diff --git a/day25/part1.lua b/day25/part1.lua
new file mode 100755
index 0000000..6d2b47c
--- /dev/null
+++ b/day25/part1.lua
@@ -0,0 +1,56 @@
+#!/usr/bin/env luajit
+require("utils")
+require("grid")
+
+local f = io.open(arg[3])
+local line = f:read("*l")
+local g = grid(0, 0, #line, 0)
+local r = 0
+while line ~= nil do
+	g:grow()
+	local c = 0
+	for char in chars(line) do
+		g:set(r, c, char)
+		c = c + 1
+	end
+	line = f:read("*l")
+	r = r + 1
+end
+
+local changed = true
+local step = 0
+while changed do
+	changed = false
+	step = step + 1
+
+	for r = g.top, g.bottom do
+		for c = g.left, g.right do
+			if g:get(r, c) == ">" and g:get(r, (c + 1) % g.width) == "." then
+				g:set(r, c, ">>")
+			end
+		end
+		for c = g.left, g.right do
+			if g:get(r, c) == ">>" then
+				changed = true
+				g:set(r, c, ".")
+				g:set(r, (c + 1) % g.width, ">")
+			end
+		end
+	end
+
+	for c = g.left, g.right do
+		for r = g.top, g.bottom do
+			if g:get(r, c) == "v" and g:get((r + 1) % g.height, c) == "." then
+				g:set(r, c, "vv")
+			end
+		end
+		for r = g.top, g.bottom do
+			if g:get(r, c) == "vv" then
+				changed = true
+				g:set(r, c, ".")
+				g:set((r + 1) % g.height, c, "v")
+			end
+		end
+	end
+end
+print(step)
diff --git a/grid.lua b/grid.lua
index 4eaeb61..5960a09 100644
--- a/grid.lua
+++ b/grid.lua
@@ -64,6 +64,11 @@ local function write(self, format)
 	end
 end
 
+local function grow(self)
+	self.height = self.height + 1
+	self.bottom = self.bottom + 1
+end
+
 function grid(top, left, width, height, default)
 	return {
 		top = top,
@@ -79,5 +84,6 @@ function grid(top, left, width, height, default)
 		iter = iter,
 		neigh9 = neigh9,
 		write = write,
+		grow = grow,
 	}
 end