defmodule Odinsea.World.Migration do
  @moduledoc """
  Character migration system for server transfers.
  Manages transfer tokens when moving between login/channel/cash shop servers.
  
  Ported from Java handling.world.CharacterTransfer and World.ChannelChange_Data
  
  In the Java version, this uses Redis for cross-server communication.
  """

  require Logger

  alias Odinsea.Database.Context

  @token_ttl 60  # Token expires after 60 seconds

  @doc """
  Creates a migration token for character transfer.
  
  Called when:
  - Player selects character (login -> channel)
  - Player changes channel (channel -> channel)
  - Player enters cash shop (channel -> cash shop)
  
  ## Parameters
  - character_id: Character ID being transferred
  - account_id: Account ID
  - source_server: :login, :channel, or :shop
  - target_channel: Target channel ID (or -10 for cash shop, -20 for MTS)
  - character_data: Optional character state to preserve during transfer
  
  ## Returns
  - {:ok, token_id} on success
  - {:error, reason} on failure
  """
  def create_migration_token(character_id, account_id, source_server, target_channel, character_data \\ %{}) do
    token_id = generate_token_id()
    
    token = %{
      id: token_id,
      character_id: character_id,
      account_id: account_id,
      source_server: source_server,
      target_channel: target_channel,
      character_data: character_data,
      created_at: System.system_time(:second),
      status: :pending
    }
    
    # Store in ETS (local cache)
    :ets.insert(:migration_tokens, {token_id, token})
    
    # Also store in Redis for cross-server visibility
    case store_in_redis(token_id, token) do
      :ok ->
        Logger.info("Created migration token #{token_id} for character #{character_id} to channel #{target_channel}")
        {:ok, token_id}
        
      {:error, reason} ->
        Logger.error("Failed to store migration token in Redis: #{inspect(reason)}")
        # Still return OK since ETS has it (for single-node deployments)
        {:ok, token_id}
    end
  end

  @doc """
  Validates a migration token when a character migrates into a server.
  
  Called by InterServerHandler.MigrateIn when player connects to channel/shop.
  
  ## Parameters
  - token_id: The migration token ID
  - character_id: Expected character ID
  - target_server: :channel or :shop
  
  ## Returns
  - {:ok, token} if valid
  - {:error, reason} if invalid/expired
  """
  def validate_migration_token(token_id, character_id, target_server) do
    # Try Redis first (for cross-server)
    token = 
      case get_from_redis(token_id) do
        {:ok, nil} ->
          # Try ETS (local fallback)
          get_from_ets(token_id)
          
        {:ok, token} ->
          token
          
        {:error, _} ->
          get_from_ets(token_id)
      end
    
    cond do
      is_nil(token) ->
        Logger.warning("Migration token #{token_id} not found")
        {:error, :token_not_found}
        
      token.character_id != character_id ->
        Logger.warning("Migration token character mismatch: expected #{character_id}, got #{token.character_id}")
        {:error, :character_mismatch}
        
      token_expired?(token) ->
        Logger.warning("Migration token #{token_id} expired")
        delete_token(token_id)
        {:error, :token_expired}
        
      token.status != :pending ->
        Logger.warning("Migration token #{token_id} already used")
        {:error, :token_already_used}
        
      true ->
        # Mark as used
        update_token_status(token_id, :used)
        {:ok, token}
    end
  end

  @doc """
  Gets pending character data for a migration.
  Returns nil if no pending migration exists.
  """
  def get_pending_character(character_id) do
    # Search for pending tokens for this character
    case :ets.select(:migration_tokens, [
      {{:_, %{character_id: character_id, status: :pending}}, [], [:_]}])
    do
      [{_key, token}] -> token
      [] -> nil
      _ -> nil
    end
  end

  @doc """
  Deletes a migration token.
  """
  def delete_token(token_id) do
    :ets.delete(:migration_tokens, token_id)
    delete_from_redis(token_id)
    :ok
  end

  @doc """
  Cleans up expired tokens.
  Should be called periodically.
  """
  def cleanup_expired_tokens do
    now = System.system_time(:second)
    
    expired = :ets.select(:migration_tokens, [
      {{:_, %{created_at: :"$1", id: :"$2"}},
       [{:<, {:+, :"$1", @token_ttl}, now}],
       [:"$2"]}
    ])
    
    Enum.each(expired, fn token_id ->
      Logger.debug("Cleaning up expired migration token #{token_id}")
      delete_token(token_id)
    end)
    
    length(expired)
  end

  @doc """
  Gets the count of pending migrations (for server capacity checking).
  """
  def pending_count do
    :ets.info(:migration_tokens, :size)
  end

  # ==================================================================================================
  # GenServer Callbacks for Token Cleanup
  # ==================================================================================================

  use GenServer

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  @impl true
  def init(_) do
    # Create ETS table for migration tokens
    :ets.new(:migration_tokens, [
      :set,
      :public,
      :named_table,
      read_concurrency: true,
      write_concurrency: true
    ])
    
    # Schedule cleanup every 30 seconds
    schedule_cleanup()
    
    {:ok, %{}}
  end

  @impl true
  def handle_info(:cleanup, state) do
    count = cleanup_expired_tokens()
    
    if count > 0 do
      Logger.debug("Cleaned up #{count} expired migration tokens")
    end
    
    schedule_cleanup()
    {:noreply, state}
  end

  defp schedule_cleanup do
    Process.send_after(self(), :cleanup, 30_000)
  end

  # ==================================================================================================
  # Private Functions
  # ==================================================================================================

  defp generate_token_id do
    :crypto.strong_rand_bytes(16)
    |> Base.encode16(case: :lower)
  end

  defp token_expired?(token) do
    now = System.system_time(:second)
    now > token.created_at + @token_ttl
  end

  defp update_token_status(token_id, status) do
    case :ets.lookup(:migration_tokens, token_id) do
      [{^token_id, token}] ->
        updated = %{token | status: status}
        :ets.insert(:migration_tokens, {token_id, updated})
        store_in_redis(token_id, updated)
        
      [] ->
        :ok
    end
  end

  defp get_from_ets(token_id) do
    case :ets.lookup(:migration_tokens, token_id) do
      [{^token_id, token}] -> token
      [] -> nil
    end
  end

  # Redis operations (for cross-server communication)
  
  defp store_in_redis(token_id, token) do
    case Redix.command(:redix, ["SETEX", "migration:#{token_id}", @token_ttl, :erlang.term_to_binary(token)]) do
      {:ok, _} -> :ok
      error -> error
    end
  rescue
    _ -> {:error, :redis_unavailable}
  end

  defp get_from_redis(token_id) do
    case Redix.command(:redix, ["GET", "migration:#{token_id}"]) do
      {:ok, nil} -> {:ok, nil}
      {:ok, data} -> {:ok, :erlang.binary_to_term(data)}
      error -> error
    end
  rescue
    _ -> {:error, :redis_unavailable}
  end

  defp delete_from_redis(token_id) do
    case Redix.command(:redix, ["DEL", "migration:#{token_id}"]) do
      {:ok, _} -> :ok
      error -> error
    end
  rescue
    _ -> {:ok}
  end
end