Add & test GraphState.from_json

We needed to implement `GraphState.from_json` so that the server can
track a copy of the client's state. That's a useful byproduct of this
work!

abp/graphstate.py

@@ -405,9 +405,6 @@ class GraphState(object):
             >>> with open("graph.json") as f:
                     json.dump(graph.to_json(True), f)
 
-        .. todo::
-            Implement ``from_json()``!
-
         """
         if stringify:
             node = {str(key): value for key, value in self.node.items()}
@@ -417,6 +414,13 @@ class GraphState(object):
         else:
             return {"node": self.node, "adj": self.adj}
 
+    def from_json(self, data):
+        """ Construct the graph from JSON data 
+        :param data: JSON data to be read.
+        """
+        self.node = data["node"]
+        self.adj = data["adj"]
+
     def to_state_vector(self):
         """ Get the full state vector corresponding to this stabilizer state. Useful for debugging, interface with other simulators.
             This method becomes very slow for more than about ten qubits!

bin/abpserver

@@ -16,6 +16,7 @@ import json
 from pkg_resources import resource_filename
 
 clients = []
+local_state = abp.GraphState()
 
 def new_message(client, server, message):
     if message.startswith("edit:"):

tests/test_json.py

@@ -0,0 +1,31 @@
+import mock
+import abp
+
+def test_json():
+    """ Test to_json and from_json """
+    a = mock.named_node_graph()
+    j = a.to_json()
+
+    b = abp.GraphState()
+    b.from_json(j)
+    assert a == b
+
+
+def test_json_again():
+    """ Test to_json and from_json """
+    # Make a random graph
+    a = abp.GraphState(10)
+    a.act_circuit(mock.random_graph_circuit())
+
+    # Dump it to JSON
+    j = a.to_json()
+
+    # Reconstruct from JSON
+    b = abp.GraphState()
+    b.from_json(j)
+
+    # Check equality
+    assert a == b
+
+
+