support for directed device graphs

[Vivek1106-04]

Add Directed Graph Support to RouteCQC

Fixes #5863

This PR adds support for directed device graphs to Cirq’s RouteCQC router.
The router now handles unidirectional couplings using a Hadamard-based decomposition when required.

---

Summary of Changes

1️⃣ Removed Directed Graph Restriction

File: route_circuit_cqc.py

• Updated __init__ to accept both directed and undirected graphs
• Removed the ValueError previously raised for directed graphs
• Updated docstring to reflect expanded device support

---

2️⃣ Implemented _emit_swap (Direction-Aware SWAP Emission)

File: route_circuit_cqc.py

Added a new classmethod _emit_swap that emits SWAP operations based on edge directionality:

• Case A: Bidirectional edge

  • Uses standard SWAP
  • Automatically decomposed by Cirq into 3 CNOTs

• Case B: Unidirectional edge (q1 → q2)

  • Uses Hadamard-based decomposition:

    CNOT(q1, q2) H(q1), H(q2) CNOT(q1, q2) H(q1), H(q2) CNOT(q1, q2) 

  • Basis change allows effective CNOT direction reversal

• Case C: Unidirectional edge (q2 → q1)

  • Same logic as Case B with qubits flipped

• Case D: No edge

  • Raises ValueError (should not occur for valid routing)

---

3️⃣ Updated Routing Logic

File: route_circuit_cqc.py

• Added device_graph parameter to _route
• Updated route_circuit to pass self.device_graph
• Replaced direct SWAP insertion with _emit_swap

---

4️⃣ Updated Tests

File: route_circuit_cqc_test.py

• test_directed_device

  • Now accepts directed graphs
  • Verifies routing of basic CNOT circuits

• test_directed_device_swap_decomposition (NEW)

  • Routes a SWAP on a directed ring device
  • Asserts presence of Hadamard gates
  • Confirms unidirectional edge handling is exercised

[Vivek1106-04]

Added additional tests to cover edge cases for directed device graphs and the tag_inserted_swaps parameter.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (16d4406) to head (bced992).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@ ## main #7810 +/- ## ======================================== Coverage 99.56% 99.57% ======================================== Files 1102 1102 Lines 98668 98873 +205 ======================================== + Hits 98242 98452 +210  + Misses 426 421 -5 

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.