=== AMB ARCHIVE FORMAT ===

An AMB file (Ancient Machines Book) is an extremely lightweight file format
meant to store any kind of hypertext documentation that may be comfortably
viewed even on the most ancient PCs: technical manuals, books, etc. Think of
it as a retro equivalent of a *.CHM help file. The AMB format is designed to
allow for some limited formatting, support internal links and require very
little processing power to read, so a reader may be run even on the oldest
IBM PC. The format also strives for simplicity of implementation.

The AMB file is a container - one could say it is a very simplistic archive
format. It starts with a 4-bytes format signature (magic value) "AMB1". Then
comes a 2-bytes number that tells how many files are present in the container,
followed by the list of all files: each file is described by a file entry.
All values are little-endian.

  offset
    0      format signature: "AMB1"
    4      files count (16-bit value)
    6      FILE ENTRY #1
           FILE ENTRY #2
           FILE ENTRY #3
           ....
           DATA

Each file entry is a 20-bytes structure:
  offset
    0      filename, 12 characters, zero-padded ("FILE.EXT\0\0\0\0")
   12      offset where this file starts (32 bits)
   16      file length, in bytes (16 bits)
   18      BSD sum (16-bit) of the file

The AMB archive is expected to contain a set of AMA (Ancient Machines Article)
files, and optionally a title file and an index dictionnary.


=== DOCUMENT TITLE ===

The AMB title is a string that may be displayed as the document's main title.
To set such title, the AMB archive has to contain a file named simply 'title'
that would contain the text. The title string should not be longer than
64 characters, anything longer may be truncated by the reader.


=== AMA FORMAT ===

The AMA format is a text-based file format. For guaranteed interoperability
with old machines, its maximum allowed size is 65535 bytes (ie. 2^16 - 1).
Larger contents must be segmented into a set of two or more AMA articles.

An AMB reader must display content with a 78-characters width, hence an AMA
article must not contain any line longer than 78 displayable characters.Lines
longer than this limit will be truncated by the client reader.

AMA articles may contain control codes. A control code is a characters pair,
where the first is a percent (%) character. Possible control codes:

  %t       normal text follows (default state)
  %h       heading follows
  %l       link follows (filename ended by a ':', followed by a description)
  %!       notice/warning follows
  %b       boring text follows (usually displayed grey on grey)
  %#       internal comment follows (hidden text)
  %%       display a percent character

It is important to note that the current text mode is reset to %t at the end
of every line, hence there is no need to prefix a line of text with %t.

Line endings may be either LF or CR/LF. The former is recommended, as it is
more compact.


=== INDEX DATA ===

On top of AMA files, the AMB archive may contain a file named DICT.IDX. This
file, if it exists, provides indexing metadata to allow the client to perform
fast and efficient full-text searches across the AMB book.

The index file contains a hash table: a serie of 256 16-bit indexes, where
each index points to a region of the index structure that contains a list of
words (LoW). The index (0..255) itself is an 8 bits hash based on the length
of the word and its characters. The checksum is made of two nibbles: LC.
The high nibble (L) is the length of the word minus 2, while the low nibble
(C) is a simple checksum of all the word's characters XORed together. This
algorithm can be formalized as follows:

  ((wordlen - 2) << 4) | ((a & 15) XOR (b & 15) XOR (...))

For example, the word "Disk" would end up being indexed under value 0x25,
because:

            ((4 - 2) << 4) | ((D & 15) XOR (i & 15) XOR (s & 15) XOR (k & 15))
translates to:    (2 << 4) | (4 XOR 9 XOR 3 XOR 11)
which leads to:         32 | 5
resulting in:           37 = 0x25

After the index we can find the pointer to the words list. A pointer is a 16
bits file offset from the index structure start.

It needs to be noted that words of less than 2 characters and more than 17
characters cannot be indexed. The presented algorithm has also the interesting
side-effect of indexing low and high caps of the ranges a..z and A..Z
identically. An important limitation is the fact that the list of words (LoW)
is restricted by the 16-bit addressing offset, which means that all LoWs must
start at an offset within the first 64 KiB of the file.

Now that we know the offset at which our LoW starts, we can read the words.
First go to the offset, and read a single 16 bits word. Its value contains the
number of words in the list. Then, read the words one after another (note that
all words in the list have the same length, and you know this length already).
Words are always written in lower case characters. Each word is followed by a
1-byte value that tells how many files the word has been found in. Then, that
many 32-bit file identifiers follow.

index format:

 * List of words *

    xx   number of words in the list
    ?    word
    x    how many files the word is present in
    xxxx file identifier 1
    xxxx file identifier 2
    ...
    xxxx file identifier n

    (other 255 lists of words follow)

 * hash table *

    xx offset of the LoW for words that match hash 0x00
    xx offset of the LoW for words that match hash 0x01
    ...
    xx offset of the LoW for words that match hash 0xff

====================================================================== EOF ===