Honor forced pairs in Murty subproblems

src/pyrecest/utils/assignment.py

@@ -249,10 +249,17 @@ def _solve_subproblem(  # pylint: disable=too-many-locals
     for row_index, col_index in subproblem.forbidden_pairs:
         modified_cost_matrix[row_index, col_index] = large_cost
 
+    forbidden_pairs = set(subproblem.forbidden_pairs)
     forced_rows = set()
     forced_cols = set()
     for row_index, col_index in subproblem.forced_pairs:
-        if row_index in forced_rows or col_index in forced_cols:
+        if (
+            row_index in forced_rows
+            or col_index in forced_cols
+            or (row_index, col_index) in forbidden_pairs
+        ):
+            return None
+        if bool(augmented_cost_matrix[row_index, col_index] >= large_cost / 2.0):
             return None
         forced_rows.add(row_index)
         forced_cols.add(col_index)

tests/utils/test_assignment_murty_forced_prefix.py

@@ -0,0 +1,32 @@
+import numpy as np
+
+from pyrecest.utils.assignment import murty_k_best_assignments
+
+
+def _assignment_tuple(solution):
+    return tuple(int(index) for index in solution["assignment"])
+
+
+def test_murty_forced_prefix_subproblems_do_not_repeat_assignments():
+    cost_matrix = np.asarray(
+        [
+            [0.0, 100.0],
+            [1.0, 2.0],
+        ]
+    )
+
+    solutions = murty_k_best_assignments(cost_matrix, k=5)
+    assignments = [_assignment_tuple(solution) for solution in solutions]
+
+    assert len(assignments) == 5
+    assert len(assignments) == len(set(assignments))
+    assert set(assignments) == {
+        (0, -1),
+        (-1, -1),
+        (-1, 0),
+        (0, 1),
+        (-1, 1),
+    }
+    assert [solution["cost"] for solution in solutions] == sorted(
+        solution["cost"] for solution in solutions
+    )