d18.rb

def reduce(n)
  while explode(n) || split(n)
  end
  n
end

def explode(n)
  i = 0
  depth = 0
  while i < n.length
    case n[i]
    when '[' then depth += 1
    when ']' then depth -= 1
    end
    if depth >= 5 && m = n[i..-1].match(/^\[(\d+),(\d+)\]/)
      l, r = n[0...i], n[i+m[0].length..-1]
      l.reverse!.sub!(/\d+/) { |c| (c.reverse.to_i + m[1].to_i).to_s.reverse }&.reverse!
      r.sub!(/\d+/) { |c| (c.to_i + m[2].to_i).to_s }
      n.replace(l + '0' + r)
      return true
    end
    i += 1
  end

  return false
end

def split(n)
  !!n.sub!(/\d\d+/) {|s| a = s.to_i/2; "[#{a},#{s.to_i - a}]"}
end

def magnitude(n)
  while n.sub!(/\[(\d+),(\d+)\]/) { $1.to_i * 3 + $2.to_i * 2 }
  end
  n.to_i
end

def add(a, b)
  reduce("[#{a},#{b}]")
end

if __FILE__ == $0
  lines = STDIN.read.split
  p magnitude(lines[1..-1].inject(lines[0]) {|a, b| add(a, b)})

  p lines.map { |a| lines.filter_map { |b| a != b && magnitude(add(a, b)) } }.flatten.max
end