defmodule DaProductApp.FeatureFlags do
  @moduledoc """
  Feature flag system for gradual migration and A/B testing.
  
  This module provides a centralized way to enable/disable features
  during development and production deployments, supporting the
  gradual migration strategy.
  """

  @doc """
  Check if a feature flag is enabled.
  
  Priority order:
  1. Environment variables (FEATURE_<FLAG_NAME>=true/false)
  2. Application config
  3. Database settings (when implemented)
  4. Default value
  """
  def enabled?(flag_name, default \\ false) do
    flag_name
    |> to_string()
    |> String.upcase()
    |> check_feature_flag(default)
  end

  @doc """
  Enable a feature flag programmatically.
  Useful for testing and dynamic feature control.
  """
  def enable(flag_name) do
    Application.put_env(:da_product_app, :feature_flags, 
      Keyword.put(get_feature_flags(), flag_name, true)
    )
  end

  @doc """
  Disable a feature flag programmatically.
  """
  def disable(flag_name) do
    Application.put_env(:da_product_app, :feature_flags, 
      Keyword.put(get_feature_flags(), flag_name, false)
    )
  end

  @doc """
  Get all currently configured feature flags.
  """
  def list_flags do
    get_feature_flags()
  end

  # Available feature flags for our gradual migration
  @available_flags %{
    # Data source flags
    use_database_settings: false,
    use_database_international_payments: false,
    use_database_settlements: false,
    
    # Performance flags  
    use_redis_caching: false,
    enable_transaction_type_filtering: true,
    
    # Audit and logging flags
    enable_api_request_logging: true,
    enable_response_logging: true,
    enable_log_viewer: false,
    
    # Fallback and reliability flags
    enable_database_fallback: true,
    enable_file_logging_on_db_failure: true,
    
    # UI and monitoring flags
    show_real_time_metrics: false,
    enable_advanced_monitoring: false
  }

  @doc """
  Get default configuration for all available flags.
  """
  def default_flags, do: @available_flags

  # Private functions

  defp check_feature_flag(flag_name, default) do
    # 1. Check environment variable first
    case System.get_env("FEATURE_#{flag_name}") do
      "true" -> true
      "false" -> false
      nil ->
        # 2. Check application config
        case get_feature_flags()[String.to_atom(String.downcase(flag_name))] do
          nil -> default
          value -> value
        end
      _ -> default
    end
  end

  defp get_feature_flags do
    Application.get_env(:da_product_app, :feature_flags, [])
  end
end