5 files changed, 81 insertions, 0 deletions
diff --git a/day12/Dockerfile b/day12/Dockerfile
new file mode 100644
index 0000000..e5adcb1
--- /dev/null
+++ b/day12/Dockerfile
@@ -0,0 +1,7 @@
+FROM ruby:2.6.5-slim
+
+WORKDIR /opt
+
+COPY . .
+
+ENTRYPOINT ["./entrypoint"]
diff --git a/day12/Makefile b/day12/Makefile
new file mode 100644
index 0000000..6d578a1
--- /dev/null
+++ b/day12/Makefile
@@ -0,0 +1,14 @@
+DAY = 12
+
+.PHONY: run clean
+
+run: build
+	docker run -it --rm aoc2019day$(DAY)
+
+build: part* input
+	docker build -t aoc2019day$(DAY) .
+	touch build
+
+clean:
+	docker image rm -f aoc2019day$(DAY)
+	rm -f build
diff --git a/day12/entrypoint b/day12/entrypoint
new file mode 100755
index 0000000..8982d21
--- /dev/null
+++ b/day12/entrypoint
@@ -0,0 +1,13 @@
+#!/bin/bash
+
+if [ -x part1 ] ; then
+	echo -ne "Part 1:\n\t"
+	time ./part1 < input
+fi
+if [ -x part1 -a -x part2 ] ; then
+	echo
+fi
+if [ -x part2 ] ; then
+	echo -ne "Part 2:\n\t"
+	time ./part2 < input
+fi
diff --git a/day12/input b/day12/input
new file mode 100644
index 0000000..14eda85
--- /dev/null
+++ b/day12/input
@@ -0,0 +1,4 @@
+<x=19, y=-10, z=7>
+<x=1, y=2, z=-3>
+<x=14, y=-4, z=1>
+<x=8, y=7, z=-6>
diff --git a/day12/part1 b/day12/part1
new file mode 100755
index 0000000..2debadf
--- /dev/null
+++ b/day12/part1
@@ -0,0 +1,43 @@
+#!/usr/bin/env ruby
+
+def apply_gravity(moons)
+  (0...moons.count).to_a.combination(2).each do |a, b|
+    3.times do |i|
+      case moons[a][0][i] <=> moons[b][0][i]
+      when -1
+        moons[a][1][i] += 1
+        moons[b][1][i] -= 1
+      when 1
+        moons[b][1][i] += 1
+        moons[a][1][i] -= 1
+      end
+    end
+  end
+  moons
+end
+
+def update(moons)
+  moons.map do |moon|
+    [moon[0].zip(moon[1]).map(&:sum), moon[1]]
+  end
+end
+
+def calculate_energy(moons)
+  moons.map { |moon|
+    moon[0].map(&:abs).sum * moon[1].map(&:abs).sum
+  }.sum
+end
+
+input = $stdin.readlines.map(&:strip)
+
+moons = input.map do |line|
+  m = line.scan(/-?\d+/)
+  [[m[0].to_i, m[1].to_i, m[2].to_i], [0, 0, 0]]
+end
+
+1000.times do
+  moons = apply_gravity(moons)
+  moons = update(moons)
+end
+
+puts calculate_energy(moons)