uint8 *CSteamNetworkConnectionBase::SNP_SerializeAckBlocks( const SNPAckSerializerHelper &helper, uint8 *pOut, const uint8 *pOutEnd, SteamNetworkingMicroseconds usecNow )
{

	// We shouldn't be called if we never received anything
	Assert( m_statsEndToEnd.m_nMaxRecvPktNum > 0 );

	// No room even for the header?
	if ( pOut + SNPAckSerializerHelper::k_cbHeaderSize > pOutEnd )
		return pOut;

	// !KLUDGE! For now limit number of blocks, and always use 16-bit ID.
	//          Later we might want to make this code smarter.
	COMPILE_TIME_ASSERT( SNPAckSerializerHelper::k_cbHeaderSize == 5 );
	uint8 *pAckHeaderByte = pOut;
	++pOut;
	uint16 *pLatestPktNum = (uint16 *)pOut;
	pOut += 2;
	uint16 *pTimeSinceLatestPktNum = (uint16 *)pOut;
	pOut += 2;

	// 10011000 - ack frame designator, with 16-bit last-received sequence number, and no ack blocks
	*pAckHeaderByte = 0x98;

	int nLogLevelPacketDecode = m_connectionConfig.m_LogLevel_PacketDecode.Get();

	#ifdef SNP_ENABLE_PACKETSENDLOG
		PacketSendLog *pLog = &m_vecSendLog[ m_vecSendLog.size()-1 ];
	#endif

	// Fast case for no packet loss we need to ack, which will (hopefully!) be a common case
	if ( m_receiverState.m_mapPacketGaps.size() == 1 )
	{
		int64 nLastRecvPktNum = m_statsEndToEnd.m_nMaxRecvPktNum;
		*pLatestPktNum = LittleWord( (uint16)nLastRecvPktNum );
		*pTimeSinceLatestPktNum = LittleWord( (uint16)SNPAckSerializerHelper::EncodeTimeSince( usecNow, m_statsEndToEnd.m_usecTimeLastRecvSeq ) );

		SpewDebugGroup( nLogLevelPacketDecode, "[%s]   encode pkt %lld last recv %lld (no loss)\n",
			GetDescription(),
			(long long)m_statsEndToEnd.m_nNextSendSequenceNumber, (long long)nLastRecvPktNum
		);
		m_receiverState.m_mapPacketGaps.rbegin()->second.m_usecWhenAckPrior = INT64_MAX; // Clear timer, we wrote everything we needed to

		#ifdef SNP_ENABLE_PACKETSENDLOG
			pLog->m_nAckBlocksSent = 0;
			pLog->m_nAckEnd = nLastRecvPktNum;
		#endif

		return pOut;
	}

	// Fit as many blocks as possible.
	// (Unless we are badly fragmented and are trying to squeeze in what
	// we can at the end of a packet, this won't ever iterate
	int nBlocks = helper.m_nBlocks;
	uint8 *pExpectedOutEnd;
	for (;;)
	{

		// Not sending any blocks at all?  (Either they don't fit, or we are waiting because we don't
		// want to nack yet.)  Just fill in the header with the oldest ack
		if ( nBlocks == 0 )
		{
			auto itOldestGap = m_receiverState.m_mapPacketGaps.begin();
			int64 nLastRecvPktNum = itOldestGap->first-1;
			*pLatestPktNum = LittleWord( uint16( nLastRecvPktNum ) );
			*pTimeSinceLatestPktNum = LittleWord( (uint16)SNPAckSerializerHelper::EncodeTimeSince( usecNow, itOldestGap->second.m_usecWhenReceivedPktBefore ) );

			SpewDebugGroup( nLogLevelPacketDecode, "[%s]   encode pkt %lld last recv %lld (no blocks, actual last recv=%lld)\n",
				GetDescription(),
				(long long)m_statsEndToEnd.m_nNextSendSequenceNumber, (long long)nLastRecvPktNum, (long long)m_statsEndToEnd.m_nMaxRecvPktNum
			);

			#ifdef SNP_ENABLE_PACKETSENDLOG
				pLog->m_nAckBlocksSent = 0;
				pLog->m_nAckEnd = nLastRecvPktNum;
			#endif

			// Acked packets before this gap.  Were we waiting to flush them?
			if ( itOldestGap == m_receiverState.m_itPendingAck )
			{
				// Mark it as sent
				m_receiverState.m_itPendingAck->second.m_usecWhenAckPrior = INT64_MAX;
				++m_receiverState.m_itPendingAck;
			}

			// NOTE: We did NOT nack anything just now
			return pOut;
		}

		int cbTotalEncoded = helper.m_arBlocks[nBlocks-1].m_cbTotalEncodedSize;
		pExpectedOutEnd = pAckHeaderByte + cbTotalEncoded; // Save for debugging below
		if ( pExpectedOutEnd <= pOutEnd )
			break;

		// Won't fit, peel off the newest one, see if the earlier ones will fit
		--nBlocks;
	}

	// OK, we know how many blocks we are going to write.  Finish the header byte
	Assert( nBlocks == uint8(nBlocks) );
	if ( nBlocks > 6 )
	{
		*pAckHeaderByte |= 7;
		*(pOut++) = uint8( nBlocks );
	}
	else
	{
		*pAckHeaderByte |= uint8( nBlocks );
	}

	// Locate the first one we will serialize.
	// (It's the newest one, which is the last one in the list).
	const SNPAckSerializerHelper::Block *pBlock = &helper.m_arBlocks[nBlocks-1];

	// Latest packet number and time
	*pLatestPktNum = LittleWord( uint16( pBlock->m_nLatestPktNum ) );
	*pTimeSinceLatestPktNum = LittleWord( pBlock->m_nEncodedTimeSinceLatestPktNum );

	// Full packet number, for spew
	int64 nAckEnd = ( m_statsEndToEnd.m_nMaxRecvPktNum & ~(int64)(~(uint32)0) ) | pBlock->m_nLatestPktNum;
	++nAckEnd;

	#ifdef SNP_ENABLE_PACKETSENDLOG
		pLog->m_nAckBlocksSent = nBlocks;
		pLog->m_nAckEnd = nAckEnd;
	#endif

	SpewDebugGroup( nLogLevelPacketDecode, "[%s]   encode pkt %lld last recv %lld (%d blocks, actual last recv=%lld)\n",
		GetDescription(),
		(long long)m_statsEndToEnd.m_nNextSendSequenceNumber, (long long)(nAckEnd-1), nBlocks, (long long)m_statsEndToEnd.m_nMaxRecvPktNum
	);

	// Check for a common case where we report on everything
	if ( nAckEnd > m_statsEndToEnd.m_nMaxRecvPktNum )
	{
		Assert( nAckEnd == m_statsEndToEnd.m_nMaxRecvPktNum+1 );
		for (;;)
		{
			m_receiverState.m_itPendingAck->second.m_usecWhenAckPrior = INT64_MAX;
			if ( m_receiverState.m_itPendingAck->first == INT64_MAX )
				break;
			++m_receiverState.m_itPendingAck;
		}
		m_receiverState.m_itPendingNack = m_receiverState.m_itPendingAck;
	}
	else
	{

		// Advance pointer to next block that needs to be acked,
		// past the ones we are about to ack.
		if ( m_receiverState.m_itPendingAck->first <= nAckEnd )
		{
			do
			{
				m_receiverState.m_itPendingAck->second.m_usecWhenAckPrior = INT64_MAX;
				++m_receiverState.m_itPendingAck;
			} while ( m_receiverState.m_itPendingAck->first <= nAckEnd );
		}

		// Advance pointer to next block that needs to be nacked, past the ones
		// we are about to nack.
		while ( m_receiverState.m_itPendingNack->first < nAckEnd )
			++m_receiverState.m_itPendingNack;
	}

	// Serialize the blocks into the packet, from newest to oldest
	while ( pBlock >= helper.m_arBlocks )
	{
		uint8 *pAckBlockHeaderByte = pOut;
		++pOut;

		// Encode ACK (number of packets successfully received)
		{
			if ( pBlock->m_nAck < 8 )
			{
				// Small block of packets.  Encode directly in the header.
				*pAckBlockHeaderByte = uint8(pBlock->m_nAck << 4);
			}
			else
			{
				// Larger block of received packets.  Put lowest bits in the header,
				// and overflow using varint.  This is probably going to be pretty
				// common.
				*pAckBlockHeaderByte = 0x80 | ( uint8(pBlock->m_nAck & 7) << 4 );
				pOut = SerializeVarInt( pOut, pBlock->m_nAck>>3, pOutEnd );
				if ( pOut == nullptr )
				{
					AssertMsg( false, "Overflow serializing packet ack varint count" );
					return nullptr;
				}
			}
		}

		// Encode NACK (number of packets dropped)
		{
			if ( pBlock->m_nNack < 8 )
			{
				// Small block of packets.  Encode directly in the header.
				*pAckBlockHeaderByte |= uint8(pBlock->m_nNack);
			}
			else
			{
				// Larger block of dropped packets.  Put lowest bits in the header,
				// and overflow using varint.  This is probably going to be less common than
				// large ACK runs, but not totally uncommon.  Losing one or two packets is
				// really common, but loss events often involve a lost of many packets in a run.
				*pAckBlockHeaderByte |= 0x08 | uint8(pBlock->m_nNack & 7);
				pOut = SerializeVarInt( pOut, pBlock->m_nNack >> 3, pOutEnd );
				if ( pOut == nullptr )
				{
					AssertMsg( false, "Overflow serializing packet nack varint count" );
					return nullptr;
				}
			}
		}

		// Debug
		int64 nAckBegin = nAckEnd - pBlock->m_nAck;
		int64 nNackBegin = nAckBegin - pBlock->m_nNack;
		SpewDebugGroup( nLogLevelPacketDecode, "[%s]   encode pkt %lld nack [%lld,%lld) ack [%lld,%lld) \n",
			GetDescription(),
			(long long)m_statsEndToEnd.m_nNextSendSequenceNumber,
			(long long)nNackBegin, (long long)nAckBegin,
			(long long)nAckBegin, (long long)nAckEnd
		);
		nAckEnd = nNackBegin;
		Assert( nAckEnd > 0 ); // Make sure we don't try to ack packet 0 or below

		// Move backwards in time
		--pBlock;
	}

	// Make sure when we were checking what would fit, we correctly calculated serialized size
	Assert( pOut == pExpectedOutEnd );

	return pOut;
}