Day 12, Part 1 only

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+#!/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)