# Polyhedron

As seen in the previous section, a polyhedron can be described in 2 ways: either using the H-representation (intersection of halfspaces) or the V-representation (convex hull of points and rays). The problem of computing the H-representation from the V-representation (or vice versa) is called the *representation conversion problem*. It can be solved by the Double-Description method

`Polyhedra.doubledescription`

— Function`doubledescription(h::HRepresentation)`

Computes the V-representation of the polyhedron represented by `h`

using the Double-Description algorithm [MRTT53, FP96]. It maintains a list of the points, rays and lines that are in the resulting representation and in the hyperplane corresponding to each halfspace and then add their intersection for each halfspace with `Polyhedra.add_intersection!`

. The resulting V-representation has no redundancy.

`doubledescription(V::VRepresentation)`

Computes the H-representation of the polyhedron represented by `v`

using the Double-Description algorithm [MRTT53, FP96]. Currently, this fallbacks to lifting it to the V-representation of a cone by homogenization, interpreting it as the H-representation of the dual cone so that it can rely on `doubledescription(::HRepresentation)`

.

[MRTT53] Motzkin, T. S., Raiffa, H., Thompson, G. L. and Thrall, R. M. The double description method *Contribution to the Theory of Games*, *Princeton University Press*, **1953**

[FP96] Fukuda, K. and Prodon, A. Double description method revisited *Combinatorics and computer science*, *Springer*, **1996**, 91-111

However, other methods exist such as the reverse search implemented by LRS and the quick hull algorithm implemented by qhull.

This motivates the creation of a type representing polyhedra, transparently handling the conversion from H-representation to V-representation when needed for some operation. Just like the abstract type `AbstractArray{N,T}`

represents an `N`

-dimensional array with elements of type `T`

, the abstract type `Polyhedron{N,T}`

represents an `N`

-dimensional polyhedron with elements of coefficient type `T`

.

There is typically one concrete subtype of `Polyhedron`

by library. For instance, the CDD library defines `CDDLib.Polyhedron`

and the LRS library defines `LRSLib.Polyhedron`

. It must be said that the type `T`

is not necessarily how the elements are stored internally by the library but the polyhedron will behave just like it is stored that way. For instance, when retreiving an H-(or V-)representation, the representation will be of type `T`

. Therefore using `Int`

for `T`

may result in `InexactError`

. For this reason, by default, the type `T`

chosen is not a subtype of `Integer`

.

A polyhedron can be created from a representation and a library using the `polyhedron`

function.

`Polyhedra.polyhedron`

— Function`polyhedron(rep::Representation{T})`

Creates a polyhedron from the representation `rep`

using the default library included in the Polyhedra package.

To illustrate the usage of the `polyhedron`

function, consider the following representations:

```
hr = HalfSpace([1, 1], 1) ∩ HalfSpace([1, -1], 0) ∩ HalfSpace([-1, 0], 0)
vre = convexhull([0, 0], [0, 1], [1//2, 1//2])
vrf = convexhull([0, 0], [0, 1], [1/2, 1/2])
```

One can use the CDD library, to create an instance of a concrete subtype of `Polyhedron`

as follows:

```
julia> using CDDLib
julia> polyf = polyhedron(hr, CDDLib.Library())
julia> typeof(polyhf)
CDDLib.CDDLib.Polyhedron{2,Float64}
```

We see that the library has choosen to deal with floating point arithmetic. This decision does not depend on the type of `hr`

but only on the instance of `CDDLib.Library`

given. `CDDLib.Library`

creates `CDDLib.Polyhedron`

of type either `Float64`

or `Rational{BigInt}`

. One can choose the first one using `CDDLib.Library(:float)`

and the second one using `CDDLib.Library(:exact)`

, by default it is `:float`

.

```
julia> poly = polyhedron(hr, CDDLib.Library(:exact))
julia> typeof(poly)
CDDLib.Polyhedron{2,Rational{BigInt}}
```

The first polyhedron `polyf`

can also be created from its V-representation using either of the 4 following lines:

```
julia> polyf = polyhedron(vrf, CDDLib.Library(:float))
julia> polyf = polyhedron(vrf, CDDLib.Library())
julia> polyf = polyhedron(vre, CDDLib.Library(:float))
julia> polyf = polyhedron(vre, CDDLib.Library())
```

and `poly`

using either of those lines:

```
julia> poly = polyhedron(vrf, CDDLib.Library(:exact))
julia> poly = polyhedron(vre, CDDLib.Library(:exact))
```

Of course, creating a representation in floating points with exact arithmetic works here because we have `0.5`

which is `0.1`

in binary but in general, is not a good idea.

```
julia> Rational{BigInt}(1/2)
1//2
julia> Rational{BigInt}(1/3)
6004799503160661//18014398509481984
julia> Rational{BigInt}(1/5)
3602879701896397//18014398509481984
```

## Retrieving a representation

One can retrieve an H-representation (resp. V-representation) from a polyhedron using `hrep`

(resp. `vrep`

). The concrete subtype of `HRepresentation`

(resp. `VRepresentation`

) returned is not necessarily the same that the one used to create the polyhedron. As a rule of thumb, it is the representation the closest to the internal representation used by the library.

```
julia> hr = hrep(poly)
julia> typeof(hr)
Polyhedra.LiftedHRepresentation{2,Rational{BigInt}}
julia> hr = MixedMatHRep(hr)
julia> typeof(hr)
Polyhedra.MixedMatHRep{2,Rational{BigInt}}
julia> hr.A
3x2 Array{Rational{BigInt},2}:
1//1 1//1
1//1 -1//1
-1//1 0//1
julia> hr.b
3-element Array{Rational{BigInt},1}:
1//1
0//1
0//1
julia> vr = vrep(poly)
julia> typeof(vr)
Polyhedra.LiftedVRepresentation{2,Rational{BigInt}}
julia> vr = MixedMatVRep(vrep)
julia> typeof(vr)
Polyhedra.MixedMatVRep{2,Rational{BigInt}}
julia> vr.V
3x2 Array{Rational{BigInt},2}:
1//2 1//2
0//1 1//1
0//1 0//1
julia> vr.R
0x2 Array{Rational{BigInt},2}
```

## Checking if a representation has been computed

`Polyhedra.hrepiscomputed`

— Function`hrepiscomputed(p::Polyhedron)`

Returns whether the H-representation of this polyhedron has been computed.

`Polyhedra.vrepiscomputed`

— Function`vrepiscomputed(p::Polyhedron)`

Returns whether the V-representation of this polyhedron has been computed.

## Incidence

Elements can be accessed in a representation or polyhedron using indices and `Base.get`

:

`Polyhedra.Index`

— Type`Index{T, ElemT}`

Index of an element of type `ElemT`

in a `Rep{T}`

.

`Polyhedra.Indices`

— Type`Indices{T, ElemT, RepT<:Rep{T}}`

Iterator over the indices of the elements of type `ElemT`

of the field `rep`

.

The list of indices can be obtained using, e.g., `eachindex(points(rep))`

. For instance, the following prints all points using indices

```
for pi in eachindex(points(rep))
@show get(rep, pi)
end
```

A point $p$ (resp. ray $r$) is incident to an halfspace $\langle a, x \rangle \le \beta$ if $\langle a, p \rangle = \beta$ (resp. $\langle a, r \rangle = \beta$).

`Polyhedra.incidenthalfspaces`

— Function`incidenthalfspaces(p::Polyhedron, idx)`

Returns the list of halfspaces incident to idx for the polyhedron `p`

.

`Polyhedra.incidenthalfspaceindices`

— Function`incidenthalfspaceindices(p::Polyhedron, idx)`

Returns the list of the indices of halfspaces incident to idx for the polyhedron `p`

.

`Polyhedra.incidentpoints`

— Function`incidentpoints(p::Polyhedron, idx)`

Returns the list of points incident to idx for the polyhedron `p`

.

`Polyhedra.incidentpointindices`

— Function`incidentpointindices(p::Polyhedron, idx)`

Returns the list of the indices of points incident to idx for the polyhedron `p`

.

`Polyhedra.incidentrays`

— Function`incidentrays(p::Polyhedron, idx)`

Returns the list of rays incident to idx for the polyhedron `p`

.

`Polyhedra.incidentrayindices`

— Function`incidentrayindices(p::Polyhedron, idx)`

Returns the list of the indices of rays incident to idx for the polyhedron `p`

.

In a polyhedron, all points and rays are incident to all hyperplanes and all halfspaces are incident to all lines. The following methods are therefore redundant, e.g. `incidenthyperplanes(p, idx)`

is equivalent to `hyperplanes(p)`

and `incidenthyperplaneindices(p, idx)`

is equivalent to `eachindex(hyperplanes(p))`

. The methods are hence only defined for consistency.

`Polyhedra.incidenthyperplanes`

— Function`incidenthyperplanes(p::Polyhedron, idx)`

Returns the list of hyperplanes incident to idx for the polyhedron `p`

.

`Polyhedra.incidenthyperplaneindices`

— Function`incidenthyperplaneindices(p::Polyhedron, idx)`

Returns the list of the indices of hyperplanes incident to idx for the polyhedron `p`

.

`Polyhedra.incidentlines`

— Function`incidentlines(p::Polyhedron, idx)`

Returns the list of lines incident to idx for the polyhedron `p`

.

`Polyhedra.incidentlineindices`

— Function`incidentlineindices(p::Polyhedron, idx)`

Returns the list of the indices of lines incident to idx for the polyhedron `p`

.

## Default libraries

The following functions allows to select a default library:

`Polyhedra.default_library`

— Function`default_library(d::FullDim, ::Type{T}) where {T}`

Returns the default polyhedral library for `d`

-dimensional polyhedron of coefficient type `T`

.

**Examples**

To obtain the default library for 2-dimensional polyhedra of eltype `Float64`

, do `default_library(2, Float64)`

.

Given an `StaticArrays.SVector`

`v`

, to obtain a default library for points of the type of `v`

in a type stable way, do `default_library(Polyhedra.FullDim(v), eltype(v))`

.

`Polyhedra.similar_library`

— Function`similar_library(lib::Library, d::FullDim, T::Type)`

Returns a library that supports polyhedra of full dimension `T`

with coefficient type `T`

. If `lib`

does not support it, this commonly calls `default_library(d, T)`

.

`Polyhedra.library`

— Function`library(p::Polyhedron)`

Returns the library used by `p`

.

`Polyhedra.default_type`

— Function`default_type(d::FullDim, ::Type{T}) where {T}`

Returns the default polyhedron type for `d`

-dimensional polyhedron of coefficient type `T`

.

The following libraries serves as fallback:

`Polyhedra.DefaultLibrary`

— Type`DefaultLibrary{T}`

Default library for polyhedra of dimension larger than 1 (`IntervalLibrary`

is the default for polyhedra of dimension 1). The library implements the bare minimum and uses the fallback implementation for all operations.

`Polyhedra.IntervalLibrary`

— Type`IntervalLibrary{T}`

Default library for polyhedra of dimension 1. Many aspect of polyhedral computation become trivial in one dimension. This library exploits this fact. The library is also used as a fallback for libraries that do not support 1-dimensional polyhedra (e.g. qhull). That is projecting a polyhedron using such library produces a polyhedron using `IntervalLibrary`

.

The type and library of the polyhedron obtained after applying an operation of several polyhedra (of possibly different type and/or library) is determined by the `similar`

function.

`Base.similar`

— Function`similar(p::Tuple{Vararg{Polyhedra.Rep}}, d::Polyhedra.FullDim, ::Type{T}, it::Polyhedra.It{T}...)`

Creates a representation with a type similar to `p`

of a polyhedron of full dimension `d`

, element type `T`

and initialize it with the iterators `it`

. The type of the result will be chosen closer to the type of `p[1]`

.