#!/usr/bin/env ruby

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 = Array.new(expanded_disk.size)

l, r = 0, expanded_disk.size - 1
while l <= r
  if expanded_disk[l] != "."
    defragmented_disk[l] = expanded_disk[l]
    l += 1
  elsif expanded_disk[r] == "."
    defragmented_disk[r] = expanded_disk[r]
    r -= 1
  else
    defragmented_disk[l] = expanded_disk[r]
    defragmented_disk[r] = "."
    l += 1
    r -= 1
  end
end

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

p checksum


__END__
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