diff --git a/18/a.py b/18/a.py
new file mode 100755
index 0000000..bd57ee9
--- /dev/null
+++ b/18/a.py
@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+
+
+from enum import Enum
+from collections import defaultdict
+
+
+class Tile(Enum):
+    EMPTY = 0
+    YARD = 1
+    TREE = 2
+
+
+def char2tile(char):
+    return {".": Tile.EMPTY,
+            "#": Tile.YARD,
+            "|": Tile.TREE}[char]
+
+
+def tile2char(tile):
+    return {Tile.EMPTY: ".",
+            Tile.YARD: "#",
+            Tile.TREE: "|"}[tile]
+
+
+def loadmap(fname):
+    field = {}
+    width = 0
+    height = 0
+    with open(fname) as f:
+        for y, line in enumerate(f.readlines()):
+            height = y
+            for x, char in enumerate(line.strip()):
+                field[(x, y)] = char2tile(char)
+                width = y
+    return field, width + 1, height + 1
+
+
+def printmap(field, width, height):
+    for y in range(0, height):
+        for x in range(0, width):
+            print(tile2char(field[(x, y)]), end="")
+        print()
+
+
+def iterneighbors(field, cx, cy):
+    for y in range(cy - 1, cy + 2):
+        for x in range(cx - 1, cx + 2):
+            if x != cx or y != cy:
+                try:
+                    yield field[(x, y)]
+                except KeyError:
+                    pass
+
+
+def tick(field, width, height):
+    """
+    Empty => Trees     >=3 adj with trees
+    Trees => Yard      >=3 adj with yards
+    Yard  => Yard      >0 adj with yard AND >0 adj with trees
+    Yard  => Empty     If above rule fails
+    """
+    newfield = {}
+    for y in range(0, height):
+        for x in range(0, width):
+            counts = defaultdict(int)
+            for tile in iterneighbors(field, x, y):
+                counts[tile] += 1
+            tile = field[(x, y)]
+            if tile == Tile.EMPTY:
+                if counts[Tile.TREE] >= 3:
+                    tile = Tile.TREE
+            elif tile == Tile.TREE:
+                if counts[Tile.YARD] >= 3:
+                    tile = Tile.YARD
+            elif tile == Tile.YARD:
+                if counts[Tile.YARD] == 0 or counts[Tile.TREE] == 0:
+                    tile = Tile.EMPTY
+            newfield[(x, y)] = tile
+    return newfield
+
+
+def counttypes(field):
+    counts = defaultdict(int)
+    for _, v in field.items():
+        counts[v] += 1
+    return counts
+
+
+def main():
+    field, width, height = loadmap("input.txt")
+    printmap(field, width, height)
+
+    generation = 0
+    while True:
+        # input()
+        generation += 1
+        field = tick(field, width, height)
+        print("Generation", generation)
+        printmap(field, width, height)
+        print()
+        if generation >= 10:
+            break
+
+    counts = counttypes(field)
+    print(counts[Tile.TREE] * counts[Tile.YARD])
+
+
+if __name__ == '__main__':
+    main()
diff --git a/18/b.py b/18/b.py
new file mode 100755
index 0000000..b138d5e
--- /dev/null
+++ b/18/b.py
@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+
+
+from collections import defaultdict
+from a import Tile, loadmap, iterneighbors, counttypes
+
+
+def tick(field, width, height):
+    """
+    Empty => Trees     >=3 adj with trees
+    Trees => Yard      >=3 adj with yards
+    Yard  => Yard      >0 adj with yard AND >0 adj with trees
+    Yard  => Empty     If above rule fails
+    """
+    newfield = {}
+    for y in range(0, height):
+        for x in range(0, width):
+            counts = defaultdict(int)
+            for tile in iterneighbors(field, x, y):
+                counts[tile] += 1
+            tile = field[(x, y)]
+            if tile == Tile.EMPTY:
+                if counts[Tile.TREE] >= 3:
+                    tile = Tile.TREE
+            elif tile == Tile.TREE:
+                if counts[Tile.YARD] >= 3:
+                    tile = Tile.YARD
+            elif tile == Tile.YARD:
+                if counts[Tile.YARD] == 0 or counts[Tile.TREE] == 0:
+                    tile = Tile.EMPTY
+            newfield[(x, y)] = tile
+    return newfield
+
+
+def main():
+    field, width, height = loadmap("input.txt")
+
+    # Generations to run:                                             1000000000
+    # I (manually) noticed it cycles after every X generations:       28
+    # 1000000000 % 28 =                                               20
+
+    generation = 0
+
+    # Run the first 500 generations
+    while generation <= 500:
+        field = tick(field, width, height)
+        generation += 1
+
+    # Run until our generation minus 20 is a multiple of 28
+    while (generation - 20) % 28 != 0:
+        field = tick(field, width, height)
+        generation += 1
+
+    counts = counttypes(field)
+    total = counts[Tile.TREE] * counts[Tile.YARD]
+    print("Generation", generation, "Total", total)
+
+
+if __name__ == '__main__':
+    main()
diff --git a/18/input.txt b/18/input.txt
new file mode 100644
index 0000000..b414c48
--- /dev/null
+++ b/18/input.txt
@@ -0,0 +1,50 @@
+#.##...#....|...#..|...#|...#....|.....|..##.|.|.#
+||#...##|.|.##....|..#.#......#.#..#......|..#.|.#
+|......#.##....||..#..||..|.....|..##....|.#.|..|#
+.|...#||..||#|#.....|#...|.||.||#...#..|.##.......
+...#.|.||.|.....|..##.#|....|#...|#.#|.#|...#.....
+##.#....|..|.|....#.#|..##..#.......|.|.||#.......
+....|..#|...|...|#..#...#....#.........#....#.|..|
+....##...|..........|.|##|##..||..|..#..#|..|.....
+........|..#.......|.#|............##.||.##.#..||#
+.|...|##..#.|..|..|.#|.#.|#...|......#.....#...##|
+............#.|.|||....|...#.#..#|.#...|.#....#|..
+.....#....##....|#..##..|......|#.........|..##..|
+#|.#||...|#...#|#|......#|#.|.#|#.|#...|.....#|.||
+..|..#..|.#||.#.#.......#..#.#...|.#...|.|.|..|..#
+.#|....#..#.|........#.....#.|....|#|#........#.|.
+###|....##|#|||#.||.....#....#.#....|.#...|#..|#..
+...#.##..#....|...||.#.|#.....||##|#.....|#|...|..
+#..|.|.|#......|.#....#||...#.|.|..|#...|#|.|#|...
+....###..|||#..##....||..|.|.#|#.....#||.......|..
+|....#...#.||.|...#.#|...##.##....||.|.|##.#.#|...
+#..#...#...|##....#.#|....|.#|#.|.....|##|||...#.|
+..||.|.#...#.#|#...#..||.|#.|.|.|.###.....|....#..
+.#|..|###.#....#.......#|.#|..#.|.||.|.||..##...#|
+|..|....|#.........#|.|.....|.#...|.#|.||..|.|||.|
+#...........|##|..#..##.......|||..#.....#.|#..##.
+.#.#..|....|.|||###..#........|..#..|..##|...|#...
+#.#..|||||....#.#..#...##....#....##.#|......##.#.
+|.|#|.....|......#.....#........|.||....##|.#...|.
+#....#.....#.#.|#......|#.|........|#|..#.|##.|.#.
+.#|##..|.||..#|....#.|..#|#.#...###|...#.#....#...
+#...#....||..#.|...||..#....|.|......|||....#.....
+.#|....#|...#.#....|...|#|#.##...|.|.#||||..|.##.#
+....#..#....|.|.#....|..|||#....#...|.#....#.|#.|.
+.#..###|..|#.....#....|.|.|||.|.|#||#.#..|.##|...#
+##.#...#|.....||.#..|.|...||.....|||.||#......|#..
+||#|.|...||......#...#.|#..|..#.......##..##.#...#
+...##.|#....#||.|.#.|#..........#...#.......#....|
+..#...#...|#..|||..#||||....#..|.#..#.#..||.##|.||
+#..|...|##||.||..|#.|.#....|.......|..#....|.#|...
+...|.#####.##|####..#.|..#....#...|#.#.#|......||.
+###......|.|..|#.|#....|.......|##.|#|...|.......|
+##|....|#..|...#..|.||......#|.....##|...|...|..#|
+..#.|.#.||...###..##.##..|....|..|....#..#...####.
+.|##.#..|..|...##......#...##||........|.|....#||.
+##.|#....|...|..|#.....|....|.|....|.|.|..|..#.#..
+..|.|.#|.|....||#.|#...............##..|...#.....#
+.#...#.......#.......#.##...#.|.|..####.||#....#||
+.||#...##.#|.||..##.|....|||##...|#.|...#.#|.#....
+.#|.||#..#.|#.......||.||#...........#....|###....
+#|....#......##.#....|.....|##.#|.....|.#..|.....#