Tooling

Tools/Terrain/internal/fluvial/hillslope_test.go

@@ -152,7 +152,12 @@ func TestDiffuseNonlinearIsStable(t *testing.T) {
 			field[i] = 100 // flat base level, the mean of the checkerboard
 		}
 	}
-	for i := 0; i < 500; i++ {
+	// Two thousand steps, not five hundred. The nine-point stencil damps the checkerboard more slowly per
+	// step than the five-point did - the diagonal faces of a checkerboard are flat, so only the 4/6 of the
+	// stencil facing the cardinals sees the mode at all - and it is run at a Courant target of 0.3 rather
+	// than 0.2 because its stability limit is 0.375 rather than 0.25. Both of those are arguments on paper.
+	// A slow instability takes hundreds of steps to show, and a solve runs a thousand.
+	for i := 0; i < 2000; i++ {
 		g.DiffuseNonlinear(field, 0.02, sc, 0.95, 1500, 24)
 	}
 	lo, hi := float32(math.Inf(1)), float32(math.Inf(-1))
@@ -170,9 +175,9 @@ func TestDiffuseNonlinearIsStable(t *testing.T) {
 			}
 		}
 	}
-	t.Logf("after 500 steps the interior spans %.3f..%.3f m, from a 200 m checkerboard", lo, hi)
+	t.Logf("after 2000 steps the interior spans %.3f..%.3f m, from a 200 m checkerboard", lo, hi)
 	if hi-lo > 1 {
-		t.Errorf("the checkerboard is still %.1f m after 500 steps: it is not being damped", hi-lo)
+		t.Errorf("the checkerboard is still %.1f m after 2000 steps: it is not being damped", hi-lo)
 	}
 }