defmodule CPSolver.ConstraintStore do
  @moduledoc """
  Constraint store is a key-value store, where `key` is a variable id,
  and `value` is a implementation-dependent structure that allows to
  update and keep track of variables' domains.
  """
  #################
  alias CPSolver.{Common, Variable}
  alias CPSolver.DefaultDomain, as: Domain

  require Logger

  @type get_operation :: Common.domain_get_operation() | nil
  @type update_operation :: Common.domain_update_operation()

  @unfixed Common.unfixed()

  def default_store() do
    CPSolver.Store.ETS
  end

  ### Callbacks

  ## Tell basic constraints (a.k.a, domains) to a constraint store
  @callback create(variables :: Enum.t(), opts :: Keyword.t()) ::
              {:ok, any()} | {:error, any()}

  ## Get variable details
  @callback get(store :: any(), variable :: Variable.t(), get_operation(), [any()]) ::
              {:ok, any()} | {:error, any()}

  @callback update(store :: any(), variable :: Variable.t(), update_operation(), [any()]) ::
              any()

  @callback update_domain(store :: any(), variable :: Variable.t(), update_operation(), [any()]) ::
              any()

  @callback dispose(store :: any(), variables :: [Variable.t()]) :: :ok | :not_found

  @callback domain(store :: any(), variable :: Variable.t()) :: {:ok, any()} | {:error, any()}

  @callback on_fail(store :: any(), variable :: Variable.t()) :: any()

  @callback on_no_change(store :: any(), variable :: Variable.t()) :: any()

  @callback on_change(
              store :: any(),
              variable :: Variable.t(),
              change :: Common.domain_change()
            ) :: any()
  @callback on_fix(store :: any(), variable :: Variable.t(), value :: any()) :: any()

  @callback get_variables(store :: any()) :: [any()]

  ### API
  defmacro __using__(_) do
    quote do
      @behaviour CPSolver.ConstraintStore
      @domain_events CPSolver.Common.domain_events()
      alias CPSolver.ConstraintStore
      require Logger

      def update(store, variable, operation, args) do
        update_domain(store, variable, operation, args)
        |> tap(fn
          :fail ->
            on_fail(store, variable)

          {:fixed, value} ->
            on_fix(store, variable, value)

          :no_change ->
            on_no_change(store, variable)

          change when change in @domain_events ->
            on_change(store, variable, change)
        end)
      end

      defoverridable update: 4
    end
  end

  def default_store_opts() do
    [space: self(), store_impl: default_store()]
  end

  def create_store(variables, opts \\ [])

  def create_store(variables, opts) do
    opts = Keyword.merge(default_store_opts(), opts)
    space = Keyword.get(opts, :space)
    store_impl = Keyword.get(opts, :store_impl)
    {:ok, store_handle} = store_impl.create(variables, opts)

    fixed_variables_store = create_fixed_vars_store(variables)

    store = %{
      space: space,
      handle: store_handle,
      store_impl: store_impl,
      fixed_variables: fixed_variables_store
    }

    {:ok,
     variables
     |> Enum.with_index(1)
     |> Enum.map(fn {%{domain: domain} = var, index} = _indexed_var ->
       var
       |> Map.put(:index, index)
       |> Map.put(:name, var.name)
       |> Map.put(:store, store)
       |> Map.put(:fixed?, Domain.fixed?(domain))
       |> tap(fn v -> register_fixed(v) end)
     end), store}
  end

  def domain(variable) do
    domain(variable.store, variable)
  end

  def domain(%{handle: handle, store_impl: store_impl} = _store, variable) do
    store_impl.domain(handle, variable)
  end

  def get(%{handle: handle, store_impl: store_impl} = _store, variable, operation, args \\ []) do
    store_impl.get(handle, variable, operation, args)
  end

  def update(
        %{handle: handle, store_impl: store_impl} = _store,
        variable,
        operation,
        args \\ []
      ) do
    store_impl.update(handle, variable, operation, args)
    |> then(fn
      {:fixed, value} ->
        # :fail
        update_fixed(variable, value)

      result ->
        result
    end)
  end

  def get_variables(%{handle: handle, store_impl: store_impl} = _store) do
    store_impl.get_variables(handle)
  end

  def dispose(%{handle: handle, store_impl: store_impl} = _store, variables) do
    store_impl.dispose(handle, variables)
  end

  def variable_id(%Variable{id: id}) do
    id
  end

  def variable_id(id) do
    id
  end

  ## There is a possible race condition for the updates that fix a variable.
  ## It goes like this:
  ## Propagators P1 and P2 run concurrently,
  ## and the filtering for each of them results
  ## in fixing the same variable.
  ## Filter calls for both P1 and P2 read the domain of the variable,
  ## but the updates are unaware that the domain may have already been fixed by
  ## another propagator.
  ##
  ## The fix: use :atomics to enforce sequential operations when updating variables to
  ## :fixed state.
  ## In the scenario above, the code checks if the variable has already been fixed
  ## by looking up variable index in :atomics list.

  def create_fixed_vars_store(variables) do
    :atomics.new(length(variables), signed: true)
  end

  ## Note: if index is not supplied, this operation is not thread-safe
  def update_fixed(%{index: nil} = variable, fixed_value) do
    domain = domain(variable)
    (Domain.fixed?(domain) && Domain.min(domain) != fixed_value && :fail) || :fixed
  end

  def update_fixed(
        %{index: index, store: %{fixed_variables: fixed_vars}} = _variable,
        fixed_value
      ) do
    case :atomics.exchange(fixed_vars, index, fixed_value) do
      prev_value when prev_value == @unfixed -> :fixed
      prev_value when prev_value != fixed_value -> :fail
      _same -> :fixed
    end
  end

  def register_fixed(
        %{index: index, domain: domain, store: %{fixed_variables: fixed_vars}} = _variable
      ) do
    value = (Domain.fixed?(domain) && Domain.min(domain)) || @unfixed
    :atomics.put(fixed_vars, index, value)
  end

  def fixed?(%{store: %{fixed_variables: fixed_vars}, index: index} = _var) do
    :atomics.get(fixed_vars, index) != @unfixed
  end
end