defmodule Raxol.Core.Session.SessionMigrator do
  @moduledoc """
  Handles session migration and failover scenarios in a distributed environment.

  The SessionMigrator provides capabilities for moving sessions between nodes,
  handling node failures, load balancing, and maintaining session continuity
  during cluster topology changes.

  ## Features

  - Live session migration with minimal downtime
  - Automatic failover on node failure
  - Load-based session redistribution
  - Rolling cluster updates with session preservation
  - Migration rollback and recovery
  - Performance impact monitoring during migrations

  ## Migration Strategies

  - **Hot Migration**: Zero-downtime migration using shadow copies
  - **Warm Migration**: Brief pause during migration commit
  - **Cold Migration**: Full session suspension during migration
  - **Bulk Migration**: Efficient batch migration for maintenance

  ## Failover Modes

  - **Immediate**: Instant failover to available replicas
  - **Graceful**: Coordinated failover with state synchronization
  - **Manual**: Administrator-controlled failover process

  ## Usage

      # Start migrator
      {:ok, pid} = SessionMigrator.start_link(
        failover_mode: :graceful,
        migration_batch_size: 50,
        max_concurrent_migrations: 5
      )

      # Migrate single session
      SessionMigrator.migrate_session(pid, session_id, target_node, :hot)

      # Migrate all sessions from a node
      SessionMigrator.evacuate_node(pid, source_node, [:target1, :target2])

      # Handle node failure
      SessionMigrator.handle_node_failure(pid, failed_node)
  """

  use Raxol.Core.Behaviours.BaseManager
  alias Raxol.Core.Runtime.Log

  alias Raxol.Core.Session.{
    DistributedSessionRegistry,
    DistributedSessionStorage
  }

  defstruct [
    :failover_mode,
    :migration_batch_size,
    :max_concurrent_migrations,
    :current_migrations,
    :migration_stats,
    :node_health,
    :failover_policies,
    :migration_history,
    :load_balancer_config
  ]

  @type migration_strategy :: :hot | :warm | :cold | :bulk
  @type failover_mode :: :immediate | :graceful | :manual
  @type migration_status ::
          :pending | :in_progress | :completed | :failed | :rolled_back

  @type migration_info :: %{
          session_id: binary(),
          source_node: node(),
          target_node: node(),
          strategy: migration_strategy(),
          status: migration_status(),
          started_at: DateTime.t(),
          completed_at: DateTime.t() | nil,
          rollback_data: term() | nil
        }

  @default_migration_batch_size 10
  @default_max_concurrent_migrations 3

  # Public API

  @spec migrate_session(pid(), binary(), node(), migration_strategy()) ::
          {:ok, migration_info()} | {:error, term()}
  def migrate_session(pid, session_id, target_node, strategy) do
    GenServer.call(pid, {:migrate_session, session_id, target_node, strategy})
  end

  @spec migrate_sessions_bulk(pid(), [binary()], node(), migration_strategy()) ::
          {:ok, [migration_info()]} | {:error, term()}
  def migrate_sessions_bulk(pid, session_ids, target_node, strategy) do
    GenServer.call(
      pid,
      {:migrate_sessions_bulk, session_ids, target_node, strategy},
      60_000
    )
  end

  @spec evacuate_node(pid(), node(), [node()]) ::
          {:ok, non_neg_integer()} | {:error, term()}
  def evacuate_node(pid, source_node, target_nodes) do
    GenServer.call(pid, {:evacuate_node, source_node, target_nodes}, 120_000)
  end

  @spec handle_node_failure(pid(), node()) :: {:ok, term()} | {:error, term()}
  def handle_node_failure(pid, failed_node) do
    GenServer.call(pid, {:handle_node_failure, failed_node})
  end

  @spec get_migration_status(pid(), binary()) ::
          {:ok, migration_info()} | {:error, term()}
  def get_migration_status(pid, session_id) do
    GenServer.call(pid, {:get_migration_status, session_id})
  end

  @spec list_active_migrations(pid()) :: {:ok, [migration_info()]}
  def list_active_migrations(pid) do
    GenServer.call(pid, :list_active_migrations)
  end

  @spec rollback_migration(pid(), binary()) :: {:ok, term()} | {:error, term()}
  def rollback_migration(pid, session_id) do
    GenServer.call(pid, {:rollback_migration, session_id})
  end

  @spec rebalance_sessions(pid(), map()) ::
          {:ok, non_neg_integer()} | {:error, term()}
  def rebalance_sessions(pid, rebalance_config) do
    GenServer.call(pid, {:rebalance_sessions, rebalance_config}, 300_000)
  end

  @spec get_migration_stats(pid()) :: %{
          total_migrations: non_neg_integer(),
          successful_migrations: non_neg_integer(),
          failed_migrations: non_neg_integer(),
          average_migration_time: float(),
          active_migrations: non_neg_integer()
        }
  def get_migration_stats(pid) do
    GenServer.call(pid, :get_migration_stats)
  end

  # BaseManager Callbacks

  @impl true
  def init_manager(opts) do
    state = %__MODULE__{
      failover_mode: Keyword.get(opts, :failover_mode, :graceful),
      migration_batch_size:
        Keyword.get(opts, :migration_batch_size, @default_migration_batch_size),
      max_concurrent_migrations:
        Keyword.get(
          opts,
          :max_concurrent_migrations,
          @default_max_concurrent_migrations
        ),
      current_migrations: %{},
      migration_stats: init_migration_stats(),
      node_health: %{},
      failover_policies:
        Keyword.get(opts, :failover_policies, default_failover_policies()),
      migration_history: [],
      load_balancer_config: Keyword.get(opts, :load_balancer_config, %{})
    }

    # Monitor cluster nodes
    :net_kernel.monitor_nodes(true)

    Log.info(
      "SessionMigrator started with failover_mode=#{state.failover_mode}"
    )

    {:ok, state}
  end

  @impl true
  def handle_call(
        {:migrate_session, session_id, target_node, strategy},
        _from,
        state
      ) do
    case can_start_migration?(state) do
      true ->
        case start_session_migration(session_id, target_node, strategy, state) do
          {:ok, migration_info, updated_state} ->
            {:reply, {:ok, migration_info}, updated_state}

          {:error, reason} = error ->
            Log.error(
              "Failed to start migration for session #{session_id}: #{inspect(reason)}"
            )

            {:reply, error, state}
        end

      false ->
        {:reply, {:error, :max_concurrent_migrations_exceeded}, state}
    end
  end

  @impl true
  def handle_call(
        {:migrate_sessions_bulk, session_ids, target_node, strategy},
        _from,
        state
      ) do
    case start_bulk_migration(session_ids, target_node, strategy, state) do
      {:ok, migration_infos, updated_state} ->
        {:reply, {:ok, migration_infos}, updated_state}

      {:error, _reason} = error ->
        {:reply, error, state}
    end
  end

  @impl true
  def handle_call({:evacuate_node, source_node, target_nodes}, _from, state) do
    case perform_node_evacuation(source_node, target_nodes, state) do
      {:ok, migrated_count, updated_state} ->
        {:reply, {:ok, migrated_count}, updated_state}

      {:error, _reason} = error ->
        {:reply, error, state}
    end
  end

  @impl true
  def handle_call({:handle_node_failure, failed_node}, _from, state) do
    case perform_failover(failed_node, state) do
      {:ok, failover_result, updated_state} ->
        {:reply, {:ok, failover_result}, updated_state}

      {:error, _reason} = error ->
        {:reply, error, state}
    end
  end

  @impl true
  def handle_call({:get_migration_status, session_id}, _from, state) do
    case Map.get(state.current_migrations, session_id) do
      nil -> {:reply, {:error, :not_found}, state}
      migration_info -> {:reply, {:ok, migration_info}, state}
    end
  end

  @impl true
  def handle_call(:list_active_migrations, _from, state) do
    active_migrations = Map.values(state.current_migrations)
    {:reply, {:ok, active_migrations}, state}
  end

  @impl true
  def handle_call({:rollback_migration, session_id}, _from, state) do
    case rollback_session_migration(session_id, state) do
      {:ok, updated_state} ->
        {:reply, :ok, updated_state}

      {:error, _reason} = error ->
        {:reply, error, state}
    end
  end

  @impl true
  def handle_call({:rebalance_sessions, rebalance_config}, _from, state) do
    case perform_session_rebalancing(rebalance_config, state) do
      {:ok, rebalanced_count, updated_state} ->
        {:reply, {:ok, rebalanced_count}, updated_state}

      {:error, _reason} = error ->
        {:reply, error, state}
    end
  end

  @impl true
  def handle_call(:get_migration_stats, _from, state) do
    {:reply, state.migration_stats, state}
  end

  @impl true
  def handle_info({:migration_completed, session_id, result}, state) do
    updated_state = handle_migration_completion(session_id, result, state)
    {:noreply, updated_state}
  end

  @impl true
  def handle_info({:migration_failed, session_id, reason}, state) do
    updated_state = handle_migration_failure(session_id, reason, state)
    {:noreply, updated_state}
  end

  @impl true
  def handle_info({:nodeup, node}, state) do
    Log.info("Node #{node} joined cluster")
    updated_health = Map.put(state.node_health, node, :healthy)
    {:noreply, %{state | node_health: updated_health}}
  end

  @impl true
  def handle_info({:nodedown, node}, state) do
    Log.warning("Node #{node} left cluster, initiating failover")

    case state.failover_mode do
      :immediate ->
        spawn(fn -> handle_node_failure(self(), node) end)

      :graceful ->
        Process.send_after(self(), {:delayed_failover, node}, 5000)

      :manual ->
        Log.info(
          "Manual failover mode - administrator intervention required for node #{node}"
        )
    end

    updated_health = Map.put(state.node_health, node, :failed)
    {:noreply, %{state | node_health: updated_health}}
  end

  @impl true
  def handle_info({:delayed_failover, node}, state) do
    case Map.get(state.node_health, node) do
      :failed ->
        spawn(fn -> handle_node_failure(self(), node) end)

      _ ->
        # Node recovered, no failover needed
        :ok
    end

    {:noreply, state}
  end

  # Private Implementation

  defp can_start_migration?(state) do
    map_size(state.current_migrations) < state.max_concurrent_migrations
  end

  defp start_session_migration(session_id, target_node, strategy, state) do
    # Determine source node
    case DistributedSessionRegistry.locate_session(session_id) do
      {:ok, source_node} when source_node != target_node ->
        migration_info = %{
          session_id: session_id,
          source_node: source_node,
          target_node: target_node,
          strategy: strategy,
          status: :pending,
          started_at: DateTime.utc_now(),
          completed_at: nil,
          rollback_data: nil
        }

        # Start migration process
        case perform_migration(migration_info, state) do
          {:ok, updated_migration_info} ->
            updated_migrations =
              Map.put(
                state.current_migrations,
                session_id,
                updated_migration_info
              )

            updated_state = %{state | current_migrations: updated_migrations}

            {:ok, updated_migration_info, updated_state}

          {:error, reason} ->
            {:error, reason}
        end

      {:ok, source_node} when source_node == target_node ->
        {:error, :session_already_on_target_node}

      {:error, reason} ->
        {:error, {:session_location_failed, reason}}
    end
  end

  defp perform_migration(migration_info, _state) do
    %{
      session_id: session_id,
      source_node: source_node,
      target_node: target_node,
      strategy: strategy
    } = migration_info

    case strategy do
      :hot ->
        perform_hot_migration(
          session_id,
          source_node,
          target_node,
          migration_info
        )

      :warm ->
        perform_warm_migration(
          session_id,
          source_node,
          target_node,
          migration_info
        )

      :cold ->
        perform_cold_migration(
          session_id,
          source_node,
          target_node,
          migration_info
        )

      :bulk ->
        perform_bulk_migration(
          [session_id],
          source_node,
          target_node,
          migration_info
        )
    end
  end

  defp perform_hot_migration(
         session_id,
         source_node,
         target_node,
         migration_info
       ) do
    # Hot migration: Create shadow copy, sync, then switch
    parent = self()

    spawn_link(fn ->
      try do
        # Step 1: Get session data from source
        case :rpc.call(source_node, DistributedSessionStorage, :get, [
               DistributedSessionStorage,
               session_id
             ]) do
          {:ok, session_data} ->
            # Step 2: Create shadow copy on target
            case :rpc.call(target_node, DistributedSessionStorage, :store, [
                   DistributedSessionStorage,
                   session_id,
                   session_data
                 ]) do
              :ok ->
                # Step 3: Update registry to point to target
                case DistributedSessionRegistry.migrate_session(
                       session_id,
                       target_node
                     ) do
                  :ok ->
                    # Step 4: Remove from source
                    :rpc.call(source_node, DistributedSessionStorage, :delete, [
                      DistributedSessionStorage,
                      session_id
                    ])

                    send(
                      parent,
                      {:migration_completed, session_id, :hot_migration_success}
                    )

                  {:error, reason} ->
                    # Cleanup shadow copy
                    :rpc.call(target_node, DistributedSessionStorage, :delete, [
                      DistributedSessionStorage,
                      session_id
                    ])

                    send(
                      parent,
                      {:migration_failed, session_id,
                       {:registry_update_failed, reason}}
                    )
                end

              {:error, reason} ->
                send(
                  parent,
                  {:migration_failed, session_id,
                   {:target_store_failed, reason}}
                )
            end

          {:error, reason} ->
            send(
              parent,
              {:migration_failed, session_id, {:source_fetch_failed, reason}}
            )
        end
      rescue
        error ->
          send(
            parent,
            {:migration_failed, session_id, {:migration_exception, error}}
          )
      end
    end)

    updated_migration_info = %{migration_info | status: :in_progress}
    {:ok, updated_migration_info}
  end

  defp perform_warm_migration(
         session_id,
         source_node,
         target_node,
         migration_info
       ) do
    # Warm migration: Brief pause during migration
    parent = self()

    spawn_link(fn ->
      try do
        # Step 1: Pause session operations
        case :rpc.call(
               source_node,
               DistributedSessionRegistry,
               :pause_session,
               [session_id]
             ) do
          :ok ->
            # Step 2: Get session data
            case :rpc.call(source_node, DistributedSessionStorage, :get, [
                   DistributedSessionStorage,
                   session_id
                 ]) do
              {:ok, session_data} ->
                # Step 3: Store on target
                case :rpc.call(target_node, DistributedSessionStorage, :store, [
                       DistributedSessionStorage,
                       session_id,
                       session_data
                     ]) do
                  :ok ->
                    # Step 4: Update registry
                    case DistributedSessionRegistry.migrate_session(
                           session_id,
                           target_node
                         ) do
                      :ok ->
                        # Step 5: Resume operations on target
                        :rpc.call(
                          target_node,
                          DistributedSessionRegistry,
                          :resume_session,
                          [session_id]
                        )

                        # Step 6: Cleanup source
                        :rpc.call(
                          source_node,
                          DistributedSessionStorage,
                          :delete,
                          [DistributedSessionStorage, session_id]
                        )

                        send(
                          parent,
                          {:migration_completed, session_id,
                           :warm_migration_success}
                        )

                      {:error, reason} ->
                        :rpc.call(
                          source_node,
                          DistributedSessionRegistry,
                          :resume_session,
                          [session_id]
                        )

                        send(
                          parent,
                          {:migration_failed, session_id,
                           {:registry_update_failed, reason}}
                        )
                    end

                  {:error, reason} ->
                    :rpc.call(
                      source_node,
                      DistributedSessionRegistry,
                      :resume_session,
                      [session_id]
                    )

                    send(
                      parent,
                      {:migration_failed, session_id,
                       {:target_store_failed, reason}}
                    )
                end

              {:error, reason} ->
                :rpc.call(
                  source_node,
                  DistributedSessionRegistry,
                  :resume_session,
                  [session_id]
                )

                send(
                  parent,
                  {:migration_failed, session_id,
                   {:source_fetch_failed, reason}}
                )
            end

          {:error, reason} ->
            send(
              parent,
              {:migration_failed, session_id, {:session_pause_failed, reason}}
            )
        end
      rescue
        error ->
          send(
            parent,
            {:migration_failed, session_id, {:migration_exception, error}}
          )
      end
    end)

    updated_migration_info = %{migration_info | status: :in_progress}
    {:ok, updated_migration_info}
  end

  defp perform_cold_migration(
         session_id,
         source_node,
         target_node,
         migration_info
       ) do
    # Cold migration: Full suspension during migration
    parent = self()

    spawn_link(fn ->
      try do
        # Step 1: Suspend session completely
        case :rpc.call(
               source_node,
               DistributedSessionRegistry,
               :suspend_session,
               [session_id]
             ) do
          :ok ->
            # Step 2-6: Same as warm migration but with full suspension
            case migrate_session_data(session_id, source_node, target_node) do
              :ok ->
                send(
                  parent,
                  {:migration_completed, session_id, :cold_migration_success}
                )

              {:error, reason} ->
                :rpc.call(
                  source_node,
                  DistributedSessionRegistry,
                  :resume_session,
                  [session_id]
                )

                send(parent, {:migration_failed, session_id, reason})
            end

          {:error, reason} ->
            send(
              parent,
              {:migration_failed, session_id, {:session_suspend_failed, reason}}
            )
        end
      rescue
        error ->
          send(
            parent,
            {:migration_failed, session_id, {:migration_exception, error}}
          )
      end
    end)

    updated_migration_info = %{migration_info | status: :in_progress}
    {:ok, updated_migration_info}
  end

  defp perform_bulk_migration(
         session_ids,
         source_node,
         target_node,
         migration_info
       )
       when is_list(session_ids) do
    # Bulk migration: Efficient batch processing
    parent = self()

    spawn_link(fn ->
      try do
        results =
          Enum.map(session_ids, fn session_id ->
            case migrate_session_data(session_id, source_node, target_node) do
              :ok -> {:ok, session_id}
              {:error, reason} -> {:error, session_id, reason}
            end
          end)

        case Enum.split_with(results, &match?({:ok, _}, &1)) do
          {successes, []} ->
            send(
              parent,
              {:migration_completed, migration_info.session_id,
               {:bulk_migration_success, length(successes)}}
            )

          {successes, failures} ->
            send(
              parent,
              {:migration_completed, migration_info.session_id,
               {:bulk_migration_partial, length(successes), failures}}
            )
        end
      rescue
        error ->
          send(
            parent,
            {:migration_failed, migration_info.session_id,
             {:bulk_migration_exception, error}}
          )
      end
    end)

    updated_migration_info = %{migration_info | status: :in_progress}
    {:ok, updated_migration_info}
  end

  defp migrate_session_data(session_id, source_node, target_node) do
    with {:ok, session_data} <-
           :rpc.call(source_node, DistributedSessionStorage, :get, [
             DistributedSessionStorage,
             session_id
           ]),
         :ok <-
           :rpc.call(target_node, DistributedSessionStorage, :store, [
             DistributedSessionStorage,
             session_id,
             session_data
           ]),
         :ok <-
           DistributedSessionRegistry.migrate_session(session_id, target_node),
         :ok <-
           :rpc.call(source_node, DistributedSessionStorage, :delete, [
             DistributedSessionStorage,
             session_id
           ]) do
      :ok
    else
      {:error, reason} -> {:error, reason}
      {:badrpc, reason} -> {:error, {:rpc_failed, reason}}
    end
  end

  defp start_bulk_migration(session_ids, target_node, strategy, state) do
    # Group sessions by source node for efficient migration
    session_groups =
      Enum.group_by(session_ids, fn session_id ->
        case DistributedSessionRegistry.locate_session(session_id) do
          {:ok, source_node} -> source_node
          {:error, _} -> :unknown
        end
      end)

    migration_infos =
      for {source_node, group_session_ids} <- session_groups,
          source_node != :unknown,
          session_id <- group_session_ids do
        %{
          session_id: session_id,
          source_node: source_node,
          target_node: target_node,
          strategy: strategy,
          status: :pending,
          started_at: DateTime.utc_now(),
          completed_at: nil,
          rollback_data: nil
        }
      end

    # Start migrations
    updated_migrations =
      Enum.reduce(migration_infos, state.current_migrations, fn migration_info,
                                                                acc ->
        Map.put(acc, migration_info.session_id, migration_info)
      end)

    updated_state = %{state | current_migrations: updated_migrations}

    # Start bulk migration process
    spawn(fn ->
      Enum.each(migration_infos, fn migration_info ->
        perform_migration(migration_info, state)
      end)
    end)

    {:ok, migration_infos, updated_state}
  end

  defp perform_node_evacuation(source_node, target_nodes, state) do
    # Get all sessions on the source node
    case :rpc.call(source_node, DistributedSessionStorage, :list_sessions, [
           DistributedSessionStorage
         ]) do
      {:ok, session_ids} ->
        # Distribute sessions across target nodes using round-robin
        session_distribution =
          distribute_sessions_round_robin(session_ids, target_nodes)

        migration_count =
          Enum.reduce(session_distribution, 0, fn {target_node, sessions},
                                                  acc ->
            case migrate_sessions_bulk(self(), sessions, target_node, :warm) do
              {:ok, _migration_infos} -> acc + length(sessions)
              {:error, _reason} -> acc
            end
          end)

        {:ok, migration_count, state}

      {:error, reason} ->
        {:error, {:session_list_failed, reason}}

      {:badrpc, reason} ->
        {:error, {:rpc_failed, reason}}
    end
  end

  defp perform_failover(failed_node, state) do
    Log.warning("Performing failover for failed node: #{failed_node}")

    # Get sessions that were on the failed node from replicas
    case find_sessions_on_failed_node(failed_node) do
      {:ok, [_ | _] = session_ids} ->
        # Find healthy nodes for failover
        healthy_nodes = get_healthy_nodes(state)

        case healthy_nodes do
          [] ->
            {:error, :no_healthy_nodes_available}

          nodes ->
            # Failover sessions to healthy nodes
            failover_count = perform_session_failover(session_ids, nodes, state)

            Log.info(
              "Completed failover for #{failover_count} sessions from node #{failed_node}"
            )

            failover_result = %{
              failed_node: failed_node,
              failover_targets: nodes,
              sessions_migrated: failover_count
            }

            {:ok, failover_result, state}
        end

      {:ok, []} ->
        Log.info("No sessions found on failed node #{failed_node}")
        {:ok, %{failed_node: failed_node, sessions_migrated: 0}, state}

      {:error, reason} ->
        {:error, {:failover_discovery_failed, reason}}
    end
  end

  # Helper Functions

  defp init_migration_stats do
    %{
      total_migrations: 0,
      successful_migrations: 0,
      failed_migrations: 0,
      average_migration_time: 0.0,
      active_migrations: 0
    }
  end

  defp default_failover_policies do
    %{
      max_failover_attempts: 3,
      failover_timeout: 30_000,
      prefer_local_replicas: true,
      load_balance_failover: true
    }
  end

  defp handle_migration_completion(session_id, result, state) do
    case Map.get(state.current_migrations, session_id) do
      nil ->
        state

      migration_info ->
        completed_migration = %{
          migration_info
          | status: :completed,
            completed_at: DateTime.utc_now()
        }

        updated_migrations = Map.delete(state.current_migrations, session_id)
        updated_history = [completed_migration | state.migration_history]

        updated_stats =
          update_migration_stats(
            state.migration_stats,
            :success,
            completed_migration
          )

        Log.debug(
          "Migration completed for session #{session_id}: #{inspect(result)}"
        )

        %{
          state
          | current_migrations: updated_migrations,
            # Keep last 1000 migrations
            migration_history: Enum.take(updated_history, 1000),
            migration_stats: updated_stats
        }
    end
  end

  defp handle_migration_failure(session_id, reason, state) do
    case Map.get(state.current_migrations, session_id) do
      nil ->
        state

      migration_info ->
        failed_migration = %{
          migration_info
          | status: :failed,
            completed_at: DateTime.utc_now()
        }

        updated_migrations = Map.delete(state.current_migrations, session_id)
        updated_history = [failed_migration | state.migration_history]

        updated_stats =
          update_migration_stats(
            state.migration_stats,
            :failure,
            failed_migration
          )

        Log.error(
          "Migration failed for session #{session_id}: #{inspect(reason)}"
        )

        %{
          state
          | current_migrations: updated_migrations,
            migration_history: Enum.take(updated_history, 1000),
            migration_stats: updated_stats
        }
    end
  end

  defp distribute_sessions_round_robin(session_ids, target_nodes) do
    session_ids
    |> Enum.with_index()
    |> Enum.group_by(
      fn {_session_id, index} ->
        Enum.at(target_nodes, rem(index, length(target_nodes)))
      end,
      fn {session_id, _index} -> session_id end
    )
  end

  defp find_sessions_on_failed_node(_failed_node) do
    # Implementation would query replicas to find sessions that were on the failed node
    {:ok, []}
  end

  defp get_healthy_nodes(state) do
    Enum.filter(Map.keys(state.node_health), fn node ->
      Map.get(state.node_health, node) == :healthy
    end)
  end

  defp perform_session_failover(_session_ids, _target_nodes, _state) do
    # Implementation would perform actual failover
    0
  end

  defp rollback_session_migration(_session_id, state) do
    # Implementation would rollback a migration using stored rollback data
    {:ok, state}
  end

  defp perform_session_rebalancing(_rebalance_config, state) do
    # Implementation would rebalance sessions across nodes based on load
    {:ok, 0, state}
  end

  defp update_migration_stats(stats, result, migration_info) do
    migration_time =
      case migration_info.completed_at do
        nil ->
          0

        completed_at ->
          DateTime.diff(completed_at, migration_info.started_at, :millisecond)
      end

    case result do
      :success ->
        total = stats.total_migrations + 1
        successful = stats.successful_migrations + 1

        new_avg =
          (stats.average_migration_time * stats.total_migrations +
             migration_time) / total

        %{
          stats
          | total_migrations: total,
            successful_migrations: successful,
            average_migration_time: new_avg
        }

      :failure ->
        %{
          stats
          | total_migrations: stats.total_migrations + 1,
            failed_migrations: stats.failed_migrations + 1
        }
    end
  end
end
