diff --git a/src/holt59/aoc/2015/day18.py b/src/holt59/aoc/2015/day18.py
new file mode 100644
index 0000000..b6de077
--- /dev/null
+++ b/src/holt59/aoc/2015/day18.py
@@ -0,0 +1,66 @@
+import itertools
+import sys
+
+import numpy as np
+from numpy.typing import NDArray
+
+grid0 = np.array([[c == "#" for c in line] for line in sys.stdin.read().splitlines()])
+
+# add an always off circle around
+grid0 = np.concatenate(
+    [
+        np.zeros((grid0.shape[0] + 2, 1), dtype=bool),
+        np.concatenate(
+            [
+                np.zeros((1, grid0.shape[1]), dtype=bool),
+                grid0,
+                np.zeros((1, grid0.shape[1]), dtype=bool),
+            ]
+        ),
+        np.zeros((grid0.shape[0] + 2, 1), dtype=bool),
+    ],
+    axis=1,
+)
+
+moves = list(itertools.product([-1, 0, 1], repeat=2))
+moves.remove((0, 0))
+
+jjs, iis = np.meshgrid(
+    np.arange(1, grid0.shape[0] - 1, dtype=int),
+    np.arange(1, grid0.shape[1] - 1, dtype=int),
+)
+iis, jjs = iis.flatten(), jjs.flatten()
+
+ins = iis[:, None] + np.array(moves)[:, 0]
+jns = jjs[:, None] + np.array(moves)[:, 1]
+
+
+def game_of_life(grid: NDArray[np.bool_]) -> NDArray[np.bool_]:
+    neighbors_on = grid[ins, jns].sum(axis=1)
+    cells_on = grid[iis, jjs]
+
+    grid = np.zeros_like(grid)
+    grid[iis, jjs] = (neighbors_on == 3) | (cells_on & (neighbors_on == 2))
+
+    return grid
+
+
+grid = grid0
+n_steps = 4 if len(grid) < 10 else 100
+for _ in range(n_steps):
+    grid = game_of_life(grid)
+
+answer_1 = grid.sum()
+print(f"answer 1 is {answer_1}")
+
+
+n_steps = 5 if len(grid) < 10 else 100
+grid = grid0
+for _ in range(n_steps):
+    grid[[1, 1, -2, -2], [1, -2, 1, -2]] = True
+    grid = game_of_life(grid)
+
+grid[[1, 1, -2, -2], [1, -2, 1, -2]] = True
+
+answer_2 = sum(cell for line in grid for cell in line)
+print(f"answer 2 is {answer_2}")
diff --git a/src/holt59/aoc/inputs/holt59/2015/day18.txt b/src/holt59/aoc/inputs/holt59/2015/day18.txt
new file mode 100644
index 0000000..e91a2a7
--- /dev/null
+++ b/src/holt59/aoc/inputs/holt59/2015/day18.txt
@@ -0,0 +1,100 @@
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diff --git a/src/holt59/aoc/inputs/tests/2015/day18.txt b/src/holt59/aoc/inputs/tests/2015/day18.txt
new file mode 100644
index 0000000..6ddce72
--- /dev/null
+++ b/src/holt59/aoc/inputs/tests/2015/day18.txt
@@ -0,0 +1,6 @@
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