Code update

PiperOrigin-RevId: 561981580

swirl_dynamics/lib/solvers/ode.py

@@ -14,6 +14,7 @@
 
 """Solvers for ordinary differential equations (ODEs)."""
 
+import dataclasses
 from typing import Any, Protocol
 
 import flax
@@ -45,8 +46,16 @@ def __call__(
     ...
 
 
+@dataclasses.dataclass
 class ScanOdeSolver:
-  """ODE solver based on `jax.lax.scan`."""
+  """ODE solver based on `jax.lax.scan`.
+
+  Attributes:
+    time_axis_pos: move the time axis to the specified position in the output
+      tensor (by default it is at the 0th position).
+  """
+
+  time_axis_pos: int = 0
 
   def step(
       self, func: OdeDynamics, x0: Array, t0: Array, dt: Array, params: PyTree
@@ -68,7 +77,10 @@ def scan_fun(
       return (x_next, t_next), x_next
 
     _, out = jax.lax.scan(scan_fun, (x0, tspan[0]), tspan[1:])
-    return jnp.concatenate([x0[None], out], axis=0)
+    out = jnp.concatenate([x0[None], out], axis=0)
+    if self.time_axis_pos:
+      out = jnp.moveaxis(out, 0, self.time_axis_pos)
+    return out
 
 
 class ExplicitEuler(ScanOdeSolver):

swirl_dynamics/lib/solvers/ode_test.py

@@ -46,6 +46,20 @@ def test_output_shape_and_value(self, solver, backward):
     self.assertEqual(out.shape, (num_steps, x_dim))
     np.testing.assert_allclose(out[-1], np.ones((x_dim,)) * tspan[-1])
 
+  def test_move_time_axis_pos(self):
+    dt = 0.1
+    num_steps = 10
+    x_dim = 5
+    batch_sz = 6
+    tspan = jnp.arange(num_steps) * dt
+    out = ode.ExplicitEuler(time_axis_pos=1)(
+        dummy_ode_dynamics, jnp.zeros((batch_sz, x_dim)), tspan, {}
+    )
+    self.assertEqual(out.shape, (batch_sz, num_steps, x_dim))
+    np.testing.assert_allclose(
+        out[:, -1], np.ones((batch_sz, x_dim)) * tspan[-1]
+    )
+
   @parameterized.parameters((np.arange(10) * -1,), (np.zeros(10),))
   def test_dopri45_backward_error(self, tspan):
     tspan = jnp.asarray(tspan)