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<Chapter Label="chap:Technicalities of the AtlasRep Package">
<Heading>Technicalities of the &AtlasRep; Package</Heading>

This chapter describes those parts of the &GAP; interface to the
&ATLAS; of Group Representations that do not belong to the user interface
(cf. Chapter <Ref Chap="chap:The User Interface of the AtlasRep Package"/>).
<P/>
Besides global variables used for administrational purposes
(see Section <Ref Sect="sect:Global Variables Used by the AtlasRep Package"/>)
and several sanity checks
(see Section <Ref Sect="sect:AGR Sanity Checks"/>),
they can be regarded as
the interface between the data actually contained in the files and
the corresponding &GAP; objects
(see
Section <Ref Sect="sect:How to Customize the Access to Data files"/>,
<Ref Sect="sect:Reading and Writing MeatAxe Format Files"/>,
<Ref Sect="sect:Reading and Writing Atlas Straight Line Programs"/>,
and <Ref Sect="sect:Data Types Used in the AGR"/>),
and the interface between the remote and the local version of the database
(see Section <Ref Sect="sect:Filenames Used in the AGR"/>
and <Ref Sect="sect:The Tables of Contents of the AGR"/>).
The former interface contains functions to read and write files in
&MeatAxe; format, which may be interesting for users familiar with
&MeatAxe; standalones (see for example <Cite Key="CMeatAxe"/>).
Other low level functions may be undocumented in the sense that they are
not described in this manual.
Users interested in them may look at the actual implementation in the
<F>gap</F> directory of the package,
but it may happen that this will be changed
in future versions of the package.


<Section Label="sect:Global Variables Used by the AtlasRep Package">
<Heading>Global Variables Used by the &AtlasRep; Package</Heading>

For debugging purposes, &AtlasRep; functions print information depending on
the info level of the info classes <Ref InfoClass="InfoAtlasRep"/>,
<Ref InfoClass="InfoCMeatAxe"/>, and <Ref InfoClass="InfoBBox"/>
(cf. <Ref Sect="Info Functions" BookName="ref"/>).

<P/>

The info level of an info class can be changed using
<Ref Func="SetInfoLevel" BookName="ref"/>.
For example, the info level of <Ref InfoClass="InfoAtlasRep"/> can be set
to the nonnegative integer <M>n</M> using
<C>SetInfoLevel( InfoAtlasRep, </C><M>n</M><C> )</C>.

<P/>

<#Include Label="InfoAtlasRep">
<#Include Label="InfoCMeatAxe">
<#Include Label="InfoBBox">

<P/>

<#Include Label="AGR">
<#Include Label="AtlasOfGroupRepresentationsInfo">

</Section>


<Section Label="sect:How to Customize the Access to Data files">
<Heading>How to Customize the Access to Data files</Heading>

<#Include Label="AccessFunctionsDefault">

</Section>


<Section Label="sect:Reading and Writing MeatAxe Format Files">
<Heading>Reading and Writing MeatAxe Format Files</Heading>

<Index Subkey="MeatAxe format">matrix</Index>
<Index Subkey="MeatAxe format">permutation</Index>

<#Include Label="ScanMeatAxeFile">
<#Include Label="MeatAxeString">
<#Include Label="FFList">
<#Include Label="CMtxBinaryFFMatOrPerm">
<#Include Label="FFMatOrPermCMtxBinary">

</Section>


<Section Label="sect:Reading and Writing Atlas Straight Line Programs">
<Heading>Reading and Writing &ATLAS; Straight Line Programs</Heading>

<#Include Label="ScanStraightLineProgram">
<#Include Label="AtlasStringOfProgram">

</Section>


<Section Label="sect:Data Types Used in the AGR">
<Heading>Data Types Used in the &AtlasRep; Package</Heading>

Each representation or program that is administrated by the
&AtlasRep; package belongs to a unique <E>data type</E>.
Informally, examples of data types are
<Q>permutation representation</Q>,
<Q>matrix representation over the integers</Q>, or
<Q>straight line program for computing class representatives</Q>.

<P/>

The idea is that for each data type, there can be
<List>
<Item>
  a column of its own in the output produced by
  <Ref Func="DisplayAtlasInfo"/>
  when called without arguments or with only argument a list of
  group names,
</Item>
<Item>
  a line format of its own for the output produced by
  <Ref Func="DisplayAtlasInfo"/>
  when called with first argument a group name,
</Item>
<Item>
  an input format of its own for <Ref Func="AtlasProgram"/>,
</Item>
<Item>
  an input format of its own for <Ref Func="OneAtlasGeneratingSetInfo"/>,
  and
</Item>
<Item>
  specific tests for the data of this data type;
  these functions are used by the global tests described in
  Section <Ref Sect="sect:AGR Sanity Checks"/>.
</Item>
</List>

<P/>

Formally, a data type is defined by a record whose components are used by
the interface functions.
The details are described in the following.

<#Include Label="AGRDeclareDataType">

</Section>


<Section Label="sect:Filenames Used in the AGR">
<Heading>Filenames Used in the &AtlasRep; Package</Heading>

<#Include Label="[1]{access}">

<P/>

<#Include Label="AGRParseFilenameFormat">
<#Include Label="AGRFileContents">

</Section>


<Section Label="sect:identifier component">
<Heading>The record component <C>identifier</C> used by the &AtlasRep; Package</Heading>

The functions
<Ref Func="AtlasGenerators"/>,
<Ref Func="AtlasProgram"/>,
<Ref Func="AtlasProgramInfo"/>,
<Ref Func="OneAtlasGeneratingSetInfo"/>, and
<Ref Func="AllAtlasGeneratingSetInfos"/>
return records which have a component <C>identifier</C>.
The value of this component describes the record
in the sense that one can reconstruct the whole record from it,
and the <C>identifier</C> value can be used as an input for
<Ref Func="AtlasGenerators"/>,
<Ref Func="AtlasProgram"/>,
<Ref Func="AtlasProgramInfo"/>,
<Ref Func="AtlasGroup" Label="for various arguments"/>, and
<Ref Func="AtlasSubgroup"
Label="for a group name (and various arguments) and a number"/>.

<P/>

The <C>identifier</C> component has the following format.

<P/>

<List>
<Item>
For records describing representations,
it is a list of the form <C>[ gapname, files, std, info ]</C>.
</Item>
<Item>
For records describing straight line programs and straight line decisions,
it is a list of the form <C>[ gapname, files, std ]</C>.
</Item>
</List>

<P/>

Here <C>gapname</C> is the &GAP; name of the group in question,
<C>files</C> defines the data files,
<C>std</C> is the standardization of its generators,
and <C>info</C> is some information that depends on the type of the
representation, for example the number of moved points in the case of a
permutation representation.

<P/>

The <C>files</C> entry has one of the following formats:

<P/>

<List>
<Item>
  a string, in the case that exactly one file is needed
  that does not belong to a private extension;
  an example of such an <C>identifier</C> value is
  <C>[ "J1", "J1G1-cycW1", 1 ]</C>
</Item>
<Item>
  a list whose entries are strings
  (which refer to files from the core part of the database)
  and pairs of the form <C>[ tocid, file ]</C>
  (which refer to files from the extension given by <C>tocid</C>);
  examples of <C>identifier</C> values are
  <C>[ "A5", [ "A5G1-p5B0.m1", "A5G1-p5B0.m2" ], 1, 5 ]</C>,
  <C>[ "2.M12", [ [ "mfer", "2M12G1-cclsW1" ] ], 1 ]</C>,
  <C>[ "2.M12", [ "M12G1-max1W1", [ "internal", "2M12G1-kerM12W1" ] ], 1 ]</C>,
  <C>[ "2.M12", [ [ "mfer", "2M12G1-p24bB0.m1" ],
  [ "mfer", "2M12G1-p24bB0.m2" ] ], 1, 24 ]</C>.
</Item>
</List>

<P/>

Up to version 1.5 of the &AtlasRep; package,
a different <C>identifier</C> format was used for files from
extensions of the database.
Namely, the first entry of the list was a pair <C>[ tocid, groupname ]</C>,
and the second entry was either a string or a list of strings.
Note that with that old format,
it was not possible to describe a combination of several files
from different sources (core part and extension, or different extensions).
The function <Ref Func="AtlasRepIdentifier"
Label="convert an old type identifier to a new type one"/>
can be used to convert between the two formats.

<#Include Label="AtlasRepIdentifier">

</Section>


<Section Label="sect:The Tables of Contents of the AGR">
<Heading>The Tables of Contents of the &AtlasRep; Package</Heading>

<#Include Label="toc">

</Section>


<Section Label="sect:AGR Sanity Checks">
<Heading>Sanity Checks for the &AtlasRep; Package</Heading>

<#Include Label="tests">

Finally, we reset the user preference and the info level
which had been set at the beginning of Chapter <Ref Chap="chap:tutorial"/>.

<P/>

<Example><![CDATA[
gap> SetUserPreference( "AtlasRep", "DisplayFunction", origpref );
gap> SetInfoLevel( InfoAtlasRep, globallevel );
]]></Example>

</Section>



</Chapter>

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