# https://github.com/JuliaLang/julia/issues/4707

#----------------- Get the A matrix
function getDivGrad(n1,n2,n3)

        # the Divergence
        D1 = kron(speye(n3),kron(speye(n2),ddx(n1)))
        D2 = kron(speye(n3),kron(ddx(n2),speye(n1)))
        D3 = kron(ddx(n3),kron(speye(n2),speye(n1)))
        # DIV from faces to cell-centers
        Div = [D1 D2 D3]

        return Div*Div';
end

#----------------- 1D finite difference on staggered grid
function ddx(n)
# generate 1D derivatives
    return d = spdiags(ones(n)*[-1 1],[0,1],n,n+1)
end

#------------- Build a diagonal matrix
function spdiags(B,d,m,n)
#   spdiags(B,d,m,n)
# creates a sparse matrix from its diagonals

    d = d[:]
    p = length(d)
    
    len = zeros(p+1,1)
    for k = 1:p
        len[k+1] = int(len[k]+length(max(1,1-d[k]):min(m,n-d[k])))
    end
    a = zeros(int(len[p+1]),3)
    for k = 1:p
        # Append new d[k]-th diagonal to compact form
        i = max(1,1-d[k]):min(m,n-d[k])
        a[(int(len[k])+1):int(len[k+1]),:] = [i i+d[k] B[i+(m>=n)*d[k],k]]
    end
    
    A = sparse(int(a[:,1]),int(a[:,2]),a[:,3],m,n);
    
    return A
    
end

