// https://tools.ietf.org/html/rfc793

// See https://en.wikipedia.org/wiki/File:Tcp_state_diagram_fixed_new.svg
#define TCP_STATE_CLOSED 0
#define TCP_STATE_LISTEN 1
#define TCP_STATE_SYN_SENT 2
#define TCP_STATE_SYN_RECEIVED 3
#define TCP_STATE_ESTABLISHED 4
#define TCP_STATE_FIN_WAIT_1 5
#define TCP_STATE_FIN_WAIT_2 6
#define TCP_STATE_CLOSE_WAIT 7
#define TCP_STATE_CLOSING 8
#define TCP_STATE_LAST_ACK 9
#define TCP_STATE_TIME_WAIT 10

#define TCP_CONNECT_TIMEOUT 10000

//#define TCP_DEFAULT_MSS         536

#define TCP_DEFAULT_MSS 1500

#define TCP_WINDOW_SIZE 65536

#define TCP_FLAG_FIN 0x01
#define TCP_FLAG_SYN 0x02
#define TCP_FLAG_RST 0x04
#define TCP_FLAG_PSH 0x08
#define TCP_FLAG_ACK 0x10
#define TCP_FLAG_URG 0x20

#define TCP_SRTT_ALPHA 0.9
#define TCP_RTO_MIN 0.2
#define TCP_RTO_MAX 10000
#define TCP_RTO_BETA 2

class CTcpHeader {
  U16 source_port;
  U16 dest_port;
  U32 seq;
  U32 ack;
  U8 data_offset;
  U8 flags;
  U16 window_size;
  U16 checksum;
  U16 urgent_pointer;
};

class CTcpSendBufHeader {
  CTcpSendBufHeader *next;

  F64 time_sent;
  U32 length;
  U32 retries;
  U32 seq_start;
  U32 seq_end;
};

class CTcpSocket {
  CSocket sock;

  I64 state;

  U32 local_addr;
  U16 local_port;

  U32 remote_addr;
  U32 remote_port;

  U32 snd_una; // seq number of first unacknowledged octet
  U32 snd_nxt; // seq number of next octet to send
  U32 snd_wnd; // allowed number of unacknowledged outgoing octets
  U32 mss;     // maximum segment size

  U32 rcv_nxt; // seq number of next octet to receive
  U32 rcv_wnd; // allowed number of unacknowledged incoming octets

  F64 conntime;
  F64 srtt;

  I64 recv_buf_size;
  U8 *recv_buf;
  I64 recv_buf_read_pos;
  I64 recv_buf_write_pos;

  CTcpSocket *backlog_next;
  CTcpSocket *backlog_first;
  CTcpSocket *backlog_last;
  I64 backlog_remaining;

  CTcpSendBufHeader *send_buf_first;
  CTcpSendBufHeader *send_buf_last;

  // I64 rcvtimeo_ms;
  // I64 recv_maxtime;
};

class CTcpPseudoHeader {
  U32 source_addr;
  U32 dest_addr;
  U8 zeros;
  U8 protocol;
  U16 tcp_length;
};

class CTcpSocketListItem {
  CTcpSocketListItem *prev;
  CTcpSocketListItem *next;
  CTcpSocket *sock;
};

static CTcpSocketListItem **tcp_socket_list;

static CTcpSocket *GetTcpSocketFromList(CIPv4Packet *packet, CTcpHeader *hdr) {
  CTcpSocketListItem *item = tcp_socket_list[ntohs(hdr->dest_port)]->next;
  while (item) {
    if (item->sock->remote_addr == packet->source_ip &&
        item->sock->remote_port == ntohs(hdr->source_port)) {
      return item->sock;
    }
    item = item->next;
  }
  return NULL;
}

U0 AddTcpSocketToList(CTcpSocket *s) {
  CTcpSocketListItem *prev = tcp_socket_list[s->local_port];
  CTcpSocketListItem *new = CAlloc(sizeof(CTcpSocketListItem));
  while (prev->next) {
    prev = prev->next;
  }
  new->prev = prev;
  new->sock = s;
  prev->next = new;
}

CTcpSocket *RemoveTcpSocketFromList(CTcpSocket *s) {
  CTcpSocketListItem *prev = NULL;
  CTcpSocketListItem *next = NULL;
  CTcpSocketListItem *item = tcp_socket_list[s->local_port]->next;
  while (item) {
    if (item->sock == s) {
      prev = item->prev;
      next = item->next;
      if (prev) {
        prev->next = next;
      }
      if (next) {
        next->prev = prev;
      }
      return s;
    }
    item = item->next;
  }
  return NULL;
}

// TODO: this takes up half a meg, change it to a binary tree or something
static CTcpSocket **tcp_bound_sockets;

static U16 tcp_next_source_port = RandU16();

static Bool TcpIsSynchronizedState(I64 state) {
  return state == TCP_STATE_ESTABLISHED || state == TCP_STATE_FIN_WAIT_1 ||
         state == TCP_STATE_FIN_WAIT_2 || state == TCP_STATE_CLOSE_WAIT ||
         state == TCP_STATE_CLOSING || state == TCP_STATE_LAST_ACK ||
         state == TCP_STATE_TIME_WAIT;
}

static U16 TcpPartialChecksum(U32 sum, U8 *header, I64 length) {
  I64 nleft = length;
  U16 *w = header;

  while (nleft > 1) {
    sum += *(w++);
    nleft -= 2;
  }

  return sum;
}

static U16 TcpFinalChecksum(U32 sum, U8 *header, I64 length) {
  I64 nleft = length;
  U16 *w = header;

  while (nleft > 1) {
    sum += *(w++);
    nleft -= 2;
  }

  // mop up an odd byte, if necessary
  if (nleft == 1) {
    sum += ((*w) & 0x00ff);
  }

  // add back carry outs from top 16 bits to low 16 bits
  sum = (sum >> 16) + (sum & 0xffff); // add hi 16 to low 16
  sum += (sum >> 16);                 // add carry
  return (~sum) & 0xffff;
}

I64 TcpPacketAlloc(U8 **frame_out, U32 source_ip, U16 source_port, U32 dest_ip,
                   U16 dest_port, U32 seq, U32 ack, U8 flags, I64 length) {
  U8 *frame;
  I64 index = IPv4PacketAlloc(&frame, IP_PROTO_TCP, source_ip, dest_ip,
                              sizeof(CTcpHeader) + length);

  if (index < 0)
    return index;

  CTcpHeader *hdr = frame;
  hdr->source_port = htons(source_port);
  hdr->dest_port = htons(dest_port);
  hdr->seq = htonl(seq);
  hdr->ack = htonl(ack);
  hdr->data_offset = (sizeof(CTcpHeader) / 4) << 4;
  hdr->flags = flags;
  hdr->window_size = htons(TCP_WINDOW_SIZE / 2); // FIXME
  hdr->checksum = 0;
  hdr->urgent_pointer = 0;

  *frame_out = frame + sizeof(CTcpHeader);
  return index;
}

I64 TcpPacketFinish(I64 index, U32 source_ip, U32 dest_ip, U8 *frame,
                    I64 length, CTcpSendBufHeader **send_buf_out) {
  CTcpHeader *hdr = frame - sizeof(CTcpHeader);

  CTcpPseudoHeader pseudo;
  pseudo.source_addr = htonl(source_ip);
  pseudo.dest_addr = htonl(dest_ip);
  pseudo.zeros = 0;
  pseudo.protocol = IP_PROTO_TCP;
  pseudo.tcp_length = htons(sizeof(CTcpHeader) + length);

  U32 sum = TcpPartialChecksum(0, &pseudo, sizeof(CTcpPseudoHeader));
  hdr->checksum = TcpFinalChecksum(sum, hdr, sizeof(CTcpHeader) + length);

  if (send_buf_out) {
    CTcpSendBufHeader *sb =
        MAlloc(sizeof(CTcpSendBufHeader) + sizeof(CTcpHeader) + length);
    sb->next = NULL;
    sb->time_sent = tS;
    sb->length = sizeof(CTcpHeader) + length;
    sb->retries = 0;
    sb->seq_start = ntohl(hdr->seq);
    sb->seq_end = 0; // NEEDS TO BE SET UPSTREAM

    MemCpy((sb(U8 *)) + sizeof(CTcpSendBufHeader), frame,
           sizeof(CTcpHeader) + length);
    *send_buf_out = sb;
  }

  return IPv4PacketFinish(index);
}

// Send a TCP frame with flags and/or data
I64 TcpSend(U32 local_addr, U16 local_port, U32 remote_addr, U16 remote_port,
            U32 seq, U32 ack, U8 flags) {
  U8 *frame;
  I64 index = TcpPacketAlloc(&frame, local_addr, local_port, remote_addr,
                             remote_port, seq, ack, flags, 0);

  if (index < 0)
    return index;

  return TcpPacketFinish(index, local_addr, remote_addr, frame, 0, NULL);
}

// Send a TCP frame with flags only, no data
I64 TcpSend2(CTcpSocket *s, U8 flags) {
  U8 *frame;
  I64 index =
      TcpPacketAlloc(&frame, s->local_addr, s->local_port, s->remote_addr,
                     s->remote_port, s->snd_nxt, s->rcv_nxt, flags, 0);

  if (index < 0)
    return index;

  if (flags & TCP_FLAG_SYN)
    s->snd_nxt++;

  if (flags & TCP_FLAG_FIN)
    s->snd_nxt++;

  //"Sent #%d, to %08X, err = %d\n", s->seq, s->remote_addr, error;
  if (flags & (TCP_FLAG_SYN | TCP_FLAG_FIN)) {
    CTcpSendBufHeader *sb;
    TcpPacketFinish(index, s->local_addr, s->remote_addr, frame, 0, &sb);
    sb->seq_end = s->snd_nxt;

    // Append to SendBuf chain
    if (s->send_buf_first)
      s->send_buf_last->next = sb;
    else
      s->send_buf_first = sb;

    s->send_buf_last = sb;
  } else {
    return TcpPacketFinish(index, s->local_addr, s->remote_addr, frame, 0,
                           NULL);
  }
}

// Send a TCP frame with flags and data
I64 TcpSendData2(CTcpSocket *s, U8 flags, U8 *data, I64 length) {
  U8 *frame;
  I64 index =
      TcpPacketAlloc(&frame, s->local_addr, s->local_port, s->remote_addr,
                     s->remote_port, s->snd_nxt, s->rcv_nxt, flags, length);

  if (index < 0)
    return index;

  if (length)
    MemCpy(frame, data, length);

  if (flags & TCP_FLAG_SYN)
    s->snd_nxt++;

  s->snd_nxt += length;

  if (flags & TCP_FLAG_FIN)
    s->snd_nxt++;

  //"Sent #%d, to %08X, err = %d\n", s->seq, s->remote_addr, error;

  CTcpSendBufHeader *sb;
  TcpPacketFinish(index, s->local_addr, s->remote_addr, frame, length, &sb);
  sb->seq_end = s->snd_nxt;

  // Append to SendBuf chain
  if (s->send_buf_first)
    s->send_buf_last->next = sb;
  else
    s->send_buf_first = sb;

  s->send_buf_last = sb;
}

I64 TcpParsePacket(CTcpHeader **header_out, U8 **data_out, I64 *length_out,
                   CIPv4Packet *packet) {
  if (packet->proto != IP_PROTO_TCP)
    return -1;

  // FIXME: validate packet->length
  // FIXME: checksum

  CTcpHeader *hdr = packet->data;
  I64 header_length = (hdr->data_offset >> 4) * 4;

  //"TCP: in hdr %d, flags %02Xh, seq %d, ack %d, len %d, chksum %d\n",
  //    header_length, hdr->flags, ntohl(hdr->seq), ntohl(hdr->ack),
  //    packet->length - header_length, ntohs(hdr->checksum);

  *header_out = hdr;
  *data_out = packet->data + header_length;
  *length_out = packet->length - header_length;
  return 0;
}

/*
class CTcpSendBufHeader {
  CTcpSendBufHeader* next;

  F64 time_sent;
  U32 length;
  U32 retries;
  U32 seq_start;
  U32 seq_end;
};
*/

static U0 TcpSocketAckSendBufs(CTcpSocket *s, U32 seg_ack) {
  F64 time = tS;

  while (s->send_buf_first) {
    CTcpSendBufHeader *sb = s->send_buf_first;

    // There's no notion of smaller/greater than in modular arithemtic,
    // we can only check if a number lies within some range.
    // Here we check that
    //   sb->seq_end <= seg_ack <= s->snd_nxt
    // because that will work for all meaningful ACKs.
    I64 seg_ack_rel = (seg_ack - sb->seq_end) & 0xffffffff;
    I64 snd_nxt_rel = (s->snd_nxt - sb->seq_end) & 0xffffffff;

    if (seg_ack_rel <= snd_nxt_rel) {
      // Update smoothed RTT
      F64 rtt = time - sb->time_sent;
      s->srtt = (s->srtt * TCP_SRTT_ALPHA) + ((1.0 - TCP_SRTT_ALPHA) * rtt);
      //"ACK'd %d->%d (RTT %f ms)", sb->seq_start, sb->seq_end, rtt * 1000;

      // Remove SendBuf from chain
      s->send_buf_first = sb->next;

      if (s->send_buf_first == NULL)
        s->send_buf_last = NULL;

      Free(sb);
    } else
      break;
  }
}

// Check unacknowledged outgoing packets and retransmit if needed
static U0 TcpSocketCheckSendBufs(CTcpSocket *s) {
  F64 time = tS;

  F64 rto = TCP_RTO_BETA * s->srtt;

  if (rto < TCP_RTO_MIN)
    rto = TCP_RTO_MIN;
  if (rto > TCP_RTO_MAX)
    rto = TCP_RTO_MAX;

  while (s->send_buf_first) {
    CTcpSendBufHeader *sb = s->send_buf_first;

    if (time > sb->time_sent + rto) {
      break;

      // Retransmit
      "Retransmit %d->%d (%f ms)!\n", sb->seq_start, sb->seq_end,
          (time - sb->time_sent) * 1000;

      U8 *frame;
      I64 index = IPv4PacketAlloc(&frame, IP_PROTO_TCP, s->local_addr,
                                  s->remote_addr, sb->length);

      if (index < 0) {
        return; // retry later I guess
      }

      MemCpy(frame, (sb(U8 *)) + sizeof(CTcpSendBufHeader), sb->length);
      IPv4PacketFinish(index);

      sb->time_sent = tS;

      // Move to the end of the chain
      s->send_buf_first = sb->next;
      sb->next = NULL;

      if (s->send_buf_first)
        s->send_buf_last->next = sb;
      else
        s->send_buf_first = sb;

      s->send_buf_last = sb;
    } else
      break;
  }
}

I64 TcpSocketAccept(CTcpSocket *s, sockaddr *addr, I64 addrlen) {
  if (s->state != TCP_STATE_LISTEN)
    return -1;

  while (1) {
    // TODO: Thread safe?
    if (s->backlog_first) {
      CTcpSocket *new_socket = s->backlog_first;
      // "Retr %p\n", new_socket;

      s->backlog_first = s->backlog_first->backlog_next;
      if (!s->backlog_first)
        s->backlog_last = NULL;

      s->backlog_remaining++;

      // TODO: this should be done in a way that doesn't block on accept()
      I64 maxtime = cnts.jiffies + TCP_CONNECT_TIMEOUT * JIFFY_FREQ / 1000;

      while (cnts.jiffies < maxtime) {
        if (new_socket->state == TCP_STATE_ESTABLISHED ||
            new_socket->state == TCP_STATE_CLOSED)
          break;
        else
          Yield;
      }

      if (new_socket->state != TCP_STATE_ESTABLISHED) {
        close(new_socket);
        return -1;
      }

      return new_socket;
    } else
      Yield;
  }

  no_warn addr; // FIXME
  no_warn addrlen;
  return -1;
}

I64 TcpSocketBind(CTcpSocket *s, sockaddr *addr, I64 addrlen) {
  if (addrlen < sizeof(sockaddr_in))
    return -1;

  if (s->state != TCP_STATE_CLOSED)
    return -1;

  sockaddr_in *addr_in = addr;

  U16 local_port = ntohs(addr_in->sin_port);

  // TODO: address & stuff
  if (tcp_bound_sockets[local_port] != NULL)
    return -1;

  tcp_bound_sockets[local_port] = s;

  s->local_addr = IPv4GetAddress();
  s->local_port = local_port;

  return 0;
}

I64 TcpSocketClose(CTcpSocket *s) {
  /* https://tools.ietf.org/html/rfc793#section-3.5
    Case 1:  Local user initiates the close

    In this case, a FIN segment can be constructed and placed on the
    outgoing segment queue.  No further SENDs from the user will be
    accepted by the TCP, and it enters the FIN-WAIT-1 state.  RECEIVEs
    are allowed in this state.  All segments preceding and including FIN
    will be retransmitted until acknowledged.  When the other TCP has
    both acknowledged the FIN and sent a FIN of its own, the first TCP
    can ACK this FIN.  Note that a TCP receiving a FIN will ACK but not
    send its own FIN until its user has CLOSED the connection also.
  */

  // Send FIN & wait for acknowledge
  if (s->state == TCP_STATE_ESTABLISHED) {
    while (TcpSend2(s, TCP_FLAG_FIN | TCP_FLAG_ACK) < 0) {
      TcpSocketCheckSendBufs(s);
      Yield;
    }

    s->state = TCP_STATE_FIN_WAIT_1;
    // "FIN-WAIT-1\n";

    // Block until all outgoing data including our FIN have been acknowledged
    // (una == nxt)
    //
    // TODO: what other states are permissible here?
    // TODO: this can block for ever if our receive buffer fills up, but the
    // other side
    //       insists on pushing more data before closing the connection
    while ((s->state == TCP_STATE_FIN_WAIT_1) && s->snd_una != s->snd_nxt) {
      TcpSocketCheckSendBufs(s);
      Yield;
    }

    if (s->state == TCP_STATE_FIN_WAIT_1) {
      s->state = TCP_STATE_FIN_WAIT_2;
      // "FIN-WAIT-2 (%d/%d)\n", s->snd_una, s->snd_nxt;
    }

    // Now we should wait for the other side's FIN and acknowledge it
    // TODO: time-out
    while (s->state == TCP_STATE_FIN_WAIT_2) {
      Yield;
    }
  } else if (s->state == TCP_STATE_CLOSE_WAIT) {
    while (TcpSend2(s, TCP_FLAG_FIN | TCP_FLAG_ACK) < 0) {
      TcpSocketCheckSendBufs(s);
      Yield;
    }

    if (s->state == TCP_STATE_CLOSE_WAIT) {
      s->state = TCP_STATE_LAST_ACK;
      // "LAST-ACK (%d/%d)\n", s->snd_una, s->snd_nxt;
    }

    // Block until all outgoing data including our FIN have been acknowledged
    // (una == nxt)
    while (s->state == TCP_STATE_LAST_ACK && s->snd_una != s->snd_nxt) {
      TcpSocketCheckSendBufs(s);
      Yield;
    }
  }

  // Still connected? RST it!
  if (TcpIsSynchronizedState(s->state)) {
    TcpSend2(s, TCP_FLAG_RST);
  }

  // Free backlog
  CTcpSocket *backlog = s->backlog_first;
  CTcpSocket *backlog2;

  while (backlog) {
    backlog2 = backlog->backlog_next;
    close(backlog);
    backlog = backlog2;
  }

  if (s->local_port)
    if (!RemoveTcpSocketFromList(s))
      tcp_bound_sockets[s->local_port] = NULL;

  Free(s->recv_buf);
  Free(s);
  return 0;
}

I64 TcpSocketConnect(CTcpSocket *s, sockaddr *addr, I64 addrlen) {
  if (addrlen < sizeof(sockaddr_in))
    return -1;

  if (s->state != TCP_STATE_CLOSED)
    return -1;

  sockaddr_in *addr_in = addr;

  U16 local_port = 0x8000 + (tcp_next_source_port & 0x7fff);
  tcp_next_source_port++;

  // TODO: address & stuff
  if (tcp_bound_sockets[local_port] != NULL)
    return -1;

  tcp_bound_sockets[local_port] = s;

  s->local_addr = IPv4GetAddress();
  s->local_port = local_port;
  s->remote_addr = ntohl(addr_in->sin_addr.s_addr);
  s->remote_port = ntohs(addr_in->sin_port);

  s->snd_una = 0;
  s->snd_nxt = 0;
  s->snd_wnd = 0;
  s->mss = TCP_DEFAULT_MSS;

  s->rcv_nxt = 0;
  s->rcv_wnd = TCP_WINDOW_SIZE;

  s->conntime = tS;

  TcpSend2(s, TCP_FLAG_SYN);
  s->state = TCP_STATE_SYN_SENT;

  // TODO: TcpSetTimeout
  I64 maxtime = cnts.jiffies + TCP_CONNECT_TIMEOUT * JIFFY_FREQ / 1000;

  while (cnts.jiffies < maxtime) {
    if (s->state == TCP_STATE_ESTABLISHED || s->state == TCP_STATE_CLOSED)
      break;
    else
      Yield;
  }

  if (s->state != TCP_STATE_ESTABLISHED)
    return -1;

  return 0;
}

I64 TcpSocketListen(CTcpSocket *s, I64 backlog) {
  if (s->state != TCP_STATE_CLOSED)
    return -1;

  // Enter listen state. If a SYN packet arrives, it will be processed by
  // TcpHandler, which opens the connection and puts the new socket into the
  // listening socket's accept backlog.
  s->state = TCP_STATE_LISTEN;
  s->backlog_remaining = backlog;

  return 0;
}

I64 TcpSocketRecvfrom(CTcpSocket *s, U8 *buf, I64 len, I64 flags,
                      sockaddr *src_addr, I64 addrlen) {
  no_warn flags;
  no_warn src_addr; // FIXME
  no_warn addrlen;
  //"TcpSocketRecvfrom\n";
  // If we are ready to receive data, but there is none currently, block until
  // we receive is some.
  // TODO: checking for FIN-WAIT-1 here is not so useful, since it only exists
  // while we are in Close()
  while (
      (s->state == TCP_STATE_ESTABLISHED || s->state == TCP_STATE_FIN_WAIT_1) &&
      s->recv_buf_read_pos == s->recv_buf_write_pos) {
    TcpSocketCheckSendBufs(s);
    Yield;
  }

  // TODO: this works for now, but we should be still able to receive data
  //       in connection-closing states
  if (((s->state != TCP_STATE_ESTABLISHED ||
        s->state == TCP_STATE_FIN_WAIT_1) &&
       s->recv_buf_read_pos == s->recv_buf_write_pos) ||
      len == 0)
    return 0;

  I64 read_pos = s->recv_buf_read_pos;
  I64 write_pos = s->recv_buf_write_pos;

  // I64 avail = (write_pos - read_pos) & (s->recv_buf_size);
  I64 read_total = 0;
  I64 step;

  if (write_pos < read_pos) {
    // We can read up to the end of the buffer
    step = s->recv_buf_size - read_pos;

    if (step > len)
      step = len;

    //"Read %d from %d..end\n", step, read_pos;
    MemCpy(buf, s->recv_buf + read_pos, step);
    buf += step;
    len -= step;
    read_pos = (read_pos + step) & (s->recv_buf_size - 1);
    read_total += step;

    // at this point, (len == 0 || read_pos == 0) must be true
  }

  if (len) {
    step = write_pos - read_pos;

    if (step > len)
      step = len;

    //"Read %d from start+%d..\n", step, read_pos;
    MemCpy(buf, s->recv_buf + read_pos, step);
    buf += step;
    len -= step;
    read_pos += step;
    read_total += step;
  }

  s->recv_buf_read_pos = read_pos;
  return read_total;
}

// This function blocks until at least some data is sent.
// Then it returns if the transmission window or outgoing buffers are full.
I64 TcpSocketSendto(CTcpSocket *s, U8 *buf, I64 len, I64 flags,
                    sockaddr_in *dest_addr, I64 addrlen) {
  no_warn dest_addr; // TODO: should be validated instead, no?
  no_warn addrlen;
  no_warn flags;

  I64 sent_total = 0;

  while (
      (s->state == TCP_STATE_ESTABLISHED || s->state == TCP_STATE_CLOSE_WAIT) &&
      len) {
    I64 can_send = (s->snd_una + s->snd_wnd - s->snd_nxt) & 0xffffffff;

    // TODO: Keep trying
    // Must be tied to a timeout; see RFC793/Managing-the-Window
    // if (s->snd_wnd == 0)
    //  can_send = 1;

    if (can_send == 0) {
      if (sent_total > 0)
        break;
      else {
        // Check unacknowledged outgoing packets, re-transmit as needed
        TcpSocketCheckSendBufs(s);
        Yield;
      }
    } else {
      if (can_send > len)
        can_send = len;

      if (can_send > s->mss)
        can_send = s->mss;

      if (TcpSendData2(s, TCP_FLAG_ACK, buf, can_send) < 0) {
        // No out-buffers available! Handle in the same way as full window:
        // stall until some of the outdoing data is acknowledged.
        if (sent_total > 0)
          break;
        else {
          // Check unacknowledged outgoing packets, re-transmit as needed
          TcpSocketCheckSendBufs(s);
          Yield;
        }
      } else {
        buf += can_send;
        len -= can_send;
      }
    }
  }

  return sent_total;
}

I64 TcpSocketSetsockopt(CTcpSocket *s, I64 level, I64 optname, U8 *optval,
                        I64 optlen) {
  /*if (level == SOL_SOCKET && optname == SO_RCVTIMEO_MS && optlen == 8) {
    s->rcvtimeo_ms = *(optval(I64*));
    return 0;
  }*/

  no_warn s;
  no_warn level;
  no_warn optname;
  no_warn optval;
  no_warn optlen;

  return -1;
}

CTcpSocket *TcpSocket(U16 domain, U16 type) {
  if (domain != AF_INET || type != SOCK_STREAM)
    return NULL;

  CTcpSocket *s = MAlloc(sizeof(CTcpSocket));
  s->sock.accept = &TcpSocketAccept;
  s->sock.bind = &TcpSocketBind;
  s->sock.close = &TcpSocketClose;
  s->sock.connect = &TcpSocketConnect;
  s->sock.listen = &TcpSocketListen;
  s->sock.recvfrom = &TcpSocketRecvfrom;
  s->sock.sendto = &TcpSocketSendto;
  s->sock.setsockopt = &TcpSocketSetsockopt;

  s->state = TCP_STATE_CLOSED;

  s->send_buf_first = NULL;
  s->send_buf_last = NULL;

  s->recv_buf_size = TCP_WINDOW_SIZE;
  s->recv_buf = MAlloc(s->recv_buf_size);
  s->recv_buf_read_pos = 0;
  s->recv_buf_write_pos = 0;

  s->backlog_next = NULL;
  s->backlog_first = NULL;
  s->backlog_last = NULL;
  s->backlog_remaining = 0;

  /*s->rcvtimeo_ms = 0;
  s->recv_maxtime = 0;

  s->recv_buf = NULL;
  s->recv_len = 0;
  s->recv_addr.sin_family = AF_INET;
  s->bound_to = 0;*/
  return s;
}

U0 TcpSocketHandle(CTcpSocket *s, CIPv4Packet *packet, CTcpHeader *hdr,
                   U8 *data, I64 length) {
  U32 seg_len = length;

  if (hdr->flags & TCP_FLAG_FIN)
    seg_len++;
  if (hdr->flags & TCP_FLAG_SYN)
    seg_len++;

  U32 seg_seq = ntohl(hdr->seq);

  if (s->state == TCP_STATE_LISTEN) {
    // A new connection is being opened.

    if ((hdr->flags & TCP_FLAG_SYN) && s->backlog_remaining > 0) {
      //"SYN in from %08X:%d => %08X:%d.\n", packet->source_ip,
      // ntohs(hdr->source_port),
      //    packet->dest_ip, ntohs(hdr->dest_port);
      CTcpSocket *new_socket = TcpSocket(AF_INET, SOCK_STREAM);

      new_socket->local_addr = IPv4GetAddress();
      new_socket->local_port = s->local_port;
      new_socket->remote_addr = packet->source_ip;
      new_socket->remote_port = ntohs(hdr->source_port);

      new_socket->snd_una = 0;
      new_socket->snd_nxt = 0;
      new_socket->snd_wnd = 0;
      new_socket->mss = TCP_DEFAULT_MSS;

      new_socket->rcv_nxt = ++seg_seq;
      new_socket->rcv_wnd = TCP_WINDOW_SIZE;

      new_socket->conntime = tS;

      TcpSend2(new_socket, TCP_FLAG_SYN | TCP_FLAG_ACK);
      new_socket->state = TCP_STATE_SYN_RECEIVED;

      AddTcpSocketToList(new_socket);

      if (s->backlog_last)
        s->backlog_last->backlog_next = new_socket;
      else
        s->backlog_first = new_socket;

      s->backlog_last = new_socket;
      s->backlog_remaining--;
    } else {
      //"REJ %08X:%d (as %08X:%d)\n", packet->source_ip,
      // ntohs(hdr->source_port),
      //    packet->dest_ip, ntohs(hdr->dest_port);
      TcpSend(packet->dest_ip, ntohs(hdr->dest_port), packet->source_ip,
              ntohs(hdr->source_port), seg_seq + 1, seg_seq + 1,
              TCP_FLAG_ACK | TCP_FLAG_RST);
    }

    return;
  }

  if (s->state == TCP_STATE_CLOSED)
    return;

  Bool must_ack = FALSE;

  // Process SYN
  if (hdr->flags & TCP_FLAG_SYN) {
    s->rcv_nxt = ++seg_seq;
    //"Reset ACK to %d\n", s->ack;

    must_ack = TRUE;
  }

  // Validate SEQ
  Bool valid_seq;

  if (seg_len == 0 && s->rcv_wnd == 0) {
    valid_seq = (seg_seq == s->rcv_nxt);
  } else {
    // At least one of these must be true:
    //   RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
    //   RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
    I64 rel_seq = ((seg_seq - s->rcv_nxt) & 0xffffffff);
    I64 rel_seq_end = ((seg_seq + seg_len - 1 - s->rcv_nxt) & 0xffffffff);

    if (rel_seq < s->rcv_wnd || rel_seq_end < s->rcv_wnd)
      valid_seq = TRUE;
    else
      valid_seq = FALSE;
  }

  if (!valid_seq)
    "SEQ error: seg_seq %d, seg_len %d, rcv_nxt %d, rcv_wnd %d\n", seg_seq,
        seg_len, s->rcv_nxt, s->rcv_wnd;

  // Process ACK
  if (hdr->flags & TCP_FLAG_ACK) {
    U32 seg_ack = ntohl(hdr->ack);
    // ACK is acceptable iff SND.UNA < SEG.ACK =< SND.NXT

    I64 rel_ack = ((seg_ack - s->snd_una) & 0xffffffff);
    I64 rel_nxt = ((s->snd_nxt - s->snd_una) & 0xffffffff);

    // RFC 793 is poorly worded in this regard, unacceptable ACK
    // is not the opposite of an acceptible (= new) ACK!
    // TODO: Instead of zero, we should compare rel_ack to some
    // NEGATIVE_CONSTANT, so that we don't unnecessarily try to correct every
    // slightly delayed ACK
    if (/*0 < rel_ack &&*/ rel_ack <= rel_nxt) {
      TcpSocketAckSendBufs(s, seg_ack);

      // Accept ACK
      s->snd_una = seg_ack;

      if (s->state == TCP_STATE_SYN_SENT && (hdr->flags & TCP_FLAG_SYN)) {
        s->state = TCP_STATE_ESTABLISHED;
        s->srtt = tS - s->conntime;
        //"Initial RTT: %f ms", s->srtt * 1000;
      } else if (s->state == TCP_STATE_SYN_RECEIVED) {
        //"Connection established.\n";
        s->state = TCP_STATE_ESTABLISHED;
        s->srtt = tS - s->conntime;
        //"Initial RTT: %f ms", s->srtt * 1000;
      }
    } else {
      // Unacceptable ACK
      "Bad ACK; state %d, seg_ack %d, snd_nxt %d\n", s->state, seg_ack,
          s->snd_nxt;

      if (s->state == TCP_STATE_LISTEN || s->state == TCP_STATE_SYN_SENT ||
          s->state == TCP_STATE_SYN_RECEIVED) {
        // Reset
        TcpSend(packet->dest_ip, ntohs(hdr->dest_port), packet->source_ip,
                ntohs(hdr->source_port), seg_ack, seg_seq + seg_len,
                TCP_FLAG_ACK | TCP_FLAG_RST);
      } else if (TcpIsSynchronizedState(s->state)) {
        // Send a 'corrective' ACK
        must_ack = TRUE;
      }
    }
  }

  // Process RST
  if (hdr->flags & TCP_FLAG_RST) {
    if ((s->state == TCP_STATE_SYN_SENT)) {
      // If acknowledged
      if (s->snd_una == s->snd_nxt) {
        "Connection refused\n";
        s->state = TCP_STATE_CLOSED;
        return;
      }
    } else {
      if (valid_seq) {
        "Connection reset by peer\n";
        s->state = TCP_STATE_CLOSED;
        return;
      }
    }

    "Spurious RST\n";
  }

  // FIXME check remote addr & port

  // Process data
  if (valid_seq) {
    s->snd_wnd = hdr->window_size;

    if (s->state == TCP_STATE_ESTABLISHED || s->state == TCP_STATE_FIN_WAIT_1) {
      I64 write_pos = s->recv_buf_write_pos;
      //"%d in @ %d", length, write_pos;

      // Skip retransmitted bytes
      while (length && seg_seq != s->rcv_nxt) {
        seg_seq = (seg_seq + 1) & 0xffffffff;
        data++;
        length--;
      }

      // ugh!
      I64 i = 0;
      for (i = 0; i < length; i++) {
        I64 next_pos = (write_pos + 1) & (s->recv_buf_size - 1);

        if (next_pos == s->recv_buf_read_pos)
          break;

        s->recv_buf[write_pos] = data[i];
        write_pos = next_pos;
      }

      s->recv_buf_write_pos = write_pos;
      s->rcv_nxt += i;
      //"; %d saved\n", i;

      if (i > 0)
        must_ack = TRUE;

      if (hdr->flags & TCP_FLAG_FIN) {
        must_ack = TRUE;
        s->rcv_nxt++;

        if (s->state == TCP_STATE_ESTABLISHED) {
          s->state = TCP_STATE_CLOSE_WAIT;
        } else if (s->state == TCP_STATE_FIN_WAIT_1 ||
                   s->state == TCP_STATE_FIN_WAIT_2) {
          s->state = TCP_STATE_TIME_WAIT;
        }
        // else { ?? }
      }
    }
  }

  if (must_ack) {
    TcpSend2(s, TCP_FLAG_ACK);
  }
}

I64 TcpHandler(CIPv4Packet *packet) {
  CTcpHeader *hdr;
  U8 *data;
  I64 length;

  I64 error = TcpParsePacket(&hdr, &data, &length, packet);

  if (error < 0)
    return error;

  U16 dest_port = ntohs(hdr->dest_port);
  //"%u => %p\n", dest_port, tcp_bound_sockets[dest_port];

  CTcpSocket *s = GetTcpSocketFromList(packet, hdr);
  if (!s)
    s = tcp_bound_sockets[dest_port];

  // FIXME: should also check that bound address is INADDR_ANY,
  //        OR packet dest IP matches bound address
  if (s != NULL) {
    TcpSocketHandle(s, packet, hdr, data, length);
  } else {
    // TODO: Send RST as per RFC793/Reset-Generation
  }

  return error;
}

U0 TcpInit() {
  I64 i;
  tcp_bound_sockets = MAlloc(65536 * sizeof(CTcpSocket *));
  MemSet(tcp_bound_sockets, 0, 65536 * sizeof(CTcpSocket *));
  tcp_socket_list = MAlloc(65536 * sizeof(CTcpSocketListItem *));
  for (i = 0; i < 65536; i++) {
    tcp_socket_list[i] = CAlloc(sizeof(CTcpSocketListItem));
  }
}

TcpInit;
RegisterL4Protocol(IP_PROTO_TCP, &TcpHandler);
RegisterSocketClass(AF_INET, SOCK_STREAM, &TcpSocket);