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127 lines
2.5 KiB
127 lines
2.5 KiB
# typed: ignore
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require "matrix"
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require "prime"
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require "set"
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V = Vector
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Moon = Struct.new(:id, :pos, :vel) do
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def potential_energy
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pos.map(&:abs).sum
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end
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def kinetic_energy
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vel.map(&:abs).sum
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end
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end
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class Moons < SimpleDelegator
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def self.from(input)
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moons = input
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.scan(/x=(-?\d+), y=(-?\d+), z=(-?\d+)/)
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.map.with_index {|x,i| Moon.new(i, V[*x.map(&:to_i)], V.zero(3)) }
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new(moons)
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end
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def step
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# apply gravity
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deltas = Hash.new {|h,k| h[k] = V.zero(3) }
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combination(2).each do |a, b|
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delta = V[*a.pos.zip(b.pos).map {|a,b| a <=> b }]
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deltas[a] -= delta
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deltas[b] += delta
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end
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# apply velocity
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Moons.new(map {|m|
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v = m.vel + deltas.fetch(m)
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Moon.new(m.id, m.pos + v, v)
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})
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end
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def energy
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map {|m| m.potential_energy * m.kinetic_energy }.sum
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end
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def fingerprint(axis)
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map {|m| [m.pos[axis], m.vel[axis]] }
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end
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end
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require "minitest"
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class TestMoons < Minitest::Test
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def setup
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@moons = Moons.from(<<~MOONS)
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<x=-1, y=0, z=2>
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<x=2, y=-10, z=-7>
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<x=4, y=-8, z=8>
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<x=3, y=5, z=-1>
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MOONS
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end
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def test_from
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assert_equal Moon.new(0, V[-1, 0, 2], V[0, 0, 0]), @moons.fetch(0)
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assert_equal Moon.new(3, V[3, 5, -1], V[0, 0, 0]), @moons.fetch(3)
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end
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def test_step
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moons = @moons.step
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assert_equal Moon.new(0, V[2, -1, 1], V[3, -1, -1]), moons.fetch(0)
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assert_equal Moon.new(3, V[2, 2, 0], V[-1, -3, 1]), moons.fetch(3)
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9.times do
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moons = moons.step
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end
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assert_equal Moon.new(0, V[2, 1, -3], V[-3, -2, 1]), moons.fetch(0)
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assert_equal Moon.new(3, V[2, 0, 4], V[1, -1, -1]), moons.fetch(3)
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end
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def test_energy
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10.times do
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@moons = @moons.step
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end
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assert_equal 179, @moons.energy
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end
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end
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def gcd(*n)
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n.map {|c| Prime.prime_division(c).to_h }
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.inject {|h,p| h.merge(p) {|_, a, b| [a, b].max }}
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.map {|n,e| n**e }
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.inject(&:*)
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end
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if ARGF.to_io.tty?
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require "minitest/autorun"
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else
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moons = Moons.from(ARGF.read)
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# 1000.times do
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# moons = moons.step
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# end
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# puts moons.energy
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seen = [{}, {}, {}]
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cycles = [nil, nil, nil]
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(0..).each do |step|
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if cycles.none?(&:nil?)
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puts gcd(*cycles.map(&:size))
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exit
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end
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(0..2).each do |i|
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seen[i][moons.fingerprint(i)] ||= step
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end
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moons = moons.step
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(0..2).each do |i|
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cycles[i] ||= (seen[i][moons.fingerprint(i)]..step) if seen[i].include?(moons.fingerprint(i))
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end
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end
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end
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