Bidirectional build: prune joint states to a fixed point

CHANGELOG.md

@@ -5,6 +5,13 @@
 A second correctness review: every fix below was reproduced against a
 brute-force reference or a closed-form value and has a regression test.
 
+### Bidirectional builds
+
+- `build_bidirectional_epsilon_machine` prunes joint states to a fixed point: dropping states
+  outside the stationary support could leave others with incomplete outgoing rows, so builds from
+  truncated reverse machines failed with "joint masses from ... sum to 0.86". Restriction to the
+  stationary support and to states with complete rows now alternate until neither removes anything.
+
 ### State labels
 
 - `sofic.states.sequential_labels` no longer stops at 26 states: after ``A``–``Z`` it continues

sofic/generators/bidirectional_construction.py

@@ -175,16 +175,17 @@ def _prune_to_stationary_support(
 
     keep = _recurrent_support(graph) or set(states)
     trimmed = _restrict_graph(graph, keep)
-    initial = _joint_pi_minimum_support(trimmed, forward, reverse, tol=tol)
-    if not initial:
-        return trimmed, {}
-
-    trimmed = _restrict_graph(trimmed, set(initial.keys()))
-    keep = _recurrent_support(trimmed) or set(initial.keys())
-    trimmed = _restrict_graph(trimmed, keep)
-    initial = _joint_pi_minimum_support(trimmed, forward, reverse, tol=tol)
-    if initial:
-        trimmed = _restrict_graph(trimmed, set(initial.keys()))
+    # Dropping states outside the stationary support can leave others with incomplete rows, and
+    # dropping those changes the support, so alternate the two until neither removes anything.
+    for _ in range(len(states) + 1):
+        initial = _joint_pi_minimum_support(trimmed, forward, reverse, tol=tol)
+        if not initial:
+            return trimmed, {}
+        current = set(trimmed.states())
+        keep = _recurrent_support(_restrict_graph(trimmed, set(initial))) or set(initial)
+        if keep == current:
+            break
+        trimmed = _restrict_graph(trimmed, keep)
     return trimmed, initial
 
 

tests/test_bidirectional_build.py

@@ -58,3 +58,27 @@ def test_survey_explosion_code_may_build_with_ladder() -> None:
     else:
         built = try_build_bidirectional(forward)
         assert built.bidir is not None
+
+
+def test_truncated_reverse_machine_prunes_to_complete_rows() -> None:
+    """Regression: dropping sub-tolerance joint states left (2, 'CE') with 86% of its outgoing mass."""
+    mats = {x: np.zeros((4, 4)) for x in (0, 1)}
+    for x, i, j, p in (
+        (0, 1, 2, 0.8848986395305858),
+        (0, 2, 3, 0.42866489513010786),
+        (0, 3, 1, 0.3568362368298324),
+        (1, 0, 1, 1.0),
+        (1, 1, 0, 0.11510136046941416),
+        (1, 2, 1, 0.571335104869892),
+        (1, 3, 2, 0.6431637631701677),
+    ):
+        mats[x][i, j] = p
+    forward = from_symbol_matrices(tuple(range(4)), (0, 1), mats)
+    built = try_build_bidirectional(forward)
+    assert built.preflight.bidir_class == "truncated"
+    assert built.bidir is not None
+    bidir_pi = built.bidir.forward_machine.stationary_distribution()
+    marginal = np.zeros(4)
+    for (alpha, _gamma), mass in built.bidir._joint_pi.items():
+        marginal[alpha] += float(mass)
+    assert np.allclose(marginal, bidir_pi, atol=1e-8)