add hypothesis test and smallest failing example

.gitignore

@@ -22,3 +22,6 @@ devenv.local.nix
 .coverage
 coverage.xml
 lcov.info
+
+# python hypothesis testing
+.hypothesis

pyproject.toml

@@ -14,12 +14,13 @@ packages = [{ include = "tket2-py" }]
 [tool.poetry.dependencies]
 python = ">=3.10"
 
-[tool.poetry.dev-dependencies]
+[tool.poetry.group.dev.dependencies]
 maturin = "^1.3.0"
 pytket = "*"
 pytest = "^7.1.2"
 pytest-cov = "^4.1.0"
 ruff = "^0.1.3"
+hypothesis = "^6.91.1"
 
 [build-system]
 requires = ["maturin~=1.3"]

tket2-py/test/test_pass.py

@@ -1,9 +1,43 @@
 from pytket import Circuit, OpType
 from dataclasses import dataclass
-
+from typing import Callable, Any
 from tket2.passes import badger_pass, greedy_depth_reduce, chunks
 from tket2.circuit import Tk2Circuit
 from tket2.pattern import Rule, RuleMatcher
+import hypothesis.strategies as st
+from hypothesis.strategies._internal import SearchStrategy
+from hypothesis import given, settings
+
+
+@st.composite
+def circuits(
+    draw: Callable[[SearchStrategy[Any]], Any],
+    n_qubits: SearchStrategy[int] = st.integers(min_value=0, max_value=8),
+    depth: SearchStrategy[int] = st.integers(min_value=5, max_value=50),
+) -> Circuit:
+    total_qubits = draw(n_qubits)
+    circuit = Circuit(total_qubits)
+    if total_qubits == 0:
+        return circuit
+    for _ in range(draw(depth)):
+        gates = [circuit.Rz, circuit.H]
+        if total_qubits > 1:
+            gates.extend([circuit.CX])
+        gate = draw(st.sampled_from(gates))
+        control = draw(st.integers(min_value=0, max_value=total_qubits - 1))
+        if gate in (circuit.CX,):
+            target = draw(
+                st.integers(min_value=0, max_value=total_qubits - 1).filter(
+                    lambda x: x != control
+                )
+            )
+            gate(control, target)
+        if gate == circuit.Rz:
+            angle = draw(st.floats(min_value=-2.0, max_value=2.0))
+            gate(angle, control)
+        if gate == circuit.H:
+            gate(control)
+    return circuit
 
 
 def test_simple_badger_pass_no_opt():
@@ -30,6 +64,27 @@ def test_depth_optimise():
     assert c.depth() == 2
 
 
+def test_depth_optimise_buggy():
+    # bug https://github.com/CQCL/tket2/issues/253
+    c = Circuit(3).H(2).CX(2, 1).CX(0, 2).CX(0, 1)
+
+    assert c.depth() == 4
+
+    c, moves = greedy_depth_reduce(c)
+    assert moves == 1
+    assert c == Circuit(3).H(2).CX(0, 1).CX(2, 1).CX(0, 2)
+    assert c.depth() == 3
+
+
+@given(circ=circuits())
+@settings(print_blob=True, deadline=30)
+def test_depth_hyp(circ: Circuit) -> None:
+    new, _ = greedy_depth_reduce(circ)
+
+    assert circ.n_gates == new.n_gates
+    assert new.depth() <= circ.depth()
+
+
 def test_chunks():
     c = Circuit(4).CX(0, 2).CX(1, 3).CX(1, 2).CX(0, 3).CX(1, 3)