Eod.

coloring/coloring.py

@@ -1,4 +1,4 @@
-from copy import deepcopy
+from copy import copy
 
 colors_max = None
 
@@ -27,6 +27,15 @@ class Node(object):
             if len(nb.colors) == 1:
                 nb.set_only_color()
 
+    def get_state(self):
+        return list(map(copy, [self.neighbors, self.colors, self.color]))
+
+    def restore_state(self, state):
+        self.neighbors = state[0]
+        self.colors = state[1]
+        self.color = state[2]
+
+
 def parse(input_data):
     # parse the input
     lines = input_data.split('\n')
@@ -66,11 +75,25 @@ def branch(nodes, color):
         return next_nodes
 
     # This is where we actually have to iterate and branch.
-    min_node.color = min_node.colors.pop()
-    for nb in min_node.neighbors:
-        nb.colors.discard(min_node.color)
-        nb.neighbors.remove(min_node)
-    return search(next_nodes, color)
+    print("THIS IS WHERE THE MAGIC HAPPENS.")
+    for min_node_color in list(min_node.colors):
+        try:
+            states = [n.get_state for n in next_nodes]
+            state = min_node.get_state()
+
+            min_node.colors.remove(min_node_color)
+            min_node.color = min_node_color
+
+            for nb in min_node.neighbors:
+                nb.colors.discard(min_node_color)
+                nb.neighbors.remove(min_node)
+            return search(next_nodes, color)
+        except ValueError:
+            print("RESTORE: {color=} did not work for {n}.")
+            min_node.restore_state(state)
+            for node, state in zip(next_nodes, states):
+                node.restore_state(state)
+        raise Exception("We should not have gotten here.")
 
 
 def prune(nodes, color):
@@ -118,7 +141,7 @@ def search(nodes, color):
 def solve_it(input_data):
     global colors_max
     nodes = parse(input_data)
-    # colors_max = 6
+    colors_max = 6
     nodes.sort(key=lambda n: len(n.neighbors), reverse=True)
     color = 0
     search(list(nodes), color)