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.

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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

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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)

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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

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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 (a8efbc5) to head (e71ea5f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7810      +/-   ##
==========================================
- Coverage   99.57%   99.57%   -0.01%     
==========================================
  Files        1102     1102              
  Lines       98772    98878     +106     
==========================================
+ Hits        98351    98456     +105     
- Misses        421      422       +1     

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[Vivek1106-04]

Introduced RoutingConfig, SwapSearchContext, and CircuitOps dataclasses to bundle related parameters and reduce function argument counts, resolving pylint too-many-arguments errors without using disable comments.