Test Integrating discontinuous flow (#1499)

Fixes #1498.

core/src/callback.jl

@@ -20,7 +20,7 @@ function create_callbacks(
     push!(callbacks, integrating_flows_cb)
 
     tstops = get_tstops(basin.time.time, starttime)
-    basin_cb = PresetTimeCallback(tstops, update_basin; save_positions = (false, false))
+    basin_cb = PresetTimeCallback(tstops, update_basin!; save_positions = (false, false))
     push!(callbacks, basin_cb)
 
     tstops = get_tstops(tabulated_rating_curve.time.time, starttime)
@@ -426,7 +426,7 @@ function save_subgrid_level(u, t, integrator)
 end
 
 "Load updates from 'Basin / time' into the parameters"
-function update_basin(integrator)::Nothing
+function update_basin!(integrator)::Nothing
     (; p, u) = integrator
     (; basin) = p
     (; storage) = u

core/test/time_test.jl

@@ -78,3 +78,27 @@ end
         basin_table.precipitation .≈ time_table.fixed_area .* time_table.precipitation,
     )
 end
+
+@testitem "Integrate over discontinuity" begin
+    import BasicModelInterface as BMI
+    using Ribasim: get_tmp
+
+    toml_path = normpath(@__DIR__, "../../generated_testmodels/level_demand/ribasim.toml")
+    @test ispath(toml_path)
+    day = 86400.0
+
+    saveat = 20day
+    config = Ribasim.Config(
+        toml_path;
+        solver_saveat = saveat,
+        solver_dt = 5day,
+        solver_algorithm = "Euler",
+    )
+    model = Ribasim.Model(config)
+    starting_precipitation =
+        get_tmp(model.integrator.p.basin.vertical_flux, 0).precipitation[1]
+    BMI.update_until(model, saveat)
+    mean_precipitation = only(model.saved.vertical_flux.saveval).precipitation[1]
+    # Given that precipitation stops after 15 of the 20 days
+    @test mean_precipitation ≈ 3 / 4 * starting_precipitation
+end