----
-- `Tag` Registry.
--
-- @classmod Registry

class Registry
--- internals for `Tag`
-- @section internals

  --- lookup the last registration.
  --
  -- @tparam number|string index the registration index
  -- @treturn any
  last: (index) => if tag = @last_map[index] then tag.builtin

  --- set the current registration.
  --
  -- @tparam Tag tag the Tag to register
  -- @treturn boolean whether the tag is new
  register: (tag) =>
    index = tag\index!

    if index and not @map[index] or ignore_dup
      L\trace "reg: registering #{index} (#{ignore_dup})"
      @map[index] = tag
      nil
    else
      if index
        L\warn "duplicate tag [#{index}], reassigning repeated occurrence"
        tag\set nil

      table.insert @pending, tag
      L\trace "reg: registered pending"
      true

--- members
-- @section members

  --- begin an evaluation cycle.
  --
  -- All calls go `begin_eval` must be matched with a call to
  -- `assign_tags` followed by either `end_eval` or `rollback_eval`.
  begin_eval: =>
    assert not @map, "unfinished evaluation cycle"
    @map, @pending = {}, {}

  --- assign unique tags to pending registrations.
  --
  -- Assigns a unique number to each tag added in this evaluation cycle.
  assign_tags: =>
    next_tag = 1

    for tag in *@pending
      if not tag\index!
        while @map[next_tag]
          next_tag += 1

        L\trace "assigned new tag #{next_tag} to #{tag}"
        tag\set next_tag

      @map[tag\index!] = tag

  --- abort an evaluation cycle.
  --
  -- Destroys all pending registrations
  rollback_eval: =>
    assert @map, "no eval cycle to abort"
    for tag in *@pending
      if builtin = tag.builtin
        builtin\destroy!

    @map, @pending = nil, nil

  --- end an evaluation cycle.
  --
  -- Register all pending `Tag`s and destroy all orphaned registrations.
  -- @treturn bool whether any changes to the AST were made
  end_eval: =>
    -- destroy old registrations
    for index, tag in pairs @last_map
      if builtin = tag.builtin
        builtin\destroy!

    -- mark all regisrations complete
    for index, tag in pairs @map
      tag.pending = nil

    dirty = #@pending > 0
    @last_map, @map, @pending = @map, nil, nil

    dirty

  --- destroy this Registry and all associated Registrations.
  -- needs to be called *after* `:eval`.
  destroy: =>
    assert not @tag, "unfinished evaluation cycle"
    for index, tag in pairs @last_map
      if builtin = tag.builtin
        builtin\destroy!

    @last_map = {}

  --- the module associated with this Registry
  -- @tfield Module module

--- static functions
-- @section static

  --- create a new Registry.
  -- @classmethod
  -- @tparam Module module
  new: (@module) =>
    @last_map = {}

{
  :Registry
}