# FiniteHorizonPOMDPs.jl

POMDPs.jl-compatible 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

1. Provide a way for value-iteration-based algorithms to start at the final-stage and work backwards
2. Be compatible with generic POMDPs.jl solvers and simulators (i.e. solvers should not have to check anything more than isterminal)
3. Provide a wrapper so that an infinite horizon POMDP can be easily made into a finite horizon one
4. 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 to stage_stateindex(mdp, s).
• 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 to stage_obsindex(w,o).