Added test for convert_to_vector

Author: MaxFroehlich1410

tests/core/data_structures/test_networks.py

@@ -520,3 +520,55 @@ def test_check_canonical_form():
     # We'll just check that it returns something.
     # If it prints "MPS is right (B) canonical." for example, that's okay.
     assert res is not None
+
+def test_convert_to_vector():
+    """
+    Tests the MPS_to_vector function for various initial states.
+    For each state, the expected full state vector is computed as the tensor
+    product of the corresponding local state vectors.
+    """
+    test_states = ["zeros", "ones", "x+", "x-", "y+", "y-", "Neel", "wall"]
+    L = 4  # Use a small number of sites for testing.
+    tol = 1e-12
+
+    def local_state_vector(state_str: str, index: int, L: int) -> np.ndarray:
+        """
+        Returns the local state vector for a given state string.
+        For 'Neel' and 'wall', the local state depends on the site index.
+        """
+        if state_str == "zeros":
+            return np.array([1, 0], dtype=complex)
+        elif state_str == "ones":
+            return np.array([0, 1], dtype=complex)
+        elif state_str == "x+":
+            return np.array([1/np.sqrt(2), 1/np.sqrt(2)], dtype=complex)
+        elif state_str == "x-":
+            return np.array([1/np.sqrt(2), -1/np.sqrt(2)], dtype=complex)
+        elif state_str == "y+":
+            return np.array([1/np.sqrt(2), 1j/np.sqrt(2)], dtype=complex)
+        elif state_str == "y-":
+            return np.array([1/np.sqrt(2), -1j/np.sqrt(2)], dtype=complex)
+        elif state_str == "Neel":
+            # According to the MPS code: if index is odd, local vector = [1, 0]; if even, [0, 1].
+            return np.array([1, 0], dtype=complex) if index % 2 == 1 else np.array([0, 1], dtype=complex)
+        elif state_str == "wall":
+            # For a "wall" state: sites with index < L//2 are |0>, else |1>.
+            return np.array([1, 0], dtype=complex) if index < L // 2 else np.array([0, 1], dtype=complex)
+        else:
+            raise ValueError("Invalid state string")
+
+    for state_str in test_states:
+        # Create an MPS for the given state.
+        mps = MPS(length=L, state=state_str)
+        psi = mps.convert_to_vector()
+        
+        # Construct the expected state vector as the Kronecker product of local states.
+        local_states = [local_state_vector(state_str, i, L) for i in range(L)]
+        expected = reduce(np.kron, local_states)
+        
+        if np.allclose(psi, expected, atol=tol):
+            print(f"Test passed for state '{state_str}'.")
+        else:
+            print(f"Test FAILED for state '{state_str}'.")
+            print("Expected:", expected)
+            print("Got     :", psi)