defmodule Philtre.Block.ContentEditable.Selection do
  @moduledoc """
  Holds current selection in a block.

  Passed from client to backend and vice-versa when executing block commands.

  The ids are ids of cells in which a selection starts or ends.
  The offests are indices within those cells where the selection starts or ends.

  That means a simple caret (a cursor somewhere in the block text) will have
  the same ids and same offsets.

  Similarly, a selection of a text within a single cell will have the same ids,
  but different offsets.

  Lastly, a selection across cells within a block will have different ids and
  different offsets.

  Selection across blocks is not possible. Only whole blocks can be selected and
  this is handled at a different level.
  """
  alias Philtre.Block.ContentEditable
  defstruct [:start_id, :end_id, :start_offset, :end_offset]

  @type t :: %__MODULE__{
          start_id: ContentEditable.Cell.id() | nil,
          end_id: ContentEditable.Cell.id() | nil,
          start_offset: non_neg_integer() | nil,
          end_offset: non_neg_integer() | nil
        }

  def normalize!(nil), do: nil

  def normalize!(%{
        "start_id" => start_id,
        "end_id" => end_id,
        "start_offset" => start_offset,
        "end_offset" => end_offset
      })
      when is_binary(start_id) and is_binary(end_id) and is_integer(start_offset) and
             is_integer(end_offset) do
    %__MODULE__{
      start_id: start_id,
      end_id: end_id,
      start_offset: start_offset,
      end_offset: end_offset
    }
  end

  def normalize!(%{"start_id" => value}) when not is_binary(value) do
    raise "selection start_id must be a valid id string, got: #{value}"
  end

  def normalize!(%{"end_id" => value}) when not is_binary(value) do
    raise "selection end_id must be a valid id string, got: #{value}"
  end

  def normalize!(%{"start_offset" => value}) when not is_integer(value) do
    raise "selection start_offset must be a valid number, got: #{value}"
  end

  def normalize!(%{"end_offset" => value}) when not is_integer(value) do
    raise "selection end_offset must be a valid number, got: #{value}"
  end

  def new_empty, do: %__MODULE__{}

  def new_start_of(%ContentEditable.Cell{id: id}) do
    %__MODULE__{
      start_id: id,
      end_id: id,
      start_offset: 0,
      end_offset: 0
    }
  end

  def new_end_of(%ContentEditable.Cell{id: id, text: text}) do
    offset = String.length(text)

    %__MODULE__{
      start_id: id,
      end_id: id,
      start_offset: offset,
      end_offset: offset
    }
  end
end