JSON importer should probably call auto_detect_io

[zickgraf]

Consider the following example importing a ZX diagram from JSON and extracting a circuit:

import pyzx as zx
import numpy as np
g = zx.Graph.from_json('{"wire_vertices": {"b0": {"annotation": {"boundary": true, "coord": [2.338, 0.253], "input": false, "output": true}}, "b1": {"annotation": {"boundary": true, "coord": [2.358, -0.787], "input": false, "output": true}}, "b2": {"annotation": {"boundary": true, "coord": [0.398, -0.827], "input": true, "output": false}}, "b3": {"annotation": {"boundary": true, "coord": [0.478, 0.333], "input": true, "output": false}}}, "node_vertices": {"v0": {"annotation": {"coord": [1.408, 0.293]}, "data": {"type": "hadamard", "is_edge": "true"}}, "v1": {"annotation": {"coord": [1.378, -0.807]}, "data": {"type": "hadamard", "is_edge": "true"}}}, "undir_edges": {"e0": {"src": "b0", "tgt": "v0"}, "e1": {"src": "b3", "tgt": "v0"}, "e2": {"src": "b1", "tgt": "v1"}, "e3": {"src": "b2", "tgt": "v1"}}, "scalar": "{\\"power2\\": 0, \\"phase\\": \\"0\\"}"}')
#g.auto_detect_io()
zx.full_reduce(g)
zx.draw(g)
circ = zx.extract_circuit(g.copy())
np.allclose(circ.to_matrix(), g.to_matrix())

The last line displays False, i.e. the matrix of the graph and the matrix of the extracted circuit do not match. Uncommenting the line g.auto_detect_io() fixes the issue.

PyZX version: 669f5a2 (current master at the time of writing)

[zickgraf]

I have analyzed this a bit more: It seems like Graph.from_json populates g.inputs() in the order in which the inputs appear in the JSON. The order in the JSON in the example above in turn matches the order in which I drew the nodes in the interactive editor. This order is different from the visual order, which g.auto_detect_io() enforces.

So there are actually two bugs:

1. Exporting and re-importing a graph does not preserve the order of g.inputs(). I guess the JSON should explicitly store the order of inputs and outputs to not have to rely on guesswork.
2. There is an inconsistency between g.to_matrix and zx.extract_circuit: g.to_matrix seems to always use the visual order while zx.extract_circuit seems to always use the order of g.inputs().

[jvdwetering]

In commit 4ad8938 I updated the jsonparser as well. It should now remember the input and output order correctly. Note that this does invalidate the json you have above (it is not backwards compatible). If this lack of backwards compatibility is a big problem I can see what I can do to save it.

[zickgraf]

In commit 4ad8938 I updated the jsonparser as well. It should now remember the input and output order correctly.

Thanks a lot, that allows to further process the JSON much more safely.

Note that this does invalidate the json you have above (it is not backwards compatible). If this lack of backwards compatibility is a big problem I can see what I can do to save it.

That's not a problem, I'm very happy with the new data structure :-)

Now everything seems to work, thank you!