added mass_matrix

src/SBP_operators.jl

@@ -368,6 +368,21 @@ function integrate(func, u::AbstractVector, D::DerivativeOperator)
 end
 
 
+"""
+    mass_matrix(D::DerivativeOperator)
+
+Create the diagonal mass matrix for the SBP derivative operator `D`.
+"""
+function mass_matrix(D::DerivativeOperator)
+    m = ones(grid(D))
+    @unpack left_weights, right_weights = D.coefficients
+
+    m[1:length(left_weights)] = left_weights
+    m[end:-1:end-length(right_weights)+1] = right_weights
+    Diagonal(D.Δx * m)
+end
+
+
 """
     derivative_operator(source_of_coefficients, derivative_order, accuracy_order,
                         xmin, xmax, N, parallel=Val{:serial}())

src/SummationByPartsOperators.jl

@@ -38,6 +38,7 @@ include("SBP_coefficients/MattssonAlmquistCarpenter2014Optimal.jl")
 # exports
 export PeriodicDerivativeOperator, DerivativeOperator, SourceOfCoefficients
 export derivative_order, accuracy_order, source_of_coeffcients, grid
+export mass_matrix
 export mul!, integrate, derivative_left, derivative_right
 export periodic_central_derivative_operator, periodic_derivative_operator, derivative_operator,
         dissipation_operator