From 2bb84ae6c6c71d51b445bf0b232c0eb6ca38e89b Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Thu, 12 Dec 2019 12:13:56 +0000
Subject: Day 12 part 2, working, using the potentially bad assumption that the
 cycle contains the initial state

---
 day12/part2                   | 38 +++++++-------------
 day12/part2.wo.bad.assumption | 80 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+), 25 deletions(-)
 create mode 100755 day12/part2.wo.bad.assumption

diff --git a/day12/part2 b/day12/part2
index 237432f..f5b0f96 100755
--- a/day12/part2
+++ b/day12/part2
@@ -38,43 +38,31 @@ moonsz = input.map do |line|
   [m[2].to_i, 0]
 end
 
-xs = [moonsx.dup]
+initx = moonsx.map { |m| [m[0], m[1]] }
+xlen = 0
 loop do
   moonsx = apply_gravity(moonsx)
   moonsx = update(moonsx)
-  if xs.include?(moonsx) then
-    break
-  else
-    xs << moonsx.dup
-  end
+  xlen += 1
+  break if initx == moonsx
 end
-xintro = xs.index(moonsx)
-xlen = xs.count - xintro
 
-ys = [moonsy.dup]
+inity = moonsy.map { |m| [m[0], m[1]] }
+ylen = 0
 loop do
   moonsy = apply_gravity(moonsy)
   moonsy = update(moonsy)
-  if ys.include?(moonsy) then
-    break
-  else
-    ys << moonsy.dup
-  end
+  ylen += 1
+  break if inity == moonsy
 end
-yintro = ys.index(moonsy)
-ylen = ys.count - yintro
 
-zs = [moonsz.dup]
+initz = moonsz.map { |m| [m[0], m[1]] }
+zlen = 0
 loop do
   moonsz = apply_gravity(moonsz)
   moonsz = update(moonsz)
-  if zs.include?(moonsz) then
-    break
-  else
-    zs << moonsz.dup
-  end
+  zlen += 1
+  break if initz == moonsz
 end
-zintro = zs.index(moonsz)
-zlen = zs.count - zintro
 
-puts [xintro, yintro, zintro].max + [xlen, ylen, zlen].reduce(1, :lcm)
+puts [xlen, ylen, zlen].reduce(1, :lcm)
diff --git a/day12/part2.wo.bad.assumption b/day12/part2.wo.bad.assumption
new file mode 100755
index 0000000..237432f
--- /dev/null
+++ b/day12/part2.wo.bad.assumption
@@ -0,0 +1,80 @@
+#!/usr/bin/env ruby
+
+def apply_gravity(moons)
+  ms = moons.map { |m| [m[0], m[1]] }
+  (0...ms.count).to_a.combination(2).each do |a, b|
+    case ms[a][0] <=> ms[b][0]
+    when -1
+      ms[a][1] += 1
+      ms[b][1] -= 1
+    when 1
+      ms[b][1] += 1
+      ms[a][1] -= 1
+    end
+  end
+  ms
+end
+
+def update(moons)
+  moons.map do |moon|
+    [moon[0] + moon[1], moon[1]]
+  end
+end
+
+input = $stdin.readlines.map(&:strip)
+
+moonsx = input.map do |line|
+  m = line.scan(/-?\d+/)
+  [m[0].to_i, 0]
+end
+
+moonsy = input.map do |line|
+  m = line.scan(/-?\d+/)
+  [m[1].to_i, 0]
+end
+
+moonsz = input.map do |line|
+  m = line.scan(/-?\d+/)
+  [m[2].to_i, 0]
+end
+
+xs = [moonsx.dup]
+loop do
+  moonsx = apply_gravity(moonsx)
+  moonsx = update(moonsx)
+  if xs.include?(moonsx) then
+    break
+  else
+    xs << moonsx.dup
+  end
+end
+xintro = xs.index(moonsx)
+xlen = xs.count - xintro
+
+ys = [moonsy.dup]
+loop do
+  moonsy = apply_gravity(moonsy)
+  moonsy = update(moonsy)
+  if ys.include?(moonsy) then
+    break
+  else
+    ys << moonsy.dup
+  end
+end
+yintro = ys.index(moonsy)
+ylen = ys.count - yintro
+
+zs = [moonsz.dup]
+loop do
+  moonsz = apply_gravity(moonsz)
+  moonsz = update(moonsz)
+  if zs.include?(moonsz) then
+    break
+  else
+    zs << moonsz.dup
+  end
+end
+zintro = zs.index(moonsz)
+zlen = zs.count - zintro
+
+puts [xintro, yintro, zintro].max + [xlen, ylen, zlen].reduce(1, :lcm)
-- 
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