Day 18: Everything is fine

This might be some of the ugliest code I've ever written, but it gets
the job done! Took me ~4 hours and there was a point about halfway
through that I started to wish I had just built out the binary tree
structure instead of trying to manipulate the strings.

Oh well.

2021/18/input.txt

@@ -0,0 +1,100 @@
+[[1,[8,[5,8]]],[[4,4],[8,[8,8]]]]
+[[[3,[2,3]],[[8,0],2]],[0,[[8,1],[7,0]]]]
+[4,[[0,3],[[6,6],[3,8]]]]
+[[[7,[6,4]],[[0,6],[2,0]]],[[[5,6],[0,4]],[[8,1],[9,1]]]]
+[[[6,3],[[6,9],4]],[[1,[4,2]],[[0,0],1]]]
+[[2,0],[3,[0,8]]]
+[[0,[5,5]],[[4,2],[3,[6,4]]]]
+[[[[9,9],[8,5]],[7,4]],[[6,9],[8,[0,8]]]]
+[[[[7,1],[2,9]],[[9,3],0]],[3,[[0,6],[7,6]]]]
+[[[[3,7],[7,1]],[[5,8],[0,1]]],3]
+[[[[4,6],[6,2]],[[9,1],7]],[[9,1],[8,0]]]
+[[[[2,7],0],[[9,4],[2,6]]],[0,[[7,4],[0,3]]]]
+[[5,[[0,2],[8,8]]],[[[4,1],9],3]]
+[[[7,1],[[3,7],[3,4]]],[[[0,7],[1,6]],1]]
+[[[6,5],[[1,8],[8,8]]],[[4,5],[3,7]]]
+[[[1,[3,3]],[[3,2],[5,7]]],[[8,[9,3]],[[5,3],4]]]
+[[[4,[2,7]],9],[9,[[5,6],4]]]
+[[[9,1],3],[[1,2],9]]
+[[[[0,0],[2,3]],[[7,8],[1,5]]],[[[8,6],7],[[8,3],9]]]
+[6,[[5,[0,8]],1]]
+[4,[[[3,0],[2,0]],[[7,2],[1,4]]]]
+[[[[4,3],[4,1]],8],[[[9,4],[1,9]],[4,[0,6]]]]
+[4,[5,6]]
+[[[0,[6,1]],[[6,1],3]],[[0,[7,8]],[1,0]]]
+[[5,[[8,7],8]],8]
+[[5,[[5,2],0]],[[1,[4,7]],[[0,9],[2,3]]]]
+[[7,[2,2]],[[6,3],[5,8]]]
+[[[0,9],5],[1,[[5,7],1]]]
+[[8,[3,[0,3]]],[[[2,2],2],[[8,8],[8,9]]]]
+[[6,[[3,2],[2,6]]],[5,1]]
+[[[[9,8],[6,8]],[0,7]],7]
+[[[7,2],[[6,3],4]],2]
+[[[5,2],[[1,6],[8,3]]],[6,5]]
+[[5,2],[0,5]]
+[[[[4,5],5],[[4,6],[1,2]]],[[[3,6],[4,9]],[1,9]]]
+[[1,[4,1]],[[9,[5,5]],[[9,0],[5,7]]]]
+[[[[8,9],[7,7]],2],[8,1]]
+[[[8,1],[8,[9,5]]],3]
+[[[2,[3,9]],[[5,4],[7,9]]],[9,8]]
+[8,[[2,[0,9]],[[5,0],4]]]
+[[[6,[4,8]],[0,6]],[[8,[1,8]],1]]
+[[6,[[1,0],[6,2]]],[[9,[3,7]],[5,[4,0]]]]
+[[8,[0,[9,1]]],8]
+[7,[4,[7,2]]]
+[[1,[[5,7],[5,4]]],[[5,[8,0]],[1,6]]]
+[[[[0,6],[6,2]],3],[[[9,3],7],[7,[1,2]]]]
+[[[6,[4,9]],8],[6,5]]
+[[[0,[1,9]],[[1,9],[3,9]]],[[[3,4],[7,5]],3]]
+[[[[9,3],5],[[0,5],[2,7]]],9]
+[[[6,[7,5]],5],[1,[[7,0],[3,4]]]]
+[[[2,1],[[1,3],[1,5]]],[4,[9,[7,9]]]]
+[[[[7,9],4],[[8,8],7]],[[[3,5],2],[[4,4],[6,5]]]]
+[[1,1],[1,1]]
+[[8,[0,2]],8]
+[[[2,[2,1]],[[1,7],[1,2]]],[[1,6],5]]
+[6,[0,[[1,0],[0,9]]]]
+[6,[[2,[8,0]],[8,[8,8]]]]
+[4,[[3,[0,3]],4]]
+[[[5,3],3],[[0,[7,6]],[2,[5,8]]]]
+[[[[8,1],[4,1]],[[5,8],[4,8]]],[[[1,7],[7,2]],[0,[2,7]]]]
+[[[2,[3,5]],3],5]
+[[7,[[9,5],[8,2]]],[[[1,8],8],5]]
+[[3,5],[[4,[9,3]],5]]
+[[[[4,6],2],[2,2]],[0,[0,4]]]
+[[[[5,8],[6,6]],[2,0]],[[[2,3],9],[[4,5],2]]]
+[[[[1,9],3],[[3,4],6]],[[3,6],[6,[0,7]]]]
+[[[0,[5,5]],[2,6]],[[[7,4],4],2]]
+[0,[[8,[6,2]],[5,[1,5]]]]
+[[[[5,5],[9,6]],[[5,2],2]],[[4,7],[[5,5],[1,6]]]]
+[[4,7],[[[1,8],[9,6]],[2,3]]]
+[5,[5,4]]
+[[[[2,1],[7,0]],[5,[7,8]]],[6,[3,1]]]
+[[[3,1],[[2,4],6]],[[[1,8],[2,1]],[[1,7],4]]]
+[[[5,[3,3]],6],[[[0,0],9],[1,[7,4]]]]
+[[[6,5],[[7,3],4]],[[9,[0,3]],[3,[6,0]]]]
+[[[3,4],7],[8,[[1,7],[9,9]]]]
+[[[[2,1],6],[2,6]],[[[8,1],[6,2]],[9,0]]]
+[[8,4],[5,2]]
+[[4,[[4,5],9]],[[3,[5,2]],[4,2]]]
+[[[8,8],[[8,0],[5,3]]],4]
+[[1,8],[0,2]]
+[[[[7,2],[9,0]],[[9,2],[1,2]]],[[[4,0],3],0]]
+[[[[1,2],[1,8]],[[4,3],[8,6]]],[[[5,1],8],[8,1]]]
+[[[[5,3],[7,2]],7],[[6,[7,9]],[[3,8],[9,4]]]]
+[[[[3,1],[2,5]],6],[[[3,2],[8,8]],[4,6]]]
+[9,[[3,[2,3]],6]]
+[[[[4,0],[5,6]],[5,4]],[[[9,0],[1,8]],[5,[3,6]]]]
+[[[[9,5],[9,4]],[[5,7],5]],[[[1,4],7],[6,1]]]
+[[2,[6,[8,2]]],[7,[1,[3,3]]]]
+[[[9,1],[0,[6,3]]],[[5,[1,5]],[7,[1,0]]]]
+[1,6]
+[[0,[2,[8,9]]],[[[4,5],[5,4]],1]]
+[[[1,[4,1]],8],[[2,[7,0]],[7,[9,9]]]]
+[[[[5,7],[3,5]],[[6,6],2]],[2,[8,[9,0]]]]
+[6,[[[3,9],8],[[4,3],[6,1]]]]
+[[[[6,7],[7,6]],[2,8]],[[9,[4,1]],6]]
+[[[[4,5],[4,5]],[[0,6],5]],[[[6,5],[7,0]],1]]
+[[[[6,7],9],[[5,5],[6,6]]],[[7,1],[[8,2],[3,1]]]]
+[[[9,6],7],[[[1,8],8],[1,7]]]
+[[5,2],[[1,9],[2,2]]]

2021/18/run.rb

@@ -0,0 +1,9 @@
+require_relative "snailfish"
+
+input = File.readlines("input.txt")
+fish = input.map{Snailfish.new(_1.chomp)}
+
+puts "Magnitude: #{fish.reduce(:+).magnitude}"
+
+magnitudes = fish.permutation(2).map{ _1.reduce(:+).magnitude }
+puts "Largest of any two: #{magnitudes.max}"

2021/18/snailfish.rb

@@ -0,0 +1,93 @@
+require "json"
+
+class Snailfish
+  attr_reader :number
+
+  # Convenience constructor to help make tests easier to read.
+  def self.[](number)
+    self.new(number)
+  end
+
+  def initialize(number)
+    @number = number
+  end
+
+  def +(other)
+    Snailfish["[#{number},#{other.number}]"].reduce!
+  end
+
+  def ==(other)
+    number == other.number
+  end
+
+  def inspect
+    number
+  end
+
+  def magnitude
+    a, b = JSON.parse(number)
+
+    a = a.is_a?(Array) ? Snailfish[a.to_json].magnitude : a
+    b = b.is_a?(Array) ? Snailfish[b.to_json].magnitude : b
+
+    3*a + 2*b
+  end
+
+  def reduce!
+    true while explode! || split!
+    self
+  end
+
+  def explode!
+    return false if max_depth <= 4
+
+    head, pair, tail = find_exploding_pair(JSON.parse(number))
+
+    if (left = head.scan(/\d{1,}/).last)
+      idx = head.rindex(left)
+      head[idx...(idx+left.length)] = (left.to_i + pair[0]).to_s
+    end
+
+    if (right = tail.scan(/\d{1,}/).first)
+      idx = tail.index(right)
+      tail[idx...(idx+right.length)] = (right.to_i + pair[1]).to_s
+    end
+
+    @number = [head,0,tail].join
+  end
+
+  def split!
+    number.match(/\d{2,}/) do |split|
+      head, tail = number.split(split[0], 2)
+      m = split[0].to_i / 2.0
+      @number = [head, [m.floor, m.ceil].to_json, tail].join
+    end
+  end
+
+  private
+
+  def max_depth
+    array = JSON.parse(number)
+    depth = 1
+
+    until array == array.flatten
+      depth +=1
+      array = array.flatten(1)
+    end
+
+    depth
+  end
+
+  def find_exploding_pair(pair, depth = 1, head = "", tail = "")
+    a, b = pair
+
+    if depth == 4
+      return ["#{head}[", a, ",#{b.to_json}]#{tail}"] if a.is_a?(Array)
+      return ["#{head}[#{a.to_json},", b, "]#{tail}"] if b.is_a?(Array)
+      return false
+    end
+
+    find_exploding_pair(a, depth + 1, "#{head}[", ",#{b.to_json}]#{tail}") ||
+      find_exploding_pair(b, depth + 1, "#{head}[#{a.to_json},", "]#{tail}")
+  end
+end

2021/18/test.rb

@@ -0,0 +1,117 @@
+require "minitest/autorun"
+require_relative "snailfish"
+
+class SnailfishTest < Minitest::Test
+  def test_addition
+    assert_equal Snailfish["[[1,2],[[3,4],5]]"], [
+      Snailfish["[1,2]"],
+      Snailfish["[[3,4],5]"]
+    ].reduce(:+)
+
+    assert_equal Snailfish["[[[[1,1],[2,2]],[3,3]],[4,4]]"], [
+      Snailfish["[1,1]"],
+      Snailfish["[2,2]"],
+      Snailfish["[3,3]"],
+      Snailfish["[4,4]"]
+    ].reduce(:+)
+
+    assert_equal Snailfish["[[[[3,0],[5,3]],[4,4]],[5,5]]"], [
+      Snailfish["[1,1]"],
+      Snailfish["[2,2]"],
+      Snailfish["[3,3]"],
+      Snailfish["[4,4]"],
+      Snailfish["[5,5]"]
+    ].reduce(:+)
+
+    assert_equal Snailfish["[[[[5,0],[7,4]],[5,5]],[6,6]]"], [
+      Snailfish["[1,1]"],
+      Snailfish["[2,2]"],
+      Snailfish["[3,3]"],
+      Snailfish["[4,4]"],
+      Snailfish["[5,5]"],
+      Snailfish["[6,6]"]
+    ].reduce(:+)
+  end
+
+  def test_complex_addition
+    assert_equal Snailfish["[[[[4,0],[5,4]],[[7,7],[6,0]]],[[8,[7,7]],[[7,9],[5,0]]]]"],
+      Snailfish["[[[0,[4,5]],[0,0]],[[[4,5],[2,6]],[9,5]]]"] +
+      Snailfish["[7,[[[3,7],[4,3]],[[6,3],[8,8]]]]"]
+
+    assert_equal Snailfish["[[[[6,7],[6,7]],[[7,7],[0,7]]],[[[8,7],[7,7]],[[8,8],[8,0]]]]"],
+      Snailfish["[[[[4,0],[5,4]],[[7,7],[6,0]]],[[8,[7,7]],[[7,9],[5,0]]]]"] +
+      Snailfish["[[2,[[0,8],[3,4]]],[[[6,7],1],[7,[1,6]]]]"]
+
+    assert_equal Snailfish["[[[[7,0],[7,7]],[[7,7],[7,8]]],[[[7,7],[8,8]],[[7,7],[8,7]]]]"],
+      Snailfish["[[[[6,7],[6,7]],[[7,7],[0,7]]],[[[8,7],[7,7]],[[8,8],[8,0]]]]"] +
+      Snailfish["[[[[2,4],7],[6,[0,5]]],[[[6,8],[2,8]],[[2,1],[4,5]]]]"]
+
+    assert_equal Snailfish["[[[[7,7],[7,8]],[[9,5],[8,7]]],[[[6,8],[0,8]],[[9,9],[9,0]]]]"],
+      Snailfish["[[[[7,0],[7,7]],[[7,7],[7,8]]],[[[7,7],[8,8]],[[7,7],[8,7]]]]"] +
+      Snailfish["[7,[5,[[3,8],[1,4]]]]"]
+
+    assert_equal Snailfish["[[[[6,6],[6,6]],[[6,0],[6,7]]],[[[7,7],[8,9]],[8,[8,1]]]]"],
+      Snailfish["[[[[7,7],[7,8]],[[9,5],[8,7]]],[[[6,8],[0,8]],[[9,9],[9,0]]]]"] +
+      Snailfish["[[2,[2,2]],[8,[8,1]]]"]
+
+    assert_equal Snailfish["[[[[6,6],[7,7]],[[0,7],[7,7]]],[[[5,5],[5,6]],9]]"],
+      Snailfish["[[[[6,6],[6,6]],[[6,0],[6,7]]],[[[7,7],[8,9]],[8,[8,1]]]]"] +
+      Snailfish["[2,9]"]
+
+    assert_equal Snailfish["[[[[7,8],[6,7]],[[6,8],[0,8]]],[[[7,7],[5,0]],[[5,5],[5,6]]]]"],
+      Snailfish["[[[[6,6],[7,7]],[[0,7],[7,7]]],[[[5,5],[5,6]],9]]"] +
+      Snailfish["[1,[[[9,3],9],[[9,0],[0,7]]]]"]
+
+    assert_equal Snailfish["[[[[7,7],[7,7]],[[8,7],[8,7]]],[[[7,0],[7,7]],9]]"],
+      Snailfish["[[[[7,8],[6,7]],[[6,8],[0,8]]],[[[7,7],[5,0]],[[5,5],[5,6]]]]"] +
+      Snailfish["[[[5,[7,4]],7],1]"]
+
+    assert_equal Snailfish["[[[[8,7],[7,7]],[[8,6],[7,7]]],[[[0,7],[6,6]],[8,7]]]"],
+      Snailfish["[[[[7,7],[7,7]],[[8,7],[8,7]]],[[[7,0],[7,7]],9]]"] +
+      Snailfish["[[[[4,2],2],6],[8,7]]"]
+  end
+
+  def test_magnitude
+    assert_equal 143, Snailfish["[[1,2],[[3,4],5]]"].magnitude
+    assert_equal 1384, Snailfish["[[[[0,7],4],[[7,8],[6,0]]],[8,1]]"].magnitude
+    assert_equal 445, Snailfish["[[[[1,1],[2,2]],[3,3]],[4,4]]"].magnitude
+    assert_equal 791, Snailfish["[[[[3,0],[5,3]],[4,4]],[5,5]]"].magnitude
+    assert_equal 1137, Snailfish["[[[[5,0],[7,4]],[5,5]],[6,6]]"].magnitude
+    assert_equal 3488, Snailfish["[[[[8,7],[7,7]],[[8,6],[7,7]]],[[[0,7],[6,6]],[8,7]]]"].magnitude
+  end
+
+  def test_split
+    assert_equal "[[5,5],0]", Snailfish["[10,0]"].split!
+    assert_equal "[0,[5,6]]", Snailfish["[0,11]"].split!
+
+    assert_equal "[0,[[6,6],0]]", Snailfish["[0,[12,0]]"].split!
+    assert_equal "[0,[[6,7],[0,14]]]", Snailfish["[0,[13,[0,14]]]"].split!
+
+    assert_equal "[[[[0,7],4],[[7,8],[0,13]]],[1,1]]",
+      Snailfish["[[[[0,7],4],[15,[0,13]]],[1,1]]"].split!
+
+    refute Snailfish["[0,[1,[3,[[4,5],6]]]]"].split!
+  end
+
+  def test_explode
+    assert_equal "[[[[0,9],2],3],4]", Snailfish["[[[[[9,8],1],2],3],4]"].explode!
+    assert_equal "[7,[6,[5,[7,0]]]]", Snailfish["[7,[6,[5,[4,[3,2]]]]]"].explode!
+    assert_equal "[[6,[5,[7,0]]],3]", Snailfish["[[6,[5,[4,[3,2]]]],1]"].explode!
+
+    assert_equal "[[3,[2,[8,0]]],[9,[5,[4,[3,2]]]]]",
+      Snailfish["[[3,[2,[1,[7,3]]]],[6,[5,[4,[3,2]]]]]"].explode!
+    assert_equal "[[3,[2,[8,0]]],[9,[5,[7,0]]]]",
+
+      Snailfish["[[3,[2,[8,0]]],[9,[5,[4,[3,2]]]]]"].explode!
+
+    assert_equal "[[[[6,7],[0,7]],[[13,0],[7,21]]],[[2,[11,10]],[[0,8],[8,0]]]]",
+      Snailfish["[[[[6,7],[0,7]],[[7,[6,7]],[0,21]]],[[2,[11,10]],[[0,8],[8,0]]]]"].explode!
+
+    refute Snailfish["[[[[0,7],4],[[7,8],[6,0]]],[8,1]]"].explode!
+  end
+
+  def test_reduce
+    assert_equal Snailfish["[[[[0,7],4],[[7,8],[6,0]]],[8,1]]"],
+      Snailfish["[[[[[4,3],4],4],[7,[[8,4],9]]],[1,1]]"].reduce!
+  end
+end