue.hpp

/*

Copyright (c) 2015-2016, Arvid Norberg
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*/

#ifndef TORRENT_HETEROGENEOUS_QUEUE_HPP_INCLUDED
#define TORRENT_HETEROGENEOUS_QUEUE_HPP_INCLUDED

#include <vector>
#include <cstdint>
#include <type_traits>

#include "libtorrent/assert.hpp"

namespace libtorrent {

	template <class T>
	struct heterogeneous_queue
	{
		heterogeneous_queue()
			: m_storage(nullptr)
			, m_capacity(0)
			, m_size(0)
			, m_num_items(0)
		{}

		template <class U, typename... Args>
		typename std::enable_if<std::is_base_of<T, U>::value, U&>::type
		emplace_back(Args&&... args)
		{
			// the size of the type rounded up to pointer alignment
			const int object_size = (sizeof(U) + sizeof(*m_storage) - 1)
				/ sizeof(*m_storage);

			// +1 for the length prefix
			if (m_size + object_size + header_size > m_capacity)
				grow_capacity(object_size);

			uintptr_t* ptr = m_storage + m_size;

			// length prefix
			header_t* hdr = reinterpret_cast<header_t*>(ptr);
			hdr->len = object_size;
			hdr->move = &move<U>;
			ptr += header_size;

			// construct in-place
			new (ptr) U(std::forward<Args>(args)...);

			// if we constructed the object without throwing any exception
			// update counters to indicate the new item is in there
			++m_num_items;
			m_size += header_size + object_size;
			return *reinterpret_cast<U*>(ptr);
		}

		void get_pointers(std::vector<T*>& out)
		{
			out.clear();

			uintptr_t* ptr = m_storage;
			uintptr_t const* const end = m_storage + m_size;
			while (ptr < end)
			{
				header_t* hdr = reinterpret_cast<header_t*>(ptr);
				ptr += header_size;
				TORRENT_ASSERT(ptr + hdr->len <= end);
				out.push_back(reinterpret_cast<T*>(ptr));
				ptr += hdr->len;
			}
		}

		void swap(heterogeneous_queue& rhs)
		{
			std::swap(m_storage, rhs.m_storage);
			std::swap(m_capacity, rhs.m_capacity);
			std::swap(m_size, rhs.m_size);
			std::swap(m_num_items, rhs.m_num_items);
		}

		int size() const { return m_num_items; }
		bool empty() const { return m_num_items == 0; }

		void clear()
		{
			uintptr_t* ptr = m_storage;
			uintptr_t const* const end = m_storage + m_size;
			while (ptr < end)
			{
				header_t* hdr = reinterpret_cast<header_t*>(ptr);
				ptr += header_size;
				TORRENT_ASSERT(ptr + hdr->len <= end);
				T* a = reinterpret_cast<T*>(ptr);
				a->~T();
				ptr += hdr->len;
			}
			m_size = 0;
			m_num_items = 0;
		}

		T* front()
		{
			if (m_size == 0) return nullptr;

			TORRENT_ASSERT(m_size > 1);
			uintptr_t* ptr = m_storage;
			TORRENT_ASSERT(reinterpret_cast<header_t*>(ptr)->len <= m_size);
			ptr += header_size;
			return reinterpret_cast<T*>(ptr);
		}

		~heterogeneous_queue()
		{
			clear();
			delete[] m_storage;
		}

	private:

		// non-copyable
		heterogeneous_queue(heterogeneous_queue const&);
		heterogeneous_queue& operator=(heterogeneous_queue const&);

		// this header is put in front of every element. It tells us
		// how many uintptr_t's it's using for its allocation, and it
		// also tells us how to move this type if we need to grow our
		// allocation.
		struct header_t
		{
			int len;
			void (*move)(uintptr_t* dst, uintptr_t* src);
		};

		static int const header_size = (sizeof(header_t) + sizeof(uintptr_t)
			- 1) / sizeof(uintptr_t);

		void grow_capacity(int const size)
		{
			int const amount_to_grow = (std::max)(size + header_size
				, (std::max)(m_capacity * 3 / 2, 128));

			uintptr_t* new_storage = new uintptr_t[m_capacity + amount_to_grow];

			uintptr_t* src = m_storage;
			uintptr_t* dst = new_storage;
			uintptr_t const* const end = m_storage + m_size;
			while (src < end)
			{
				header_t* src_hdr = reinterpret_cast<header_t*>(src);
				header_t* dst_hdr = reinterpret_cast<header_t*>(dst);
				*dst_hdr = *src_hdr;
				src += header_size;
				dst += header_size;
				TORRENT_ASSERT(src + src_hdr->len <= end);
				// TODO: if this throws, should we do anything?
				src_hdr->move(dst, src);
				src += src_hdr->len;
				dst += src_hdr->len;
			}

			delete[] m_storage;
			m_storage = new_storage;
			m_capacity += amount_to_grow;
		}

		template <class U>
		static void move(uintptr_t* dst, uintptr_t* src)
		{
			U& rhs = *reinterpret_cast<U*>(src);
			new (dst) U(static_cast<U&&>(rhs));
			rhs.~U();
		}

		uintptr_t* m_storage;
		// number of uintptr_t's allocated under m_storage
		int m_capacity;
		// the number of uintptr_t's used under m_storage
		int m_size;
		// the number of objects allocated under m_storage
		int m_num_items;
	};
}

#endif