#!/usr/bin/env ruby

require "debug"

problem = 9
input = File.read("#{problem}.input")
#input = DATA.read
  .chars
  .map(&:to_i)

expanded_disk = []

id = 0
type = :file
input.each do
  if type == :free
    _1.times { expanded_disk << "." }

    type = :file
  else
    _1.times { expanded_disk << id }
    id += 1
    type = :free
  end
end

defragmented_disk = expanded_disk.dup

start = Time.now
r = expanded_disk.size - 1
while r > 0
  if expanded_disk[r] == "."
    r -= 1
  else
    rr = r
    rr -= 1 while expanded_disk[rr] == expanded_disk[r]
    space = r - rr

    l = expanded_disk.index(".")
    while l < expanded_disk.size
      if expanded_disk[l] != "."
        l += 1
        next
      end

      ll = l
      ll += 1 while defragmented_disk[ll] == "."
      free = ll - l

      if l > r
        break
      elsif free >= space
        (l..(l + space - 1)).each { defragmented_disk[_1] = expanded_disk[r] }
        ((rr + 1)..r).each { defragmented_disk[_1] = "." }
        break
      else
        l = ll + 1
      end
    end

    r = rr
  end
end
finish = Time.now
puts "Finished in #{finish - start}"

checksum = defragmented_disk
  .map.with_index { _1.to_i * _2.to_i }
  .sum

p checksum


__END__
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