[bug/helpers_typo] Adding tests for the helpers that we expose to python

tests/fast_pauli/test_helpers.py

@@ -0,0 +1,81 @@
+"""Test the helpers module."""
+
+import itertools
+
+import pytest
+
+import fast_pauli as fp
+
+
+def calculate_trusted_pauli_strings(n_qubits: int, weight: int) -> list[str]:
+    """Calculate all possible Pauli strings for a given number of qubits and weight.
+
+    Parameters
+    ----------
+    n_qubits : int
+        The number of qubits.
+    weight : int
+        The Pauli weight.
+
+    Returns
+    -------
+    list[str]
+        All possible Pauli strings for the given number of qubits and weight.
+    """
+    strings = []
+    nontrivial_matrix_elements = list(itertools.product(["X", "Y", "Z"], repeat=weight))
+    for indices in itertools.combinations(range(n_qubits), weight):  # n(n-1)/2 terms
+        for elements in nontrivial_matrix_elements:
+            pauli_string = []
+            for qbit in range(n_qubits):
+                for el_position, i in enumerate(indices):
+                    if i == qbit:
+                        pauli_string.append(elements[el_position])
+                        break
+                else:
+                    pauli_string.append("I")
+            strings.append("".join(pauli_string))
+    return strings
+
+
+@pytest.mark.parametrize("weight", [0, 1, 2, 3])
+def test_get_nontrivial_paulis(weight: int) -> None:
+    """Test the get_nontrivial_paulis function."""
+    res = fp.helpers.get_nontrivial_paulis(weight)
+    trusted = [
+        "".join(x) for x in itertools.product("XYZ", repeat=weight) if len(x) > 0
+    ]
+    assert len(res) == len(trusted)
+    assert set(res) == set(trusted)
+
+
+@pytest.mark.parametrize("n_qubits", [1, 2, 3, 4], ids=lambda x: f"nq={x}")
+@pytest.mark.parametrize("weight", [1, 2, 3], ids=lambda x: f"w={x}")
+def test_calculate_pauli_strings(n_qubits: int, weight: int) -> None:
+    """Test the calculate_pauli_strings function."""
+    if n_qubits < weight:
+        pytest.skip("n_qubits must be greater than or equal to weight")
+
+    res = [str(x) for x in fp.helpers.calculate_pauli_strings(n_qubits, weight)]
+    trusted = calculate_trusted_pauli_strings(n_qubits, weight)
+
+    assert len(res) == len(trusted)
+    assert set(res) == set(trusted)
+
+
+@pytest.mark.parametrize("n_qubits", [1, 2, 3, 4], ids=lambda x: f"nq={x}")
+@pytest.mark.parametrize("weight", [1, 2, 3], ids=lambda x: f"w={x}")
+def test_calculate_pauli_strings_max_weight(n_qubits: int, weight: int) -> None:
+    """Test the calculate_pauli_strings_max_weight function."""
+    if n_qubits < weight:
+        pytest.skip("n_qubits must be greater than or equal to weight")
+
+    res = [
+        str(x) for x in fp.helpers.calculate_pauli_strings_max_weight(n_qubits, weight)
+    ]
+    trusted = ["I" * n_qubits]
+    for i in range(1, weight + 1):
+        trusted.extend(calculate_trusted_pauli_strings(n_qubits, i))
+
+    assert len(res) == len(trusted)
+    assert set(res) == set(trusted)