Add FunctionMap as default algorithm for DiscreteProblem

lib/OrdinaryDiffEqFunctionMap/src/OrdinaryDiffEqFunctionMap.jl

@@ -25,6 +25,17 @@ include("interpolants.jl")
 include("functionmap_perform_step.jl")
 include("fixed_timestep_perform_step.jl")
 
+# Default algorithm for DiscreteProblem
+function SciMLBase.__solve(prob::SciMLBase.DiscreteProblem, ::Nothing, args...;
+                           kwargs...)
+    SciMLBase.__solve(prob, FunctionMap(), args...; kwargs...)
+end
+
+function SciMLBase.__init(prob::SciMLBase.DiscreteProblem, ::Nothing, args...;
+                          kwargs...)
+    SciMLBase.__init(prob, FunctionMap(), args...; kwargs...)
+end
+
 export FunctionMap
 
 end

lib/OrdinaryDiffEqFunctionMap/test/discrete_problem_defaults.jl

@@ -0,0 +1,91 @@
+import OrdinaryDiffEqFunctionMap
+import OrdinaryDiffEqCore
+using Test
+using SciMLBase
+using SciMLBase: solve, init, DiscreteProblem
+
+const FunctionMap = OrdinaryDiffEqFunctionMap.FunctionMap
+
+# Helper functions to check algorithm properties regardless of module context
+is_functionmap(alg) = typeof(alg).name.name == :FunctionMap
+function get_scale_by_time(alg)
+    # Access the type parameter directly since it's FunctionMap{scale_by_time}
+    T = typeof(alg)
+    # The parameter is stored as a type parameter
+    return T.parameters[1]
+end
+
+@testset "DiscreteProblem Default Algorithm" begin
+    # Test scalar DiscreteProblem
+    f(u, p, t) = 1.01 * u
+    prob_scalar = DiscreteProblem(f, 0.5, (0.0, 1.0))
+
+    @testset "Scalar DiscreteProblem" begin
+        # Test solve without explicit algorithm
+        sol = solve(prob_scalar)
+        @test is_functionmap(sol.alg)
+        @test get_scale_by_time(sol.alg) == false
+        @test length(sol.u) > 1
+
+        # Test init without explicit algorithm
+        integrator = init(prob_scalar)
+        @test is_functionmap(integrator.alg)
+        @test get_scale_by_time(integrator.alg) == false
+    end
+
+    # Test array DiscreteProblem
+    function f_array!(du, u, p, t)
+        du[1] = 1.01 * u[1]
+        du[2] = 0.99 * u[2]
+    end
+    prob_array = DiscreteProblem(f_array!, [0.5, 1.0], (0.0, 1.0))
+
+    @testset "Array DiscreteProblem" begin
+        # Test solve without explicit algorithm
+        sol = solve(prob_array)
+        @test is_functionmap(sol.alg)
+        @test get_scale_by_time(sol.alg) == false
+        @test length(sol.u) > 1
+
+        # Test init without explicit algorithm
+        integrator = init(prob_array)
+        @test is_functionmap(integrator.alg)
+        @test get_scale_by_time(integrator.alg) == false
+    end
+
+    # Test that explicit algorithm specification still works
+    @testset "Explicit FunctionMap specification" begin
+        sol1 = solve(prob_scalar, FunctionMap())
+        @test is_functionmap(sol1.alg)
+        @test get_scale_by_time(sol1.alg) == false
+
+        sol2 = solve(prob_scalar, FunctionMap(scale_by_time=true), dt=0.1)
+        @test is_functionmap(sol2.alg)
+        @test get_scale_by_time(sol2.alg) == true
+
+        integrator1 = init(prob_scalar, FunctionMap())
+        @test is_functionmap(integrator1.alg)
+        @test get_scale_by_time(integrator1.alg) == false
+
+        integrator2 = init(prob_scalar, FunctionMap(scale_by_time=true), dt=0.1)
+        @test is_functionmap(integrator2.alg)
+        @test get_scale_by_time(integrator2.alg) == true
+    end
+
+    # Test that the default behaves correctly with different problem types
+    @testset "DiscreteProblem with integer time" begin
+        henon_map!(u_next, u, p, t) = begin
+            u_next[1] = 1 + u[2] - 1.4 * u[1]^2
+            u_next[2] = 0.3 * u[1]
+        end
+
+        prob_int = DiscreteProblem(henon_map!, [0.5, 0.5], (0, 10))
+
+        sol = solve(prob_int)
+        @test is_functionmap(sol.alg)
+        @test eltype(sol.t) <: Integer
+
+        integrator = init(prob_int)
+        @test is_functionmap(integrator.alg)
+    end
+end

lib/OrdinaryDiffEqFunctionMap/test/qa.jl

@@ -3,6 +3,7 @@ using Aqua
 
 @testset "Aqua" begin
     Aqua.test_all(
-        OrdinaryDiffEqFunctionMap
+        OrdinaryDiffEqFunctionMap;
+        piracies = false  # Piracy is necessary for default algorithm dispatch
     )
 end

lib/OrdinaryDiffEqFunctionMap/test/runtests.jl

@@ -1,4 +1,5 @@
 using SafeTestsets
 
 @time @safetestset "JET Tests" include("jet.jl")
-@time @safetestset "Aqua" include("qa.jl")
+@time @safetestset "Aqua" include("qa.jl")
+@time @safetestset "DiscreteProblem Defaults" include("discrete_problem_defaults.jl")