`RoutingPass` runs forever with `Unitary2qBox` about tket

[CalMacCQ]

Thanks for making an issue,

Is the call to generate_random_op making a random 2 qubit unitary matrix? I've replaced it with a specific unitary below to ensure reproducibility.

I have actually reporoduced this... It does seem like RoutingPass gets stuck for me. I think its a problem with RoutingPass not working with Unitary2qBox in general. As a user its fair to expect this to either succeed or raise an informative error.

from pytket.circuit import Circuit, Unitary2qBox
from pytket.placement import LinePlacement
from pytket.passes import PlacementPass, RoutingPass
from pytket.architecture import Architecture
import numpy as np


# Define 2q unitary subroutine
unitary_2q = np.asarray([[0, 1, 0, 0],
                        [0, 0, 0, -1],
                        [1, 0, 0, 0],
                        [0, 0, -1j, 0]])

box_2q = Unitary2qBox(unitary_2q)


# Build Circuit
circuit = Circuit(4)
circuit.add_unitary2qbox(box_2q, 0, 1)
circuit.add_unitary2qbox(box_2q, 2, 3)
circuit.add_unitary2qbox(box_2q, 1, 2)
circuit.add_unitary2qbox(box_2q, 0, 3)


# Define architecture
arch = Architecture([(0, 1), (1, 2), (2, 3), (3, 4)])

# Perform placement pass
PlacementPass(LinePlacement(arch)).apply(circuit)

All of the above works fine.. If I now do

RoutingPass(arch).apply(circuit)

This seems to not terminate with python 3.11.1 and pytket v 1.27 which is indeed puzzling... we should look into that. If however I perform the DecomposeBoxes pass to decompose the unitaries to simpler gates and then do the RoutingPass everything works fine. It seems that RoutingPass is not playing nicely with the higher level box operations.

Seems like a bug to me.

Workaround below

from pytket.passes import DecomposeBoxes

# Define architecture
arch = Architecture([(0, 1), (1, 2), (2, 3), (3, 4)])

# Route circuit
PlacementPass(LinePlacement(arch)).apply(circuit)

# Decompose the Unitary boxes to primitive gates - this is the new step
DecomposeBoxes().apply(circuit)

# Route Circuit
RoutingPass(arch).apply(circuit) # Now working