day 7

day07/part1.lua

@@ -0,0 +1,14 @@
+#!/usr/bin/env luajit
+require("utils")
+
+local positions = split(io.open(arg[3]):read("*a"), ",")
+foreach(positions, tonumber)
+table.sort(positions)
+
+local median = positions[#positions/2]
+local cost = 0
+for k, v in pairs(positions) do
+	cost = cost + math.abs(median - v)
+end
+
+print(cost)

day07/part2.lua

@@ -0,0 +1,28 @@
+#!/usr/bin/env luajit
+require("utils")
+
+local positions = split(io.open(arg[3]):read("*a"), ",")
+foreach(positions, tonumber)
+
+local min, max = math.huge, -math.huge
+for k, v in pairs(positions) do
+	if v < min then min = v end
+	if v > max then max = v end
+end
+
+function gauss(n)
+	return n * (n + 1) / 2
+end
+
+local minimum = math.huge
+for i = min, max do
+	local cost = 0
+	for k, v in pairs(positions) do
+		cost = cost + gauss(math.abs(i - v))
+	end
+	if cost < minimum then
+		minimum = cost
+	end
+end
+
+print(minimum)

utils.lua

@@ -17,3 +17,9 @@ function sign(n)
 	else return 1
 	end
 end
+
+function foreach(t, f)
+	for k, v in pairs(t) do
+		t[k] = f(v)
+	end
+end