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+--- Day 22: Sporifica Virus ---
+
+Diagnostics indicate that the local grid computing cluster has been
+contaminated with the Sporifica Virus. The grid computing cluster is a
+seemingly-infinite two-dimensional grid of compute nodes. Each node is
+either clean or infected by the virus.
+
+To prevent overloading the nodes (which would render them useless to
+the virus) or detection by system administrators, exactly one virus
+carrier moves through the network, infecting or cleaning nodes as it
+moves. The virus carrier is always located on a single node in the
+network (the current node) and keeps track of the direction it is
+facing.
+
+To avoid detection, the virus carrier works in bursts; in each burst,
+it wakes up, does some work, and goes back to sleep. The following
+steps are all executed in order one time each burst:
+
+    If the current node is infected, it turns to its right. Otherwise,
+    it turns to its left. (Turning is done in-place; the current node
+    does not change.)
+
+    If the current node is clean, it becomes infected. Otherwise, it
+    becomes cleaned. (This is done after the node is considered for
+    the purposes of changing direction.)
+
+    The virus carrier moves forward one node in the direction it is
+    facing.
+
+Diagnostics have also provided a map of the node infection status
+(your puzzle input). Clean nodes are shown as .; infected nodes are
+shown as #. This map only shows the center of the grid; there are many
+more nodes beyond those shown, but none of them are currently
+infected.
+
+The virus carrier begins in the middle of the map facing up.
+
+For example, suppose you are given a map like this:
+
+..#
+#..
+...
+
+Then, the middle of the infinite grid looks like this, with the virus
+carrier's position marked with [ ]:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . # . . .
+. . . #[.]. . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+The virus carrier is on a clean node, so it turns left, infects the
+node, and moves left:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . # . . .
+. . .[#]# . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+The virus carrier is on an infected node, so it turns right, cleans
+the node, and moves up:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . .[.]. # . . .
+. . . . # . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+Four times in a row, the virus carrier finds a clean, infects it,
+turns left, and moves forward, ending in the same place and still
+facing up:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . #[#]. # . . .
+. . # # # . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+Now on the same node as before, it sees an infection, which causes it
+to turn right, clean the node, and move forward:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . # .[.]# . . .
+. . # # # . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+After the above actions, a total of 7 bursts of activity had taken
+place. Of them, 5 bursts of activity caused an infection.
+
+After a total of 70, the grid looks like this, with the virus carrier
+facing up:
+
+. . . . . # # . .
+. . . . # . . # .
+. . . # . . . . #
+. . # . #[.]. . #
+. . # . # . . # .
+. . . . . # # . .
+. . . . . . . . .
+. . . . . . . . .
+
+By this time, 41 bursts of activity caused an infection (though most
+of those nodes have since been cleaned).
+
+After a total of 10000 bursts of activity, 5587 bursts will have
+caused an infection.
+
+Given your actual map, after 10000 bursts of activity, how many bursts
+cause a node to become infected? (Do not count nodes that begin
+infected.)
+
+Your puzzle answer was 5399.
+
+--- Part Two ---
+
+As you go to remove the virus from the infected nodes, it evolves to
+resist your attempt.
+
+Now, before it infects a clean node, it will weaken it to disable your
+defenses. If it encounters an infected node, it will instead flag the
+node to be cleaned in the future. So:
+
+    Clean nodes become weakened.
+
+    Weakened nodes become infected.
+
+    Infected nodes become flagged.
+
+    Flagged nodes become clean.
+
+Every node is always in exactly one of the above states.
+
+The virus carrier still functions in a similar way, but now uses the
+following logic during its bursts of action:
+
+    Decide which way to turn based on the current node:
+
+        If it is clean, it turns left.
+
+        If it is weakened, it does not turn, and will continue moving
+        in the same direction.
+
+        If it is infected, it turns right.
+
+        If it is flagged, it reverses direction, and will go back the
+        way it came.
+
+    Modify the state of the current node, as described above.
+
+    The virus carrier moves forward one node in the direction it is
+    facing.
+
+
+Start with the same map (still using . for clean and # for infected)
+and still with the virus carrier starting in the middle and facing up.
+
+Using the same initial state as the previous example, and drawing
+weakened as W and flagged as F, the middle of the infinite grid looks
+like this, with the virus carrier's position again marked with [ ]:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . # . . .
+. . . #[.]. . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+This is the same as before, since no initial nodes are weakened or
+flagged. The virus carrier is on a clean node, so it still turns left,
+instead weakens the node, and moves left:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . # . . .
+. . .[#]W . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+The virus carrier is on an infected node, so it still turns right,
+instead flags the node, and moves up:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . .[.]. # . . .
+. . . F W . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+This process repeats three more times, ending on the
+previously-flagged node and facing right:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . W W . # . . .
+. . W[F]W . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+Finding a flagged node, it reverses direction and cleans the node:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . W W . # . . .
+. .[W]. W . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+The weakened node becomes infected, and it continues in the same
+direction:
+
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . W W . # . . .
+.[.]# . W . . . .
+. . . . . . . . .
+. . . . . . . . .
+. . . . . . . . .
+
+Of the first 100 bursts, 26 will result in infection. Unfortunately,
+another feature of this evolved virus is speed; of the first 10000000
+bursts, 2511944 will result in infection.
+
+Given your actual map, after 10000000 bursts of activity, how many
+bursts cause a node to become infected? (Do not count nodes that begin
+infected.)
+
+Your puzzle answer was 2511776.
+
+Both parts of this puzzle are complete! They provide two gold stars: **