Packet Hook Implementation Notes
--------------------------------
WARNING: Packethooks all run as critical scripts. Be sure to consider this
when using them for packets that are used a lot by the client or server!


Racalac 4/17/2004
-Version 3
Added realm parameter to SendAreaPacket

Racalac 8/12/2003
-Version 2: 
Offsets are now 0-based, changed example to reflect plus const offsets.
encoded size for variable length packets is now maintained automatically
changed CreatePacket param list
Added SubPacket entry stuff and example

Racalac 7/13/2003
-Initial Version 1
--------------------------------

Packet Hooks allow the scripter to intercept incoming client messages and
outgoing server messages. This allows the scripter to decide how to implement
the UO client's feature-of-the-week, without having to wait for the POL team to
create a customizable solution for each packet. You will need a UO Network
Protocol document to successfully create packet hook scripts. We will try to
provide one with our future releases that is more up to date than most. Note not 
all client features can be completely implemented in packet hook scripts, some
will require core support, which we will endevour to provide.


Configuration:

To define a packet hook, create a packaged uopacket.cfg: () = arbitrary value,
[] = optional entry.
Packet (packet ID byte)
{
  Length (fixed integer length or 'variable' without quotes)
  [ReceiveFunction (scriptname:functionname)]
  [SendFunction    (scriptname:functionname)]
  [SubCommandOffset (integer)]
  [SubCommandLength (1, 2 or 4)]
}
[Packet...]

Packet ID must be a byte integer, i.e. 12 or 0xAE.

Length MUST be exactly correct for fixed-length packets and in bytes (i.e.
message 0x20 is 19 bytes), or the string 'variable' for variable-length packets
(i.e. 0xAE unicode speech).

ReceiveFunction is the function to intercept a packet coming from a client.

SendFunction is to intercept an outgoing packet created by the core (i.e. player
status update). Normally a Packet element will only define one of these two, 
unless the packet is bi-directional AND the core currently sends the packet. For
example, 0xB8 is the character profile packet, it is bi-directional with
different structure for the client and server versions. Since the core currently
never sends this packet, a SendFunction for this packet is meaningless. A
ReceiveFunction is all you would need to implement the character profile (see
below for example implementation). The method of specifying the function is
exactly the same as vitals.cfg and the vital max, regen, etc functions.

SubCommandOffset is the 0-based offset into the packet that contains the
sub-command ID number, if applicable for this packet. SubCommandLength is the
number of bytes to extract to determine the sub command (ex. 2 for 0xBF, 1 for 
0x12).

To define a SubPacket (an entry for hooking a sub-command of a packet), put this
in a packaged uopacket.cfg:

SubPacket (main packet ID byte)
{
  SubCommandID (sub-command id)
  [ReceiveFunction (scriptname:functionname)]
  [SendFunction    (scriptname:functionname)]
}

SubCommandID is the 2-byte ID to be found at the parent packet's SubCommandOffset.
You need not define Receive and Send functions for the parent packet if you define
subpacket entries. If a subcommand is received or being sent that is not hooked, 
the default behavior will occur.
As normal, the parent packet entry must only be defined once.
Please do not try to hook Sub-Sub-Commands (like the 0xBF 0x06 Party System 
subsubcommands), instead use a case statement in the subcommand hook.


Script Prototype:

As with other syshooks, you must define a "program" named scriptname (from above
cfg examples) in the file scriptname. This program should perform any substantive
initialization for supporting the individual packet hooks in the file. Return 1
to install the hooks, or 0 to disable. This will be run only once on startup.

A Packet Hook Script has the following prototype:

exported function functionname( character, byref packet )

If this is a ReceiveFunction, 'character' is a MobileRef (not and NPC) to the
character that sent the packet. If it is a SendFunction, it is the character the 
packet is being sent to. In either case, 'packet' is a Packet Object (detailed 
below) that provides an interface to the data received or being sent. Also if 
there is no character logged in yet for this client (i.e. you hooked a character 
select packet) this can be uninitialized.

This function should return 0 if you wish POL to perform its normal data
processing on the packet (if any default packet handler exists for this packet
ID), or 1 if you wish POL to do no processing on the packet (i.e. your script
handled it fully). For example, the character profile packet has no internal
packet handler, so returning 0 would have no effect.

It is important that the packet be passed 'byref', especially if you wish POL to
act on a modified packet. Also make sure it is passed byref to any helper
functions you might write. If it is not passed byref, a copy is made, so any
changes you might make to the packet would not be in the calling function's
reference.

Remember, these functions are Run-to-Completion, so be very careful about which
packets you hook (i.e. Walk Request would be insane to hook), and write the
fastest code you can manage to keep your sysload down.


Packet Object Reference

This object should totally replace the UO.EM::SendPacket function which forces 
you to make a text representation of a packet. The packet object allows you to 
set binary data without unnecessary text processing. The Packet Scripting Object 
implements the following methods:

SendPacket(character)
  Sends this packet to character.
  Returns 1 if sent, 0 if NPC or client not ready

SendAreaPacket(x,y,range,realm)
  Sends this packet to all the players in range of x,y
  Returns integer number of clients this packet was successfully sent to
 
GetSize()
  Returns size of packet data in bytes

GetInt8(offset)
  Gets 8-bit (1 byte) value at offset (0-based).
  Returns integer value, or error if offset too high.

GetInt16(offset)
  Gets 16-bit (2 byte) value at offset (0-based).
  Returns integer value, or error if offset too high, or any of the bytes
referenced are past the size of packet. Automatically converts to Big-Endian.
 
GetInt32(offset)
  Gets 32-bit (4 byte) value at offset (0-based).
  Returns integer value, or error if offset too high, or any of the bytes
referenced are past the size of packet. Automatically converts to Big-Endian.
 
GetInt16Flipped(offset)
  Gets 16-bit (2 byte) value at offset (0-based).
  Returns integer value, or error if offset too high, or any of the bytes
referenced are past the size of packet. Automatically converts to Little-Endian.
 
GetInt32Flipped(offset)
  Gets 32-bit (4 byte) value at offset (0-based).
  Returns integer value, or error if offset too high, or any of the bytes
referenced are past the size of packet. Automatically converts to Little-Endian.
 
GetString(offset,length)
  Gets a string (1-byte characters) at offset (0-based) for length characters.
  Returns string, or error if offset too high, or any of the bytes referenced
are past the size of packet.
 
GetUnicodeString(offset,length)
  Gets a 'unicode string' (2-byte characters) at offset (0-based) for length
characters (NOT BYTES).
  Returns a 'unicode string', or error if offset too high, or any of the bytes 
referenced are past the size of packet. Automatically converts to Big-Endian.
 
GetUnicodeStringFlipped(offset,length)
  Gets a 'unicode string' (2-byte characters) at offset (0-based) for length
characters (NOT BYTES).
  Returns a 'unicode string', or error if offset too high, or any of the bytes 
referenced are past the size of packet. Automatically converts to Little-Endian.
 
SetSize(newsize)
  Sets the new size of the packet, possibly destroying data if packet size was
decreased. Updates the encoded size for variable length packets, not allowed
for fixed-length packets.
  Returns the old size of the packet.
 
SetInt8(offset,value)
  Sets an 8-bit (1-byte) value at offset (0-based). If offset is greater than
current size, the packet is resized to fit the new data and the encoded size is 
updated for variable length packets. Resizing is not allowed for fixed-length 
packets.
  Returns 1.

SetInt16(offset,value)
  Sets a 16-bit (2-byte) value at offset (0-based). If offset is greater than
current size, the packet is resized to fit the new data and the encoded size is 
updated for variable length packets. Resizing is not allowed for fixed-length 
packets. Automatically converts to Big-Endian.
  Returns 1.
 
SetInt32(offset,value)
  Sets a 32-bit (4-byte) value at offset (0-based). If offset is greater than
current size, the packet is resized to fit the new data and the encoded size is 
updated for variable length packets. Resizing is not allowed for fixed-length 
packets. Automatically converts to Big-Endian. NOTE: currently there's a compiler 
problem setting 0xFFFFFFFF, it gets converted to 0x7FFFFFFF. Use multiple SetInt16 
or SetInt8 calls for now.
  Returns 1.
 
SetInt16Flipped(offset,value)
  Sets a 16-bit (2-byte) value at offset (0-based). If offset is greater than
current size, the packet is resized to fit the new data and the encoded size is 
updated for variable length packets. Resizing is not allowed for fixed-length 
packets. Automatically converts to Little-Endian.
  Returns 1.
 
SetInt32Flipped(offset,value)
  Sets a 32-bit (4-byte) value at offset (0-based). If offset is greater than
current size, the packet is resized to fit the new data and the encoded size is 
updated for variable length packets. Resizing is not allowed for fixed-length 
packets. Automatically converts to Little-Endian. NOTE: currently there's a compiler 
problem setting 0xFFFFFFFF, it gets converted to 0x7FFFFFFF. Use multiple SetInt16 
or SetInt8 calls for now.
  Returns 1.
 
SetString(offset,string,nullterminate)
  Sets a string value at offset (0-based). If offset plus length of string is
greater than current size, the packet is resized to fit the new data and the 
encoded size is updated for variable length packets. Resizing is not allowed for 
fixed-length packets. Set nullterminate to 1 if you want to automatically append 
a 0 terminator.
  Returns 1.
 
SetUnicodeString(offset,unicode character array,nullterminate)
  Sets a unicode string at offset (0-based). If offset plus length of string
(2*number of members in the array) is greater than current size, the packet is
resized to fit the new data and the encoded size is updated for variable length 
packets. Resizing is not allowed for fixed-length packets. Set nullterminate to 
1 if you want to automatically append a double 0 terminator. Unicode strings in 
eScript are arrays of 2-byte values. See unicode.em for useful functions. CAscZ 
in basic.em is useful for character sets that use the ascii/ansi standard.
Automatically converts to Big-Endian.
  Returns 1.
 
SetUnicodeStringFlipped(offset,unicode character array,nullterminate)
  Sets a unicode string at offset (0-based). If offset plus length of string
(2*number of members in the array) is greater than current size, the packet is
resized to fit the new data and the encoded size is updated for variable length 
packets. Resizing is not allowed for fixed-length packets. Set nullterminate to 
1 if you want to automatically append a double 0 terminator. Unicode strings in 
eScript are arrays of 2-byte values. See unicode.em for useful functions. CAscZ 
in basic.em is useful for character sets that use the ascii/ansi standard.
Automatically converts to Little-Endian.
  Returns 1.
 
TypeOf(packet)
  Returns "Packet"
 
print(packet)
  Returns string of packet contents, i.e. B800140012345678....
 
polsys.em: CreatePacket(type, size)
  Returns a new packet object. Type is the byte command id that always is set as
the first byte (no need to set it yourself). Size is the fixed-length size for 
this packet, or MSGLEN_VARIABLE if it is variable length (no need to figure out 
the size in advance, the packet buffer will be resized as need by using the Set* 
methods).


Using the Packets: ReceiveFunction

Here is a sample implementation of the character profile packet.

Packet 0xB8
{
  Length variable
  ReceiveFunction charprofile:HandleCharProfileRequest
}

use uo;
use os;
use polsys;
use unicode;
use util;

program charprofile()
  Print( "Hooking Character Profile..." );
  return 1;
endprogram

const PROFILE_MSGTYPE := 0xB8;
const PROFILE_TITLE := "Profile for ";
const PROFILE_UPDATE_MODE := 1;
const PROFILE_REQUEST_MODE := 0;
const HEADER_SIZE := 7;
const NULL_SIZE := 1;
const UNULL_SIZE := 2;
const UCHAR_SIZE := 2;

const OFFSET_MSGTYPE := 0;
const OFFSET_MSGLEN := 1;
const OFFSET_MODE := 3;
const OFFSET_SERIAL_OUT := 3;
const OFFSET_SERIAL_IN := 4;
const OFFSET_TITLE_STR := 7;
const OFFSET_CMDTYPE := 8;
const OFFSET_NEW_PROFILE_TEXTLEN := 10;
const OFFSET_NEW_PROFILE := 12;

exported function HandleCharProfileRequest( character, byref packet )
  var size := packet.GetSize();
  var mode := packet.GetInt8(OFFSET_MODE); //mode 0 for request, 1 for update
  var id := packet.GetInt32(OFFSET_SERIAL_IN);  //serial of requested profile 
                                                //character
  var chr := SystemFindObjectBySerial(id);
    if(!chr || !chr.isa(POLCLASS_MOBILE))
      return; //don't bother working on nonexistant or items :P
    endif 

  if(mode == PROFILE_UPDATE_MODE)
    //profile update
    var cmdtype := packet.GetInt16(OFFSET_CMDTYPE); //only 1 command, update
    
    //number of unicode characters
    var msglen := packet.GetInt16(OFFSET_NEW_PROFILE_TEXTLEN);
    
    //updated profile str
    var uctext := packet.GetUnicodeString(OFFSET_NEW_PROFILE,msglen);
    if(chr.serial == character.serial) //don't allow setting profile for others
      SetObjProperty(chr,"profile_uctext",uctext); //set my profile
    endif
  elseif(mode == PROFILE_REQUEST_MODE)
    //profile request, send profile back
    var profile_uctext := GetObjProperty(chr,"profile_uctext");
    if(profile_uctext == error)
      profile_uctext := array; //empty array if no profile was set prev.
    endif
  
    //create the response packet
    var title_str := PROFILE_TITLE + chr.name; //goes at the top of scroll
    
    var outpkt := CreatePacket(PROFILE_MSGTYPE, MSGLEN_VARIABLE);
                
    outpkt.SetInt16(OFFSET_MSGLEN, outpkt.GetSize()); //set the size of the packet
    outpkt.SetInt32(OFFSET_SERIAL_OUT, chr.serial); //set the serial of the character
    outpkt.SetString(OFFSET_TITLE_STR,title_str,1); //set the title string+terminator
  
    //profile packet includes an uneditable string and an editable one.
    //if this is "my" profile, put the profile text in the editable
    //location. if this is another character's profile, put it in the
    //uneditable location. This is why we reserved 2 bytes twice for the
    //terminators. only one of the strings will be filled, the other will
    //only include a 2-byte terminator.
    var uneditable_profile_offset := OFFSET_TITLE_STR+len(title_str)+NULL_SIZE; //edit comes first
    var editable_profile_offset;
  
    if(chr.serial != character.serial)
      //not me, set the string at the uneditable offset
      outpkt.SetUnicodeString(uneditable_profile_offset,profile_uctext,1);
      //calculate the editable string offset
      editable_profile_offset := uneditable_profile_offset +
                                 (profile_uctext.size()*UCHAR_SIZE);
      //set just a double terminator at the editable offset                  
      outpkt.SetInt16(editable_profile_offset,0);
    else
      //it's my profile, no text at uneditable
      outpkt.SetInt16(uneditable_profile_offset,0);
      //editable offset past the 2 byte terminator
      editable_profile_offset := uneditable_profile_offset + UNULL_SIZE;
      //set the unicode text
      outpkt.SetUnicodeString(editable_profile_offset,profile_uctext,1);
    endif

    //send the packet to the _requesting_ character, not the character
    //whose profile this is.
    outpkt.SendPacket(character);
  else
    SysLog("Unknown profile mode: " + mode);
  endif
 
  return 1;
endfunction


Using the Packets: SendFunction

The SendFunction hook is for looking at or modifying a packet POL is sending
before it is actually sent. You must be careful when using this function,
especially making sure the packet's size (from CreatePacket, SetSize, or any of
the Set data functions) is what you expect, as the packet's encoded length (for
variable-length packets) will be set to this size. Also, you MUST NOT use the
SendPacket or SendAreaPacket methods on the passed-in packet with SendFunction.
Rather, edit the packet as you wish, and return 0. POL will send the packet as
normal. You may create and send a different packet (NOT the same Packet ID, else
the send functions will loop) from within this function, but note the sending
order will be created packet, then hooked packet (if you return 0).

The serial of the "source" object is often encoded into the packet. Extract the
32-bit serial and use SystemFindObjectBySerial to get a reference to the object.

Packets sent to all players in a certain area will have their SendFunction
called multiple times. For example, when a player changes warmode, the update is
sent to all the players around him. If you hook this packet with a SendFunction,
it will be called for each player around the changing player.

Example Implementation of Speech LOS Checking:

Packet 0xAE
{
  Length variable
  SendFunction speech_hooks:HandleUCOutgoing
}

use uo;

program speech_hooks()
  Print( "Hooking Outgoing Speech..." );
  return 1;
endprogram

const OFFSET_SERIAL := 3;

exported function HandleUCOutgoing( character, byref packet )
  var serial := packet.GetInt32(OFFSET_SERIAL);
  var source := SystemFindObjectBySerial(serial);
  if(CheckLineOfSight(source, character))
    return 0; //I didn't handle it, send this packet on to character
  else
    return 1; //I handled it, don't send the speech
  endif
endfunction



Example of hooking subcommands:
uopacket.cfg:
Packet 0xBF
{
	Length variable
	SubCommandOffset 3
	ReceiveFunction command_bf:HandleBF  //not necessary, but you can do it
}
SubPacket 0xBF
{
	SubCommandID 0x5 //client screen size
	ReceiveFunction command_bf:HandleSub5
}
SubPacket 0xBF
{
	SubCommandID 0x6 //party scroll commands
	ReceiveFunction command_bf:HandlePartySystem
}

command_bf.src:

use os;

program command_bf()
	Print( "Hooking Command 0xBF..." );
	return 1;
endprogram

//the main HandleBF will catch any subcommand that are not hooked, but
//since you should hook those seperately, it is a good idea to either
//not define this, or simply return 0 to let POL handle it.
exported function HandleBF( character, byref packet )
	print(packet);
	return 0;
endfunction

exported function HandleSub5( character, byref packet )
	print("handled sub5");
	return 1;
endfunction

const OFFSET_PARTY_CMD := 5;
exported function HandlePartySystem( character, byref packet )
	var party_cmd := packet.GetInt8(OFFSET_PARTY_CMD);
	case(party_cmd)
		//.....implementation
	endcase
	return 1;
endfunction


Things To Do:

-packing a packet?
-make uopacket.cfg and packet hook scripts unloadable
-make sure negative numbers are handled correctly. What packet even expects
negative number data?
