Packfile format information.





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============ Introduction and scope ============
================================================

   This document describes the format of Allegro's packfiles from a
   decompression point of view. It does not describe how to do the
   compression (read the source, or ask Google about LZSS).

   file.c says "This compression algorithm is based on the ideas of Lempel
   and Ziv, with the modifications suggested by Storer and Szymanski.", if
   that means anything to you.


============================================
============ Packfile signature ============
============================================

   All compressed packfiles begin with a four byte signature "slh!" (ASCII),
   which in hexadecimal is 0x73, 0x6C, 0x68, 0x21 (in that order).

   Another form of packfiles are uncompressed packfiles, which begin with the
   four byte signature "slh." (ASCII), in hexadecimal 0x73, 0x6C, 0x68, 0x2B.
   The rest of the file is then completely raw. Uncompressed packfiles will
   not be discussed further.


============================================================
============ Format and decompression algorithm ============
============================================================

   Decompression requires a ring buffer of 4096 bytes and a ring index (which
   indexes into the ring buffer). The ring index starts at 4078, assuming
   0-based indices. New bytes may be stored into in the position in the ring
   buffer that the index points to. When a byte is stored, the index is
   incremented by one, wrapping around to zero as necessary.

   Packfiles must be decompressed sequentially. The first byte following the
   signature in the packfile is the "flags" byte, which determines how the
   following "tokens" in the file are to be interpreted.

   Bit 1 (the least significant bit) corresponds to the first token following
   the flags byte. Bit 2 corresponds to the second token, etc. After every
   eight tokens, there is another flag byte and another eight tokens, and so
   on, until the end of file.

   If the bit I of the flags byte is set, then token I is a single byte to be
   sent to the output. In addition, it must put onto the ring buffer and the
   ring index incremented.

   Otherwise, if bit I of the flags byte is not set, then token I is a two
   byte sequence containing an index and a length. The index is the first
   byte OR'd together with the higher 4 bits of the second byte as the higher
   order bits, thus making a 12-bit unsigned number (0-4095). The length is
   the lower 4 bits of the second byte, plus 3.
   In less hand-wavy C syntax:

      index  = byte1 | ((byte2 & 0xF0) << 4);
      length = (byte2 & 0x0F) + 3;

   Then, "length" number of bytes from the ring buffer, starting at "index",
   are to be sent to the output. In addition, all these bytes must be put
   onto the ring buffer, and the ring buffer incremented accordingly.

   Decompression ends at the end of the file. There is no end-of-file
   marker. If the number of tokens is not a multiple of eight, the unused
   bits in the latest flags byte are always zero.


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============ Summary ============
=================================

   In brief, compressed packfiles take the form:

      signature -- 4 bytes

      flags     -- 1 byte
      token 1   -- each token 1 or 2 bytes
      token 2
       ...
      token 8
      flags
       ...
      (repeat flags and tokens 1-8 until EOF)