[comment {-*- tcl -*- doctools manpage}]
[vset VERSION 0.3]
[manpage_begin bee n [vset VERSION]]
[keywords bee]
[keywords BitTorrent]
[keywords bittorrent]
[keywords serialization]
[keywords torrent]
[copyright {2004 Andreas Kupries <andreas_kupries@users.sourceforge.net>}]
[moddesc   {BitTorrent}]
[titledesc {BitTorrent Serialization Format Encoder/Decoder}]
[category  Networking]
[require Tcl "8.5 9"]
[require bee [opt [vset VERSION]]]
[description]
[para]

The [package bee] package provides de- and encoder commands for data
in bencoding (speak 'bee'), the serialization format for data and
messages used by the BitTorrent protocol.

[para]

[section {PUBLIC API}]

[subsection ENCODER]

The package provides one encoder command for each of the basic forms,
and two commands per container, one taking a proper tcl data structure
to encode in the container, the other taking the same information as
several arguments.

[para]
[list_begin definitions]

[call [cmd ::bee::encodeString] [arg string]]

Returns the bee-encoding of the [arg string].

[call [cmd ::bee::encodeNumber] [arg integer]]

Returns the bee-encoding of the [arg integer] number.

[call [cmd ::bee::encodeListArgs] [arg value]...]

Takes zero or more bee-encoded values and returns the bee-encoding of
their list.

[call [cmd ::bee::encodeList] [arg list]]

Takes a list of bee-encoded values and returns the bee-encoding of the
list.

[call [cmd ::bee::encodeDictArgs] [arg key] [arg value]...]

Takes zero or more pairs of keys and values and returns the
bee-encoding of the dictionary they form. The values are expected to
be already bee-encoded, but the keys must not be. Their encoding will
be done by the command itself.

[call [cmd ::bee::encodeDict] [arg dict]]

Takes a dictionary list of string keys and bee-encoded values and
returns the bee-encoding of the list. Note that the keys in the input
must not be bee-encoded already. This will be done by the command
itself.

[list_end]
[para]

[subsection DECODER]

The package provides two main decoder commands, one for decoding a
string expected to contain a complete data structure, the other for
the incremental decoding of bee-values arriving on a channel. The
latter command is asynchronous and provides the completed decoded
values to the user through a command callback.

[para]
[list_begin definitions]

[call [cmd ::bee::decode] [arg string] [opt [arg endvar]] [opt [arg start]]]

Takes the bee-encoding in the string and returns one decoded value. In
the case of this being a container all contained values are decoded
recursively as well and the result is a properly nested tcl list
and/or dictionary.

[para]

If the optional [arg endvar] is set then it is the name of a variable
to store the index of the first character [emph after] the decoded
value into. In other words, if the string contains more than one value
then [arg endvar] can be used to obtain the position of the bee-value
after the bee-value currently decoded. together with [arg start], see
below, it is possible to iterate over the string to extract all
contained values.

[para]

The optional [arg start] index defaults to [const 0], i.e. the
beginning of the string. It is the index of the first character of the
bee-encoded value to extract.

[call [cmd ::bee::decodeIndices] [arg string] [opt [arg endvar]] [opt [arg start]]]

Takes the same arguments as [cmd ::bee::decode] and returns the same
information in [arg endvar]. The result however is different. Instead
of the tcl value contained in the [arg string] it returns a list
describing the value with respect to type and location (indices for
the first and last character of the bee-value). In case of a container
the structure also contains the same information for all the embedded
values.

[para]

Formally the results for the various types of bee-values are:

[list_begin definitions]
[def string]

A list containing three elements:

[list_begin itemized]
[item]
The constant string [const string], denoting the type of the value.

[item]
An integer number greater than or equal to zero. This is the index of
the first character of the bee-value in the input [arg string].

[item]
An integer number greater than or equal to zero. This is the index of
the last character of the bee-value in the input [arg string].

[list_end]
[para]

[emph Note] that this information is present in the results for all
four types of bee-values, with only the first element changing
according to the type of the value.

[def integer]

The result is like for strings, except that the type element contains
the constant string [const integer].

[def list]

The result is like before, with two exceptions: One, the type element
contains the constant string [const list]. And two, the result
actually contains four elements. The last element is new, and contains
the index data as described here for all elements of the bee-list.

[def dictionary]

The result is like for strings, except that the type element contains
the constant string [const dict]. A fourth element is present as well,
with a slightly different structure than for lists. The element is a
dictionary mapping from the strings keys of the bee-dictionary to a
list containing two elements. The first of them is the index
information for the key, and the second element is the index
information for the value the key maps to. This structure is the only
which contains not only index data, but actual values from the
bee-string. While the index information of the keys is unique enough,
i.e. serviceable as keys, they are not easy to navigate when trying to
find particular element. Using the actual keys makes this much easier.

[list_end]
[para]

[call [cmd ::bee::decodeChannel] [arg chan] \
	[option -command] [arg cmdprefix] \
	[opt [option -exact]] \
	[opt "[option -prefix] [arg data]"] \
]

The command creates a decoder for a series of bee-values arriving on
the channel [arg chan] and returns its handle. This handle can be used
to remove the decoder again.

Setting up another bee decoder on [arg chan] while a bee decoder is
still active will fail with an error message.

[para]
[list_begin definitions]
[def [option -command]]

The command prefix [arg cmdprefix] specified by the [emph required]
option [option -command] is used to report extracted values and
exceptional situations (error, and EOF on the channel).

The callback will be executed at the global level of the interpreter,
with two or three arguments. The exact call signatures are

[para]
[list_begin definitions]
[call [cmd cmdprefix] [method eof] [arg token]]

The decoder has reached eof on the channel [arg chan]. No further
invocations of the callback will be made after this. The channel has
already been closed at the time of the call, and the [arg token] is
not valid anymore as well.

[call [cmd cmdprefix] [method error] [arg token] [arg message]]

The decoder encountered an error, which is not eof. For example a
malformed bee-value. The [arg message] provides details about the
error. The decoder token is in the same state as for eof,
i.e. invalid. The channel however is kept open.

[call [cmd cmdprefix] [method value] [arg token] [arg value]]

The decoder received and successfully decoded a bee-value.

The format of the equivalent tcl [arg value] is the same as returned
by [cmd ::bee::decode]. The channel is still open and the decoder
token is valid. This means that the callback is able to remove the
decoder.

[list_end]
[para]

[def [option -exact]]

By default the decoder assumes that the remainder of the data in the
channel consists only of bee-values, and reads as much as possible per
event, without regard for boundaries between bee-values. This means
that if the the input contains non-bee data after a series of
bee-value the beginning of that data may be lost because it was
already read by the decoder, but not processed.

[para]

The [option -exact] was made for this situation. When specified the
decoder will take care to not read any characters behind the currently
processed bee-value, so that any non-bee data is kept in the channel
for further processing after removal of the decoder.

[para]

[def [option -prefix]]

If this option is specified its value is assumed to be the beginning
of the bee-value and used to initialize the internal decoder
buffer. This feature is required if the creator of the decoder used
data from the channel to determine if it should create the decoder or
not. Without the option this data would be lost to the decoding.

[list_end]
[para]

[call [cmd ::bee::decodeCancel] [arg token]]

This command cancels the decoder set up by [cmd ::bee::decodeChannel]
and represented by the handle [arg token].

[call [cmd ::bee::decodePush] [arg token] [arg string]]

This command appends the [arg string] to the internal decoder
buffer. It is the runtime equivalent of the option [option -prefix] of
[cmd ::bee::decodeChannel]. Use it to push data back into the decoder
when the [method value] callback used data from the channel to
determine if it should decode another bee-value or not.

[list_end]
[para]

[section {FORMAT DEFINITION}]

Data in the bee serialization format is constructed from two basic
forms, and two container forms. The basic forms are strings and
integer numbers, and the containers are lists and dictionaries.

[para]
[list_begin definitions]
[def "String [arg S]"]

A string [arg S] of length [arg L] is encoded by the string

"[arg L][const :][arg S]", where the length is written out in textual
form.

[def "Integer [arg N]"]

An integer number [arg N] is encoded by the string

"[const i][arg N][const e]".

[def "List [arg v1] ... [arg vn]"]

A list of the values [arg v1] to [arg vn] is encoded by the string

"[const l][arg BV1]...[arg BVn][const e]"

where "BV[var i]" is the bee-encoding of the value "v[var i]".

[def "Dict [arg k1] -> [arg v1] ..."]

A dictionary mapping the string key [arg k][var i] to the value

[arg v][var i], for [var i] in [const 1] ... [var n]
is encoded by the string

"[const d][arg BK][var i][arg BV][ var i]...[const e]"

for i in [const 1] ... [var n], where "BK[var i]" is the bee-encoding
of the key string "k[var i]".  and "BV[var i]" is the bee-encoding of
the value "v[var i]".

[para]

[emph Note]: The bee-encoding does not retain the order of the keys in
the input, but stores in a sorted order. The sorting is done for the
"raw strings".

[list_end]
[para]

Note that the type of each encoded item can be determined immediately
from the first character of its representation:

[para]
[list_begin definitions]
[def i]
Integer.
[def l]
List.
[def d]
Dictionary.
[def "[lb]0-9[rb]"]
String.
[list_end]
[para]

By wrapping an integer number into [const i]...[const e] the format
makes sure that they are different from strings, which all begin with
a digit.

[section EXAMPLES]

[vset CATEGORY bee]
[include ../common-text/feedback.inc]
[manpage_end]
