FiniteHorizonPOMDPs.jl
POMDPs.jlcompatible interface for defining MDPs and POMDPs with finite horizons
This package aims to provide a standard interface for defining problems with finite horizons.
The goals are to
 Provide a way for valueiterationbased algorithms to start at the finalstage and work backwards
 Be compatible with generic POMDPs.jl solvers and simulators (i.e. solvers should not have to check anything more than
isterminal
)  Provide a wrapper so that an infinite horizon POMDP can be easily made into a finite horizon one
 Be compatible with other interface extensions like constrained POMDPs and mixed observability problems
Notably, in accordance with goal (4), this package does not define something like AbstractFiniteHorizonPOMDP
.
Usage
Package offers interface for finite horizon POMDPs. Solver currently supports only MDPs. User can either implement:
 finite horizon MDP using both POMDPs.jl and FiniteHorizonPOMDPs.jl interface functions or
 infinite horizon MDP and transform it to finite horizon one using
fixhorizon
utility
using FiniteHorizonPOMDPs
using POMDPModels
gw = SimpleGridWorld() # initialize Infinite Horizon model
fhgw = fixhorizon(gw, 2) # use fixhorizon utility to transform it to Finite Horizon
Interface

HorizonLength(::Type{<:Union{MDP,POMDP})
 Checks whether MDP is Finite or Infinite Horizon and return corresponding struct (FiniteHorizon or InfiniteHorizon).

horizon(m::Union{MDP,POMDP})::Int
 Returns the number of steps that will be taken in the (PO)MDP, given it is Finite Horizon.

stage(m::Union{MDP,POMDP}, ss)::Int
 Returns the number of input variable's stage.

stage_states(m::Union{MDP,POMDP}, stage::Int)
 Creates (PO)MDP's states for given stage.

stage_stateindex(m::Union{MDP,POMDP}, state)
 Computes the index of the given state in the corresponding stage.

ordered_stage_states(w::FHWrapper, stage::Int)
 Returns an
AbstractVector
of states from given stage ordered according tostage_stateindex(mdp, s)
.
 Returns an

stage_observations(m::Union{MDP,POMDP}, stage::Int)
 Creates (PO)MDP's observations for given stage.

stage_obsindex(m::Union{MDP,POMDP}, o::stage::Int)
 Computes the index of the given observation in the corresponding stage.

ordered_stage_observations(w::FHWrapper, stage::Int)
 Returns an
AbstractVector
of observations from given stage ordered according tostage_obsindex(w,o)
.
 Returns an