GAPGroups.jl

# TODO: as soon as GAP packages like `polycyclic` or `rcwa` are loaded,
# the custom group types and isos they define should be added to the arrays
# _gap_group_types resp. _iso_function


group_element(G::T, x::GapObj) where T <: GAPGroup = BasicGAPGroupElem{T}(G, x)


#TODO: document `check_parent`!
# If `check_parent(a, b)` yields `false` then `a` and `b` do not fit
# together.
# A `true` result does *not* imply that the objects will fit together,
# since we check only the Julia side of the data.
#
# The current situation is that GAP knows whether an operation for two
# group elements or for a group and a group element makes sense;
# for example, think about an element and a subgroup of some f.p. group.
#
# The Oscar objects may contain additional information:
# Oscar permutation groups have a degree, and conjugating a permutation group
# by an element of another permutation group shall be allowed only if
# both have the same degree, in order to assign this degree to the result.
# Analogously, Oscar matrix groups have `deg` and `ring`.

check_parent(g::GAPGroupElem, h::GAPGroupElem) = (parent(g) === parent(h))
#TODO: Insert such checks in the code, check for failures!

# default: compare only types
check_parent(G::T, g::GAPGroupElem) where T <: GAPGroup = (T === typeof(g.parent))

# `PermGroup`: compare types and degrees
check_parent(G::PermGroup, g::PermGroupElem) = (degree(G) == degree(parent(g)))

# `MatrixGroup`: compare types, dimensions, and coefficient rings
# (This cannot be defined here because `MatrixGroup` is not yet defined.)


# TODO: ideally the `elements` method below would be turned into a method for
# `collect`, as it is much faster than plain `collect`. Unfortunately, though,
# for some groups (e.g. permutation groups) the order of elements computed
# by this function may differ from that computed by an iterator over G. So
# this is not an option right now.
function elements(G::GAPGroup)
  els = GAPWrap.AsList(GapObj(G))
  return [group_element(G, x::GapObj) for x in els]
end

function parent(x::GAPGroupElem)
  return x.parent
end

# coercion embeds a group element into a different parent
function (G::GAPGroup)(x::BasicGAPGroupElem{T}) where T<:GAPGroup
   @req GapObj(x) in GapObj(G) "the element does not embed in the group"
   return group_element(G, GapObj(x))
end

"""
    is_finite(G::GAPGroup) -> Bool

Return `true` if `G` is finite, and `false` otherwise.

# Examples
```jldoctest
julia> is_finite(symmetric_group(5))
true

julia> is_finite(free_group(2))
false

```
"""
@gapattribute is_finite(G::GAPGroup) = GAP.Globals.IsFinite(GapObj(G))::Bool

# Base.is_finite(G::PcGroup) = true

"""
    is_finite_order(g::GAPGroupElem) -> Bool

Return `true` if `g` has finite order, and `false` otherwise.

# Examples
```jldoctest
julia> is_finite_order(gen(symmetric_group(5), 1))
true

julia> is_finite_order(gen(free_group(2), 1))
false

```
"""
is_finite_order(x::GAPGroupElem) = GAPWrap.IsInt(GAPWrap.Order(GapObj(x)))


"""
    order(::Type{T} = ZZRingElem, x::Union{GAPGroupElem, GAPGroup}) where T <: IntegerUnion

Return the order of `x`, as an instance of `T`.

For a group element `x` in the group `G`, the order of `x` is the smallest
positive integer `n` such that `x^n` is the identity of `G`.
For a group `x`, the order of `x` is the number of elements in `x`.

An exception is thrown if the order of `x` is infinite,
use [`is_finite`](@ref) for checking the finiteness of a group,
and [`is_finite_order`](@ref) for checking whether a group element
has finite order.

# Examples
```jldoctest
julia> g = symmetric_group(3);

julia> order(g)
6

julia> order(gen(g, 1))
3

julia> g = free_group(1);

julia> is_finite(g)
false

julia> is_finite_order(gen(g, 1))
false
```
"""
function order(::Type{T}, x::Union{GAPGroupElem, GAPGroup}) where T <: IntegerUnion
   ord = GAPWrap.Order(GapObj(x))
   if ord === GAP.Globals.infinity
      throw(InfiniteOrderError(x))
   end
   return T(ord)
end

has_order(G::GAPGroup) = GAPWrap.HasSize(GapObj(G))
set_order(G::GAPGroup, val::T) where T<:IntegerUnion = GAPWrap.SetSize(GapObj(G), GAP.Obj(val))


"""
    is_trivial(G::GAPGroup)

Return `true` if `G` has order `1`, and `false` otherwise.

# Examples
```jldoctest
julia> is_trivial(symmetric_group(1))
true

julia> is_trivial(symmetric_group(2))
false
```
"""
@gapattribute is_trivial(G::GAPGroup) = GAP.Globals.IsTrivial(GapObj(G))::Bool


@doc raw"""
    exponent(::Type{T} = ZZRingElem, G::GAPGroup) where T <: IntegerUnion

Return the exponent of `G`, as an instance of `T`,
i.e., the smallest positive integer $e$ such that
$g^e$ is the identity of `G` for every $g$ in `G`.

# Examples
```jldoctest
julia> exponent(symmetric_group(3))
6

julia> exponent(symmetric_group(13))
360360
```
"""
@gapattribute exponent(x::GAPGroup) = ZZRingElem(GAP.Globals.Exponent(GapObj(x))::GapInt)

Base.exponent(::Type{T}, G::GAPGroup) where T <: IntegerUnion = T(GAP.Globals.Exponent(GapObj(G))::GapInt)

"""
    rand(rng::Random.AbstractRNG = Random.GLOBAL_RNG, G::Group)

Return a random element of `G`, using the random number generator `rng`.
"""
Base.rand(G::GAPGroup) = Base.rand(Random.GLOBAL_RNG, G)

function Base.rand(rng::Random.AbstractRNG, G::GAPGroup)
   s = GAP.Globals.Random(GAP.wrap_rng(rng), GapObj(G))::GapObj
   return group_element(G, s)
end

function Base.rand(rng::Random.AbstractRNG, rs::Random.SamplerTrivial{Gr}) where Gr<:GAPGroup
   return rand(rng, rs[])
end

"""
    rand_pseudo(G::GAPGroup)

Return a pseudo random element of `G`.  This works faster than `rand`,
but the returned elements are not necessarily uniformly distributed.

It is sometimes necessary to work with finite groups that we cannot
effectively enumerate, e.g. matrix groups over finite fields. We may not even
know the size of these groups. Yet many algorithms need to sample elements
from the group "as randomly as possible", whatever that means; but also they
need this *fast*.

The function `rand_pseudo` returns elements that are cheap to compute and
somehow random, but makes no guarantees about their distribution.

For finitely presented groups, it returns random words of bounded length.

For finite permutation and matrix groups, it uses a variant of the product
replacement algorithm. For most inputs, the resulting stream of elements
relatively quickly converges to a uniform distribution.

"""
function rand_pseudo(G::GAPGroup; radius::Int = 10)
  return group_element(G, GAP.Globals.PseudoRandom(GapObj(G); radius = radius)::GapObj)
end


# We allow arithmetic operations between two group elements with
# *nonidentical parents*.
# In this case, the parent of the resulting element is set according to
# the following rules, depending on the types of the parents.
#
# - `PermGroup`, `PermGroup`:
#   The operation is allowed whenever the parents have the same degree,
#   then the parent of the result is the symmetric group of that degree.
#
# - `PcGroup`, `PcGroup` and
#   `FPGroup`, `FPGroup`:
#   The operation is allowed whenever the parents have the same `GapObj`
#   (in the sense of `===`),
#   then the first of the two groups is taken as the parent of the result.
#
# - `SubPcGroup`, `SubPcGroup` and
#   `SubFPGroup`, `SubFPGroup`:
#   The operation is allowed whenever the `full_group` fields of the parents
#   have the same `GapObj`
#   (in the sense of `===`),
#   then the first of the two groups is taken as the parent of the result.
#
# - `SubPcGroup`, `PcGroup` and
#   `PcGroup`, `SubPcGroup` and
#   `SubFPGroup`, `FPGroup` and
#   `FPGroup`, `SubFPGroup`:
#   The operation is allowed whenever the `full_group` of the `SubPcGroup`
#   (`SubFPGroup`) and the `PcGroup` (`FPGroup`) have the same `GapObj`
#   (in the sense of `===`),
#   then the `full_group` of the `SubPcGroup` (`SubFPGroup`) is taken
#   as the parent of the result.
#   Note that we take a group of type `SubPcGroup` (`SubFpGroup`) in order
#   to achieve type stability:
#   Multiplying two `SubPcGroupElem`s with identical parent yields an element
#   with this parent, of type `SubPcGroup`, hence the product of two
#   `SubPcGroupElem`s with different parents is also a `SubPcGroupElem`.
#
# - `MatrixGroup`, `MatrixGroup`:
#   The operation is allowed whenever the two groups have the same `degree`
#   and `base_ring`,
#   then the general linear group of that degree over that ring
#   is taken as the parent of the result.
#
# - `AutomorphismGroup`, `AutomorphismGroup`:
#   The operation is allowed whenever the two groups have the same `.G` field,
#   then the full automorphism group of that group
#   is taken as the parent of the result.
#
# - `DirectProductGroup`, `DirectProductGroup` and
#   `SemidirectProductGroup`, `SemidirectProductGroup` and
#   `WreathProductGroup`, `WreathProductGroup`:
#   The operation is allowed whenever the two groups have the same `.Xfull`
#   field,
#   then the direct/semidirect/wreath product of these groups
#   is taken as the parent of the result.
#
# For other types of groups, we throw an exception if the parent groups are
# not equal and their `GapObj`s are not equal.
#
# Note that we do not want to perform `==` checks that are more expensive
# then `===` checks.
# In general, we cannot guarantee that the Oscar groups objects in question
# are *identical* because the same GAP group can be wrapped several times,
# but we want to force that their `GapObj`s are identical.
# (Permutation groups are an exception,
# we want to force only that the degrees are equal.)
# Thus we regard it as an error for example to ask for the product of two
# `PcGroupElem`s whose parents are equal in the sense of `==`
# but whose `GapObj`s are not identical.
#
function _common_parent_group(x::PermGroup, y::PermGroup)
  x === y && return x
  @req degree(x) == degree(y) "the groups have different degrees"
  return symmetric_group(degree(x))
end

function _common_parent_group(x::PcGroup, y::PcGroup)
  GapObj(x) === GapObj(y) && return x
  throw(ArgumentError("the groups are not compatible"))
end

function _common_parent_group(x::FPGroup, y::FPGroup)
  GapObj(x) === GapObj(y) && return x
  throw(ArgumentError("the groups are not compatible"))
end

function _common_parent_group(x::SubPcGroup, y::SubPcGroup)
  x === y && return x
  @req GapObj(x.full_group) === GapObj(y.full_group) "the groups belong to different full groups"
  return as_sub_pc_group(x.full_group)
end

function _common_parent_group(x::SubPcGroup, y::PcGroup)
  @req GapObj(x.full_group) === GapObj(y) "the groups belong to different full groups"
  return as_sub_pc_group(x.full_group)
end

function _common_parent_group(x::PcGroup, y::SubPcGroup)
  @req GapObj(y.full_group) === GapObj(x) "the groups belong to different full groups"
  return as_sub_pc_group(y.full_group)
end

function _common_parent_group(x::SubFPGroup, y::SubFPGroup)
  x === y && return x
  @req GapObj(x.full_group) === GapObj(y.full_group) "the groups belong to different full groups"
  return as_sub_fp_group(x.full_group)
end

function _common_parent_group(x::SubFPGroup, y::FPGroup)
  @req GapObj(x.full_group) === GapObj(y) "the groups belong to different full groups"
  return as_sub_fp_group(x.full_group)
end

function _common_parent_group(x::FPGroup, y::SubFPGroup)
  @req GapObj(y.full_group) === GapObj(x) "the groups belong to different full groups"
  return as_sub_fp_group(y.full_group)
end

function _common_parent_group(x::AutomorphismGroup{T}, y::AutomorphismGroup{T}) where T <: GAPGroup
  x === y && return x
  @req x.G === y.G "the groups belong to different full groups"
  return automorphism_group(x.G)
end

function _common_parent_group(x::DirectProductGroup, y::DirectProductGroup)
  x === y && return x
  @req x.Xfull === y.Xfull "the groups belong to different full groups"
  return DirectProductGroup(x.Xfull, x.L, x.Xfull, true)
end

function _common_parent_group(x::SemidirectProductGroup, y::SemidirectProductGroup)
  x === y && return x
  @req x.Xfull === y.Xfull "the groups belong to different full groups"
  return SemidirectProductGroup{typeof(x.N), typeof(x.H)}(x.Xfull, x.N, x.H, x.f, x.Xfull, true)
end

function _common_parent_group(x::WreathProductGroup, y::WreathProductGroup)
  x === y && return x
  @req x.Xfull === y.Xfull "the groups belong to different full groups"
  return WreathProductGroup(x.Xfull, x.G, x.H, x.a, x.Xfull, true)
end

# generic method
function _common_parent_group(x::T, y::T) where T <: GAPGroup
  (x === y || GapObj(x) == GapObj(y)) && return x
  throw(ArgumentError("the groups are not compatible"))
end

function _prod(x::GAPGroupElem, y::GAPGroupElem)
  G = _common_parent_group(parent(x), parent(y))
  return group_element(G, GapObj(x)*GapObj(y))
end

Base.:*(x::GAPGroupElem, y::GAPGroupElem) = _prod(x, y)

==(x::GAPGroup, y::GAPGroup) = GapObj(x) == GapObj(y)

==(x::BasicGAPGroupElem, y::BasicGAPGroupElem ) = GapObj(x) == GapObj(y)

"""
    one(G::GAPGroup) -> elem_type(G)

Return the identity of the group `G`.
"""
Base.one(x::GAPGroup) = group_element(x, GAPWrap.Identity(GapObj(x)))

"""
    one(x::GAPGroupElem{T}) -> GAPGroupElem{T}

Return the identity of the parent group of `x`.
"""
Base.one(x::GAPGroupElem) = one(parent(x))

Base.show(io::IO, x::GAPGroupElem) = print(io, String(GAPWrap.StringViewObj(GapObj(x))))

# Printing GAP groups
function Base.show(io::IO, G::GAPGroup)
  @show_name(io, G)
  @show_special(io, G)
  print(io, "Group")
  if !is_terse(io)
    if has_order(G)
      if is_finite(G)
        print(io, " of order ", order(G))
      else
        print(io, " of infinite order")
      end
    end
  end
end

function Base.show(io::IO, G::Union{FPGroup, SubFPGroup})
  @show_name(io, G)
  @show_special(io, G)
  if GAPWrap.IsFreeGroup(GapObj(G))
    print(io, "Free group")
    if !is_terse(io) && GAP.Globals.HasRankOfFreeGroup(GapObj(G))::Bool
      print(io, " of rank ", GAP.Globals.RankOfFreeGroup(GapObj(G))::Int)
    end
  else
    T = typeof(G) == FPGroup ? "Finitely presented group" : "Sub-finitely presented group"
    print(io, T)  # FIXME: actually some of these groups are *not* finitely presented
    if !is_terse(io)
    if has_order(G)
      if is_finite(G)
        print(io, " of order ", order(G))
      else
        print(io, " of infinite order")
      end
    end
    end
  end
end

function Base.show(io::IO, G::PermGroup)
  @show_name(io, G)
  @show_special(io, G)

  # Treat groups specially which know that they are nat. symmetric/alternating.
  io = pretty(io)
  if has_is_natural_symmetric_group(G) && is_natural_symmetric_group(G) &&
     number_of_moved_points(G) == degree(G)
    print(io, LowercaseOff(), "Sym(", degree(G), ")")
  elseif has_is_natural_alternating_group(G) && is_natural_alternating_group(G) &&
     number_of_moved_points(G) == degree(G)
    print(io, LowercaseOff(), "Alt(", degree(G), ")")
  else
    print(io, "Permutation group")
    if !is_terse(io)
      print(io, " of degree ", degree(G))
      if has_order(G)
        if is_finite(G)
          print(io, " and order ", order(G))
        else
          print(io, " and infinite order")
        end
      elseif GAP.Globals.HasStabChainMutable(GapObj(G))
        # HACK: to show order in a few more cases where it is trivial to get
        # but really, GAP should be using this anyway?
        s = GAP.Globals.SizeStabChain( GAP.Globals.StabChainMutable( GapObj(G) ) )
        print(io, " and order ", ZZRingElem(s))
      end
    end
  end
end

function Base.show(io::IO, G::Union{PcGroup,SubPcGroup})
  @show_name(io, G)
  @show_special(io, G)
  T = typeof(G) == PcGroup ? "Pc group" : "Sub-pc group"
  print(io, T)
  if !is_terse(io)
    if isfinite(G)
      print(io, " of order ", order(G))
    else
      print(io, " of infinite order")
    end
  end
end


Base.isone(x::GAPGroupElem) = GAPWrap.IsOne(GapObj(x))

Base.inv(x::GAPGroupElem) = group_element(parent(x), GAPWrap.Inverse(GapObj(x)))

Base.:^(x::GAPGroupElem, y::Int) = group_element(parent(x), (GapObj(x) ^ y)::GapObj)

Base.:^(x::GAPGroupElem, y::ZZRingElem) = Nemo._generic_power(x, y) # TODO: perhaps  let GAP handle this; also handle arbitrary Integer subtypes?

div_right(x::GAPGroupElem, y::GAPGroupElem) = group_element(parent(x), (GapObj(x) / GapObj(y))::GapObj)
div_left(x::GAPGroupElem, y::GAPGroupElem) = group_element(parent(x), (GapObj(y) \ GapObj(x))::GapObj)

Base.conj(x::GAPGroupElem, y::GAPGroupElem) = group_element(_common_parent_group(parent(x), parent(y)), (GapObj(x) ^ GapObj(y))::GapObj)

# AbstractAlgebra defines `x^y` for group elements of the *same* type only.
Base.:^(x::GAPGroupElem, y::GAPGroupElem) = Base.conj(x, y)


"""
    comm(x::GAPGroupElem, y::GAPGroupElem)

Return the commutator of `x` and `y`,
which is defined as `x^-1*y^-1*x*y`,
and usually denoted as `[x,y]` in the literature.
"""
comm(x::GAPGroupElem, y::GAPGroupElem) = x^-1*x^y

Base.IteratorSize(::Type{<:GAPGroup}) = Base.SizeUnknown()
Base.IteratorSize(::Type{PermGroup}) = Base.HasLength()

Base.iterate(G::GAPGroup) = iterate(G, GAPWrap.Iterator(GapObj(G)))

function Base.iterate(G::GAPGroup, state)
  GAPWrap.IsDoneIterator(state) && return nothing
  i = GAPWrap.NextIterator(state)::GapObj
  return group_element(G, i), state
end

# need this function just for the iterator
Base.length(x::GAPGroup)::Int = order(Int, x)

"""
    Base.in(g::GAPGroupElem, G::GAPGroup)

Return whether `g` is an element of `G`.
The parent of `g` need not be equal to `G`.
"""
Base.in(g::GAPGroupElem, G::GAPGroup) = GapObj(g) in GapObj(G)

"""
    gens(G::Group)

Return a vector of generators of `G`.
To get the `i`-th generator,
use `G[i]` or `gen(G,i)` (see [`gen`](@ref)) instead of `gens(G)[i]`,
as that is more efficient.

# Examples
```jldoctest
julia> g = symmetric_group(5);  gens(g)
2-element Vector{PermGroupElem}:
 (1,2,3,4,5)
 (1,2)

julia> g[2]
(1,2)

```

!!! note
    The output of `gens(G)` is not, in general, the minimal list of generators for `G`.
"""
function gens(G::GAPGroup)
   L = GAPWrap.GeneratorsOfGroup(GapObj(G))::GapObj
   res = Vector{elem_type(G)}(undef, length(L))
   for i = 1:length(res)
     res[i] = group_element(G, L[i]::GapObj)
   end
   return res
end

"""
    has_gens(G::Group)

Return whether generators for the group `G` are known.

# Examples
```jldoctest
julia> F = free_group(2)
Free group of rank 2

julia> has_gens(F)
true

julia> H = derived_subgroup(F)[1]
Free group

julia> has_gens(H)
false
```
"""
has_gens(G::GAPGroup) = GAP.Globals.HasGeneratorsOfGroup(GapObj(G))::Bool

"""
    gen(G::GAPGroup, i::Int)

Return `one(G)` if `i == 0`,
the `i`-th element of the vector `gens(G)` if `i` is positive,
and the inverse of the `i`-th element of `gens(G)` if `i` is negative.

For positive `i`, this is equivalent to `G[i]`, and returns `gens(G)[i]`
but may be more efficient than the latter.

An exception is thrown if `abs(i)` is larger than the length of `gens(G)`.

# Examples
```jldoctest
julia> g = symmetric_group(5);  gen(g, 1)
(1,2,3,4,5)

julia> g[-1]
(1,5,4,3,2)
```
"""
function gen(G::GAPGroup, i::Int)
   i == 0 && return one(G)
   L = GAPWrap.GeneratorsOfGroup(GapObj(G))::GapObj
   0 < i && i <= length(L) && return group_element(G, L[i]::GapObj)
   i < 0 && -i <= length(L) && return group_element(G, inv(L[-i])::GapObj)
   @req false "i must be in the range -$(length(L)):$(length(L))"
end

"""
    number_of_generators(G::GAPGroup) -> Int

Return the length of the vector [`gens`](@ref)`(G)`.

!!! warning "WARNING:"
    this is *NOT*, in general, the minimum number of generators for G.
"""
number_of_generators(G::GAPGroup) = length(GAPWrap.GeneratorsOfGroup(GapObj(G)))

"""
    small_generating_set(G::GAPGroup)

Return a reasonably short vector of elements in `G` that generate `G`;
in general the length of this vector is not minimal.

# Examples
```jldoctest
julia> length(small_generating_set(abelian_group(SubPcGroup, [2,3,4])))
2

julia> length(small_generating_set(abelian_group(PermGroup, [2,3,4])))
3
```
"""
@gapattribute function small_generating_set(G::GAPGroup)
   L = GAP.Globals.SmallGeneratingSet(GapObj(G))::GapObj
   res = Vector{elem_type(G)}(undef, length(L))
   for i = 1:length(res)
     res[i] = group_element(G, L[i]::GapObj)
   end
   return res
end

"""
    minimal_generating_set(G::GAPGroup)

Return a vector of minimal length of elements in `G` that generate `G`.

# Examples
```jldoctest
julia> length(minimal_generating_set(abelian_group(SubPcGroup, [2,3,4])))
2

julia> length(minimal_generating_set(abelian_group(PermGroup, [2,3,4])))
2

julia> minimal_generating_set(symmetric_group(5))
2-element Vector{PermGroupElem}:
 (1,2,3,4,5)
 (1,2)
```
"""
@gapattribute function minimal_generating_set(G::GAPGroup)
   L = GAP.Globals.MinimalGeneratingSet(GapObj(G))::GapObj
   res = Vector{elem_type(G)}(undef, length(L))
   for i = 1:length(res)
     res[i] = group_element(G, L[i]::GapObj)
   end
   return res
end


################################################################################
#
#   Conjugacy Classes
#
################################################################################

@attributes mutable struct GAPGroupConjClass{T<:GAPGroup, S<:Union{GAPGroupElem,GAPGroup}} <: GroupConjClass{T, S}
   X::T
   repr::S
   CC::GapObj

   function GAPGroupConjClass(G::T, obj::S, C::GapObj) where T<:GAPGroup where S<:Union{GAPGroupElem, GAPGroup}
     return new{T, S}(G, obj, C, Dict{Symbol,Any}())
   end
end

GAP.julia_to_gap(obj::GAPGroupConjClass) = obj.CC

Base.eltype(::Type{GAPGroupConjClass{T,S}}) where {T,S} = S

Base.hash(x::GAPGroupConjClass, h::UInt) = h # FIXME

function Base.show(io::IO, ::MIME"text/plain", x::GAPGroupConjClass)
  println(io, "Conjugacy class of")
  io = pretty(io)
  print(io, Indent())
  println(io, Lowercase(), x.repr, " in")
  print(io, Lowercase(), acting_group(x))
  print(io, Dedent())
end

function Base.show(io::IO, x::GAPGroupConjClass{T, S}) where T where S
  if is_terse(io)
    if S <: GAPGroupElem
      print(io, "Conjugacy class of group elements")
    else
      print(io, "Conjugacy class of subgroups")
    end
  else
    print(io, "Conjugacy class of ")
    io = pretty(io)
    print(terse(io), Lowercase(), x.repr, " in ", Lowercase(), acting_group(x))
  end
end

action_function(C::GAPGroupConjClass) = ^

==(a::GAPGroupConjClass{T, S}, b::GAPGroupConjClass{T, S}) where S where T = a.CC == b.CC

function Base.length(::Type{T}, C::GAPGroupConjClass) where T <: IntegerUnion
   return T(GAPWrap.Size(C.CC))
end

Base.length(C::GroupConjClass) = length(ZZRingElem, C)
Base.lastindex(C::GroupConjClass) = length(C)

Base.keys(C::GroupConjClass) = keys(1:length(C))

is_transitive(C::GroupConjClass) = true

orbit(G::GAPGroup, g::T) where T<: Union{GAPGroupElem, GAPGroup} = conjugacy_class(G, g)

orbits(C::GAPGroupConjClass) = [C]

function permutation(C::GAPGroupConjClass, g::GAPGroupElem)
  pi = GAP.Globals.Permutation(GapObj(g), C.CC, GAP.Globals.OnPoints)::GapObj
  return group_element(action_range(C), pi)
end

@attr GAPGroupHomomorphism{T, PermGroup} function action_homomorphism(C::GAPGroupConjClass{T}) where T
  G = acting_group(C)
  acthom = GAP.Globals.ActionHomomorphism(GapObj(G), C.CC, GAP.Globals.OnPoints)::GapObj

  # See the comment about `SetJuliaData` in the `action_homomorphism` method
  # for `GSetByElements`.
  GAP.Globals.SetJuliaData(acthom, GAP.Obj([C, G]))

  return GAPGroupHomomorphism(G, action_range(C), acthom)
end


"""
    representative(C::GroupConjClass)

Return a representative of the conjugacy class `C`.

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> C = conjugacy_class(G, G([2, 1, 3, 4]))
Conjugacy class of
  (1,2) in
  Sym(4)

julia> representative(C)
(1,2)
```
"""
representative(C::GroupConjClass) = C.repr

"""
    acting_group(C::GroupConjClass)

Return the acting group of `C`.

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> C = conjugacy_class(G, G([2, 1, 3, 4]))
Conjugacy class of
  (1,2) in
  Sym(4)

julia> acting_group(C) === G
true
```
"""
acting_group(C::GroupConjClass) = C.X

# START elements conjugation

"""
    conjugacy_class(G::Group, g::GAPGroupElem) -> GroupConjClass

Return the conjugacy class `cc` of `g` in `G`, where `g` = `representative`(`cc`).

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> C = conjugacy_class(G, G([2, 1, 3, 4]))
Conjugacy class of
  (1,2) in
  Sym(4)
```
"""
function conjugacy_class(G::GAPGroup, g::GAPGroupElem)
   return GAPGroupConjClass(G, g, GAPWrap.ConjugacyClass(GapObj(G),GapObj(g)))
end

function Base.rand(C::GroupConjClass{S,T}) where S where T<:GAPGroupElem
   return Base.rand(Random.GLOBAL_RNG, C)
end

function Base.rand(rng::Random.AbstractRNG, C::GAPGroupConjClass{S,T}) where S where T<:GAPGroupElem
   return group_element(acting_group(C), GAP.Globals.Random(GAP.wrap_rng(rng), C.CC)::GapObj)
end

Base.in(g::GAPGroupElem, C::GAPGroupConjClass) = GapObj(g) in C.CC
Base.in(G::GAPGroup, C::GAPGroupConjClass) = GapObj(G) in C.CC

Base.IteratorSize(::Type{<:GAPGroupConjClass}) = Base.SizeUnknown()

Base.iterate(cc::GAPGroupConjClass) = iterate(cc, GAPWrap.Iterator(cc.CC))

function Base.iterate(cc::GAPGroupConjClass{S,T}, state::GapObj) where {S,T}
  GAPWrap.IsDoneIterator(state) && return nothing
  i = GAPWrap.NextIterator(state)::GapObj
  if T <: GAPGroupElem
     return group_element(acting_group(cc), i), state
  else
     return _as_subgroup(acting_group(cc), i)[1], state
  end
end


"""
    number_of_conjugacy_classes(G::GAPGroup)

Return the number of conjugacy classes of elements in `G`.
"""
@gapattribute number_of_conjugacy_classes(G::GAPGroup) = ZZRingElem(GAP.Globals.NrConjugacyClasses(GapObj(G))::GapInt)

number_of_conjugacy_classes(::Type{T}, G::GAPGroup) where T <: IntegerUnion = T(GAPWrap.NrConjugacyClasses(GapObj(G)))

"""
    conjugacy_classes(G::Group)

Return a vector of all conjugacy classes of elements in `G`.
It is guaranteed that the class of the identity is in the first position.
"""
function conjugacy_classes(G::GAPGroup)
   L=Vector{GapObj}(GAPWrap.ConjugacyClasses(GapObj(G)))
   return [GAPGroupConjClass(G, group_element(G, GAPWrap.Representative(cc)), cc) for cc in L]
end

@doc raw"""
    is_conjugate(G::GAPGroup, x::GAPGroupElem, y::GAPGroupElem)

Return whether `x` and `y` are conjugate elements in `G`,
i.e., there is an element `z` in `G` such that `x^z` equals `y`.
To also return the element `z`, use [`is_conjugate_with_data`](@ref).
"""
function is_conjugate(G::GAPGroup, x::GAPGroupElem, y::GAPGroupElem)
   if isdefined(G,:descr) && (G.descr == :GL || G.descr == :SL)
     return is_conjugate_with_data_in_gl_or_sl(G, x, y)[1]
   end
   return GAPWrap.IsConjugate(GapObj(G), GapObj(x), GapObj(y))
end

"""
    is_conjugate_with_data(G::Group, x::GAPGroupElem, y::GAPGroupElem)

If `x` and `y` are conjugate in `G`,
return `(true, z)`, where `x^z == y` holds;
otherwise, return `(false, nothing)`.
If the conjugating element `z` is not needed,
use [`is_conjugate`](@ref).
"""
function is_conjugate_with_data(G::GAPGroup, x::GAPGroupElem, y::GAPGroupElem)
   if isdefined(G,:descr) && (G.descr == :GL || G.descr == :SL)
     return is_conjugate_with_data_in_gl_or_sl(G, x, y)
   end
   conj = GAPWrap.RepresentativeAction(GapObj(G), GapObj(x), GapObj(y))
   if conj != GAP.Globals.fail
      return true, group_element(G, conj)
   else
      return false, nothing
   end
end
# END elements conjugation

# START subgroups conjugation
"""
    conjugacy_class(G::Group, H::Group) -> GroupConjClass

Return the subgroup conjugacy class `cc` of `H` in `G`, where `H` = `representative`(`cc`).
"""
function conjugacy_class(G::GAPGroup, H::GAPGroup)
#T _check_compatible
   return GAPGroupConjClass(G, H, GAPWrap.ConjugacyClassSubgroups(GapObj(G),GapObj(H)))
end

function Base.rand(C::GroupConjClass{S,T}) where S where T<:GAPGroup
   return Base.rand(Random.GLOBAL_RNG, C)
end

function Base.rand(rng::Random.AbstractRNG, C::GroupConjClass{S,T}) where S where T<:GAPGroup
   return _oscar_subgroup(GAP.Globals.Random(GAP.wrap_rng(rng), C.CC), acting_group(C))
end

"""
    subgroup_classes(G::GAPGroup; order::T = ZZRingElem(-1)) where T <: IntegerUnion

Return a vector of all conjugacy classes of subgroups of `G` or,
if `order` is positive, the classes of subgroups of this order.

# Examples
```jldoctest
julia> G = symmetric_group(3)
Sym(3)

julia> subgroup_classes(G)
4-element Vector{GAPGroupConjClass{PermGroup, PermGroup}}:
 Conjugacy class of permutation group in G
 Conjugacy class of permutation group in G
 Conjugacy class of permutation group in G
 Conjugacy class of permutation group in G

julia> subgroup_classes(G, order = ZZRingElem(2))
1-element Vector{GAPGroupConjClass{PermGroup, PermGroup}}:
 Conjugacy class of permutation group in G
```
"""
function subgroup_classes(G::GAPGroup; order::T = ZZRingElem(-1)) where T <: IntegerUnion
  L = Vector{GapObj}(GAPWrap.ConjugacyClassesSubgroups(GapObj(G)))
  res = [GAPGroupConjClass(G, _as_subgroup_bare(G, GAPWrap.Representative(cc)), cc) for cc in L]
  if order != -1
    filter!(x -> AbstractAlgebra.order(representative(x)) == order, res)
  end
  return res
end

"""
    subgroups(G::GAPGroup)

Return an iterator over all subgroups in `G`.
Very likely it is better to use [`subgroup_classes`](@ref) instead.

# Examples
```jldoctest
julia> println([order(H) for H in subgroups(symmetric_group(3))])
ZZRingElem[1, 2, 2, 2, 3, 6]

julia> println([order(H) for H in subgroups(quaternion_group(8))])
ZZRingElem[1, 2, 4, 4, 4, 8]
```
"""
subgroups(G::GAPGroup) = Iterators.flatten(subgroup_classes(G))

"""
    maximal_subgroup_classes(G::Group)

Return a vector of all conjugacy classes of maximal subgroups of `G`.

# Examples
```jldoctest
julia> G = symmetric_group(3);

julia> maximal_subgroup_classes(G)
2-element Vector{GAPGroupConjClass{PermGroup, PermGroup}}:
 Conjugacy class of permutation group in G
 Conjugacy class of permutation group in G
```
"""
@gapattribute function maximal_subgroup_classes(G::GAPGroup)
  L = Vector{GapObj}(GAP.Globals.ConjugacyClassesMaximalSubgroups(GapObj(G))::GapObj)
  TG = typeof(G)
  TS = sub_type(TG)
  LL = [GAPGroupConjClass(G, _as_subgroup_bare(G, GAPWrap.Representative(cc)), cc) for cc in L]
  return Vector{GAPGroupConjClass{TG, TS}}(LL)
end

"""
    maximal_subgroups(G::Group)

Return an iterator over the maximal subgroups in `G`.
Very likely it is better to use [`maximal_subgroup_classes`](@ref) instead.

# Examples
```jldoctest
julia> println([order(H) for H in maximal_subgroups(symmetric_group(3))])
ZZRingElem[3, 2, 2, 2]

julia> println([order(H) for H in maximal_subgroups(quaternion_group(8))])
ZZRingElem[4, 4, 4]
```
"""
maximal_subgroups(G::T) where T <: Union{GAPGroup, FinGenAbGroup} = Iterators.flatten(maximal_subgroup_classes(G))

"""
    low_index_subgroup_classes(G::GAPGroup, n::Int)

Return a vector of conjugacy classes of subgroups of index at most `n` in `G`.

# Examples
```jldoctest
julia> G = symmetric_group(5);

julia> low_index_subgroup_classes(G, 5)
3-element Vector{GAPGroupConjClass{PermGroup, PermGroup}}:
 Conjugacy class of Sym(5) in G
 Conjugacy class of Alt(5) in G
 Conjugacy class of permutation group in G
```
"""
function low_index_subgroup_classes(G::GAPGroup, n::Int)
  @req (n > 0) "index must be positive"
  ll = GAP.Globals.LowIndexSubgroups(GapObj(G), n)::GapObj
  return [conjugacy_class(G, H) for H in _as_subgroups(G, ll)]
end

"""
    low_index_subgroups(G::Group, n::Int)

Return an iterator over the subgroups of index at most `n` in `G`.
Very likely it is better to use [`low_index_subgroup_classes`](@ref) instead.

# Examples
```jldoctest
julia> G = alternating_group(6);

julia> length(collect(low_index_subgroups(G, 6)))
13
```
"""
low_index_subgroups(G::T, n::Int) where T <: Union{GAPGroup, FinGenAbGroup} = Iterators.flatten(low_index_subgroup_classes(G, n))

"""
    conjugate_group(G::T, x::GAPGroupElem) where T <: GAPGroup

Return the group `G^x` that consists of the elements `g^x`, for `g` in `G`.

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> H = sylow_subgroup(G, 3)[1]
Permutation group of degree 4 and order 3

julia> conjugate_group(H, gen(G, 1))
Permutation group of degree 4 and order 3

```
"""
function conjugate_group(G::T, x::GAPGroupElem) where T <: GAPGroup
  @req check_parent(G, x) "G and x are not compatible"
  return _oscar_subgroup(GAPWrap.ConjugateSubgroup(GapObj(G), GapObj(x)), G)
end

Base.:^(H::GAPGroup, y::GAPGroupElem) = conjugate_group(H, y)

# This function was never exported but may have been used somewhere.
# (The name is confusing because it is not clear *of which group* the result
# shall be a subgroup.)

@doc raw"""
    is_conjugate(G::GAPGroup, H::GAPGroup, K::GAPGroup)

Return whether `H` and `K` are conjugate subgroups in `G`,
i.e., whether there exists an element `z` in  `G` such that
`H^z` equals `K`. To also return the element `z`
use [`is_conjugate_with_data`](@ref).

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> H = sub(G, [G([2, 1, 3, 4])])[1]
Permutation group of degree 4

julia> K = sub(G, [G([1, 2, 4, 3])])[1]
Permutation group of degree 4

julia> is_conjugate(G, H, K)
true

julia> K = sub(G, [G([2, 1, 4, 3])])[1]
Permutation group of degree 4

julia> is_conjugate(G, H, K)
false

```
"""
is_conjugate(G::GAPGroup, H::GAPGroup, K::GAPGroup) = GAPWrap.IsConjugate(GapObj(G),GapObj(H),GapObj(K))

"""
    is_conjugate_with_data(G::Group, H::Group, K::Group)

If `H` and `K` are conjugate subgroups in `G`, return `(true, z)`
where `H^z = K`; otherwise, return `(false, nothing)`.
If the conjugating element `z` is not needed, use
[`is_conjugate`](@ref).

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> H = sub(G, [G([2, 1, 3, 4])])[1]
Permutation group of degree 4

julia> K = sub(G, [G([1, 2, 4, 3])])[1]
Permutation group of degree 4

julia> is_conjugate_with_data(G, H, K)
(true, (1,3)(2,4))

julia> K = sub(G, [G([2, 1, 4, 3])])[1]
Permutation group of degree 4

julia> is_conjugate_with_data(G, H, K)
(false, nothing)

```
"""
function is_conjugate_with_data(G::GAPGroup, H::GAPGroup, K::GAPGroup)
   conj = GAPWrap.RepresentativeAction(GapObj(G), GapObj(H), GapObj(K))
   if conj != GAP.Globals.fail
      return true, group_element(G, conj)
   else
      return false, nothing
   end
end

"""
    is_conjugate_subgroup(G::T, U::T, V::T) where T <: GAPGroup

Return `true` if a conjugate of `V` by some element in `G` is a subgroup of `U`,
and `false` otherwise.

If one needs a conjugating element then one can use
 [`is_conjugate_subgroup_with_data`](@ref).

In order to check whether `U` and `V` are conjugate in `G`.
use [`is_conjugate`](@ref) or [`is_conjugate_with_data`](@ref).

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> U = derived_subgroup(G)[1]
Alt(4)

julia> V = sub(G, [G([2,1,3,4])])[1]
Permutation group of degree 4

julia> is_conjugate_subgroup(G, U, V)
false

julia> V = sub(G, [G([2, 1, 4, 3])])[1]
Permutation group of degree 4

julia> is_conjugate_subgroup(G, U, V)
true
```
"""
is_conjugate_subgroup(G::T, U::T, V::T) where T <: GAPGroup = is_conjugate_subgroup_with_data(G, U, V)[1]


"""
    is_conjugate_subgroup_with_data(G::T, U::T, V::T) where T <: GAPGroup

If a conjugate of `V` by some element in `G` is a subgroup of `U`,
return `true, z` where `V^z` is a subgroup of `U`;
otherwise, return `false, one(G)`.

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> U = derived_subgroup(G)[1]
Alt(4)

julia> V = sub(G, [G([2,1,3,4])])[1]
Permutation group of degree 4

julia> is_conjugate_subgroup_with_data(G, U, V)
(false, ())

julia> V = sub(G, [G([2, 1, 4, 3])])[1]
Permutation group of degree 4

julia> is_conjugate_subgroup_with_data(G, U, V)
(true, ())
```
"""
function is_conjugate_subgroup_with_data(G::T, U::T, V::T) where T <: GAPGroup
  if order(V) == 1
    return true, one(U)
  end
  local sigma
  while true
    sigma = rand(V)
    if order(sigma) > 1
      break
    end
  end
  s = short_right_transversal(G, U, sigma)
  for t = s
    if is_subset(V^inv(t), U)
      return true, inv(t)
    end
  end
  return false, one(G)
end

@doc raw"""
    short_right_transversal(G::PermGroup, H::PermGroup, s::PermGroupElem)

Return an array of representatives `g` for all those right cosets of `H` in `G`
such that `H^g` contains the element `s`.

# Examples
```jldoctest
julia> G = symmetric_group(4);

julia> H = sylow_subgroup(G, 3)[1]
Permutation group of degree 4 and order 3

julia> short_right_transversal(G, H, G([2, 1, 3, 4]))
PermGroupElem[]

julia> short_right_transversal(G, H, G([2, 3, 1, 4]))
2-element Vector{PermGroupElem}:
 ()
 (2,3)

```
"""
function short_right_transversal(G::PermGroup, H::PermGroup, s::PermGroupElem)
  C = conjugacy_classes(H)
  cs = cycle_structure(s)
  can = PermGroupElem[]
  for c in C
    r = representative(c)
    if cs == cycle_structure(r)
      push!(can, r)
    end
  end

  R = PermGroupElem[]
  for c in can
    success, d = is_conjugate_with_data(G, c, s)
    if success
      push!(R, d)
      @assert c^R[end] == s
    end
  end

  S = PermGroupElem[]
  C = centralizer(G, s)[1]
  for r in R
    CH = centralizer(H^r, s)[1]
    for t = right_transversal(C, CH)
      push!(S, r*t)
    end
  end

  return S
end

# END subgroups conjugation


################################################################################
#
# Normal Structure
#
################################################################################

"""
    normalizer(G::Group, H::Group)

Return `N, f`, where `N` is the normalizer of `H` in `G`,
i.e., the largest subgroup of `G` in which `H` is normal,
and `f` is the embedding morphism of `N` into `G`.
"""
normalizer(G::GAPGroup, H::GAPGroup) = _as_subgroup(G, GAPWrap.Normalizer(GapObj(G), GapObj(H)))

"""
    normalizer(G::Group, x::GAPGroupElem)

Return `N, f`, where `N` is the normalizer of the cyclic subgroup generated
by `x` in `G` and `f` is the embedding morphism of `N` into `G`.
"""
normalizer(G::GAPGroup, x::GAPGroupElem) = _as_subgroup(G, GAPWrap.Normalizer(GapObj(G), GapObj(x)))

"""
    core(G::Group, H::Group)

Return `C, f`, where `C` is the normal core of `H` in `G`,
that is, the largest normal subgroup of `G` that is contained in `H`,
and `f` is the embedding morphism of `C` into `G`.
"""
core(G::GAPGroup, H::GAPGroup) = _as_subgroup(G, GAPWrap.Core(GapObj(G), GapObj(H)))

"""
    normal_closure(G::Group, H::Group)

Return `N, f`, where `N` is the normal closure of `H` in `G`,
that is, the smallest normal subgroup of `G` that contains `H`,
and `f` is the embedding morphism of `N` into `G`.

Note that `H` must be a subgroup of `G`.
"""
normal_closure(G::GAPGroup, H::GAPGroup) = _as_subgroup(G, GAPWrap.NormalClosure(GapObj(G), GapObj(H)))

# Note:
# GAP admits `NormalClosure` also when `H` is not a subgroup of `G`,
# and in this case the result is not contained in `G`.
# (We should test whether `H` is a subgroup of `G`,
# but then the user should have the possibility to omit this check.)

"""
    pcore(G::Group, p::IntegerUnion)

Return `C, f`, where `C` is the `p`-core
(i.e. the largest normal `p`-subgroup) of `G`
and `f` is the embedding morphism of `C` into `G`.
"""
function pcore(G::GAPGroup, p::IntegerUnion)
   @req is_prime(p) "p is not a prime"
   return _as_subgroup(G, GAPWrap.PCore(GapObj(G), GAP.Obj(p)))
end



################################################################################
#
# Specific Subgroups
#
################################################################################

# commutator_subgroup(G::T, H::T) where T<:GAPGroup = T(GAP.Globals.CommutatorSubgroup(GapObj(G),GapObj(H)))
# In the literature, the name commutator subgroup is often used as a synonym
# of derived subgroup.
# GAP defines `CommutatorSubgroup( G, H )` for arbitrary groups `G`, `H` in
# the same family; thus the result is in general not contained in `G` or `H`,
# and we do not know into which group the result should be embedded.
# Is this function useful at all?
# (The name `Commutator*Subgroup*` is irritating, isn't it?)

"""
    fitting_subgroup(G::GAPGroup)

Return the Fitting subgroup of `G`, i.e.,
the largest nilpotent normal subgroup of `G`.
"""
@gapattribute fitting_subgroup(G::GAPGroup) = _as_subgroup(G, GAP.Globals.FittingSubgroup(GapObj(G)))

"""
    frattini_subgroup(G::GAPGroup)

Return the Frattini subgroup of `G`, i.e.,
the intersection of all maximal subgroups of `G`.
"""
@gapattribute frattini_subgroup(G::GAPGroup) = _as_subgroup(G, GAP.Globals.FrattiniSubgroup(GapObj(G)))

"""
    solvable_radical(G::GAPGroup)

Return the solvable radical of `G`, i.e.,
the largest solvable normal subgroup of `G`.
"""
@gapattribute solvable_radical(G::GAPGroup) = _as_subgroup(G, GAP.Globals.SolvableRadical(GapObj(G)))

"""
    socle(G::GAPGroup)

Return the socle of `G`, i.e.,
the subgroup generated by all minimal normal subgroups of `G`,
see [`minimal_normal_subgroups`](@ref).
"""
@gapattribute socle(G::GAPGroup) = _as_subgroup(G, GAP.Globals.Socle(GapObj(G)))


################################################################################
#
# Sylow & Hall Subgroups
#
################################################################################

"""
    sylow_subgroup(G::Group, p::IntegerUnion)

Return a Sylow `p`-subgroup of the finite group `G`, for a prime `p`.
This is a subgroup of `p`-power order in `G`
whose index in `G` is coprime to `p`.

# Examples
```jldoctest
julia> g = symmetric_group(4); order(g)
24

julia> s = sylow_subgroup(g, 2); order(s[1])
8

julia> s = sylow_subgroup(g, 3); order(s[1])
3

```
"""
function sylow_subgroup(G::GAPGroup, p::IntegerUnion)
   @req is_prime(p) "p is not a prime"
   return _as_subgroup(G, GAPWrap.SylowSubgroup(GapObj(G), GAP.Obj(p)))
end

"""
    hall_subgroup_classes(G::Group, P::AbstractVector{<:IntegerUnion})

Return a vector that contains the conjugacy classes of
Hall `P`-subgroups of the finite group `G`, for a vector `P` of primes.
A Hall `P`-subgroup of `G` is a subgroup the order of which is only divisible
by primes in `P` and whose index in `G` is coprime to all primes in `P`.

For solvable `G`, Hall `P`-subgroups exist and are unique up to conjugacy.
For nonsolvable `G`, Hall `P`-subgroups may not exist or may not be unique
up to conjugacy.

# Examples
```jldoctest
julia> g = dihedral_group(30);

julia> h = hall_subgroup_classes(g, [2, 3]);

julia> (length(h), order(representative(h[1])))
(1, 6)

julia> g = GL(3, 2)
GL(3,2)

julia> h = hall_subgroup_classes(g, [2, 3]);

julia> (length(h), order(representative(h[1])))
(2, 24)

julia> h = hall_subgroup_classes(g, [2, 7]); length(h)
0
```
"""
function hall_subgroup_classes(G::GAPGroup, P::AbstractVector{<:IntegerUnion})
   P = unique(P)
   @req all(is_prime, P) "The integers must be prime"
   res_gap = GAP.Globals.HallSubgroup(GapObj(G), GAP.Obj(P; recursive = true))::GapObj
   if res_gap == GAP.Globals.fail
     T = typeof(G)
     return GAPGroupConjClass{T, T}[]
   elseif GAPWrap.IsList(res_gap)
     return [conjugacy_class(G, H) for H in _as_subgroups(G, res_gap)]
   else
     return [conjugacy_class(G, _as_subgroup_bare(G, res_gap))]
   end
end

"""
    hall_subgroups(G::Group, P::AbstractVector{<:IntegerUnion})

Return an iterator over the Hall `P`-subgroups in `G`.
Very likely it is better to use [`hall_subgroup_classes`](@ref) instead.

# Examples
```jldoctest
julia> g = GL(3, 2);

julia> describe(first(hall_subgroups(g, [2, 3])))
"S4"
```
"""
hall_subgroups(G::T, P::AbstractVector{<:IntegerUnion}) where T <: Union{GAPGroup, FinGenAbGroup} = Iterators.flatten(hall_subgroup_classes(G, P))

@doc raw"""
    sylow_system(G::Group)

Return a vector of Sylow $p$-subgroups of the finite group `G`,
where $p$ runs over the prime factors of the order of `G`,
such that every two such subgroups commute with each other (as subgroups).

Sylow systems exist only for solvable groups,
an exception is thrown if `G` is not solvable.
"""
@gapattribute function sylow_system(G::GAPGroup)
   @req is_solvable(G) "The group is not solvable"
   return _as_subgroups(G, GAP.Globals.SylowSystem(GapObj(G)))
end

@doc raw"""
    complement_classes(G::GAPGroup, N::GAPGroup)

Return a vector of the conjugacy classes of complements
of the normal subgroup `N` in `G`.
This function may throw an error exception if both `N` and `G/N` are
nonsolvable.

A complement is a subgroup of `G` which intersects trivially with `N` and
together with `N` generates `G`.

# Examples
```jldoctest
julia> G = symmetric_group(3);

julia> complement_classes(G, derived_subgroup(G)[1])
1-element Vector{GAPGroupConjClass{PermGroup, PermGroup}}:
 Conjugacy class of permutation group in G

julia> G = dihedral_group(8)
Pc group of order 8

julia> complement_classes(G, center(G)[1])
GAPGroupConjClass{PcGroup, SubPcGroup}[]
```
"""
function complement_classes(G::T, N::GAPGroup) where T <: GAPGroup
   res_gap = GAP.Globals.ComplementClassesRepresentatives(GapObj(G), GapObj(N))::GapObj
   if length(res_gap) == 0
     return GAPGroupConjClass{T, sub_type(T)}[]
   else
     return [conjugacy_class(G, H) for H in _as_subgroups(G, res_gap)]
   end
end

@doc raw"""
    complements(G::GAPGroup, N::GAPGroup)

Return an iterator over the complements of the normal subgroup `N` in `G`.
Very likely it is better to use [`complement_classes`](@ref) instead.

# Examples
```jldoctest
julia> G = symmetric_group(3);

julia> describe(first(complements(G, derived_subgroup(G)[1])))
"C2"
```
"""
complements(G::GAPGroup, N::GAPGroup) = Iterators.flatten(complement_classes(G, N))

@doc raw"""
    complement_system(G::Group)

Return a vector of Hall $p'$-subgroups of the finite group `G`,
where $p$ runs over the prime factors of the order of `G`.

Complement systems exist only for solvable groups,
an exception is thrown if `G` is not solvable.
"""
@gapattribute function complement_system(G::GAPGroup)
   @req is_solvable(G) "The group is not solvable"
   return _as_subgroups(G, GAP.Globals.ComplementSystem(GapObj(G)))
end

@doc raw"""
    hall_system(G::Group)

Return a vector of Hall $P$-subgroups of the finite group `G`,
where $P$ runs over the subsets of prime factors of the order of `G`.

Hall systems exist only for solvable groups,
an exception is thrown if `G` is not solvable.
"""
@gapattribute function hall_system(G::GAPGroup)
   @req is_solvable(G) "The group is not solvable"
   return _as_subgroups(G, GAP.Globals.HallSystem(GapObj(G)))
end


################################################################################
#
# Some Properties
#
################################################################################

"""
    is_perfect(G::GAPGroup)

Return whether `G` is a perfect group, i.e., equal to its derived subgroup.

# Examples
```jldoctest
julia> is_perfect(special_linear_group(2, 5))
true

julia> is_perfect(symmetric_group(5))
false

```
"""
@gapattribute is_perfect(G::GAPGroup) = GAP.Globals.IsPerfectGroup(GapObj(G))::Bool

"""
    is_simple(G::GAPGroup)

Return whether `G` is a simple group, i.e.,
`G` is not trivial and has no non-trivial normal subgroups.

# Examples
```jldoctest
julia> is_simple(alternating_group(5))
true

julia> is_simple(symmetric_group(5))
false

```
"""
@gapattribute is_simple(G::GAPGroup) = GAP.Globals.IsSimpleGroup(GapObj(G))::Bool

@doc raw"""
    is_almost_simple(G::GAPGroup)

Return whether `G` is an almost simple group,
i.e., `G` is isomorphic to a group $H$ with the property
$S \leq H \leq Aut(S)$, for some non-abelian simple group $S$.

# Examples
```jldoctest
julia> is_almost_simple(symmetric_group(5))
true

julia> is_almost_simple(special_linear_group(2, 5))
false

```
"""
@gapattribute is_almost_simple(G::GAPGroup) = GAP.Globals.IsAlmostSimpleGroup(GapObj(G))::Bool

@doc raw"""
    is_quasisimple(G::GAPGroup)

Return whether `G` is a quasisimple group,
i.e., `G` is perfect such that the factor group modulo its center is
a non-abelian simple group.

# Examples
```jldoctest
julia> is_quasisimple(special_linear_group(2, 5))
true

julia> is_quasisimple(symmetric_group(5))
false

```
"""
@gapattribute is_quasisimple(G::GAPGroup) = GAP.Globals.IsQuasisimpleGroup(GapObj(G))::Bool

@doc raw"""
    is_sporadic_simple(G::GAPGroup)

Return whether `G` is a sporadic simple group.

# Examples
```jldoctest
julia> is_sporadic_simple(mathieu_group(11))
true

julia> is_sporadic_simple(mathieu_group(10))
false

```
"""
@gapattribute is_sporadic_simple(G::GAPGroup) = GAP.Globals.IsSporadicSimpleGroup(GapObj(G))::Bool

"""
    is_pgroup(G)

Return `true` if `G` is a ``p``-group for some prime ``p``, that is, if the
order of every element in `G` is a power of ``p``.

Note that a finite group is a ``p``-group if and only if its order is a prime
power. In particular, the trivial group is a ``p``-group for every prime.

# Examples
```jldoctest
julia> is_pgroup(symmetric_group(1))
true

julia> is_pgroup(symmetric_group(2))
true

julia> is_pgroup(symmetric_group(3))
false

```
"""
@gapattribute is_pgroup(G::GAPGroup) = GAP.Globals.IsPGroup(GapObj(G))::Bool


"""
    is_pgroup_with_prime(::Type{T} = ZZRingElem, G::GAPGroup) where T <: IntegerUnion

Return `(true, nothing)` if `G` is the trivial group,
`(true, p)` if the order of every element in `G` is a power of a prime `p`,
and `(false, nothing)` otherwise.

For finite groups `G`, the first return value is `true` if and only if
the order of `G` is a prime power.

# Examples
```jldoctest
julia> is_pgroup_with_prime(symmetric_group(1))
(true, nothing)

julia> is_pgroup_with_prime(symmetric_group(2))
(true, 2)

julia> is_pgroup_with_prime(symmetric_group(3))
(false, nothing)

```
"""
function is_pgroup_with_prime(::Type{T}, G::GAPGroup) where T <: IntegerUnion
  is_trivial(G) && return true, nothing
  if is_pgroup(G)
    p = GAPWrap.PrimePGroup(GapObj(G))
    return true, T(p)
  end
  return false, nothing
end

is_pgroup_with_prime(G::GAPGroup) = is_pgroup_with_prime(ZZRingElem, G)


# helper for prime_of_pgroup: this helper is efficient thanks to
# @gapattribute, but we also want prime_of_pgroup to accept an optional
# type argument; so we cannot use @gapattribute directly. Instead we set
# up this auxiliary _prime_of_pgroup which then is called by the real
# prime_of_pgroup.
# TODO: enhance @gapattribute so this is not necessary
@gapattribute function _prime_of_pgroup(G::GAPGroup)
  @req (!is_trivial(G) && is_pgroup(G)) "only supported for non-trivial p-groups"
  return GAP.Globals.PrimePGroup(GapObj(G))
end


"""
    prime_of_pgroup(::Type{T} = ZZRingElem, G::GAPGroup) where T <: IntegerUnion

Return the prime ``p`` if `G` is a non-trivial ``p``-group.

An exception is thrown if `G` is not a ``p``-group or
is a trivial group.

# Examples
```jldoctest
julia> prime_of_pgroup(quaternion_group(8))
2

julia> prime_of_pgroup(UInt16, quaternion_group(8))
0x0002

julia> prime_of_pgroup(symmetric_group(1))
ERROR: ArgumentError: only supported for non-trivial p-groups

julia> prime_of_pgroup(symmetric_group(3))
ERROR: ArgumentError: only supported for non-trivial p-groups

```
"""
function prime_of_pgroup(::Type{T}, G::GAPGroup) where T <: IntegerUnion
  return T(_prime_of_pgroup(G))
end

# set default value for first argument T to ZZRingElem
prime_of_pgroup(G::GAPGroup) = prime_of_pgroup(ZZRingElem, G)

"""
    has_prime_of_pgroup(G::GAPGroup)

Return `true` if the value for `prime_of_pgroup(G)` has already been computed.
"""
has_prime_of_pgroup(G::GAPGroup) = has__prime_of_pgroup(G)

"""
    set_prime_of_pgroup(G::GAPGroup, p::IntegerUnion)

Set the value for `prime_of_pgroup(G)` to `p` if it hasn't been set already.
"""
function set_prime_of_pgroup(G::GAPGroup, p::IntegerUnion)
  set__prime_of_pgroup(G, GAP.Obj(p))
end

# TODO/FIXME: the rank method below is disabled because it conflicts
# with semantics of  the `rank` method for FinGenAbGroup. We'll have
# to resolve this first; afterwards we can uncomment this code,
# and possibly rename it to whatever we agreed on (if it is different from `rank`)
#"""
#    rank(G::GAPGroup)
#
#Return the rank of the group `G`, i.e., the minimal size of a generating set.
#
## Examples
#```jldoctest
#julia> rank(symmetric_group(5))
#2
#
#julia> rank(free_group(5))
#5
#```
#`"""
#function rank(G::GAPGroup)
#  is_trivial(G) && return 0
#  is_cyclic(G) && return 1
#  if is_free(G) || (has_is_finite(G) && is_finite(G) && is_pgroup(G))
#    return GAP.Globals.Rank(GapObj(G))::Int
#  end
#  has_is_finite(G) && is_finite(G) && return length(minimal_generating_set(G))
#  error("not yet supported")
#end

"""
    is_finitely_generated(G::GAPGroup)

Return whether `G` is a finitely generated group.

# Examples
```jldoctest
julia> F = free_group(2)
Free group of rank 2

julia> is_finitely_generated(F)
true

julia> H = derived_subgroup(F)[1]
Free group

julia> is_finitely_generated(H)
false
```
"""
@gapattribute is_finitely_generated(G::GAPGroup) = GAP.Globals.IsFinitelyGeneratedGroup(GapObj(G))::Bool


# TODO/FIXME: is_free is disabled for now as it is not universal; it only
# really works for fp groups, and then also only for those without relators;
# it returns `false` for a the quotient of the free group on x,y by y, which
# is mathematically a free group, but maybe not in a pure "technical" sense
#@doc raw"""
#    is_free(G::GAPGroup)
#
#Return whether `G` is a free group.
#
## Examples
#```jldoctest
#julia> F = free_group(2)
#<free group on the generators [ f1, f2 ]>
#
#julia> is_free(F)
#true
#
#julia> H = derived_subgroup(F)[1]
#Group(<free, no generators known>)
#
#julia> is_free(H)
#true
#```
#"""
#@gapattribute is_free(G::GAPGroup) = GAP.Globals.IsFreeGroup(GapObj(G))::Bool


@doc raw"""
    full_group(G::T) where T <: Union{SubFPGroup, SubPcGroup}
    full_group(G::T) where T <: Union{FPGroup, PcGroup}

Return `F, emb` where `F` is the full pc group of f.p. group of which `G`
is a subgroup, and `emb` is an embedding of `G` into `F`.

# Examples
```jldoctest
julia> G = perfect_group(FPGroup, 60, 1);

julia> H = sylow_subgroup(G, 2)[1];

julia> full_group(H)[1] == G
true

julia> full_group(G)[1] == G
true
```
"""
function full_group(G::Union{SubFPGroup, SubPcGroup})
  F = G.full_group
  return F, embedding(G, F)
end

# for convenience
function full_group(G::Union{FPGroup, PcGroup})
  return G, identity_map(G)
end


@doc raw"""
    relators(G::FPGroup)

Return a vector of relators for the full finitely presented group `G`, i.e.,
elements $[x_1, x_2, \ldots, x_n]$ in $F =$ `free_group(ngens(G))` such that
`G` is isomorphic with $F/[x_1, x_2, \ldots, x_n]$.

# Examples
```jldoctest
julia> f = free_group(2);  (x, y) = gens(f);

julia> q = quo(f, [x^2, y^2, comm(x, y)])[1];  relators(q)
3-element Vector{FPGroupElem}:
 f1^2
 f2^2
 f1^-1*f2^-1*f1*f2
```
"""
function relators(G::FPGroup)
  L = GAPWrap.RelatorsOfFpGroup(GapObj(G))::GapObj
  F = free_group(G)
  return [group_element(F, L[i]::GapObj) for i in 1:length(L)]
end


@doc raw"""
    map_word(g::FPGroupElem, genimgs::Vector; genimgs_inv::Vector = Vector(undef, length(genimgs)), init = nothing)
    map_word(v::Vector{Union{Int, Pair{Int, Int}}}, genimgs::Vector; genimgs_inv::Vector = Vector(undef, length(genimgs)), init = nothing)

Return the product $R_1 R_2 \cdots R_n$
that is described by `g` or `v`, respectively.

If `g` is an element of a free group $G$, say, then the rank of $G$ must be
equal to the length of `genimgs`, `g` is a product of the form
$g_{i_1}^{e_1} g_{i_2}^{e_2} \cdots g_{i_n}^{e_n}$
where $g_i$ is the $i$-th generator of $G$ and the $e_i$ are nonzero integers,
and $R_j =$ `imgs[`$i_j$`]`$^{e_j}$.

If `g` is an element of (a subgroup of) a finitely presented group
then the result is defined as `map_word` applied to a representing element
of the underlying free group.

If the first argument is a vector `v` of integers $k_i$ or pairs `k_i => e_i`,
respectively,
then the absolute values of the $k_i$ must be at most the length of `genimgs`,
and $R_j =$ `imgs[`$|k_i|$`]`$^{\epsilon_i}$
where $\epsilon_i$ is the `sign` of $k_i$ (times $e_i$).

If a vector `genimgs_inv` is given then its assigned entries are expected
to be the inverses of the corresponding entries in `genimgs`,
and the function will use (and set) these entries in order to avoid
calling `inv` (more than once) for entries of `genimgs`.

If `v` has length zero then `init` is returned if also `genimgs` has length
zero, otherwise `one(genimgs[1])` is returned.
In all other cases, `init` is ignored.

# Examples
```jldoctest
julia> F = free_group(2);  F1 = gen(F, 1);  F2 = gen(F, 2);

julia> imgs = gens(symmetric_group(4))
2-element Vector{PermGroupElem}:
 (1,2,3,4)
 (1,2)

julia> map_word(F1^2, imgs)
(1,3)(2,4)

julia> map_word(F2, imgs)
(1,2)

julia> map_word(one(F), imgs)
()

julia> invs = Vector(undef, 2);

julia> map_word(F1^-2*F2, imgs, genimgs_inv = invs)
(1,3,2,4)

julia> invs
2-element Vector{Any}:
    (1,4,3,2)
 #undef

```
"""
function map_word(g::Union{FPGroupElem, SubFPGroupElem}, genimgs::Vector; genimgs_inv::Vector = Vector(undef, length(genimgs)), init = nothing)
  G = parent(g)
  Ggens = gens(G)
  if length(Ggens) == 0
    @req init !== nothing "use '; init =...' if there are no generators"
    return init
  end
  gX = GapObj(g)
  if !GAPWrap.IsAssocWord(gX)
    # element of a f.p. group
    gX = GAPWrap.UnderlyingElement(gX)
  end
  @assert length(GAP.getbangproperty(GAPWrap.FamilyObj(gX), :names)) == length(genimgs)
  @assert GAPWrap.IsAssocWord(gX)
  if GAPWrap.IsLetterAssocWordRep(gX)
    # `GAPWrap.ExtRepOfObj` would create a syllable representation,
    # which is unnecessary.
    ll = Vector{Int}(GAPWrap.LetterRepAssocWord(gX))
  elseif GAPWrap.IsSyllableAssocWordRep(gX)
    # Here we take the available syllable representation.
    l = GAPWrap.ExtRepOfObj(gX)
    ll = Pair{Int, Int}[l[i] => l[i+1] for i in 1:2:length(l)]
  else
    error("do not know the type of the element $gX")
  end
  return map_word(ll, genimgs, genimgs_inv = genimgs_inv, init = init)
end


@doc raw"""
    map_word(g::PcGroupElem, genimgs::Vector; genimgs_inv::Vector = Vector(undef, length(genimgs)), init = nothing)

Return the product $R_1 R_2 \cdots R_n$ that is described by `g`,
which is a product of the form
$g_{i_1}^{e_1} g_{i_2}^{e_2} \cdots g_{i_n}^{e_n}$
where $g_i$ is the $i$-th entry in the defining polycyclic generating sequence
of $G$ and the $e_i$ are nonzero integers,
and $R_j =$ `imgs[`$i_j$`]`$^{e_j}$.

# Examples
```jldoctest
julia> G = dihedral_group(10)
Pc group of order 10

julia> x, y = gens(G);  g = x * y^4
f1*f2^4

julia> map_word(g, gens(free_group(:x, :y)))
x*y^4
```
"""
function map_word(g::PcGroupElem, genimgs::Vector; genimgs_inv::Vector = Vector(undef, length(genimgs)), init = nothing)
  G = parent(g)
  Ggens = gens(G)
  if length(Ggens) == 0
    return init
  end
  gX = GapObj(g)

  if GAPWrap.IsPcGroup(GapObj(G))
    l = GAP.Globals.ExponentsOfPcElement(GAP.Globals.FamilyPcgs(GapObj(G)), gX)
  else  # GAP.Globals.IsPcpGroup(GapObj(G))
    l = GAP.Globals.Exponents(gX)
  end
  @assert length(l) == length(genimgs)
  ll = Pair{Int, Int}[i => l[i] for i in 1:length(l)]
  return map_word(ll, genimgs, genimgs_inv = genimgs_inv, init = init)
end

function map_word(v::Union{Vector{Int}, Vector{Pair{Int, Int}}, Vector{Any}}, genimgs::Vector; genimgs_inv::Vector = Vector(undef, length(genimgs)), init = nothing)
  length(genimgs) == 0 && (@assert length(v) == 0; return init)
  length(v) == 0 && return one(genimgs[1])
  return prod(i -> _map_word_syllable(i, genimgs, genimgs_inv), v)
end

function _map_word_syllable(vi::Int, genimgs::Vector, genimgs_inv::Vector)
  vi > 0 && (@assert vi <= length(genimgs); return genimgs[vi])
  vi = -vi
  @assert vi <= length(genimgs)
  isassigned(genimgs_inv, vi) && return genimgs_inv[vi]
  res = inv(genimgs[vi])
  genimgs_inv[vi] = res
  return res
end

function _map_word_syllable(vi::Pair{Int, Int}, genimgs::Vector, genimgs_inv::Vector)
  x = vi[1]
  @assert (x > 0 && x <= length(genimgs))
  e = vi[2]
  e > 1 && return genimgs[x]^e
  e == 1 && return genimgs[x]
  isassigned(genimgs_inv, x) && return genimgs_inv[x]^-e
  res = inv(genimgs[x])
  genimgs_inv[x] = res
  e == -1 && return res
  return res^-e
end


@doc raw"""
    syllables(g::Union{FPGroupElem, SubFPGroupElem})

Return the syllables of `g` as a list of pairs `gen => exp` where
`gen` is the index of a generator and `exp` is an exponent.

# Examples
```jldoctest
julia> F = free_group(2);  F1, F2 = gens(F);

julia> syllables(F1^5*F2^-3)
2-element Vector{Pair{Int64, Int64}}:
 1 => 5
 2 => -3

julia> syllables(one(F))
Pair{Int64, Int64}[]

julia> G, epi = quo(F, [F1^10, F2^10]);

julia> syllables(epi(F1^5*F2^-3))
2-element Vector{Pair{Int64, Int64}}:
 1 => 5
 2 => -3
```
"""
function syllables(g::Union{FPGroupElem, SubFPGroupElem})
  l = GAPWrap.ExtRepOfObj(GapObj(g))
  return Pair{Int, Int}[l[i] => l[i+1] for i in 1:2:length(l)]
end


@doc raw"""
    letters(g::FPGroupElem)

Return the letters of `g` as a list of integers, each entry corresponding to
a group generator. Inverses of the generators are represented by negative
numbers.

# Examples
```jldoctest
julia> F = free_group(2);  F1, F2 = gens(F);

julia> letters(F1^5*F2^-3)
8-element Vector{Int64}:
  1
  1
  1
  1
  1
 -2
 -2
 -2

julia> letters(one(F))
Int64[]

julia> G, epi = quo(F, [F1^10, F2^10]);

julia> letters(epi(F1^5*F2^-3))
8-element Vector{Int64}:
  1
  1
  1
  1
  1
 -2
 -2
 -2
```
"""
function letters(g::FPGroupElem)
  w = GAPWrap.UnderlyingElement(GapObj(g))
  return Vector{Int}(GAPWrap.LetterRepAssocWord(w))
end


@doc raw"""
    length(g::Union{FPGroupElem, SubFPGroupElem})

Return the length of `g` as a word in terms of the generators of its parent
or of the full group of its parent if `g` is an element of a free group,
otherwise an exception is thrown.

# Examples
```jldoctest
julia> F = free_group(2);  F1 = gen(F, 1);  F2 = gen(F, 2);

julia> length(F1*F2^-2)
3

julia> length(one(F))
0

julia> length(one(quo(F, [F1])[1]))
ERROR: ArgumentError: the element does not lie in a free group
```
"""
function length(g::Union{FPGroupElem, SubFPGroupElem})
  gX = GapObj(g)
  @req GAPWrap.IsAssocWord(gX) "the element does not lie in a free group"
  return length(gX)
end


"""
    (G::FPGroup)(pairs::AbstractVector{Pair{T, S}}) where {T <: IntegerUnion, S <: IntegerUnion}

Return the element `x` of the full finitely presented group `G`
that is described by `pairs` in the sense that `x` is the product
of the powers `gen(G, i_j) ^ e_j`
where the `pairs[j]` is equal to `i_j => e_j`.
If the `i_j` in adjacent entries of `pairs` are different and the `e_j` are
nonzero then `pairs` is the vector of syllables of `x`, see [`syllables`](@ref).

# Examples
```jldoctest
julia> G = free_group(2);  pairs = [1 => 3, 2 => -1];

julia> x = G(pairs)
f1^3*f2^-1

julia> syllables(x) == pairs
true
```
"""
function (G::FPGroup)(pairs::AbstractVector{Pair{T, S}}) where {T <: IntegerUnion, S <: IntegerUnion}
   n = ngens(G)
   ll = IntegerUnion[]
   for p in pairs
     @req 0 < p.first && p.first <= n "generator number is at most $n"
     if p.second != 0
       push!(ll, p.first)
       push!(ll, p.second)
     end
   end
   return G(ll)
end

# This format is used in the serialization of `FPGroupElem`.
function (G::FPGroup)(extrep::AbstractVector{T}) where T <: IntegerUnion
   famG = GAPWrap.ElementsFamily(GAPWrap.FamilyObj(GapObj(G)))
   if GAP.Globals.IsFreeGroup(GapObj(G))
     w = GAPWrap.ObjByExtRep(famG, GapObj(extrep, true))
   else
     # For quotients of free groups, `GAPWrap.ObjByExtRep` is not defined.
     F = GAP.getbangproperty(famG, :freeGroup)
     famF = GAPWrap.ElementsFamily(GAPWrap.FamilyObj(F))
     w1 = GAPWrap.ObjByExtRep(famF, GapObj(extrep, true))
     w = GAPWrap.ElementOfFpGroup(famG, w1)
   end

   return FPGroupElem(G, w)
end


function describe(G::FinGenAbGroup)
   l = elementary_divisors(G)
   length(l) == 0 && return "0"   # trivial group
   l_tor = filter(x -> x != 0, l)
   free = length(l) - length(l_tor)
   res = length(l_tor) == 0 ? "" : "Z/" * join([string(x) for x in l_tor], " + Z/")
   return free == 0 ? res : ( res == "" ? ( free == 1 ? "Z" : "Z^$free" ) : ( free == 1 ? "$res + Z" : "$res + Z^$free" ) )
end

@doc raw"""
    describe(G::GAPGroup)

Return a string that describes some aspects of the structure of `G`.

For finite groups, the function works well if `G` is an abelian group or a
finite simple group or a group in one of the following series: symmetric,
dihedral, quasidihedral, generalized quaternion, general linear, special
linear.

For other finite groups, the function tries to decompose `G` as a direct
product or a semidirect product, and if this is not possible as a
non-splitting extension of a normal subgroup $N$ with the factor group
`G`$/N$, where $N$ is the center or the derived subgroup or the Frattini
subgroup of `G`.

For infinite groups, if the group is known to be finitely generated and
abelian or free, a reasonable description is printed.

For general infinite groups, or groups for which finiteness is not (yet) known,
not much if anything can be done. In particular we avoid potentially expensive
checks such as computing the size of the group or whether it is abelian.
While we do attempt a few limited fast checks for finiteness and
commutativity, these will not detect all finite or commutative groups.

Thus calling `describe` again on the same group after additional information
about it becomes known to Oscar may yield different outputs.

!!! note
    - for finitely presented groups, even deciding if the group is trivial
      is impossible in general; the same holds for most other properties,
      like whether the group is finite, abelian, etc.,
    - there is in general no "nice" decomposition of `G`,
    - there may be several decompositions of `G`,
    - nonisomorphic groups may yield the same `describe` result,
    - isomorphic groups may yield different `describe` results,
    - the computations can take a long time (for example in the case of
      large $p$-groups), and the results are still often not very helpful.

The following notation is used in the returned string.

| Description | Syntax |
| ----------- | ----------- |
| trivial group | 1 |
| finite cyclic group | C<size> |
| infinite cyclic group | Z |
| alternating group | A<degree> |
| symmetric group | S<degree> |
| dihedral group | D<size> |
| quaternion group | Q<size> |
| quasidihedral group | QD<size> |
| projective special linear group | PSL(<n>,<q>) |
| special linear group | SL(<n>,<q>) |
| general linear group | GL(<n>,<q>) |
| proj. special unitary group | PSU(<n>,<q>) |
| orthogonal group, type B | O(2<n>+1,<q>) |
| orthogonal group, type D | O+(2<n>,<q>) |
| orthogonal group, type 2D | O-(2<n>,<q>) |
| proj. special symplectic group | PSp(2<n>,<q>) |
| Suzuki group (type 2B) | Sz(<q>) |
| Ree group (type 2F or 2G) | Ree(<q>) |
| Lie group of exceptional type | E(6,<q>), E(7,<q>), E(8,<q>), 2E(6,<q>), F(4,<q>), G(2,<q>) |
| Steinberg triality group | 3D(4,<q>) |
| sporadic simple group | M11, M12, M22, M23, M24, J1, J2, J3, J4, Co1, Co2, Co3, Fi22, Fi23, Fi24', Suz, HS, McL, He, HN, Th, B, M, ON, Ly, Ru |
| Tits group | 2F(4,2)' |
| the indicated group from the library of perfect groups | PerfectGroup(<size>,<id>) |
| direct product | A x B |
| semidirect product | N : H |
| non-split extension | Z(G) . G/Z(G) = G' . G/G', Phi(G) . G/Phi(G) |

# Examples
```jldoctest
julia> g = symmetric_group(6);

julia> describe(g)
"S6"

julia> describe(sylow_subgroup(g,2)[1])
"C2 x D8"

julia> describe(sylow_subgroup(g, 3)[1])
"C3 x C3"

julia> describe(free_group(3))
"a free group of rank 3"

```
"""
function describe(G::GAPGroup)
   is_finitely_generated(G) || return "a non-finitely generated group"

   # force some checks in some cases
   if G isa MatrixGroup && is_infinite(base_ring(G))
      is_finite(G)
   end

   # handle groups whose finiteness is known
   if has_is_finite(G)
      # finite groups: pass them to GAP
      if is_finite(G)
         return String(GAPWrap.StructureDescription(GapObj(G)))
      end

      # infinite groups known to be abelian can still be dealt with by GAP
      if has_is_abelian(G) && is_abelian(G)
         return String(GAPWrap.StructureDescription(GapObj(G)))
      end

      return "an infinite group"
   end

   return "a group"
end

function describe(G::Union{FPGroup, SubFPGroup})
   # despite the name, there are non-finitely generated (and hence non-finitely presented)
   # FPGroup instances
   is_finitely_generated(G) || return "a non-finitely generated group"

   if GAPWrap.IsFreeGroup(GapObj(G))
      r = GAP.Globals.RankOfFreeGroup(GapObj(G))::GapInt
      r >= 2 && return "a free group of rank $(r)"
      r == 1 && return "Z"
      r == 0 && return "1"
   end

   if !GAP.Globals.IsFpGroup(GapObj(G))
     # `G` is a subgroup of an f.p. group
     G = FPGroup(GAPWrap.Range(GAPWrap.IsomorphismFpGroup(GapObj(G))))
   end

   # check for free groups in disguise
   isempty(relators(G)) && return describe(free_group(G))

   # attempt to simplify presentation
   H = simplified_fp_group(G)[1]
   ngens(H) < ngens(G) && return describe(H)

   # abelian groups can be dealt with by GAP
   extra = ""
   if !has_is_abelian(G)
      if is_obviously_abelian(G)
         set_is_abelian(G, true) # TODO: Claus won't like this...
         return String(GAPWrap.StructureDescription(GapObj(G)))
      end
   elseif is_abelian(G)
      return String(GAPWrap.StructureDescription(GapObj(G)))
   else
      extra *= " non-abelian"
   end

   if !has_is_finite(G)
      # try to obtain an isomorphic permutation group, but don't try too hard
#TODO: With GAP 4.13.0, the prescribed bound 100000 will cause a test failure.
#      This regression will hopefully be fixed in GAP 4.13.1,
#      see https://github.com/gap-system/gap/issues/5697
#      and https://github.com/gap-system/gap/pull/5698.
#     iso = GAP.Globals.IsomorphismPermGroupOrFailFpGroup(GapObj(G), 100000)::GapObj
      iso = GAP.Globals.IsomorphismPermGroupOrFailFpGroup(GapObj(G))::GapObj
      iso != GAP.Globals.fail && return describe(PermGroup(GAPWrap.Range(iso)))
   elseif is_finite(G)
      return describe(PermGroup(G))
   else
      extra *= " infinite"
   end

   return "a finitely presented$(extra) group"

end

function is_obviously_abelian(G::FPGroup)
    rels = relators(G)
    fgens = gens(free_group(G))
    signs = [(e1,e2,e3) for e1 in (-1,1) for e2 in (-1,1) for e3 in (-1,1)]
    for i in 1:length(fgens)
        a = fgens[i]
        for j in i+1:length(fgens)
            b = fgens[j]
            any(t -> comm(a^t[1],b^t[2])^t[3] in rels, signs) || return false
        end
    end
    return true
end

describe(G::MultTableGroup) = describe(PermGroup(G))