module Laplacian

"""
    laplacian(U::AbstractMatrix{<:Real}, dx::Real)

    laplacian operator implemented in code

    # Arguments:
    - `U`: activator or inhibitor matrix
    - `dx`: dx
"""
function laplacian(U::AbstractMatrix{<:Real}, dx::Real)
    h2 = dx^2
    center = U[2:end-1, 2:end-1]
    up     = U[1:end-2, 2:end-1]
    down   = U[3:end,   2:end-1]
    left   = U[2:end-1, 1:end-2]
    right  = U[2:end-1, 3:end]

    return (up .+ down .+ left .+ right .- 4 .* center) ./ h2
end

export laplacian

end # module Laplacian