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◆ main openmsx / src / utils / SerializeBuffer.hh
6.2 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
e8555c89 Andy McCall 1 
#ifndef SERIALIZEBUFFER_HH
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#define SERIALIZEBUFFER_HH
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#include "MemBuffer.hh"
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#include "inline.hh"
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#include <algorithm>
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#include <cassert>
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#include <cstdint>
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#include <cstring>
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#include <span>
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#include <tuple>
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namespace openmsx {
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/** Memory output buffer
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 *
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  * Acts as a replacement for std::vector<uint8_t>. You can insert data in the
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  * buffer and the buffer will automatically grow. Like std::vector it manages
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  * an internal memory buffer that will automatically reallocate and grow
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  * exponentially.
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  *
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  * This class is much less general than std::vector and optimized for the case
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  * of lots of (small) inserts at the end of the buffer (the main use case of
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  * in-memory savestates). This makes it more efficient than std::vector.
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  * std::vector is far from inefficient, but for savestates this is used A LOT,
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  * so even small improvements matter a lot.
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  */
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class OutputBuffer
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{
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public:
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	/** Create an empty output buffer.
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	 */
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	OutputBuffer();
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	/** Insert data at the end of this buffer.
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	  * This will automatically grow this buffer.
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	  */
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	void insert(const void* __restrict data, size_t len)
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	{
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#ifdef __GNUC__
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		if (__builtin_constant_p(len)) {
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			if        (len == 1) {
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				insertN<1>(data); return;
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			} else if (len == 2) {
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				insertN<2>(data); return;
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			} else if (len == 4) {
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				insertN<4>(data); return;
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			} else if (len == 8) {
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				insertN<8>(data); return;
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			}
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		}
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#endif
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		insertN(data, len);
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	}
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#ifdef __GNUC__
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	template<size_t N> void insertN(const void* __restrict data);
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#endif
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	void insertN(const void* __restrict data, size_t len);
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	/** Insert all the elements of the given tuple.
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	  * Equivalent to repeatedly calling insert() for all the elements of
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	  * the tuple. Though using this method the implementation only has to
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	  * check once whether enough memory is allocated.
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	  */
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	template<typename TUPLE> ALWAYS_INLINE void insert_tuple_ptr(const TUPLE& tuple)
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	{
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		size_t len = 0;
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		auto accum = [&](auto* p) { len += sizeof(*p); };
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		std::apply([&](auto&&... args) { (accum(args), ...); }, tuple);
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		if (const uint8_t* newEnd = end + len; newEnd > buf.end()) [[unlikely]] {
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			grow(len); // reallocates, thus newEnd is no longer valid.
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		}
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		uint8_t* dst = end;
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		auto write = [&](auto* src) {
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			memcpy(dst, src, sizeof(*src));
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			dst += sizeof(*src);
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		};
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		std::apply([&](auto&&... args) { (write(args), ...); }, tuple);
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		assert(dst == (end + len));
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		end = dst;
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	}
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	template<typename T> ALWAYS_INLINE void insert_tuple_ptr(const std::tuple<T*>& tuple)
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	{
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		// single-element tuple -> use insert() because it's better tuned
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		insert(std::get<0>(tuple), sizeof(T));
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	}
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	/** Insert data at a given position. This will overwrite the old data.
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	  * It's not possible to grow the buffer via this method (so the buffer
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	  * must already be big enough to hold the new data).
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	  */
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	void insertAt(size_t pos, const void* __restrict data, size_t len)
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	{
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		assert(buf.data() + pos + len <= buf.end());
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		memcpy(buf.data() + pos, data, len);
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	}
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	/** Reserve space to insert the given number of bytes.
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	  * The returned pointer is only valid until the next internal
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	  * reallocate, so till the next call to insert() or allocate().
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	  *
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	  * If you don't know yet exactly how much memory to allocate (e.g.
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	  * when the buffer will be used for gzip output data), you can request
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	  * the maximum size and deallocate the unused space later.
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	  */
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	[[nodiscard]] std::span<uint8_t> allocate(size_t len)
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	{
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		auto* newEnd = end + len;
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		// Make sure the next OutputBuffer will start with an initial size
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		// that can hold this much space plus some slack.
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		size_t newSize = newEnd - buf.data();
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		lastSize = std::max(lastSize, newSize + 1000);
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		if (newEnd <= buf.end()) {
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			uint8_t* result = end;
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			end = newEnd;
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			return {result, len};
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		} else {
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			return {allocateGrow(len), len};
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		}
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	}
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	/** Free part of a previously allocated buffer.
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	 *
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	  * The parameter must point right after the last byte of the used
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	  * portion of the buffer. So it must be in the range [buf, buf+num]
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	  * with buf and num respectively the return value and the parameter
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	  * of the last allocate() call.
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	  *
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	  * See comment in allocate(). This call must be done right after the
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	  * allocate() call, there cannot be any other (non-const) call to this
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	  * object in between.
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	  */
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	/*void deallocate(uint8_t* pos)
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	{
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		assert(buf.data() <= pos);
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		assert(pos <= end);
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		end = pos;
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	}*/
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	/** Get the current size of the buffer.
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	 */
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	[[nodiscard]] size_t getPosition() const
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	{
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		return end - buf.data();
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	}
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	/** Release ownership of the buffer.
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	 * Returns both the buffer and its size.
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	 */
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	[[nodiscard]] MemBuffer<uint8_t> release() &&;
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private:
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	void insertGrow(const void* __restrict data, size_t len);
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	[[nodiscard]] uint8_t* allocateGrow(size_t len);
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	void grow(size_t len);
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	MemBuffer<uint8_t> buf; // allocated memory
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	uint8_t* end;           // points right after the last used byte
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	                        // so   end - buf == size_in_use
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	static inline size_t lastSize = 50000; // initial estimate
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};
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/** This class is complementary to the OutputBuffer class.
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  * Instead of filling an initially empty buffer it starts from a filled buffer
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  * and allows to retrieve items starting from the start of the buffer.
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  */
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class InputBuffer
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{
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public:
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	/** Construct new InputBuffer, typically the buf_ parameter
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	  * will come from a MemBuffer object.
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	  */
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	explicit InputBuffer(std::span<const uint8_t> buf_)
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		: buf(buf_) {}
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	/** Read the given number of bytes.
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	  * This 'consumes' the read bytes, so a future read() will continue
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	  * where this read stopped.
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	  */
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	void read(void* __restrict result, size_t len)
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	{
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		assert(buf.size() >= len);
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		memcpy(result, buf.data(), len);
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		buf = buf.subspan(len);
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	}
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	/** Skip the given number of bytes.
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	  * This is similar to a read(), but it will only consume the data, not
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	  * copy it.
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	  */
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	void skip(size_t len)
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	{
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		assert(buf.size() >= len);
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		buf = buf.subspan(len);
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	}
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	/** Return a pointer to the current position in the buffer.
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	  * This is useful if you don't want to copy the data, but e.g. use it
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	  * as input for an uncompress algorithm. You can later use skip() to
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	  * actually consume the data.
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	  */
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	[[nodiscard]] const uint8_t* getCurrentPos() const { return buf.data(); }
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private:
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	std::span<const uint8_t> buf;
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};
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} // namespace openmsx
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#endif