Add option to use as preconditioner

Author: ranjanan

src/AMG.jl

@@ -19,4 +19,7 @@ export solve
 include("classical.jl")
 export ruge_stuben
 
+include("preconditioner.jl")
+export aspreconditioner
+
 end # module

src/multilevel.jl

@@ -65,6 +65,9 @@ function solve{T}(ml::MultiLevel, b::Vector{T}; maxiter = 100,
     residuals = Vector{T}()
     A = ml.levels[1].A
     normb = norm(b)
+    if normb != 0
+        tol *= normb
+    end
     push!(residuals, norm(b - A*x))
 
     lvl = 1
@@ -74,7 +77,7 @@ function solve{T}(ml::MultiLevel, b::Vector{T}; maxiter = 100,
         else
             x = __solve(cycle, ml, x, b, lvl)
         end
-        push!(residuals, norm(A*x - b))
+        push!(residuals, norm(b - A * x))
     end
     x
 end

src/preconditioner.jl

@@ -0,0 +1,13 @@
+import Base: \, *, A_ldiv_B!, A_mul_B!
+
+struct Preconditioner
+    ml::MultiLevel
+end
+
+aspreconditioner(ml::MultiLevel) = Preconditioner(ml)
+
+\(p::Preconditioner, b) = p * b
+*(p::Preconditioner, b) = solve(p.ml, b; cycle = V(), maxiter = 1, tol = 1e-12)
+
+A_ldiv_B!(x, p::Preconditioner, b) = copy!(x, p \ b)
+A_mul_B!(b, p::Preconditioner, x) = A_mul_B!(b, p.ml.levels[1].A, x)