defmodule CPSolver.Space do
  @moduledoc """
  Computation space.
  The concept is taken from Chapter 12, "Concepts, Techniques, and Models
  of Computer Programming" by Peter Van Roy and Seif Haridi.
  """

  alias CPSolver.Variable.Interface
  alias CPSolver.DefaultDomain, as: Domain
  alias CPSolver.Search, as: Search
  alias CPSolver.Solution, as: Solution
  alias CPSolver.Propagator.ConstraintGraph
  alias CPSolver.Propagator
  alias CPSolver.Space.Propagation
  alias CPSolver.Objective

  alias CPSolver.Shared
  alias CPSolver.Distributed
  alias CPSolver.Utils

  require Logger

  @behaviour GenServer

  def default_space_opts() do
    [
      solution_handler: Solution.default_handler(),
      search: Search.default_strategy(),
      space_threads: :erlang.system_info(:logical_processors),
      postpone: false,
      distributed: false
    ]
  end

  ## Top space creation
  def create(variables, propagators, space_opts \\ default_space_opts()) do
    propagators =
      propagators
      |> assign_unique_ids()
      |> maybe_add_objective_propagator(space_opts[:objective])

    initial_constraint_graph = ConstraintGraph.create(propagators)

    space_data = %{
      parent_id: nil,
      id: make_ref(),
      variables: variables,
      propagators: propagators,
      constraint_graph: initial_constraint_graph,
      opts: space_opts
    }

    search = initialize_search(space_opts[:search], space_data)
    ## Save initial constraint graph in shared data
    ## (for shared search strategies etc.)
    create(
      space_data
      |> Map.put(:search, search)
      |> put_shared(:initial_constraint_graph, initial_constraint_graph)
    )
    |> tap(fn {:ok, space_pid} ->
      shared = get_shared(space_data)
      Shared.increment_node_counts(shared)
      Shared.add_active_spaces(shared, [space_pid])
    end)
  end

  ## Child space creation
  def create(data) do
    GenServer.start(__MODULE__, data)
  end

  defp initialize_search(search, space_data) do
    Search.initialize(search, space_data)
  end

  defp assign_unique_ids(propagators) do
    Enum.map(propagators, fn p -> Map.put(p, :id, make_ref()) end)
  end

  defp maybe_add_objective_propagator(propagators, nil) do
    propagators
  end

  defp maybe_add_objective_propagator(propagators, objective) do
    [objective.propagator | propagators]
  end

  def start_propagation(space_pid) when is_pid(space_pid) do
    try do
      :done = GenServer.call(space_pid, :propagate, :infinity)
    catch
      :exit, {:normal, {GenServer, :call, _}} = _reason ->
        :ignore
    end
  end

  defp spawn_space(data) do
    solver = get_shared(data)
    worker_node = Distributed.choose_worker_node(solver.distributed)
    checked_out? = Shared.checkout_space_thread(solver, worker_node)
    run_space(worker_node, solver, data, checked_out?)
  end

  def run_space(worker_node, solver, data, checked_out?) do
    Shared.increment_node_counts(solver)

    (worker_node == Node.self() &&
       run_space(data, checked_out?)) ||
      :erpc.call(worker_node, __MODULE__, :run_space, [prepare_remote(data), checked_out?])
  end

  def run_space(data, checked_out?) do
    (checked_out? &&
       spawn(fn ->
         run_space(data)
         Shared.checkin_space_thread(get_shared(data))
       end)) ||
      run_space(data)
  end

  def run_space(data) do
    solver = get_shared(data)

    case create(
           data
           |> Map.put(:opts, Keyword.put(data.opts, :postpone, true))
         ) do
      {:ok, space_pid} ->
        Shared.add_active_spaces(solver, [space_pid])
        start_propagation(space_pid)

      {:error, _} ->
        :ignore
    end
  end

  ## Prepare local data to be used on remote node
  ## Currently we add the raw domain values to the opts,
  ## so the domains could be rebuilt on the remote nodes
  defp prepare_remote(data) do
    data
    |> Map.put(
      :domains,
      Map.new(data.variables, fn var ->
        {Interface.id(var), Utils.domain_values(var)}
      end)
    )
  end

  @impl true
  def init(%{domains: domains, variables: variables} = data) do
    updated_variables =
      Enum.map(variables, fn var ->
        domain = Map.get(domains, Interface.id(var))
        Map.put(var, :domain, Domain.new(domain))
      end)

    data
    |> Map.put(:variables, updated_variables)
    |> init_impl()
  end

  def init(data) do
    init_impl(data)
  end

  defp init_impl(%{variables: variables, opts: space_opts, constraint_graph: graph} = data) do
    space_data =
      data
      |> Map.put(:constraint_graph, update_constraint_graph(graph, variables))
      |> Map.put(:objective, update_objective(space_opts[:objective], variables))
      |> Map.put(:changes, Keyword.get(space_opts, :branch_constraint, %{}))

    (space_opts[:postpone] &&
       {:ok, space_data}) || {:ok, space_data, {:continue, :propagate}}
  end

  @impl true
  def handle_continue(:propagate, data) do
    (data.opts[:postpone] && {:noreply, data}) ||
      data
      |> propagate()
      |> tap(fn _ ->
        caller = Map.get(data, :caller)
        caller && GenServer.reply(caller, :done)
      end)
  end

  @impl true
  def handle_call(:propagate, caller, data) do
    propagate(Map.put(data, :caller, caller))
  end

  defp propagate(
         %{
           constraint_graph: constraint_graph,
           changes: changes
         } =
           data
       ) do
    try do
      case Propagation.run(constraint_graph, changes) do
        {:fail, _propagator_id} = failure ->
          handle_failure(data, failure)

        :solved ->
          handle_solved(data)

        {:stable, reduced_constraint_graph} ->
          Map.put(
            data,
            :constraint_graph,
            reduced_constraint_graph
          )
          |> handle_stable()
      end
    catch
      {:error, error} ->
        handle_error(error, data)
    end
  end

  defp handle_failure(data, {:fail, _p_id} = failure) do
    Shared.add_failure(get_shared(data), failure)
    shutdown(data, :failure)
  end

  defp handle_solved(data) do
    case checkpoint(data.propagators, data.constraint_graph) do
      {:fail, _propagator_id} = failure ->
        handle_failure(data, failure)

      :ok ->
        maybe_tighten_objective_bound(data[:objective])

        ## Generate solutions and run them through solution handler.
        solutions(data)

        shutdown(data, :solved)
    end
  end

  defp handle_error(exception, data) do
    Logger.error(inspect(exception))
    Shared.set_complete(get_shared(data))
    shutdown(data, :error)
  end

  defp solutions(%{variables: variables} = data) do
    try do
      Enum.map(variables, fn var ->
        Utils.domain_values(var)
      end)
      |> Utils.lazy_cartesian(fn values ->
        Enum.reduce(values, {0, Map.new()}, fn val, {idx_acc, map_acc} ->
          {idx_acc + 1, Map.put(map_acc, Arrays.get(variables, idx_acc).name, val)}
        end)
        |> elem(1)
        |> Solution.run_handler(data.opts[:solution_handler])
        ## Stop producing solutions if the solving is complete
        |> tap(fn _ -> CPSolver.complete?(get_shared(data)) && throw(:complete) end)
      end)
    catch
      :complete -> :complete
    end
  end

  defp maybe_tighten_objective_bound(nil) do
    :ok
  end

  defp maybe_tighten_objective_bound(objective) do
    Objective.tighten(objective)
  end

  defp update_constraint_graph(graph, variables) do
    graph
    |> ConstraintGraph.copy()
    |> ConstraintGraph.update(variables)
  end

  defp update_objective(nil, _vars) do
    nil
  end

  defp update_objective(%{variable: variable} = objective, variables) do
    updated_var = update_domain(variable, variables)
    Map.put(objective, :variable, updated_var)
  end

  defp update_domain(variable, space_variables) do
    var_domain =
      Arrays.get(space_variables, Interface.variable(variable).index - 1)
      |> Interface.domain()

    Interface.update(variable, :domain, var_domain)
  end

  def checkpoint(propagators, constraint_graph) do
    Enum.reduce_while(propagators, :ok, fn p, acc ->
        bound_p = Propagator.bind(p, constraint_graph, :domain)

        case Propagator.filter(bound_p, reset: false, constraint_graph: constraint_graph) do
          :fail -> {:halt, {:fail, p.id}}
          _ -> {:cont, acc}
        end
    end)

    # :ok
  end

  defp handle_stable(data) do
    distribute(data)
  end

  def distribute(
        %{
          id: id,
          variables: variables,
          constraint_graph: _graph,
          search: search
        } = data
      ) do
    try do
      variables
      |> Search.branch(search, data)
      |> Enum.take_while(fn {branch_variables, constraint} ->
        !CPSolver.complete?(get_shared(data)) &&
          spawn_space(
            data
            |> Map.put(:parent_id, id)
            |> Map.put(:id, make_ref())
            |> Map.put(:variables, branch_variables)
            |> put_in([:opts, :branch_constraint], constraint)
          )
      end)

      shutdown(data, :distribute)
    catch
      :all_vars_fixed ->
        handle_solved(data)
    end
  end

  defp shutdown(data, reason) do
    {:stop, :normal, (!data[:finalized] && finalize(data, reason)) || data}
  end

  def get_shared(data) do
    data.opts[:solver_data]
  end

  def put_shared(data, key, value) do
    data
    |> tap(fn _ -> Shared.put_auxillary(get_in(data, [:opts, :solver_data]), key, value) end)
  end

  defp finalize(data, reason) do
    caller = data[:caller]
    caller && GenServer.reply(caller, :done)

    Map.put(data, :finalized, true)
    |> tap(fn _ -> Shared.finalize_space(get_shared(data), data, self(), reason) end)
  end

end