declare symbol r, θ: MathExpr

-- Parameters and metrics

def N : Integer := 2

def x : Vector MathExpr := [|r, θ|]

def g_i_j : Matrix MathExpr := [| [| 1, 0 |], [| 0, r^2 |] |]_i_j
def g~i~j : Matrix MathExpr := [| [| 1, 0 |], [| 0, 1 / r^2 |] |]~i~j

-- Hodge Laplacian

def d (A: Tensor MathExpr) : Tensor MathExpr := !(flip ∂/∂) x A

def hodge (A: Tensor MathExpr) : Tensor MathExpr :=
  let k := dfOrder A in
    withSymbols [i, j]
      (sqrt (M.det g_#_#)) * (foldl (.) ((subrefs A (map 1#j_$1 (between 1 k))) . (subrefs (ε' N k) (map 1#i_$1 (between 1 N))))
                                        (map 1#g~(i_$1)~(j_$1) [1..k]))


def δ (A: Tensor MathExpr) : Tensor MathExpr :=
  let k := dfOrder A in
    -1^(N * (k + 1) + 1) * (hodge (d (hodge A)))

def Δ (A: Tensor MathExpr) : Tensor MathExpr :=
  match (dfOrder A) as integer with
  | #0 -> δ (d A)
  | #N -> d (δ A)
  | _  -> d (δ A) + δ (d A)

def f : MathExpr := function (r, θ)

assertEqual "Laplacian" (Δ f) ((-1 / r^2) * ((∂/∂ (∂/∂ f θ) θ) + r * (∂/∂ f r) + (r^2 * (∂/∂ (∂/∂ f r) r))))