module Geometry

export angle2

using StaticArrays: SVector

"""
    angle2(a::SVector{2,Float64}, b::SVector{2,Float64})

Returns the angle φ from vector a to b while φ ∈ [-π, π].
"""
function angle2(a::SVector{2,Float64}, b::SVector{2,Float64})
    θ_a = atan(a[2], a[1])
    θ_b = atan(b[2], b[1])

    θ = θ_b - θ_a

    return rem2pi(θ, RoundNearest)
end

end # module