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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Alexander Hulpke.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the declarations of operations for factor group maps
##

##
## To implement new factor group methods, one does not need to deal with
## most of the following operations (which are only used to cache known
## homomorphisms and extend them to subdirect factors). Instead only methods
## for the following three operations might need to be supplied:
## If a suitable homomorphism cannot be found from the cached homomorphisms
## pool, `NaturalHomomorphismByNormalSubgroupOp(<G>,<N>)' is called to
## construct one.
## The default method for `NaturalHomomorphismByNormalSubgroupOp' then uses
## two other operations: `DoCheapActionImages' computes actions that come
## naturally from a groups representation (for example permutation action
## on orbits and blocks) and can be computed quickly. This is intended
## as a first test to avoid hard work for homomorphisms that are easy to
## get.
## If this fails, `FindActionKernel' is called which will try to find some
## action which will give a suitable homomorphism. (This can be very time
## consuming.)
## The existing methods seem to work reasonably well for permutation groups
## and pc groups, for other kinds of groups it might be necessary to
## implement completely new methods.
##

#############################################################################
##
#O DoCheapActionImages(<G>)
##
## <ManSection>
## <Oper Name="DoCheapActionImages" Arg='G'/>
##
## <Description>
## computes natural actions for <A>G</A> and stores the resulting
## <C>NaturalHomomorphismByNormalSubgroup</C>. The type of the natural actions
## varies with the representation of <A>G</A>, for permutation groups it are for
## example constituent and block homomorphisms.
## A method for <C>DoCheapActionImages</C> must register all found actions with
## <C>AddNaturalHomomorphismsPool</C> so they become available.
## </Description>
## </ManSection>
##
DeclareOperation("DoCheapActionImages",[IsGroup]);
DeclareSynonym("DoCheapOperationImages",DoCheapActionImages);

#############################################################################
##
#O FindActionKernel( <G>, <N> ) . . . . . . . . . . . . . . . . local
##
## <ManSection>
## <Oper Name="FindActionKernel" Arg='G, N'/>
##
## <Description>
## This operation tries to find a suitable action for the group <A>G</A> such
## that its kernel is <A>N</A>. This is used to construct faithful permutation
## representations for the factor group.
## </Description>
## </ManSection>
##
DeclareOperation( "FindActionKernel",[IsGroup,IsGroup]);
DeclareSynonym( "FindOperationKernel",FindActionKernel);

#############################################################################
##
#V InfoFactor
##
## <ManSection>
## <InfoClass Name="InfoFactor"/>
##
## <Description>
## </Description>
## </ManSection>
##
DeclareInfoClass("InfoFactor");

#############################################################################
##
#A NaturalHomomorphismsPool(<G>)
##
## <ManSection>
## <Attr Name="NaturalHomomorphismsPool" Arg='G'/>
##
## <Description>
## The <C>NaturalHomomorphismsPool</C> is a record which contains the following
## components:
## <C>group</C> is the corresponding group.
## <C>ker</C> is a list of normal subgroups, which defines the arrangements.
## It is sorted.
## <C>ops</C> is a list which gives the best know actions for each normal
## subgroup. Its entries are either Homomorphisms from G or
## generator lists (G.generators images) or lists of integers. In the
## latter case the factor is subdirect product of the factors with
## the given numbers.
## <C>cost</C> gives the difficulty for each actions (degree of permgroup). It
## is used to check whether a new actions is better.
## <C>lock</C> is a bitlist, which indicates whether certain actions are
## locked. If this happens, a better new actions is not entered.
## This allows a computation to access the pool several times and to
## be guaranteed to be returned the same object. Usually a routine
## initially locks and finally unlocks.
## 
## <C>GopDone</C> indicates whether all <C>obvious</C> actions have been tried
## already
## <C>intersects</C> is a list of all intersections that have already been
## formed.
## <C>blocksdone</C> indicates if the actions already has been improved
## using blocks
## <C>in_code</C> can be set by the code to avoid addition of new actions
## (and thus resorting)
## </Description>
## </ManSection>
##
DeclareAttribute("NaturalHomomorphismsPool",IsGroup,
 "mutable");

#############################################################################
##
#O FactorCosetAction( <G>, [, <N>] ) action on the right cosets Ug
##
## <#GAPDoc Label="FactorCosetAction">
## <ManSection>
## <Oper Name="FactorCosetAction" Arg='G, U[, N]'
## Label="for a group and subgroup"/>
## <Oper Name="FactorCosetAction" Arg='G, L'
## Label="for a group and list of subgroups"/>
##
## <Description>
## This command computes the action of the group <A>G</A> on the
## right cosets of the subgroup <A>U</A>.
## If a normal subgroup <A>N</A> of <A>G</A> is given,
## it is stored as kernel of this action.
## When calling <C>FactorCosetAction</C> with a list of subgroups as the
## second argument, an action with image isomorphic to the subdirect
## product of the coset actions of all subgroups is computed. (However a
## degree reduction may take place if some of the actions are redundant, i.e.
## there is no guarantee that every subgroup in the list is represented by an
## orbit.)
## <Example><![CDATA[
## gap> g:=Group((1,2,3,4,5),(1,2));;u:=SylowSubgroup(g,2);;Index(g,u);
## 15
## gap> FactorCosetAction(g,u);
## [ (1,2,3,4,5), (1,2) ] -> [ (1,4,7,10,13)(2,5,8,11,14)(3,6,9,12,15),\
## (1,4)(2,6)(3,5)(7,8)(10,12)(13,14) ]
## gap> StructureDescription(Range(last));
## "S5"
## gap> FactorCosetAction(g,[u,SylowSubgroup(g,3)]);;
## gap> Size(Image(last));
## 120
## ]]></Example>
## The correspondence of points with cosets will, for performance reasons,
## depend on the method used. It is not guaranteed that it will be the same
## as used by <C>RightTransversal</C> or <C>RightCosets</C>.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareOperation( "FactorCosetAction", [IsGroup,IsGroup] );

DeclareGlobalFunction( "DoFactorCosetAction" );

#############################################################################
##
#F ImproveActionDegreeByBlocks( <G>, <N> , <hom> [,forceblocks] )
#F ImproveActionDegreeByBlocks( <G>, <N> , [,forceblocks] )
##
## <ManSection>
## <Func Name="ImproveActionDegreeByBlocks" Arg='G, N , hom [,forceblocks]'/>
## <Func Name="ImproveActionDegreeByBlocks" Arg='G, N , U [,forceblocks]'/>
##
## <Description>
## In the first usage, <A>N</A> is a normal subgroup of <A>G</A> and <A>hom</A> a
## homomorphism from <A>G</A> to a permutation group with kernel <A>N</A>. In the second
## usage, <A>hom</A> is taken to be the action of <A>G</A> on the cosets of <A>U</A> by right
## multiplication.
## The function tries to find another homomorphism with the same kernel but
## image group of smaller degree by looking for block systems of the image
## group. An improved result is stored in the <C>NaturalHomomorphismsPool</C>, the
## function returns the degree of this image (or the degree of the original
## image).
## If the image degree is larger than 500, only one block system is tested by
## standard. A test of all block systems is enforced by the optional boolean
## parameter <A>forceblocks</A>
## </Description>
## </ManSection>
##
DeclareGlobalFunction( "ImproveActionDegreeByBlocks" );
DeclareSynonym( "ImproveOperationDegreeByBlocks",
 ImproveActionDegreeByBlocks );

#############################################################################
##
#F SmallerDegreePermutationRepresentation( <G> )
##
## <#GAPDoc Label="SmallerDegreePermutationRepresentation">
## <ManSection>
## <Func Name="SmallerDegreePermutationRepresentation" Arg='G'/>
##
## <Description>
## Let <A>G</A> be a permutation group.
## <Ref Func="SmallerDegreePermutationRepresentation"/> tries to find a
## faithful permutation representation of smaller degree.
## The result is a group homomorphism onto a permutation group,
## in the worst case this is the identity mapping on <A>G</A>.
## 
## If the <C>cheap</C> option is given, the function only tries to reduce
## to orbits or actions on blocks, otherwise also actions on cosets of
## random subgroups are tried.
## 
## Note that the result is not guaranteed to be a faithful permutation
## representation of smallest degree,
## or of smallest degree among the transitive permutation representations
## of <A>G</A>.
## Using &GAP; interactively, one might be able to choose subgroups
## of small index for which the cores intersect trivially;
## in this case, the actions on the cosets of these subgroups give rise to
## an intransitive permutation representation
## the degree of which may be smaller than the original degree.
## 
## The methods used might involve the use of random elements and the
## permutation representation (or even the degree of the representation) is
## not guaranteed to be the same for different calls of
## <Ref Func="SmallerDegreePermutationRepresentation"/>.
## 
## If the option cheap is given less work is spent on trying to get a small
## degree representation, if the value of this option is set to the string
## "skip" the identity mapping is returned. (This is useful if a function
## called internally might try a degree reduction.)
## 
## <Example><![CDATA[
## gap> iso:=RegularActionHomomorphism(SymmetricGroup(4));;
## gap> image:= Image( iso );; NrMovedPoints( image );
## 24
## gap> small:= SmallerDegreePermutationRepresentation( image );;
## gap> Image( small );
## Group([ (2,5,4,3), (1,4)(2,6)(3,5) ])
## gap> g:=Image(IsomorphismPermGroup(GL(4,5)));;
## gap> sm:=SmallerDegreePermutationRepresentation(g:cheap);;
## gap> NrMovedPoints(Range(sm));
## 624
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareGlobalFunction( "SmallerDegreePermutationRepresentation" );

#############################################################################
##
#F AddNaturalHomomorphismsPool(G,N,op[,cost[,blocksdone]])
##
## <ManSection>
## <Func Name="AddNaturalHomomorphismsPool" Arg='G,N,op[,cost[,blocksdone]]'/>
##
## <Description>
## This function stores a computed action of <A>G</A> with kernel <A>N</A> in the
## <C>NaturalHomomorphismsPool</C> of <A>G</A>, unless a <Q>better</Q> action is already
## known. <A>op</A> usually is a homomorphism of <A>G</A> with kernel <A>N</A>. It may also
## be a subgroup of <A>G</A>, in which case the action of <A>G</A> on its cosets is
## taken.
## If the optional parameter <A>cost</A> is not given, <A>cost</A> is taken to be the
## degree of the image representation (or 1 if the image is a pc group). This
## <A>cost</A> is stored with the action to determine later whether another
## action is <Q>better</Q>.
## The optional boolean parameter <A>blocksdone</A> indicates if set to true, that
## all block systems of the image of <A>op</A> have already been computed and the
## resulting (lower degree, but not necessarily faithful for <M>G/N</M>) actions
## have been already considered. (Otherwise such a test may be done later by
## <C>DoCheapActionImages</C>.)
## The function internally re-sorts the list of normal subgroups to permit
## binary search among them. If a new action is returns the re-sorting
## permutation applied there. If returns <K>false</K> if a <Q>better</Q> action was
## already known, it returns <Q>fail</Q> if this factor is locked.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("AddNaturalHomomorphismsPool");

#############################################################################
##
#F LockNaturalHomomorphismsPool(<G>,<N>) . . store flag to prohibit changes
##
## <ManSection>
## <Func Name="LockNaturalHomomorphismsPool" Arg='G,N'/>
##
## <Description>
## Calling this function stores a flag in the <C>NaturalHomomorphismsPool</C> of
## <A>G</A> to prohibit it to store new (even better) faithful actions for <M>G/N</M>.
## This can be used in algorithms to ensure that
## <C>NaturalHomomorphismByNormalSubgroup(<A>G</A>,<A>N</A>)</C> will always return the same
## mapping, even if in the meantime other homomorphisms are computed anew,
## which –as a side effect– obtained a better action for <M>G/N</M> which &GAP;
## normally would store.
## The locking can be reverted by <C>UnlockNaturalHomomorphismsPool(<A>G</A>,<A>N</A>)</C>.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("LockNaturalHomomorphismsPool");

#############################################################################
##
#F UnlockNaturalHomomorphismsPool(<G>,<N>) . clear flag to allow changes of
##
## <ManSection>
## <Func Name="UnlockNaturalHomomorphismsPool" Arg='G,N'/>
##
## <Description>
## clears the flag set by <C>LockNaturalHomomorphismsPool(<A>G</A>,<A>N</A>)</C>.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("UnlockNaturalHomomorphismsPool");

#############################################################################
##
#F KnownNaturalHomomorphismsPool(<G>,<N>) . . . check whether Hom is stored
##
## <ManSection>
## <Func Name="KnownNaturalHomomorphismsPool" Arg='G,N'/>
##
## <Description>
## This function tests whether an homomorphism for
## <C>NaturalHomomorphismByNormalSubgroup(<A>G</A>,<A>N</A>)</C> is already known (or
## computed trivially for <M>G=N</M> or <M>N=\langle1\rangle</M>).
## </Description>
## </ManSection>
##
DeclareGlobalFunction("KnownNaturalHomomorphismsPool");

#############################################################################
##
#F GetNaturalHomomorphismsPool(<G>,<N>) . . . get action for G/N if known
##
## <ManSection>
## <Func Name="GetNaturalHomomorphismsPool" Arg='G,N'/>
##
## <Description>
## returns a <C>NaturalHomomorphismByNormalSubgroup(<A>G</A>,<A>N</A>)</C> if one is
## stored already in the <C>NaturalHomomorphismsPool</C> of <A>G</A>.
## (As the homomorphism may be stored by a <Q>recipe</Q> this command can
## still take some time when called the first time.)
## </Description>
## </ManSection>
##
DeclareGlobalFunction("GetNaturalHomomorphismsPool");

#############################################################################
##
#F DegreeNaturalHomomorphismsPool(<G>,<N>) degree for action for G/N
##
## <ManSection>
## <Func Name="DegreeNaturalHomomorphismsPool" Arg='G,N'/>
##
## <Description>
## returns the cost (see <Ref Func="AddNaturalHomomorphismsPool"/>) of a stored action
## for <M>G/N</M> and fail if no such action is stored.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("DegreeNaturalHomomorphismsPool");

#############################################################################
##
#F CloseNaturalHomomorphismsPool(<G>[,<N>]) . . calc intersections of known
##
## <ManSection>
## <Func Name="CloseNaturalHomomorphismsPool" Arg='G[,N]'/>
##
## <Description>
## This command tries to build actions for (new) factor groups from the
## already known actions in the <C>NaturalHomomorphismsPool(<A>G</A>)</C> by considering
## intransitive representations for subdirect products. Any new or better
## homomorphism obtained this way is stored (see
## <Ref Func="AddNaturalHomomorphismsPool"/>).
## If the optional parameter <A>N</A> is given, only actions which have <A>N</A> in their
## kernel are considered.
## The function keeps track of already considered subdirect products, thus
## there is no overhead in calling it several times.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("CloseNaturalHomomorphismsPool");

#############################################################################
##
#F PullBackNaturalHomomorphismsPool(<hom>) . . transfer nathoms of image
##
## <ManSection>
## <Func Name="PullBackNaturalHomomorphismsPool" Arg='hom'/>
##
## <Description>
## If <A>hom</a> is a homomorphism, this command transfers the natural
## homomorphisms of the image of <A>hom</A> to the source of <A>hom</A>.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("PullBackNaturalHomomorphismsPool");

#############################################################################
##
#F TryQuotientsFromFactorSubgroups(<hom>,<ker>,<bound>)
##
## <ManSection>
## <Func Name="TryQuotientsFromFactorSubgroups" Arg='hom,ker,bound'/>
##
## <Description>
## For a homomorphism <A>hom</A>, this command iterates through subgroups
## of the image, up to index <A>bound</A>,
## trying to find derived subgroups that expose more of the
## factor modulo <A>ker</A>.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("TryQuotientsFromFactorSubgroups");

#############################################################################
##
#F EraseNaturalHomomorphismsPool(<G>)
##
## <ManSection>
## <Func Name="EraseNaturalHomomorphismsPool" Arg='G'/>
##
## <Description>
## This command erases all stored natural homomorphisms associated to the
## group <A>G</A>. It is used to recover memory.
## </Description>
## </ManSection>
##
DeclareGlobalFunction("EraseNaturalHomomorphismsPool");

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