
"""
`module  JAC.MultiPhotonDoubleIon`  
... a submodel of JAC that contains all methods for computing multi-photon two-electron (double) ionization cross sections.
"""
module MultiPhotonDoubleIon

    using ..Basics, ..ManyElectron, ..Radial


    """
    `struct  MultiPhotonDoubleIon.Settings`  
        ... defines a type for the settings in estimating multi-photon two-electron (double) ionization cross sections.

        + multipoles              ::Array{EmMultipole}           ... Specifies the multipoles of the radiation field that are to be included.
        + gauges                  ::Array{UseGauge}              ... Specifies the gauges to be included into the computations.
        + NoPhotons               ::Int64                        ... Number of photons in the multi-photon ionization
        + photonEnergies          ::Array{Float64,1}             ... List of photon energies.
        + printBeforeComputation  ::Bool                         ... True, if all energies and lines are printed before their evaluation.
        + selectLines             ::Bool                         ... True, if lines are selected individually for the computations.
        + selectedLines           ::Array{Tuple{Int64,Int64},1}  ... List of lines, given by tupels (inital-level, final-level).

    """
    struct Settings 
        multipoles                ::Array{EmMultipole}
        gauges                    ::Array{UseGauge}
        NoPhotons                 ::Int64
        photonEnergies            ::Array{Float64,1} 
        printBeforeComputation    ::Bool
        selectLines               ::Bool
        selectedLines             ::Array{Tuple{Int64,Int64},1} 
    end 


    """
    `MultiPhotonDoubleIon.Settings()`  ... constructor for the default values of multi-photon two-electron (double) ionization estimates.
    """
    function Settings()
        Settings(EmMultipole[], UseGauge[], 0, Float64[], false, false, Tuple{Int64,Int64}[])
    end


    # `Base.show(io::IO, settings::MultiPhotonDoubleIon.Settings)`  
    #		... prepares a proper printout of the variable settings::MultiPhotonDoubleIon.Settings.
    function Base.show(io::IO, settings::MultiPhotonDoubleIon.Settings) 
        println(io, "multipoles:               $(settings.multipoles)  ")
        println(io, "gauges:                   $(settings.gauges)  ")
        println(io, "NoPhotons:                $(settings.NoPhotons)  ")
        println(io, "photonEnergies:           $(settings.photonEnergies)  ")
        println(io, "printBeforeComputation:   $(settings.printBeforeComputation)  ")
        println(io, "selectLines:              $(settings.selectLines)  ")
        println(io, "selectedLines:            $(settings.selectedLines)  ")
    end


    """
    `struct  MultiPhotonDoubleIon.Channel`  
        ... defines a type for a multi-photon channel to help characterize multi-photon (single-electron) ionization with 
            well-defined multipolarities.

        + NoPhotons      ::EmMultipole            ... Number of photons in the multi-photon ionization
        + multipoles     ::Array{EmMultipole,1}   ... Multipoles of all N incoming photons.
        + gauge          ::EmGauge                ... Gauge for dealing with the (coupled) radiation field.
        + kappa1         ::Int64                  ... partial-wave of the first free electron
        + kappa2         ::Int64                  ... partial-wave of the second free electron
        + phase1         ::Float64                ... phase of the first partial wave
        + phase2         ::Float64                ... phase of the second partial wave
        + symmetry1      ::LevelSymmetry          ... total angular momentum and parity of scattering state after coupling of first electron
        + symmetry2      ::LevelSymmetry          ... total angular momentum and parity of scattering state after coupling of second electron
        + amplitude      ::Complex{Float64}       ... multi-photon ionization amplitude associated with the given channel.
   """
    struct  Channel
        NoPhotons        ::EmMultipole 
        multipoles       ::Array{EmMultipole,1}
        gauge            ::EmGauge 
        kappa1           ::Int64 
        kappa2           ::Int64   
        phase1           ::Float64 
        phase2           ::Float64 
        symmetry1        ::LevelSymmetry 
        symmetry2        ::LevelSymmetry
        amplitude        ::Complex{Float64}
    end




    # MultiPhotonDoubleIon line between initial and final (bound-state) levels
    """
    `struct  MultiPhotonDoubleIon.Line`  ... defines a type for a multi-photon ionization line that may include the definition of channels.

        + initialLevel     ::Level           ... initial-(state) level
        + finalLevel       ::Level           ... final-(state) level
        + NoPhotons        ::Int64           ... Number of photons in the multi-photon process
        + photonEnergy     ::Float64         ... Energy of the incoming photons; all photons are assumed to have equal energy.
        + crossSection     ::EmProperty      ... Cross section for this multi-photon ionization.
        + hasChannels      ::Bool            ... Determines whether the individual (sub-) channels are defined in terms of 
                                                 their multipolarities, etc., or not.
        + channels         ::Array{MultiPhotonDoubleIon.Channel,1}  ... List of MultiPhotonDoubleIon.Channels of this line.
    """
    struct  Line
        initialLevel       ::Level
        finalLevel         ::Level
        NoPhotons          ::Int64 
        photonEnergy       ::Float64
        crossSection       ::EmProperty
        hasChannels        ::Bool
        channels           ::Array{MultiPhotonDoubleIon.Channel,1}
    end


    # `Base.show(io::IO, line::MultiPhotonDoubleIon.Line)`  ... prepares a proper printout of the variable line::MultiPhotonDoubleIon.Line.
    function Base.show(io::IO, line::MultiPhotonDoubleIon.Line) 
        println(io, "initialLevel:      $(line.initialLevel)  ")
        println(io, "finalLevel:        $(line.finalLevel)  ")
        println(io, "NoPhotons:         $(line.NoPhotons)  ")
        println(io, "photonEnergy:      $(line.photonEnergy)  ")
        println(io, "crossSection:      $(line.crossSection)  ")
        println(io, "hasChannels:       $(line.hasChannels)  ")
        println(io, "channels:          $(line.channels)  ")
    end



    """
    `MultiPhotonDoubleIon.computeLines(finalMultiplet::Multiplet, initialMultiplet::Multiplet, grid::Radial.Grid, 
                                       settings::MultiPhotonDoubleIon.Settings; output=true)` 
        ... to compute the multiphoton transition amplitudes and all properties as requested by the given settings. 
            A list of lines::Array{MultiPhotonDoubleIon.Lines} is returned.
    """
    function  computeLines(finalMultiplet::Multiplet, initialMultiplet::Multiplet, grid::Radial.Grid, 
                           settings::MultiPhotonDoubleIon.Settings; output=true)
        println("")
        printstyled("MultiPhotonDoubleIon.computeLines(): The computation of multi-photon double ionization amplitudes starts now ... \n", color=:light_green)
        printstyled("----------------------------------------------------- ---------------------------------------------------------- \n", color=:light_green)
        println("")
        #
        #
        pathways = "Not yet implemented !"
        #
        if    output    return( pathways )
        else            return( nothing )
        end
    end

end # module
