
   __  ___ ___ ___ ___ _ _   ___ ___
   | \ |_|  |  |_| |__ | |   |__ [_
   |_/ | |  |  | | |   | |__ |__ __]


	  by Shawn Hargreaves



An Allegro datafile is a bit like a zip file in that it consists of lots of 
different pieces of data stuck together one after another, and optionally 
compressed. This means that your game doesn't have to clutter up the disk 
with hundreds of tiny files, and it makes programming easier because you can 
load everything with a single function call at program startup. Another 
benefit is that the LZSS file compression algorithm works much better with 
one large file than with many small ones.

Datafiles have the extension .dat, and can be created and edited with the 
graphical grabber program or the command line dat utility. They can be 
stored as separate files and loaded into memory by the load_datafile() 
function, or you can use dat2s to convert them into asm code which can then 
be linked directly into your executable.

Each datafile contains a number of objects, of varying types. Object types 
are represented by 32 bit integer ID's, which are interpreted as four 
character ASCII strings. These ID's can be constructed with the DAT_ID() 
macro, for example a DATA object is represented by DAT_ID('D','A','T','A'), 
or you can use the predefined DAT_* constants for the standard data types:

   DAT_FILE - "FILE"
      A datafile, which contains a list of other objects. Datafile objects 
      can be nested inside other datafiles, allowing you to create 
      hierarchical structures of any depth.

   DAT_DATA - "DATA"
      A block of binary data. Allegro treats all unknown types as binary 
      data objects, so you don't need to use this ID: you can create custom 
      object formats using whatever ID's you like.

   DAT_FONT - "FONT"
      A font.

   DAT_SAMPLE - "SAMP"
      A digital sound sample.

   DAT_MIDI - "MIDI"
      A MIDI file.

   DAT_PATCH - "PAT "
      A Gravis patch (MIDI instrument).

   DAT_FLI - "FLIC"
      An FLI or FLC animation.

   DAT_BITMAP - "BMP "
      A bitmap.

   DAT_RLE_SPRITE - "RLE "
      A run length encoded sprite.

   DAT_C_SPRITE - "CMP "
      A compiled sprite.

   DAT_XC_SPRITE - "XCMP"
      A mode-X compiled sprite.

   DAT_PALETTE - "PAL "
      A 256 color palette.

   DAT_PROPERTY - "prop"
      An object property (see below). You will never directly encounter this 
      object type, but you should be aware that it is treated specially by 
      the datafile code.

   DAT_INFO - "info"
      The grabber utility uses this object to store information about the 
      datafile. Like property objects, you ought never to encounter it, but 
      you should avoid using the ID for any custom object formats you create.

   DAT_END - -1
      Special marker used to indicate the end of a datafile.

Each object can have any number of properties attached to it. These are 
ASCII strings describing attributes of the object, such as its name and 
where it came from. Like the objects themselves, properties are identified 
by 32 bit integer ID's which are constructed from four character strings by 
the DAT_ID() macro. Allegro defines the standard properties:

   "NAME"
      The name of the object.

   "ORIG"
      The object's origin, ie. the name of the file from which it was 
      grabbed.

   "DATE"
      A timestamp, used by the update command in the grabber and dat 
      utilities. This is the modification time of the file from which the 
      object was grabbed, in "mm-dd-yy, hh:mm" format.

   "XPOS"
      For bitmap objects which were grabbed from part of a larger image, the 
      x position of the origin within the parent bitmap.

   "YPOS"
      For bitmap objects which were grabbed from part of a larger image, the 
      y position of the origin within the parent bitmap.

   "XSIZ"
      For bitmap objects which were grabbed from part of a larger image, the 
      width of the selected region.

   "YSIZ"
      For bitmap objects which were grabbed from part of a larger image, the 
      height of the selected region.

The properties "PACK", "HNAM", "HPRE", "BACK", "XGRD", and "YGRD" are also 
used internally by the grabber, but you can use whatever other ID's you like 
to store custom information about your objects.



===================================
======== Using the Grabber ========
===================================


Various options can be set using the buttons and text fields at the top of 
the screen. You can edit the name of the datafile, the name of the header 
file for exporting object indexes (leave this blank if you don't want to 
output a header), and the prefix string for the header file definitions. You 
can change the grid settings for grabbing bitmaps, and alter the compression 
mode (see below). You can also enable or disable backups: if this box is 
checked, the old version will be renamed to a .bak extension when datafiles 
are saved.

The contents of the datafile are listed in the box at the bottom left of the 
screen, and can be selected with the mouse or arrow keys. Multiple objects 
can be selected by holding down the shift or control keys while you click 
on the list or move the cursor. The selected object can be edited with 
commands from the Object menu, or using the shortcut menu produced by 
pressing Esc or right-clicking on an object. Double-clicking on an object 
performs a function which varies depending on the type of the object. 
Bitmaps and fonts are displayed full-screen (use the plus and minus keys to 
zoom in and out), samples and MIDI files are played, and palettes are 
selected (meaning that they will be used when displaying and exporting 
bitmaps).

New objects can be created using the menus or by pressing Insert while the 
item list has the input focus. To make nested datafiles, create a FILE 
object (New/Datafile), and select it before inserting other objects.

To insert information into a datafile you must first create an object of the 
appropriate type, and then select the grab command (keyboard shortcut 
ctrl+G). For most objects this will bring up a file selector for you to 
select the file to read, but for graphic objects (bitmaps, RLE sprites, 
compiled sprites, and palettes) it will grab from the current contents of 
the image buffer, so you must previously have read a picture into this 
buffer with the File/Read Bitmap command (keyboard shortcut ctrl+G). You can 
use the mouse or arrow keys to select which portion of the image to grab. 
With the mouse, select the top left corner, click and hold the left mouse 
button, and move to the bottom right corner before releasing the button. 
Press the right mouse button to cancel. With the keyboard, use the arrow 
keys to select the top left corner, press Space or Enter, adjust the size 
with the arrow keys, and press Space or Enter again. By default the position 
will snap to a 16x16 grid, which can be adjusted by changing the values in 
the X-grid and Y-grid fields. Alternatively you can use the Box Grab 
command, in which case you should draw a bounding box in color #255 around 
your sprites, and can then just click once inside a box to grab the contents.

Note that palette data is not stored along with bitmap and sprite objects. 
To store the entire contents of a PCX or BMP file, you will need to create 
both bitmap and palette objects, and grab data into both of them. When you 
reload the datafile the bitmap will probably be displayed with the wrong 
palette. This can be corrected by double-clicking on the palette object, 
which will select its contents as the current palette, meaning that it will 
be used when displaying and exporting bitmaps.

The properties of the selected object are listed in the box to the right of 
the item list. These can be edited by double-clicking on one of the 
properties, and deleted by selecting one and pressing Del. You can insert 
new properties by pressing Insert while the property list has the input 
focus, or using the Object/Set Property command. Object names are stored as 
NAME properties, so the rename command is simply a shortcut for editing this 
property.

To simplify the process of grabbing several related images from a single 
bitmap (for example a set of frames which form an animation), you can use 
the File/Grab from Grid command. Like the normal bitmap grab command, this 
uses data from the image buffer, so you must read in a bitmap before you use 
it. You will then be able to adjust the grabbing parameters, enter a name 
for the new objects, and choose the type of object to create (bitmap, RLE 
sprite, or compiled sprite). Because several objects may be created, their 
names will be formed by adding a number to the end of the name you supply, 
for example if you enter "a_picture", the grabber will create the objects 
"a_picture000", "a_picture001", etc. There are two grabbing modes: using 
cutouts of color 255, and using a regular grid. The regular grid option 
simply divides the bitmap up into a set of equally sized tiles, using the 
specified grid size, and grabs each of these as a separate object. If you 
set the Skip Empties flag, the grabber will ignore tiles that don't contain 
any data (ie. those that are a single solid color). The color 255 option is 
more flexible. It expects the bitmap to contain information describing the 
position and size of each tile, in the form of a bounding box drawn in color 
255. The most reliable way to do this is to fill all the image except the 
parts you want with color 255, but the grabber should be able to understand 
more complicated layouts, even if you simply draw color 255 lines along the 
top and left edges of the area you want to be grabbed. For truecolor images 
where color 255 is not particularly meaningful, use a cyan bounding box 
(maximum blue and green, zero red), with a single yellow (maximum red and 
green, zero blue) pixel in the top left corner of the box.

By default, the grabber will run in a 640x480 resolution, using the highest 
color depth possible on your graphics card. If you want to override this, 
you can specify an alternative color depth or resolution on the commandline, 
eg. "grabber -8" for a 256 color mode, "grabber -16" for 16 bit hicolor 
graphics, "grabber -320x200" to use VGA mode 13h, or "grabber 1024x768". 
Warning: editing datafiles that contain truecolor graphics is very slow in 
256 color video modes, and the grabber is not really usable in resolutions 
lower than 640x400.

You can configure the grabber to use external tools for editing data, by 
setting some variables in the allegro.cfg file. These are in the form 
"type=command", where the type is a four letter object ID, and the command 
is whatever program you want to be invoked to edit this kind of data. For 
these variables to be seen, the allegro.cfg file must either be in the same 
directory as the grabber executable, or in the directory pointed to by your 
ALLEGRO environment variable. To invoke this feature, select the Shell Edit 
command or press ctrl+Z. The grabber will try to invoke the tool on the 
original version of the file if it knows where you grabbed the data from in 
the first place, and otherwise it will write the object out into a temporary 
file prior to editing.



==================================
======== The DAT archiver ========
==================================


As an alternative to the graphical grabber program, you can use the command 
line dat utility. This accepts the following options:

   '-a <files>'
      Adds the named files to the datafile, for example:

	 dat myfile.dat -a title.pcx scream.wav

      If the objects are already present in the datafile, their current 
      contents will be replaced. Names for the new objects will be generated 
      from the input filenames, and the object type will be detected from 
      the file extensions. In some cases this is ambiguous, for example a 
      PCX file can be read as a bitmap, RLE sprite, compiled sprite, or font 
      object, so you may need to explicitly specify the object type with the 
      '-t' flag. For example, to insert alien.pcx as an RLE sprite, use the 
      command:

	 dat myfile.dat -a alien.pcx -t RLE

   '-bpp colordepth'
      Specifies which color format bitmap data should be grabbed in (valid 
      depths are 8, 15, 16, 24, and 32 bits per pixel).

   '-c0' - no compression
   '-c1' - compress objects individually
   '-c2' - global compression on the entire datafile
      Sets the compression mode (see below). These can be used on their own 
      to convert a datafile from one format to another, or in combination 
      with any other options.

   '-d <objects>'
      Deletes the named objects from the datafile.

   '-dither'
      Dithers graphics when reducing from truecolor to hicolor formats.

   '-e <objects>'
      Extracts the named objects from the datafile. To extract everything, 
      use the wildcard * as the object name. To set the output filename or 
      directory, use the '-o filename' option. For example, to extract an 
      object called TITLE_SCREEN to the file title.pcx, use the command:

	 dat myfile.dat -e title_screen -o title.pcx

      To extract the entire contents of the datafile to the directory 
      c:\output, use:

	 dat myfile.dat -e * -o c:\output\

   '-h outputfile.h'
      Sets the output header file, for exporting object index definitions. 
      This may be used on its own to produce a header file from an existing 
      datafile, or in combination with any other commands. You can also use 
      the '-p prefixstring' option to set a prefix string for the object 
      definitions.

   '-k'
      Keep original names while grabbing objects. Without this switch, a 
      file called image.pcx will be imported as an object called IMAGE_PCX, 
      to ensure that all the object names are valid symbols for the output 
      header defines.

   '-l'
      Lists the contents of the datafile. This can be combined with the '-v' 
      option to list object properties along with the names, and you can 
      specify particular objects to produce a partial listing.

   '-m dependencyfile'
      Writes a set of makefile dependencies into the specified file, which 
      can be used to automatically update the file whenever any of the 
      source data changes.

   '-s0' - no strip: save everything
   '-s1' - strip grabber specific information from the file
   '-s2' - strip all object properties and names from the file
      Sets the strip mode (see below). These can be used on their own to 
      strip properties from the datafile, or in combination with any other 
      options.

   '-u'
      Updates the contents of the datafile. See below.

   '-v'
      Selects verbose mode. This can be used in combination with any other 
      options to produce more detailed output.

   '<objects> PROP=value'
      Sets properties for the specified objects. This works like environment 
      variables, in that setting a property to an empty string removes it. 
      Because object names are stored as NAME properties, you can use this 
      command to rename objects. For example, to rename MY_OBJECT to 
      WHAT_A_SILLY_NAME, use the command:

	 dat myfile.dat my_object NAME=what_a_silly_name

      You can use the wildcard * to apply the property to everything in the 
      file, so to remove the ORIG property from the entire datafile, you 
      could execute:

	 dat myfile.dat * ORIG=

You can create hierarchical nested datafiles by inserting one datafile into 
another with the '-a' command. Objects in the nested datafile can then be 
referred to by as "parentname/objectname". For example if the datafile 
myfile.dat contains a nested datafile called nestedfile, which contains a 
bitmap called thepicture, you could export the bitmap with the command:

   dat myfile.dat -e nestedfile/thepicture -o output.pcx



==================================
======== Misc Information ========
==================================


Datafiles can be saved using any of three compression types, selected from 
the list at the top right of the grabber screen, or with the '-c0', '-c1', 
and '-c2' options to dat. With type 0, the data is not compressed at all. 
Type 1 compresses each object individually, while type 2 uses global 
compression over the entire file. As a rule, global compression will give 
slightly better results than per-object compression, but it should not be 
used if you intend to dynamically load specific objects with the 
load_datafile_object() function.

There are also three strip modes for saving datafiles, selected with the 
File/Save Stripped command in the grabber, or using the '-s0', '-s1', and 
'-s2' options to dat. With zero stripping, all object properties are written 
to the datafile, which is normally what you will want. With strip mode 1, 
properties specific to the grabber (the ones describing the origins and 
dates of each object) are removed, which will marginally reduce the file 
size, but will prevent the update command from working. For the smallest 
possible file sizes, strip mode 2 removes all properties, including object 
names and any custom properties you have added. This level of stripping 
should obviously be used with extreme caution, although in some cases it may 
be possible to recover the object names even after they have been stripped 
out of the datafile. If the grabber and dat utilities cannot find any name 
properties in a datafile, they will look for a header (.h) file with the 
same name, and attempt to parse this to recover the names. This is far from 
foolproof, and will not work for nested datafiles, but in some situations it 
allows the names to be read from the index definition header.

Both the grabber and the dat utility have an update command, which works 
through the datafile checking if any objects have changed, and replacing 
those which are out of date. This depends on the origin and date properties 
which were set when the data was grabbed in the first place, so it won't 
work if these properties have been stripped out of the file. This command 
can be very useful if you build a datafile containing hundreds of objects 
grabbed from external bitmaps, and later go back and change some of these 
bitmaps. Rather than trying to figure out which objects are out of date and 
needing to manually re-grab all the affected data, the update command will 
automatically repair the outdated objects.

Fonts can be read from libgrx .FNT files, from 8x8 or 8x16 BIOS .FNT files, 
and from PCX and BMP bitmaps. When reading a font from a bitmap file the 
size of each character is determined by the layout of the image, which 
should be a rectangular grid containing all the ASCII characters from space 
(32) up to the tilde (126). The spaces between each letter should be filled 
with color 255. If each character is sized exactly 8x8 or 8x16 the grabber 
will create a fixed size font, otherwise it will make a proportional font. 
Probably the easiest way to get to grips with how this works is to load up 
the demo.dat file and export the TITLE_FONT into a PCX file. Have a look at 
the resulting picture in your paint program: that is the format a font 
should be in...

Datafiles can be encrypted with a password, by typing it into the 
"Password:" field in the grabber, or using the '-007 password' option to the 
dat utility. Passwords may be up to 256 characters in length, and are case 
sensitive. Encrypted files _cannot_ be read without the password, so please 
don't forget it and then come crying to me for help :-) To read an encrypted 
file into your program, call the packfile_password() function before 
load_datafile(). It is also a good idea to call packfile_password(NULL) 
afterwards, to set everything back to normal.



=====================================
======== Accessing Datafiles ========
=====================================


In order to access the contents of a datafile, you will need to know where 
each object is located. The easiest way to do this is by integer index, 
using an automatically generated header file. With the grabber, type a name 
into the "Header:" field, and the object indexes will be written to this 
file whenever the datafile is saved. With the dat utility, use the '-h' 
option, eg. "dat filename.dat -h filename.h". The header will define C 
preprocessor symbols for each object in the datafile, for example:

   #define SOME_DATA                        0        /* DATA */
   #define SOME_MORE_DATA                   1        /* DATA */

To prevent name conflicts, you can specify a prefix string for these 
definitions by typing it into the "Prefix:" field in the grabber or using 
the '-p' option to dat.

To load a datafile into memory, call the function:

   DATAFILE *load_datafile(char *filename);

This will load the entire file, returning a pointer to it, or NULL on error. 
When the data is no longer required, the entire thing can be destroyed by 
calling:

   void unload_datafile(DATAFILE *dat);

When you load a datafile, you will obtain a pointer to an array of DATAFILE 
structures:

   typedef struct DATAFILE
   {
      void *dat;                    - pointer to the actual data
      int type;                     - object type ID
      long size;                    - size of the data, in bytes
      DATAFILE_PROPERTY *prop;      - list of object properties
   } DATAFILE;

The only really important piece of information here is the dat field, which 
points to the contents of the object. What type of data this is will depend 
on the type of object: for bitmaps it will be an Allegro BITMAP structure, 
for RLE sprites an RLE_SPRITE, for fonts a FONT structure, etc. If you are 
programming in C you can pass this pointer directly to the relevant Allegro 
library functions, but if you are using C++ you will need to cast it to the 
appropriate type to prevent the compiler giving a warning.

For example, if you have a datafile called myfile.dat, which contains a 
bitmap called COOL_PICTURE, and you have used it to produce a header called 
myfile.h, you could display the bitmap with the code:

   #include "myfile.h"

   void show_the_bitmap()
   {
      DATAFILE *dat;
      BITMAP *bmp;

      dat = load_datafile("myfile.dat");
      if (!dat) {
	 /* report an error! */
	 return;
      }

      bmp = (BITMAP *)dat[COOL_PICTURE].dat;
      blit(bmp, screen, 0, 0, 0, 0, bmp->w, bmp->h);
      unload_datafile(dat);
   }

If a datafile contains nested child datafiles, the header will prefix the 
names of objects in the sub-files with the name of their parent datafile. It 
will also define a count of the number of objects in the child file, which 
may be useful if for example the child datafile contains several bitmaps 
which form a 'run' animation, and you want your code to automatically 
adjust to the number of frames in the datafile. 

For example, the datafile:

   "FILE" - NESTED_FILE
	    |- "BMP" - A_BITMAP
	    |- "FONT" - A_FONT
   "DATA" - SOME_DATA
   "DATA" - SOME_MORE_DATA

produces the header:

   #define NESTED_FILE                      0        /* FILE */

   #define NESTED_FILE_A_BITMAP             0        /* BMP  */
   #define NESTED_FILE_A_FONT               1        /* FONT */
   #define NESTED_FILE_COUNT                2

   #define SOME_DATA                        1        /* DATA */
   #define SOME_MORE_DATA                   2        /* DATA */

The main datafile contains three objects (NESTED_FILE, SOME_DATA, and 
SOME_MORE_DATA) with consecutive indexes, while the child datafile contains 
the two objects A_BITMAP and A_FONT. To access these objects you need to 
reference both the parent and child datafiles, eg:

   DATAFILE *dat = load_datafile("whatever.dat");
   DATAFILE *nested = (DATAFILE *)dat[NESTED_FILE].dat;
   FONT *thefont = (FONT *)nested[NESTED_FILE_A_FONT].dat;

If you need to access object property strings from within your program, you 
can use the function:

   char *get_datafile_property(DATAFILE *dat, int type);

This will return a pointer to the property string if it can be found, and an 
empty string (not null!) if it does not exist. One possible use of this 
function is to locate objects by name, rather than using the indexes from a 
header file. The datafile array is ended by an object of type DAT_END, so to 
search the datafile dat for the object "my_object" you could use the code:

   for (i=0; dat[i].type != DAT_END; i++) {
      if (stricmp(get_datafile_property(dat+i, DAT_ID('N','A','M','E')),
		  "my_object") == 0) {
	 /* found the object at index i */
      }
   }
   /* not found... */

It is also possible to selectively load individual objects from a datafile, 
with the function:

   DATAFILE *load_datafile_object(char *filename, char *objectname);

This searches the datafile for an object with the specified name, so 
obviously it won't work if you strip the name properties out of the file. 
Because this function needs to seek through the data, it will be extremely 
slow if you have saved the file with global compression. If you are planning 
to load objects individually, you should save the file uncompressed or with 
individual compression per-object. Because the returned datafile points to a 
single object rather than an array of objects, you should access it with the 
syntax datafile->dat, rather than datafile[index].dat, and when you are done 
you should free the object with the function:

   void unload_datafile_object(DATAFILE *dat);

Alternatively, the packfile functions can open and read directly from the 
contents of a datafile object. You do this by calling pack_fopen() with a 
fake filename in the form "filename.dat#object_name". The contents of the 
object can then be read in an identical way to a normal disk file, so any of 
the file access functions in Allegro (eg. load_pcx() and set_config_file()) 
can be used to read from datafile objects. Note that you can't write to 
datafiles in this way: the fake file is read only. Also, you should save the 
file uncompressed or with per-object compression if you are planning on 
using this feature. Finally, be aware that the special Allegro object types 
aren't the same format as the files you import the data from, so if for 
example you want to use load_pcx to read an image from a datafile, you 
should import it as a binary data chunk rather than as a BITMAP object.

If you have appended a datafile to the end of your executable with the 
exedat utility, use load_datafile("#") to read the entire thing into memory, 
load_datafile_object("#", "object_name") to load a specific object, and 
pack_fopen("#object_name", F_READ) to read one of the objects directly with 
your own code.

By default, all graphic objects loaded from a datafile will be converted 
into the current color depth. This conversion may be both lossy and very 
slow, particularly when reducing from truecolor to 256 color formats, so you 
may wish to disable it by calling set_color_conversion(COLORCONV_NONE) or 
set_color_conversion(COLORCONV_PARTIAL) before your call to load_datafile().



=====================================
======== Compiling Datafiles ========
=====================================


The utility dat2s can be used to convert a datafile into an asm (.s) source 
file, which can then be assembled and linked into your program. This avoids 
the need for a separate datafile to accompany your program, and means the 
data will automatically be loaded into memory at startup. You should be 
aware, though, that large datafiles can take a long time to compile, and 
that it is not possible to compress data which is compiled in this way.

The simplest way to invoke dat2s is with the command:

   dat2s filename.dat -o output.s

The resulting asm file can then be assembled with the command:

   gcc -c output.s

This will produce an object module called output.o, which can be linked into 
your program, for example:

   gcc myprog.c -o myprog.exe output.o -lalleg

Your program can then access the contents of the datafile as simple global 
variables. Definitions for these variables can be obtained by telling dat2s 
to output a header file as well as the asm file, with the '-h' option. You 
can also use '-p' to set a prefix string for all the object names. For 
example, when applied to the datafile:

   "BMP"  - A_BITMAP
   "BMP"  - ANOTHER_BITMAP
   "SAMP" - EXPLODE
   "PAL"  - SOME_COLORS
   "FONT" - THE_FONT

the command:

   dat2s filename.dat -o output.s -h output.h -p item

produces the header:

   extern BITMAP item_a_bitmap;
   extern BITMAP item_another_bitmap;
   extern SAMPLE item_explode;
   extern PALETTE item_some_colors;
   extern FONT item_the_font;
   extern DATAFILE item_data[];

You can refer to these objects directly, for example:

   blit(&item_a_bitmap, screen, 0, 0, 0, 0, SCREEN_W, SCREEN_H);

Alternatively, you can use the datafile array for compatibility with code 
that was originally written for separately loaded datafiles, with the 
standard syntax item_data[index].dat.



================================
======== Custom Objects ========
================================


Some of the objects in a datafile, for example palettes and FLI animations, 
are simply treated as blocks of binary data, but others are loaded into 
special formats such as bitmap structures or compiled sprites. It is 
possible to extend the datafile system to support your own custom object 
types, eg. map objects for a tile based engine, or level data for a platform 
game. Obviously the grabber has no way of understanding this data, but it 
will allow you to import binary data from external files, so you can grab 
information produced by your own utilities. If you are happy with the data 
being loaded as a simple binary block, that is all you need to do, but if 
you need to load it into a specific structure, read on...

Your custom objects must be given a unique type ID, which is formed from 
four ASCII characters (by convention all uppercase A-Z). If you don't use 
all four characters, the string should be padded with spaces (ASCII 32). You 
should use this ID when creating the objects in the grabber (select 
New/Other and type in the ID string), and in your code you should define an 
identifier for the type, eg:

   #define DAT_MAPDATA  DAT_ID('M','A','P','D')

You then need to write functions for loading and destroying objects of this 
type, in the form:

   void *load_mapdata(PACKFILE *f, long size)
   {
      /* Allegro will call this function whenever an object of your custom 
       * type needs to be loaded from a datafile. It will be passed a 
       * pointer to the file from which the data is to be read, and the size 
       * of the object in bytes. It should return a pointer to the loaded 
       * data, which will be stored in the dat field of the datafile object 
       * structure, or NULL if an error occurs. The file will have been 
       * opened as a sub-chunk of the main datafile, so it is safe to read 
       * past the end of the object (if you attempt this, Allegro will 
       * return EOF), and it is also safe to return before reading all the 
       * data in the chunk (if you do this, Allegro will skip any unused 
       * bytes before starting to read the next object). You should _not_ 
       * close the file when you are done: this will be handled by the 
       * calling function. To clarify how all this works, here's an example 
       * implementation of a null-terminated string object:
       */

      #define MAX_LEN  256

      char buf[MAX_LEN];
      char *p;
      int c;

      for (c=0; (c<MAX_LEN-1) && (!pack_feof(f)); c++)
	 buf[c] = pack_getc(f);

      buf[c] = 0;

      p = malloc(c+1);
      strcpy(p, buf);

      return p;
   }

   void destroy_mapdata(void *data)
   {
      /* Allegro will call this function whenever an object of your custom 
       * type needs to be destroyed. It will be passed a pointer to the 
       * object (as returned by the load function), and should free whatever 
       * memory the object is using. For example, the simple string object 
       * returned by the above loader could be destroyed with the code:
       */

      if (data)
	 free(data);
   }

Finally, before you load your datafile you must tell Allegro about the 
custom format, by calling:

   register_datafile_object(DAT_MAPDATA, load_mapdata, destroy_mapdata);

It is also possible to integrate support for custom object types directly 
into the grabber and dat utilities, by copying some special files into the 
tools/plugins directory. This can be used to add whole new object types and 
menu commands, or to provide additional import/export routines for the 
existing formats. See tools/plugins/plugins.txt for an overview of how to 
write your own grabber plugins.



=================================
======== The File Format ========
=================================


In case anyone wants to do some serious hackery, and for my own future 
reference, here are some details of the innards of the datafile format.

Note that this is different to the datafile format used by Allegro versions 
2.1 and earlier. Allegro can still load files from the old format, but it 
was much less flexible and didn't support nice things like object 
properties, so you should load any old files into the grabber and save them 
out again to convert to the new format.

Nb. if all you want to do is write a utility that manipulates datafiles in 
some way, the easiest approach is probably to use the helper functions in 
datedit.c, which are currently shared by the dat, dat2s, and grabber 
programs. These functions handle loading, saving, inserting and deleting 
objects, and modifying the contents of datafiles in various ways, but life 
is too short for me to bother documenting them all here. Look at the 
source...

Anyway. All numbers are stored in big-endian (Motorola) format. A datafile 
begins with one of the 32 bit values F_PACK_MAGIC or F_NOPACK_MAGIC, which 
are defined in allegro.h. If it starts with F_PACK_MAGIC the rest of the 
file is compressed with the LZSS algorithm, otherwise it is uncompressed.
This magic number and optional decompression can be handled automatically by 
using the packfile functions and opening the file in F_READ_PACKED mode. 
After this comes the 32 bit value DAT_MAGIC, followed by the number of 
objects in the root datafile (not including objects nested inside child 
datafiles), followed by each of those objects in turn.

Each object is in the format:

   OBJECT =
      var    - <property list>      - any properties relating to the object
      32 bit - <type ID>            - object type ID
      32 bit - <compressed size>    - size of the raw data in the file
      32 bit - <uncompressed size>  - see below
      var    - <data>               - the contents of the object

The property list can contain zero or more object properties, in the form:

   PROPERTY =
      32 bit - <magic>              - "prop"
      32 bit - <type ID>            - property type ID
      32 bit - <size>               - size of the property string, in bytes
      var    - <data>               - property string, _not_ null-terminated

If the uncompressed size field in an object is positive, the contents of the 
object are not compressed (ie. the raw and compressed sizes should be the 
same). If the uncompressed size is negative, the object is LZSS compressed, 
and will expand into -<uncompressed size> bytes of data. The easiest way to 
handle this is to use the pack_fopen_chunk() function to read both the raw 
and compressed sizes and the contents of the object.

The contents of an object vary depending on the type. Allegro defines the 
standard types:

   DAT_FILE =
      32 bit - <object count>       - number of objects in the sub-file
      var    - <object list>        - objects in the same format as above

   DAT_FONT =
      16 bit - <font size>          - 8, 16, or -1 (proportional)

      for 8x8 fonts:
	 unsigned char[95][8]       - 8x8 bit-packed font data

      for 8x16 fonts:
	 unsigned char[95][16]      - 8x16 bit-packed font data

      for proportional fonts:
	 95x {
	    16 bit - <width>        - character width 
	    16 bit - <height>       - character height
	    var    - <data>         - character data (8 bit pixels)
	 }

   DAT_SAMP =
      16 bit - <bits>               - sample bits (negative for stereo)
      16 bit - <freq>               - sample frequency
      32 bit - <length>             - sample length
      var    - <data>               - sample data

   DAT_MIDI =
      16 bit - <divisions>          - MIDI beat divisions
      32x {
	 32 bit - <length>          - track length, in bytes
	 var    - <data>            - MIDI track data
      }

   DAT_FLI =
      var - <data>                  - FLI or FLC animation, standard format

   DAT_BITMAP =
   DAT_C_SPRITE =
   DAT_XC_SPRITE =
      16 bit - <bits>               - bitmap color depth
      16 bit - <width>              - bitmap width
      16 bit - <height>             - bitmap height
      var    - <data>               - bitmap data

      Valid color depths are 8, 15, 16, 24, and 32. Both 15 and 16 bit 
      images are stored in 5.6.5 RGB format, and 24 and 32 bit images as 
      8.8.8 RGB.

   DAT_RLE_SPRITE =
      16 bit - <bits>               - sprite color depth
      16 bit - <width>              - sprite width
      16 bit - <height>             - sprite height
      32 bit - <size>               - data size, in bytes
      var    - <data>               - RLE compressed sprite data

      Valid color depths are 8, 15, 16, 24, and 32. Both 15 and 16 bit 
      images are stored in 5.6.5 RGB format with 16 bit skip counts and EOL 
      markers, and 24 and 32 bit images as 8.8.8 RGB. with 32 bit skip 
      counts and markers.

   DAT_PALETTE =
      256 x {
	 8 bit - <red>              - red component, 0-63
	 8 bit - <green>            - green component, 0-63
	 8 bit - <blue>             - blue component, 0-63
	 8 bit - <pad>              - alignment padding
      }

I think that covers everything.
