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 <cstdlib> // for malloc
#include <type_traits>
#include <memory>

#include "libtorrent/assert.hpp"
#include "libtorrent/aux_/throw.hpp"

namespace libtorrent {

	namespace aux {
		struct free_deleter
		{ void operator()(char* ptr) { return std::free(ptr); } };

		inline std::size_t calculate_pad_bytes(char* inptr, std::size_t alignment)
		{
			std::uintptr_t const ptr = reinterpret_cast<std::uintptr_t>(inptr);
			std::uintptr_t const offset = ptr & (alignment - 1);
			return (alignment - offset) & (alignment - 1);
		}
	}

	template <class T>
	struct heterogeneous_queue
	{
		heterogeneous_queue() : m_storage(nullptr, aux::free_deleter()) {}
		heterogeneous_queue(heterogeneous_queue const&) = delete;
		heterogeneous_queue& operator=(heterogeneous_queue const&) = delete;

		template <class U, typename... Args>
		typename std::enable_if<std::is_base_of<T, U>::value, U&>::type
		emplace_back(Args&&... args)
		{
			// make the conservative assumption that we'll need the maximum padding
			// for this object, just for purposes of growing the storage
			if (std::size_t(m_size) + sizeof(header_t) + alignof(U) + sizeof(U) > std::size_t(m_capacity))
				grow_capacity(sizeof(header_t) + alignof(U) + sizeof(U));

			char* ptr = m_storage.get() + m_size;

			std::size_t const pad_bytes = aux::calculate_pad_bytes(ptr + sizeof(header_t), alignof(U));

			// pad_bytes is only 8 bits in the header, so types that need more than
			// 256 byte alignment may not be suppored
			static_assert(alignof(U) <= 256
				, "heterogeneous_queue does not support types with alignment requirements > 256");

			// if this assert triggers, the type being added to the queue has
			// alignment requirements stricter than what malloc() returns. This is
			// not supported
			TORRENT_ASSERT((reinterpret_cast<std::uintptr_t>(m_storage.get())
				& (alignof(U) - 1)) == 0);

			// make sure the current position in the storage is aligned for
			// creating a heder_t object
			TORRENT_ASSERT((reinterpret_cast<std::uintptr_t>(ptr)
				& (alignof(header_t) - 1)) == 0);

			// length prefix
			header_t* hdr = new (ptr) header_t;
			hdr->pad_bytes = static_cast<std::uint8_t>(pad_bytes);
			hdr->move = &move<U>;
			ptr += sizeof(header_t) + pad_bytes;
			hdr->len = static_cast<std::uint16_t>(sizeof(U)
				+ aux::calculate_pad_bytes(ptr + sizeof(U),  alignof(header_t)));

			// make sure ptr is correctly aligned for the object we're about to
			// create there
			TORRENT_ASSERT((reinterpret_cast<std::uintptr_t>(ptr)
				& (alignof(U) - 1)) == 0);

			// construct in-place
			U* const ret = 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 += int(sizeof(header_t) + pad_bytes + hdr->len);
			return *ret;
		}

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

			char* ptr = m_storage.get();
			char const* const end = m_storage.get() + m_size;
			while (ptr < end)
			{
				header_t* hdr = reinterpret_cast<header_t*>(ptr);
				ptr += sizeof(header_t) + hdr->pad_bytes;
				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()
		{
			char* ptr = m_storage.get();
			char const* const end = m_storage.get() + m_size;
			while (ptr < end)
			{
				header_t* hdr = reinterpret_cast<header_t*>(ptr);
				ptr += sizeof(header_t) + hdr->pad_bytes;
				TORRENT_ASSERT(ptr + hdr->len <= end);
				T* a = reinterpret_cast<T*>(ptr);
				a->~T();
				ptr += hdr->len;
				hdr->~header_t();
			}
			m_size = 0;
			m_num_items = 0;
		}

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

			TORRENT_ASSERT(m_size > 1);
			char* ptr = m_storage.get();
			header_t* hdr = reinterpret_cast<header_t*>(ptr);
			TORRENT_ASSERT(sizeof(header_t) + hdr->pad_bytes + hdr->len
				<= std::size_t(m_size));
			ptr += sizeof(header_t) + hdr->pad_bytes;
			return reinterpret_cast<T*>(ptr);
		}

		~heterogeneous_queue() { clear(); }

	private:

		// this header is put in front of every element. It tells us
		// how many bytes 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
		{
			// the size of the object. From the start of the object, skip this many
			// bytes to get to the next header. Meaning this includes sufficient
			// padding to have the next entry be appropriately aligned for header_t
			std::uint16_t len;

			// the number of pad bytes between the end of this
			// header and the start of the object. This supports allocating types with
			// stricter alignment requirements
			std::uint8_t pad_bytes;

			void (*move)(char* dst, char* src);
		};

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

			// we use malloc() to guarantee alignment
			std::unique_ptr<char, aux::free_deleter> new_storage(
				static_cast<char*>(std::malloc(std::size_t(m_capacity + amount_to_grow)))
				, aux::free_deleter());

			if (new_storage.get() == nullptr)
				aux::throw_ex<std::bad_alloc>();

			char* src = m_storage.get();
			char* dst = new_storage.get();
			char const* const end = m_storage.get() + m_size;
			while (src < end)
			{
				header_t* src_hdr = reinterpret_cast<header_t*>(src);
				new (dst) header_t(*src_hdr);
				src += sizeof(header_t) + src_hdr->pad_bytes;
				dst += sizeof(header_t) + src_hdr->pad_bytes;
				int const len = src_hdr->len;
				TORRENT_ASSERT(src + len <= end);
				// TODO: if this throws, we should technically destruct the elements
				// we've constructed so far, but maybe we should just disallow
				// throwing move instead.
				src_hdr->move(dst, src);
				src_hdr->~header_t();
				src += len ;
				dst += len;
			}

			m_storage.swap(new_storage);
			m_capacity += amount_to_grow;
		}

		template <class U>
		static void move(char* dst, char* src)
		{
			U& rhs = *reinterpret_cast<U*>(src);

			TORRENT_ASSERT((reinterpret_cast<std::uintptr_t>(dst) & (alignof(U) - 1)) == 0);

			new (dst) U(std::move(rhs));
			rhs.~U();
		}

		std::unique_ptr<char, aux::free_deleter> m_storage;
		// number of bytes of storage allocated
		int m_capacity = 0;
		// the number of bytes used under m_storage
		int m_size = 0;
		// the number of objects allocated in m_storage
		int m_num_items = 0;
	};
}

#endif