using ForwardDiff, LinearAlgebra, StaticArrays


foo2(A) = SVector{4}(norm(r, 2) for r = eachrow(A))

function foo1!(out, x; μ = 0.8 / √2)
    λ = SVector{3}(@view x[1:3])
    K = SMatrix{3,3}(@view x[4:12])

    A_λ = [ 1   0  -1   0 ;
            0   1   0  -1 ;
            μ   μ   μ   μ ]

    out[1:4] = A_λ' * λ + foo2(A_λ' * K)

    out
end

function foo1_s!(out, x; μ = 0.8 / √2)
    λ = SVector{3}(@view x[1:3])
    K = SMatrix{3,3}(@view x[4:12])

    A_λ = @SMatrix [ 1   0  -1   0 ;
                     0   1   0  -1 ;
                     μ   μ   μ   μ ]

    out[1:4] = A_λ' * λ + foo2(A_λ' * K)

    out
end

x = zeros(12);
out = zeros(4);
display(ForwardDiff.jacobian(foo1!, out, x))
display(ForwardDiff.jacobian(foo1_s!, out, x))

nothing