add protection for full traceout

Author: refraction-ray

CHANGELOG.md

@@ -14,6 +14,8 @@
 
 - Fix quafu provider
 
+- Raise ValueError for `FGSSimulator` when traced out sites correspond the full system
+
 ## 1.0.1
 
 ### Fixed

tensorcircuit/fgs.py

@@ -181,6 +181,8 @@ def get_reduced_cmatrix(self, subsystems_to_trace_out: List[int]) -> Tensor:
             subsystems_to_trace_out = []
         keep = [i for i in range(self.L) if i not in subsystems_to_trace_out]
         keep += [i + self.L for i in range(self.L) if i not in subsystems_to_trace_out]
+        if len(keep) == 0:  # protect from empty keep
+            raise ValueError("the full system is traced out, no subsystems to keep")
         keep = backend.convert_to_tensor(keep)
 
         def slice_(a: Tensor) -> Tensor:

tests/test_fgs.py

@@ -59,6 +59,12 @@ def asymmetric_hopping_jw(J, gamma, i, j, L):
     )
 
 
+def test_full_trace():
+    c = tc.FGSSimulator(4)
+    with pytest.raises(ValueError):
+        c.entropy([0, 1, 2, 3])
+
+
 @pytest.mark.parametrize("backend", [lf("npb"), lf("tfb"), lf("jaxb")])
 def test_otoc(backend, highp):
     c = tc.FGSSimulator(4, [0, 1])