Predeclare zero-damping pinball development cases

tests/unit/test_steady_pinball.py

@@ -26,7 +26,7 @@ def test_actual_loader_and_paper_geometry(tmp_path):
 
 
 def test_structure_rest_equilibrium_and_energy_decay():
-    s = runner.Structure()
+    s = runner.Structure(zeta=.01)
     assert s.candidate(0.) == (0., 0.)
     assert s.mass == pytest.approx(10*3*math.pi*100)
     s.y, s.v = .2, 0.
@@ -52,3 +52,33 @@ def test_report_uses_completed_rows_and_net_force():
     assert got[0]['end_step'] == 4
     assert got[0]['y_rms_D'] == .1
     assert got[0]['fy_rms'] == 6.
+
+
+@pytest.mark.parametrize('case', ['Z0', 'M2', 'R2'])
+def test_zero_damping_cases_conserve_unforced_energy_and_match_forced_solution(case):
+    s = runner.Structure(*runner.STRUCTURES[case])
+    assert s.zeta == s.damping == 0.
+    s.y, s.v = .2, 1.e-5
+    y0, v0 = s.y, s.v
+    energy = .5*s.mass*v0**2 + .5*s.stiffness*y0**2
+    for _ in range(12345):
+        s.y, s.v = s.candidate(0.)
+    assert .5*s.mass*s.v**2 + .5*s.stiffness*s.y**2 == pytest.approx(energy, rel=2e-11)
+    force, elapsed = 1.e-4, 3210
+    s.y, s.v = y0, v0
+    for _ in range(elapsed):
+        s.y, s.v = s.candidate(force)
+    phase = s.omega*elapsed
+    equilibrium = force/s.stiffness
+    expected_y = equilibrium+(y0-equilibrium)*math.cos(phase)+v0*math.sin(phase)/s.omega
+    expected_v = -(y0-equilibrium)*s.omega*math.sin(phase)+v0*math.cos(phase)
+    assert (s.y, s.v) == pytest.approx((expected_y, expected_v), abs=2e-11)
+
+
+def test_predeclared_comparisons_change_structure_only():
+    z, m, r = [runner.Structure(*runner.STRUCTURES[k]) for k in ['Z0', 'M2', 'R2']]
+    assert m.mass/z.mass == pytest.approx(.2)
+    assert m.stiffness/z.stiffness == pytest.approx(.2)
+    assert r.mass == m.mass
+    assert r.stiffness/m.stiffness == pytest.approx(.36)
+    assert [runner.STEPS*s.frequency for s in [z, m, r]] == pytest.approx([60., 60., 36.])