FeynmanDiagrams.jl/src/generic_process_def.jl

_assert_particle_type_tuple(::Tuple{}) = nothing
function _assert_particle_type_tuple(t::Tuple{AbstractParticleType,Vararg})
    return _assert_particle_type_tuple(t[2:end])
end
function _assert_particle_type_tuple(t::Any)
    throw(
        InvalidInputError(
            "invalid input, provide a tuple of AbstractParticleTypes to construct a GenericQEDProcess",
        ),
    )
end

mutable struct GenericQEDProcess{INT,OUTT,INSP,OUTSP} <: AbstractProcessDefinition where {
    INT<:Tuple,OUTT<:Tuple,INSP<:Tuple,OUTSP<:Tuple
}
    incoming_particles::INT
    outgoing_particles::OUTT

    incoming_spins_pols::INSP
    outgoing_spins_pols::OUTSP

    virtual_particles_cache::Vector

    function GenericQEDProcess(
        in_particles::INT, out_particles::OUTT, in_sp::INSP, out_sp::OUTSP
    ) where {INT<:Tuple,OUTT<:Tuple,INSP<:Tuple,OUTSP<:Tuple}
        _assert_particle_type_tuple(in_particles)
        _assert_particle_type_tuple(out_particles)

        return new{INT,OUTT,INSP,OUTSP}(in_particles, out_particles, in_sp, out_sp, [])
    end

    """
        GenericQEDProcess(in_ph::Int, out_ph::Int, in_el::Int, out_el::Int, in_po::Int, out_po::Int)

    Convenience constructor from numbers of input/output photons, electrons and positrons.
    Uses `AllSpin()` and `AllPol()` for every particle's spin/pol by default.
    """
    function GenericQEDProcess(
        in_ph::Int, out_ph::Int, in_el::Int, out_el::Int, in_po::Int, out_po::Int
    )
        in_p = ntuple(i -> if i <= in_ph
            Photon()
        elseif i <= in_ph + in_el
            Electron()
        else
            Positron()
        end, in_ph + in_el + in_po)
        out_p = ntuple(i -> if i <= out_ph
            Photon()
        elseif i <= out_ph + out_el
            Electron()
        else
            Positron()
        end, out_ph + out_el + out_po)
        in_sp = tuple([i <= in_ph ? AllPol() : AllSpin() for i in 1:length(in_p)]...)
        out_sp = tuple([i <= out_ph ? AllPol() : AllSpin() for i in 1:length(out_p)]...)
        return GenericQEDProcess(in_p, out_p, in_sp, out_sp)
    end
end

function spin_or_pol(
    process::GenericQEDProcess,
    dir::ParticleDirection,
    species::AbstractParticleType,
    n::Int,
)
    i = 0
    c = n
    for p in particles(process, dir)
        i += 1
        if p == species
            c -= 1
        end
        if c == 0
            break
        end
    end

    if c != 0 || n <= 0
        throw(
            InvalidInputError(
                "could not get $n-th spin/pol of $dir $species, does not exist"
            ),
        )
    end

    if is_incoming(dir)
        return process.incoming_spins_pols[i]
    elseif is_outgoing(dir)
        return process.outgoing_spins_pols[i]
    else
        throw(InvalidInputError("unknown direction $(dir) given"))
    end
end

function QEDprocesses.incoming_particles(proc::GenericQEDProcess{INT,OUTT}) where {INT,OUTT}
    return proc.incoming_particles
end
function QEDprocesses.outgoing_particles(proc::GenericQEDProcess{INT,OUTT}) where {INT,OUTT}
    return proc.outgoing_particles
end

function isphysical(proc::GenericQEDProcess)
    return (
        number_particles(proc, Incoming(), Electron()) +
        number_particles(proc, Outgoing(), Positron()) ==
        number_particles(proc, Incoming(), Positron()) +
        number_particles(proc, Outgoing(), Electron())
    ) && number_particles(proc, Incoming()) + number_particles(proc, Outgoing()) >= 2
end

function matrix_element(proc::GenericQEDProcess, psp::PhaseSpacePoint)
    return nothing
end