fix: validate logical register parameters

backend/qec/register.py

@@ -6,12 +6,27 @@
 
 This models the idle register between gates — no inter-patch surgery.
 """
+import math
 import random
 
 from .simulator import run_shot
 
 
+def _validate_register_inputs(n: int, m: int, d: int, p: float, seed: int | None) -> None:
+    """Reject invalid register dimensions and noise parameters before simulation."""
+    for name, value in (("n", n), ("m", m)):
+        if isinstance(value, bool) or not isinstance(value, int) or value < 1:
+            raise ValueError(f"{name} must be a positive integer")
+    if isinstance(d, bool) or not isinstance(d, int) or d < 3 or d % 2 == 0:
+        raise ValueError("d must be an odd integer greater than or equal to 3")
+    if isinstance(p, bool) or not isinstance(p, (int, float)) or not math.isfinite(p) or not 0 <= p <= 1:
+        raise ValueError("p must be a finite probability between 0 and 1")
+    if seed is not None and (isinstance(seed, bool) or not isinstance(seed, int)):
+        raise ValueError("seed must be an integer or None")
+
+
 def run_register(n: int, m: int, d: int, p: float, seed: int | None = None) -> dict:
+    _validate_register_inputs(n, m, d, p, seed)
     rng = random.Random(seed)
     patches = []
     logical_errors = 0

backend/tests/test_register.py

@@ -0,0 +1,44 @@
+"""Input-validation tests for logical register simulation."""
+
+import pytest
+
+from qec import register
+
+
+@pytest.mark.parametrize(
+    ("kwargs", "message"),
+    [
+        ({"n": 0, "m": 1, "d": 3, "p": 0.0}, "n must be a positive integer"),
+        ({"n": 1, "m": 0, "d": 3, "p": 0.0}, "m must be a positive integer"),
+        ({"n": 1, "m": 1, "d": 2, "p": 0.0}, "d must be an odd integer"),
+        ({"n": 1, "m": 1, "d": 3, "p": -0.1}, "p must be a finite probability"),
+        ({"n": 1, "m": 1, "d": 3, "p": 1.1}, "p must be a finite probability"),
+        ({"n": 1, "m": 1, "d": 3, "p": float("nan")}, "p must be a finite probability"),
+        ({"n": 1, "m": 1, "d": 3, "p": 0.0, "seed": "7"}, "seed must be an integer"),
+        ({"n": True, "m": 1, "d": 3, "p": 0.0}, "n must be a positive integer"),
+        ({"n": 1, "m": False, "d": 3, "p": 0.0}, "m must be a positive integer"),
+        ({"n": 1, "m": 1, "d": True, "p": 0.0}, "d must be an odd integer"),
+        ({"n": 1, "m": 1, "d": 3, "p": True}, "p must be a finite probability"),
+        ({"n": 1, "m": 1, "d": 3, "p": 0.0, "seed": True}, "seed must be an integer"),
+    ],
+)
+def test_run_register_rejects_invalid_inputs(kwargs, message):
+    with pytest.raises(ValueError, match=message):
+        register.run_register(**kwargs)
+
+
+def test_run_register_validates_before_simulating(monkeypatch):
+    monkeypatch.setattr(register, "run_shot", lambda d, p, seed: {
+        "logical_error": False,
+        "logical_x_error": False,
+        "logical_z_error": False,
+        "errors_x": [],
+        "errors_z": [],
+        "syndromes": {},
+    })
+
+    result = register.run_register(n=1, m=1, d=3, p=0.0, seed=7)
+
+    assert result["total_patches"] == 1
+    assert result["total_physical"] == 18
+    assert result["logical_error_rate"] == 0