local capabilities = require("clever_f.capabilities")
local domain = require("clever_f.domain")
local text_topology = require("clever_f.text_topology")

local M = {}
local MemoryHost = {}
MemoryHost.__index = MemoryHost
M.MemoryHost = MemoryHost
local unpack_values = table.unpack or unpack

local function is_integer(value)
  return type(value) == "number"
    and value > -math.huge
    and value < math.huge
    and value == math.floor(value)
end

local function copy(value, seen)
  if type(value) ~= "table" or domain.type_of(value) ~= nil then
    return value
  end
  seen = seen or {}
  if seen[value] ~= nil then
    return seen[value]
  end
  local result = {}
  seen[value] = result
  for key, item in pairs(value) do
    result[copy(key, seen)] = copy(item, seen)
  end
  return result
end

local function list_copy(values)
  local result = {}
  for index = 1, #values do
    result[index] = values[index]
  end
  return result
end

local function text_snapshot(value)
  if domain.TextSnapshot.is(value) then
    return value
  end
  if type(value) == "table" and value.lines ~= nil then
    value = value.lines
  end
  return domain.TextSnapshot.new(value)
end

local function selection_value(value)
  if value == nil then
    return domain.Selection.inactive()
  end
  return domain.Selection.new(value)
end

local function macro_state(value)
  if type(value) == "table" and not domain.MacroState.is(value) then
    value = value.register
  end
  return domain.MacroState.new(value)
end

local function fold_state(options)
  if domain.FoldState.is(options.fold_state) then
    return options.fold_state
  end
  return domain.FoldState.new(
    options.fold_open_policy or {},
    options.closed_fold_levels or 0
  )
end

local function input_packet(value)
  return domain.InputPacket.from_table(value)
end

local function normalize_event_names(event_names)
  if type(event_names) == "string" then
    event_names = { event_names }
  end
  if type(event_names) ~= "table" or #event_names < 1 then
    error("event names must be a nonempty list", 3)
  end
  local result = {}
  local seen = {}
  for index = 1, #event_names do
    local name = event_names[index]
    if type(name) ~= "string" or name == "" then
      error("event name must be a nonempty string", 3)
    end
    if not seen[name] then
      seen[name] = true
      result[#result + 1] = name
    end
  end
  return result, seen
end

local function normalize_modes(modes)
  if type(modes) == "string" then
    modes = { modes }
  end
  if type(modes) ~= "table" or #modes < 1 then
    error("mapping modes must be a nonempty list", 3)
  end
  local result = {}
  for index = 1, #modes do
    if type(modes[index]) ~= "string" or modes[index] == "" then
      error("mapping mode must be a nonempty string", 3)
    end
    result[index] = modes[index]
  end
  return result
end

function MemoryHost.new(options)
  options = options or {}
  if type(options) ~= "table" then
    error("memory host options must be a table", 2)
  end

  local cursor_presentation_support = options.cursor_presentation_support
  if cursor_presentation_support == nil then
    cursor_presentation_support = options.cmdline_cursor_support
  end
  if cursor_presentation_support == nil then
    cursor_presentation_support = true
  end

  local raw_mode = options.mode or "n"
  if domain.ModeContext.is(raw_mode) then
    raw_mode = raw_mode.full_mode
  end
  domain.ModeContext.from_full_mode(raw_mode)

  local self = setmetatable({
    _text = text_snapshot(options.text or options.buffer_lines or { "" }),
    _buffer = options.buffer or "buffer-1",
    _cursor = domain.Position.coerce(options.cursor or { line = 1, byte_column = 1 }),
    _window = options.window or "window-1",
    _mode = raw_mode,
    _selection = selection_value(options.selection),
    _count = domain.Count.new(options.count),
    _configuration = copy(options.configuration or {}),
    _encoding = options.encoding or options.effective_encoding or "utf-8",
    _macro_state = macro_state(options.macro_state or options.macro_register),
    _fold_state = fold_state(options),
    _pending_operator = options.pending_operator,
    _time_values = list_copy(options.time_values_ms or {}),
    _time_index = 1,
    _current_time = options.time_ms or 0,
    _input_packets = {},
    _input_index = 1,
    _timer_support = options.timer_support ~= false,
    _cursor_presentation_support = cursor_presentation_support,
    _cursor_presentation = copy(options.cursor_presentation or {
      hidden = false,
    }),
    _emit_movement_events = options.emit_movement_events ~= false,
    _operator_inclusive = false,
    _operations = {},
    _prompts = {},
    _redraws = {},
    _diagnostics = {},
    _highlight_groups = copy(options.highlight_groups or {}),
    _highlights = {},
    _timers = {},
    _event_registrations = {},
    _event_registration_order = {},
    _actions = {},
    _mappings = {},
    _cursor_leases = {},
    _dot_repeat = nil,
    _identity_counters = {},
  }, MemoryHost)

  for index, packet in ipairs(options.input_packets or {}) do
    self._input_packets[index] = input_packet(packet)
  end

  self._event_queue = capabilities.EventQueue.new(function(name, payload)
    self:_deliver_event_now(name, payload)
  end)

  return capabilities.assert_implements(self)
end

function M.new(options)
  return MemoryHost.new(options)
end

setmetatable(M, {
  __call = function(_, options)
    return MemoryHost.new(options)
  end,
})

function MemoryHost:_next_identity(prefix)
  local next_value = (self._identity_counters[prefix] or 0) + 1
  self._identity_counters[prefix] = next_value
  return prefix .. "-" .. tostring(next_value)
end

function MemoryHost:_record(operation, details)
  local entry = { operation = operation }
  for key, value in pairs(details or {}) do
    entry[key] = copy(value)
  end
  self._operations[#self._operations + 1] = entry
end

function MemoryHost:operations()
  return copy(self._operations)
end

function MemoryHost:clear_operations()
  self._operations = {}
end

function MemoryHost:read_text()
  self:_record("read_text")
  return self._text
end

function MemoryHost:read_cursor()
  self:_record("read_cursor")
  return self._cursor
end

function MemoryHost:read_buffer()
  self:_record("read_buffer", { buffer = self._buffer })
  return self._buffer
end

function MemoryHost:read_window()
  self:_record("read_window", { window = self._window })
  return self._window
end

function MemoryHost:read_mode()
  self:_record("read_mode", { mode = self._mode })
  return self._mode
end

function MemoryHost:read_mode_context()
  return domain.ModeContext.from_full_mode(self:read_mode())
end

function MemoryHost:read_pending_operator()
  self:_record("read_pending_operator", { operator = self._pending_operator })
  return self._pending_operator
end

function MemoryHost:read_selection()
  self:_record("read_selection")
  return self._selection
end

function MemoryHost:read_count()
  self:_record("read_count", { count = self._count.value })
  return self._count
end

function MemoryHost:configuration_present(name)
  if type(name) ~= "string" or name == "" then
    error("configuration name must be a nonempty string", 2)
  end
  local present = self._configuration[name] ~= nil
  self:_record("configuration_present", { name = name, present = present })
  return present
end

function MemoryHost:read_configuration(name)
  if type(name) ~= "string" or name == "" then
    error("configuration name must be a nonempty string", 2)
  end
  local value = copy(self._configuration[name])
  self:_record("read_configuration", { name = name, value = value })
  return value
end

function MemoryHost:write_configuration(name, value)
  if type(name) ~= "string" or name == "" then
    error("configuration name must be a nonempty string", 2)
  end
  self._configuration[name] = copy(value)
  self:_record("write_configuration", { name = name, value = value })
end

function MemoryHost:read_encoding()
  self:_record("read_encoding", { encoding = self._encoding })
  return self._encoding
end

function MemoryHost:read_macro_state()
  self:_record("read_macro_state", { executing = self._macro_state.executing })
  return self._macro_state
end

function MemoryHost:read_fold_state()
  self:_record("read_fold_state", { closed_levels = self._fold_state.closed_levels })
  return self._fold_state
end

function MemoryHost:read_time_ms()
  local value = self._time_values[self._time_index]
  if value ~= nil then
    self._time_index = self._time_index + 1
    self._current_time = value
  else
    value = self._current_time
  end
  if type(value) ~= "number" then
    error("time value must be a number", 2)
  end
  self:_record("read_time_ms", { value = value })
  return value
end

function MemoryHost:set_text(value)
  self._text = text_snapshot(value)
end

function MemoryHost:set_cursor(position)
  self._cursor = domain.Position.coerce(position)
end

function MemoryHost:set_buffer(buffer)
  if buffer == nil then
    error("buffer identity must be present", 2)
  end
  self._buffer = buffer
end

function MemoryHost:set_window(window)
  if window == nil then
    error("window identity must be present", 2)
  end
  self._window = window
end

function MemoryHost:set_mode(full_mode)
  if domain.ModeContext.is(full_mode) then
    full_mode = full_mode.full_mode
  end
  domain.ModeContext.from_full_mode(full_mode)
  self._mode = full_mode
end

function MemoryHost:set_selection(selection)
  self._selection = selection_value(selection)
end

function MemoryHost:set_count(count)
  self._count = domain.Count.new(count)
end

function MemoryHost:set_configuration(name, value)
  if type(name) ~= "string" or name == "" then
    error("configuration name must be a nonempty string", 2)
  end
  self._configuration[name] = copy(value)
end

function MemoryHost:unset_configuration(name)
  self._configuration[name] = nil
end

function MemoryHost:set_encoding(encoding)
  if type(encoding) ~= "string" or encoding == "" then
    error("encoding must be a nonempty string", 2)
  end
  self._encoding = encoding
end

function MemoryHost:set_macro_state(state)
  self._macro_state = macro_state(state)
end

function MemoryHost:set_fold_state(state, closed_levels)
  if domain.FoldState.is(state) then
    self._fold_state = state
  else
    self._fold_state = domain.FoldState.new(state, closed_levels)
  end
end

function MemoryHost:set_pending_operator(operator)
  self._pending_operator = operator
end

function MemoryHost:push_time_ms(value)
  if type(value) ~= "number" then
    error("time value must be a number", 2)
  end
  self._time_values[#self._time_values + 1] = value
end

function MemoryHost:push_input(packet)
  self._input_packets[#self._input_packets + 1] = input_packet(packet)
end

function MemoryHost:_emit_movement_event(previous)
  if self._emit_movement_events and not domain.Position.equal(previous, self._cursor) then
    self:deliver_event("CursorMoved", {
      cursor = self._cursor,
    })
  end
end

local function motion_descriptor(motion)
  if domain.Descriptor.is(motion) then
    return motion
  end
  if type(motion) == "table" and motion.descriptor ~= nil then
    return domain.Descriptor.from_string(motion.descriptor)
  end
  return nil
end

local function character_boundary(view, position)
  if view:line_is_empty(position.line) then
    return 1
  end
  return view:character_index_for_position(position)
end

local function character_lines(snapshot)
  local result = {}
  for line_number, line in ipairs(snapshot:lines()) do
    result[line_number] = text_topology.split_editor_characters(line)
  end
  return result
end

local function joined_range(characters, first, last)
  local result = {}
  for index = first, last do
    result[#result + 1] = characters[index]
  end
  return table.concat(result)
end

local function delete_character_range(
  snapshot,
  start_line,
  start_index,
  finish_line,
  finish_index
)
  local source = character_lines(snapshot)
  local lines = snapshot:lines()
  local result = {}

  for line_number = 1, start_line - 1 do
    result[#result + 1] = lines[line_number]
  end

  local prefix = joined_range(source[start_line], 1, start_index - 1)
  if start_line == finish_line then
    result[#result + 1] = prefix
      .. joined_range(
        source[start_line],
        finish_index,
        #source[start_line]
      )
  else
    result[#result + 1] = prefix
      .. joined_range(
        source[finish_line],
        finish_index,
        #source[finish_line]
      )
  end

  for line_number = finish_line + 1, #lines do
    result[#result + 1] = lines[line_number]
  end
  return domain.TextSnapshot.new(result)
end

local function normalized_cursor(snapshot, encoding, position)
  local line_number = math.min(position.line, snapshot.line_count)
  local view = text_topology.new(snapshot, encoding)
  return view:normalize_endpoint(line_number, position.byte_column)
end

function MemoryHost:_apply_pending_delete(origin, destination, descriptor)
  if self._pending_operator ~= "delete" and self._pending_operator ~= "d" then
    return false
  end

  local view = text_topology.new(self._text, self._encoding)
  local origin_index = character_boundary(view, origin)
  local destination_index = character_boundary(view, destination)
  local start_line
  local start_index
  local finish_line
  local finish_index
  local final_cursor

  if descriptor.direction == domain.Direction.FORWARD then
    start_line = origin.line
    start_index = origin_index
    finish_line = destination.line
    finish_index = destination_index + (self._operator_inclusive and 1 or 0)
    final_cursor = origin
  elseif descriptor.family == domain.Family.FIND then
    start_line = destination.line
    start_index = destination_index + 1
    finish_line = origin.line
    finish_index = origin_index + 1
    final_cursor = destination
  else
    start_line = destination.line
    start_index = destination_index
    finish_line = origin.line
    finish_index = origin_index
    final_cursor = view:predecessor(destination) or destination
  end

  self._text = delete_character_range(
    self._text,
    start_line,
    start_index,
    finish_line,
    finish_index
  )
  self._cursor = normalized_cursor(self._text, self._encoding, final_cursor)
  self:_record("apply_operator", {
    operator = self._pending_operator,
    descriptor = descriptor.value,
    origin = origin,
    endpoint = destination,
    position = self._cursor,
  })
  return true
end

function MemoryHost:apply_cursor(position, motion)
  position = domain.Position.coerce(position)
  local previous = self._cursor
  local descriptor = motion_descriptor(motion)
  self._cursor = position
  self:_record("apply_cursor", {
    position = position,
    descriptor = descriptor and descriptor.value or nil,
  })
  if descriptor ~= nil then
    local origin = type(motion) == "table" and motion.origin or previous
    self:_apply_pending_delete(domain.Position.coerce(origin), position, descriptor)
  end
  self:_emit_movement_event(previous)
end

function MemoryHost:apply_selection(position, kind)
  local previous = self._cursor
  local next_selection
  if domain.Selection.is(position) then
    next_selection = position
    position = next_selection.focus
  else
    position = domain.Position.coerce(position)
    if kind == nil then
      kind = self._selection.kind
    end
    kind = domain.SelectionKind.from_string(kind)
    if kind == domain.SelectionKind.NONE then
      error("selection movement requires a Visual selection kind", 2)
    end
    local anchor = self._selection.active and self._selection.anchor or previous
    next_selection = domain.Selection.active(
      kind,
      anchor,
      position,
      self._selection.option
    )
  end

  self._selection = next_selection
  self._cursor = position
  self:_record("apply_selection", {
    position = position,
    kind = next_selection.kind.value,
  })
  self:_emit_movement_event(previous)
end

function MemoryHost:set_operator_inclusive(enabled)
  if type(enabled) ~= "boolean" then
    error("operator inclusivity must be a Boolean", 2)
  end
  self._operator_inclusive = enabled
  self:_record("set_operator_inclusive", { enabled = enabled })
end

function MemoryHost:operator_inclusive()
  return self._operator_inclusive
end

function MemoryHost:read_input()
  local packet = self._input_packets[self._input_index]
  if packet == nil then
    error("in-memory input queue is empty", 2)
  end
  self._input_index = self._input_index + 1
  self:_record("read_input", { packet = packet:to_table() })
  if packet.kind == domain.InputPacketKind.ERROR then
    error(packet.message, 0)
  end
  return packet
end

function MemoryHost:open_fold(position)
  position = position and domain.Position.coerce(position) or self._cursor
  local closed_levels = self._fold_state.closed_levels
  if closed_levels == 0 then
    self:_record("open_fold", { position = position, opened = false })
    return false
  end
  self:_record("open_fold", {
    position = position,
    fold_level = closed_levels,
    opened = true,
  })
  self._fold_state = domain.FoldState.new(
    self._fold_state:policies(),
    closed_levels - 1
  )
  return true
end

function MemoryHost:show_prompt(text)
  if type(text) ~= "string" then
    error("prompt must be a string", 2)
  end
  self._prompts[#self._prompts + 1] = text
  self:_record("show_prompt", { text = text })
end

function MemoryHost:prompts()
  return list_copy(self._prompts)
end

function MemoryHost:redraw(kind)
  if kind ~= "screen" and kind ~= "full" and kind ~= "suppressed" then
    error("redraw kind must be screen, full, or suppressed", 2)
  end
  self._redraws[#self._redraws + 1] = kind
  self:_record("redraw", { kind = kind })
end

function MemoryHost:redraws()
  return list_copy(self._redraws)
end

function MemoryHost:emit_diagnostic(level, text)
  if level ~= "error" and level ~= "warning" and level ~= "info" then
    error("diagnostic level must be error, warning, or info", 2)
  end
  if type(text) ~= "string" or text == "" then
    error("diagnostic text must be a nonempty string", 2)
  end
  local diagnostic = { level = level, text = text }
  self._diagnostics[#self._diagnostics + 1] = diagnostic
  self:_record("emit_diagnostic", diagnostic)
end

function MemoryHost:diagnostics()
  return copy(self._diagnostics)
end

function MemoryHost:read_highlight_group(name)
  if type(name) ~= "string" or name == "" then
    error("highlight group name must be a nonempty string", 2)
  end
  local definition = self._highlight_groups[name]
  self:_record("read_highlight_group", {
    name = name,
    defined = definition ~= nil,
  })
  return copy(definition)
end

function MemoryHost:highlight_groups()
  return copy(self._highlight_groups)
end

function MemoryHost:define_highlight_group(name, definition, options)
  if type(name) ~= "string" or name == "" then
    error("highlight group name must be a nonempty string", 2)
  end
  if type(definition) ~= "table" then
    error("highlight group definition must be a table", 2)
  end
  options = options or {}
  if type(options) ~= "table" then
    error("highlight group options must be a table", 2)
  end
  if options.default ~= nil and type(options.default) ~= "boolean" then
    error("highlight group default option must be a Boolean", 2)
  end
  if options.force ~= nil and type(options.force) ~= "boolean" then
    error("highlight group force option must be a Boolean", 2)
  end

  local exists = self._highlight_groups[name] ~= nil
  local applied = not (exists and options.default)
  if applied then
    self._highlight_groups[name] = copy(definition)
  end
  self:_record("define_highlight_group", {
    name = name,
    definition = definition,
    options = options,
    applied = applied,
  })
  return applied
end

function MemoryHost:create_highlight(specification)
  if type(specification) ~= "table" then
    error("highlight specification must be a table", 2)
  end
  if type(specification.group) ~= "string" or specification.group == "" then
    error("highlight group must be a nonempty string", 2)
  end
  local identity = specification.identity or self:_next_identity("highlight")
  if self._highlights[identity] ~= nil then
    error("highlight identity is already active", 2)
  end
  local stored = copy(specification)
  stored.identity = identity
  self._highlights[identity] = stored
  self:_record("create_highlight", stored)
  return identity
end

function MemoryHost:remove_highlight(identity)
  if type(identity) ~= "string" or identity == "" then
    error("highlight identity must be a nonempty string", 2)
  end
  local removed = self._highlights[identity] ~= nil
  self._highlights[identity] = nil
  self:_record("remove_highlight", { identity = identity, removed = removed })
  return removed
end

function MemoryHost:highlights()
  return copy(self._highlights)
end

function MemoryHost:supports_cursor_presentation()
  self:_record("supports_cursor_presentation", {
    supported = self._cursor_presentation_support,
  })
  return self._cursor_presentation_support
end

function MemoryHost:suppress_cursor_presentation()
  if not self._cursor_presentation_support then
    self:_record("suppress_cursor_presentation", { supported = false })
    return nil
  end
  local identity = self:_next_identity("cursor-presentation")
  self._cursor_leases[identity] = copy(self._cursor_presentation)
  local suppressed = copy(self._cursor_presentation)
  suppressed.hidden = true
  self._cursor_presentation = suppressed
  self:_record("suppress_cursor_presentation", {
    identity = identity,
    supported = true,
  })
  return identity
end

function MemoryHost:restore_cursor_presentation(identity)
  if identity == nil then
    self:_record("restore_cursor_presentation", { restored = false })
    return false
  end
  local saved = self._cursor_leases[identity]
  if saved == nil then
    error("cursor presentation lease is inactive", 2)
  end
  self._cursor_presentation = saved
  self._cursor_leases[identity] = nil
  self:_record("restore_cursor_presentation", {
    identity = identity,
    restored = true,
  })
  return true
end

function MemoryHost:cursor_presentation()
  return copy(self._cursor_presentation)
end

function MemoryHost:supports_timers()
  self:_record("supports_timers", { supported = self._timer_support })
  return self._timer_support
end

function MemoryHost:start_timer(delay_ms, callback)
  if not is_integer(delay_ms) or delay_ms < 0 then
    error("timer delay must be a nonnegative integer", 2)
  end
  if type(callback) ~= "function" then
    error("timer callback must be a function", 2)
  end
  if not self._timer_support then
    self:_record("start_timer", { delay_ms = delay_ms, supported = false })
    return nil
  end
  local identity = self:_next_identity("timer")
  self._timers[identity] = {
    identity = identity,
    delay_ms = delay_ms,
    callback = callback,
    active = true,
  }
  self:_record("start_timer", {
    identity = identity,
    delay_ms = delay_ms,
    supported = true,
  })
  return identity
end

function MemoryHost:stop_timer(identity)
  if type(identity) ~= "string" or identity == "" then
    error("timer identity must be a nonempty string", 2)
  end
  local timer = self._timers[identity]
  local stopped = timer ~= nil and timer.active
  if timer ~= nil then
    timer.active = false
  end
  self:_record("stop_timer", { identity = identity, stopped = stopped })
  return stopped
end

function MemoryHost:fire_timer(identity)
  local timer = self._timers[identity]
  if timer == nil then
    error("timer identity is unknown", 2)
  end
  if not timer.active then
    self:_record("ignore_timer", { identity = identity })
    return false
  end
  timer.active = false
  self:_record("fire_timer", { identity = identity })
  timer.callback(identity)
  return true
end

function MemoryHost:timers()
  local result = {}
  for identity, timer in pairs(self._timers) do
    result[identity] = {
      identity = identity,
      delay_ms = timer.delay_ms,
      active = timer.active,
    }
  end
  return result
end

function MemoryHost:register_events(event_names, callback, options)
  local names, name_set = normalize_event_names(event_names)
  if type(callback) ~= "function" then
    error("event callback must be a function", 2)
  end
  local identity = self:_next_identity("event-registration")
  self._event_registrations[identity] = {
    identity = identity,
    names = names,
    name_set = name_set,
    callback = callback,
    options = copy(options or {}),
    active = true,
  }
  self._event_registration_order[#self._event_registration_order + 1] = identity
  self:_record("register_events", {
    identity = identity,
    names = names,
    options = options or {},
  })
  return identity
end

function MemoryHost:remove_event_registration(identity)
  local registration = self._event_registrations[identity]
  local removed = registration ~= nil and registration.active
  if registration ~= nil then
    registration.active = false
  end
  self:_record("remove_event_registration", {
    identity = identity,
    removed = removed,
  })
  return removed
end

function MemoryHost:_deliver_event_now(name, payload)
  self:_record("event", { name = name, payload = payload })
  local order = list_copy(self._event_registration_order)
  for _, identity in ipairs(order) do
    local registration = self._event_registrations[identity]
    if registration.active and registration.name_set[name] then
      registration.callback(name, payload)
    end
  end
end

function MemoryHost:deliver_event(name, payload)
  if type(name) ~= "string" or name == "" then
    error("event name must be a nonempty string", 2)
  end
  payload = copy(payload or {})
  local queued = self._event_queue:is_transition_active()
  self:_record(queued and "queue_event" or "deliver_event", {
    name = name,
    payload = payload,
  })
  return self._event_queue:emit(name, payload)
end

function MemoryHost:begin_action_transition()
  local token = self._event_queue:begin_transition()
  self:_record("begin_action_transition", { identity = token })
  return token
end

function MemoryHost:commit_action_transition(token)
  self:_record("commit_action_transition", { identity = token })
  self._event_queue:commit_transition(token)
end

function MemoryHost:pending_event_count()
  return self._event_queue:pending_count()
end

function MemoryHost:event_registrations()
  local result = {}
  for identity, registration in pairs(self._event_registrations) do
    result[identity] = {
      identity = identity,
      names = list_copy(registration.names),
      options = copy(registration.options),
      active = registration.active,
    }
  end
  return result
end

function MemoryHost:register_action(name, callback)
  if type(name) ~= "string" or name == "" then
    error("action name must be a nonempty string", 2)
  end
  if type(callback) ~= "function" then
    error("action callback must be a function", 2)
  end
  if self._actions[name] ~= nil then
    error("action is already registered", 2)
  end
  self._actions[name] = callback
  self:_record("register_action", { name = name })
  return name
end

function MemoryHost:invoke_action(name, ...)
  local callback = self._actions[name]
  if callback == nil then
    error("action is not registered", 2)
  end
  local arguments = { ... }
  local argument_count = select("#", ...)
  local token = self:begin_action_transition()
  local results = {
    pcall(function()
      return callback(unpack_values(arguments, 1, argument_count))
    end),
  }
  self:commit_action_transition(token)
  local succeeded = table.remove(results, 1)
  if not succeeded then
    error(results[1], 0)
  end
  return unpack_values(results)
end

function MemoryHost:register_mapping(modes, lhs, action, options)
  modes = normalize_modes(modes)
  if type(lhs) ~= "string" or lhs == "" then
    error("mapping lhs must be a nonempty string", 2)
  end
  if type(action) ~= "string" and type(action) ~= "function" then
    error("mapping action must be an action name or function", 2)
  end
  local identity = self:_next_identity("mapping")
  self._mappings[identity] = {
    identity = identity,
    modes = modes,
    lhs = lhs,
    action = action,
    options = copy(options or {}),
  }
  self:_record("register_mapping", {
    identity = identity,
    modes = modes,
    lhs = lhs,
    action = type(action) == "string" and action or "<function>",
    options = options or {},
  })
  return identity
end

function MemoryHost:mappings()
  return copy(self._mappings)
end

function MemoryHost:register_dot_repeat(payload, callback)
  if not domain.DotPayload.is(payload) then
    error("dot-repeat payload must be a DotPayload", 2)
  end
  if callback ~= nil and type(callback) ~= "function" then
    error("dot-repeat callback must be a function", 2)
  end
  self._dot_repeat = {
    payload = payload,
    callback = callback,
    operator = self._pending_operator,
    mode = self._mode,
  }
  self:_record("register_dot_repeat", { payload = payload:to_table() })
  return payload
end

function MemoryHost:dot_repeat_payload()
  return self._dot_repeat and self._dot_repeat.payload or nil
end

function MemoryHost:replay_dot(count)
  if self._dot_repeat == nil or self._dot_repeat.callback == nil then
    error("dot repeat is not executable", 2)
  end
  self._pending_operator = self._dot_repeat.operator
  self._mode = self._dot_repeat.mode
  return self._dot_repeat.callback(self._dot_repeat.payload, domain.Count.new(count))
end

return M