class TransformableList

# File lib/transformable_list.rb, line 5
def initialize(items, matcher: -> a, b { a == b })
  @items = items
  @matcher = matcher
end

# File lib/transformable_list.rb, line 10
def transform(new_items)
  build_remaining_items_index!

  moves = []
  creates = []
  deletes = []

  indexes_to_keep = Set.new

  new_items.map.with_index do |new_item, new_index|
    if existing_index = find_index_and_eliminate_match(new_item)
      if new_index != existing_index
        moves << [:move, @items[existing_index], new_index]
      end

      indexes_to_keep << existing_index
    else
      creates << [:create, new_item, new_index]
    end
  end

  (0...@items.size).each do |index|
    unless indexes_to_keep.include?(index)
      deletes << [:delete, @items[index], index]
    end
  end

  deletes.sort_by(&:last) + creates.sort_by(&:last) + moves.sort_by(&:last)
end

# File lib/transformable_list.rb, line 42
def build_remaining_items_index!
  @remaining_items = @items.map.with_index{|item, i| [item, i]}
end

# File lib/transformable_list.rb, line 46
def find_index_and_eliminate_match(new_item)
  remaining_index =
    @remaining_items.index{|item, _| item.object_id == new_item.object_id} ||
      @remaining_items.index{|item, _| match?(item, new_item)}

  _, original_index = if remaining_index
    result = @remaining_items[remaining_index]
    @remaining_items.delete_at(remaining_index)
    result
  end

  original_index
end

# File lib/transformable_list.rb, line 60
def match?(a, b)
  @matcher.(a, b)
end