defmodule ConfigCat.Rollout do
  @moduledoc false

  alias ConfigCat.Config
  alias ConfigCat.Config.ComparisonContext
  alias ConfigCat.Config.Condition
  alias ConfigCat.Config.PercentageOption
  alias ConfigCat.Config.PrerequisiteFlagComparator
  alias ConfigCat.Config.PrerequisiteFlagCondition
  alias ConfigCat.Config.Segment
  alias ConfigCat.Config.SegmentComparator
  alias ConfigCat.Config.SegmentCondition
  alias ConfigCat.Config.Setting
  alias ConfigCat.Config.SettingType
  alias ConfigCat.Config.TargetingRule
  alias ConfigCat.Config.UserComparator
  alias ConfigCat.Config.UserCondition
  alias ConfigCat.EvaluationDetails
  alias ConfigCat.EvaluationLogger
  alias ConfigCat.EvaluationWarnings
  alias ConfigCat.User

  require ConfigCat.ConfigCatLogger, as: ConfigCatLogger

  defmodule CircularDependencyError do
    @moduledoc false
    @enforce_keys [:prerequisite_key, :visited_keys]
    defexception [:prerequisite_key, :visited_keys]

    @type option :: {:prerequisite_key, String.t()} | {:visited_keys, [String.t()]}
    @type t :: %__MODULE__{
            prerequisite_key: String.t(),
            visited_keys: [String.t()]
          }

    @impl Exception
    def exception(options) do
      struct!(__MODULE__, options)
    end

    @impl Exception
    def message(%__MODULE__{} = error) do
      depending_flags =
        [error.prerequisite_key | error.visited_keys]
        |> Enum.reverse()
        |> Enum.map_join(" -> ", &"'#{&1}'")

      "Circular dependency detected between the following depending flags: #{depending_flags}"
    end
  end

  defmodule EvaluationError do
    @moduledoc false
    @enforce_keys [:message]
    defexception [:message]

    @type t :: %__MODULE__{
            message: String.t()
          }
  end

  defmodule Context do
    @moduledoc false
    use TypedStruct

    alias ConfigCat.Config.SettingType

    typedstruct enforce: true do
      field :config, Config.t()
      field :default_value, Config.value() | nil
      field :default_variation_id, Config.variation_id() | nil
      field :key, Config.key()
      field :logger, pid() | nil
      field :percentage_option_attribute, String.t()
      field :salt, Config.salt()
      field :setting_type, SettingType.t()
      field :user, User.t(), enforce: false
      field :visited_keys, [String.t()]
      field :warnings, pid()
    end
  end

  @default_percentage_option_attribute "Identifier"
  @missing_user_error "cannot evaluate, User Object is missing"

  @spec evaluate(
          Config.key(),
          User.t() | nil,
          Config.value() | nil,
          Config.variation_id() | nil,
          Config.t(),
          pid() | nil,
          [String.t()]
        ) :: EvaluationDetails.t()
  def evaluate(key, user, default_value, default_variation_id, config, logger \\ nil, visited_keys \\ []) do
    settings = Config.settings(config)

    case setting(settings, key, default_value) do
      {:ok, setting} ->
        {:ok, warnings} = EvaluationWarnings.start()

        try do
          validated_user =
            case user do
              nil ->
                nil

              %User{} = user ->
                user

              _ ->
                EvaluationWarnings.warn_invalid_user(warnings, key)
                nil
            end

          context = %Context{
            config: config,
            default_value: default_value,
            default_variation_id: default_variation_id,
            key: key,
            logger: logger,
            percentage_option_attribute:
              Setting.percentage_option_attribute(setting) || @default_percentage_option_attribute,
            salt: Setting.salt(setting),
            setting_type: Setting.setting_type(setting),
            user: validated_user,
            visited_keys: visited_keys,
            warnings: warnings
          }

          evaluate_setting(setting, context)
        after
          EvaluationWarnings.stop(warnings)
        end

      {:error, message} ->
        ConfigCatLogger.error(message, event_id: 1001)

        EvaluationDetails.new(
          default_value?: true,
          error: message,
          key: key,
          value: default_value,
          variation_id: default_variation_id
        )
    end
  end

  defp evaluate_setting(setting, %Context{} = context) do
    %Context{
      default_value: default_value,
      default_variation_id: default_variation_id,
      key: key,
      logger: logger,
      user: user,
      visited_keys: visited_keys
    } = context

    root_flag_evaluation? = visited_keys == []
    percentage_options = Setting.percentage_options(setting)
    targeting_rules = Setting.targeting_rules(setting)

    try do
      if root_flag_evaluation? do
        logger
        |> EvaluationLogger.log_evaluating(key, user)
        |> EvaluationLogger.increase_indent()
      end

      case evaluate_rules(targeting_rules, percentage_options, context) do
        {:none, _variation_id, _matching_rule, _matching_option} ->
          value = Setting.value(setting)

          if root_flag_evaluation? do
            EvaluationLogger.log_return_value(logger, value)
          end

          EvaluationDetails.new(
            key: key,
            user: user,
            value: value,
            variation_id: Setting.variation_id(setting, default_variation_id)
          )

        {value, variation_id, rule, percentage_option} ->
          if root_flag_evaluation? do
            EvaluationLogger.log_return_value(logger, value)
          end

          EvaluationDetails.new(
            key: key,
            matched_targeting_rule: rule,
            matched_percentage_option: percentage_option,
            user: user,
            value: value,
            variation_id: variation_id
          )
      end
    rescue
      error ->
        if root_flag_evaluation? do
          message =
            "Failed to evaluate setting '#{key}'. (#{Exception.message(error)}). " <>
              "Returning the default_value parameter that you specified in your application: '#{default_value}'."

          ConfigCatLogger.error(message, event_id: 2001)

          EvaluationDetails.new(
            default_value?: true,
            error: message,
            key: key,
            value: default_value,
            variation_id: default_variation_id
          )
        else
          reraise(error, __STACKTRACE__)
        end
    end
  end

  defp setting(settings, key, default_value) do
    case Map.fetch(settings, key) do
      {:ok, setting} ->
        {:ok, setting}

      :error ->
        available_keys =
          settings
          |> Map.keys()
          |> Enum.map_join(", ", &"'#{&1}'")

        message =
          "Failed to evaluate setting '#{key}' (the key was not found in config JSON). " <>
            "Returning the `default_value` parameter that you specified in your application: '#{default_value}'. " <>
            "Available keys: [#{available_keys}]."

        {:error, message}
    end
  end

  defp evaluate_rules([], [], _context), do: {:none, nil, nil, nil}

  defp evaluate_rules(targeting_rules, percentage_options, context) do
    case evaluate_targeting_rules(targeting_rules, context) do
      {:none, _, _, _} ->
        {value, variation, option} = evaluate_percentage_options(percentage_options, context)
        {value, variation, nil, option}

      {value, variation, rule, option} ->
        {value, variation, rule, option}
    end
  end

  defp evaluate_targeting_rules([], _context), do: {:none, nil, nil, nil}

  defp evaluate_targeting_rules(rules, %Context{} = context) do
    EvaluationLogger.log_evaluating_targeting_rules(context.logger)

    Enum.reduce_while(rules, {:none, nil, nil, nil}, fn rule, acc ->
      case evaluate_targeting_rule(rule, context) do
        {:none, _, _, _} -> {:cont, acc}
        result -> {:halt, result}
      end
    end)
  end

  defp evaluate_targeting_rule(rule, %Context{} = context) do
    %Context{logger: logger} = context
    conditions = TargetingRule.conditions(rule)
    value = TargetingRule.value(rule, context.setting_type)

    if evaluate_conditions(conditions, value, context) do
      case TargetingRule.simple_value(rule) do
        nil ->
          EvaluationLogger.increase_indent(logger)
          percentage_options = TargetingRule.percentage_options(rule)
          {value, variation_id, option} = evaluate_percentage_options(percentage_options, context)

          if value == :none do
            EvaluationLogger.log_ignored_targeting_rule(logger)
          end

          EvaluationLogger.decrease_indent(logger)
          {value, variation_id, rule, option}

        _ ->
          variation_id = TargetingRule.variation_id(rule, context.default_variation_id)
          {value, variation_id, rule, nil}
      end
    else
      {:none, nil, nil, nil}
    end
  end

  defp evaluate_conditions(conditions, value, context) do
    condition_count = length(conditions)

    {result, newline_before_then?} =
      conditions
      |> Enum.with_index()
      |> Enum.reduce_while({:ok, true}, fn {condition, index}, _acc ->
        EvaluationLogger.log_evaluating_condition_start(context.logger, index)

        case evaluate_condition(condition, condition_count, context) do
          {{:ok, true}, _newline?} = result -> {:cont, result}
          result -> {:halt, result}
        end
      end)

    EvaluationLogger.log_evaluating_condition_final_result(context.logger, result, newline_before_then?, value)

    case result do
      {:ok, result} -> result
      {:error, _error} -> false
    end
  end

  defp evaluate_condition(condition, condition_count, %Context{} = context) do
    %Context{logger: logger} = context
    prerequisite_flag_condition = Condition.prerequisite_flag_condition(condition)
    segment_condition = Condition.segment_condition(condition)
    user_condition = Condition.user_condition(condition)

    {result, newline?} =
      cond do
        user_condition ->
          {
            evaluate_user_condition(user_condition, context.key, context),
            condition_count > 1
          }

        segment_condition ->
          case evaluate_segment_condition(segment_condition, context) do
            {:ok, _value} = result ->
              {result, true}

            {:error, @missing_user_error} = result ->
              {result, condition_count > 1}

            result ->
              {result, true}
          end

        prerequisite_flag_condition ->
          {
            evaluate_prerequisite_flag_condition(prerequisite_flag_condition, context),
            true
          }
      end

    if condition_count > 1 do
      EvaluationLogger.log_evaluating_condition_result(logger, result)
    end

    EvaluationLogger.decrease_indent(logger)
    {result, newline?}
  end

  defp evaluate_user_condition(condition, _context_salt, %Context{user: nil} = context) do
    EvaluationLogger.log_evaluating_user_condition_start(context.logger, condition)
    EvaluationWarnings.warn_missing_user(context.warnings, context.key)
    {:error, @missing_user_error}
  end

  defp evaluate_user_condition(condition, context_salt, %Context{} = context) do
    %Context{logger: logger, user: user} = context

    EvaluationLogger.log_evaluating_user_condition_start(logger, condition)

    case UserCondition.fetch_comparison_attribute(condition) do
      {:error, :not_found} ->
        raise EvaluationError, "Comparison attribute name missing"

      {:ok, comparison_attribute} ->
        case User.get_attribute(user, comparison_attribute) do
          missing when is_nil(missing) or missing == "" ->
            EvaluationWarnings.warn_missing_user_attribute(context.warnings, context.key, condition, comparison_attribute)
            {:error, "cannot evaluate, the User.#{comparison_attribute} attribute is missing"}

          user_value ->
            compare(condition, user_value, context_salt, context)
        end
    end
  end

  defp compare(condition, user_value, context_salt, %Context{} = context) do
    %Context{key: key, salt: salt} = context

    comparison_context = %ComparisonContext{
      condition: condition,
      context_salt: context_salt,
      key: key,
      salt: salt
    }

    comparator = UserCondition.comparator(condition)
    comparison_value = UserCondition.comparison_value(condition)

    case UserComparator.compare(comparator, user_value, comparison_value, comparison_context) do
      {:ok, result} ->
        {:ok, result}

      {:error, :invalid_datetime} ->
        message = "'#{user_value}' is not a valid Unix timestamp (number of seconds elapsed since Unix epoch)"
        handle_invalid_user_attribute(condition, message, context)

      {:error, :invalid_float} ->
        message = "'#{user_value}' is not a valid decimal number"
        handle_invalid_user_attribute(condition, message, context)

      {:error, :invalid_string_list} ->
        message = "'#{user_value}' is not a valid string array"
        handle_invalid_user_attribute(condition, message, context)

      {:error, :invalid_version} ->
        trimmed = user_value |> to_string() |> String.trim()
        message = "'#{trimmed}' is not a valid semantic version"
        handle_invalid_user_attribute(condition, message, context)
    end
  end

  defp evaluate_segment_condition(condition, %Context{user: nil} = context) do
    EvaluationWarnings.warn_missing_user(context.warnings, context.key)
    EvaluationLogger.log_skipping_segment_condition_missing_user(context.logger, condition)
    {:error, "cannot evaluate, User Object is missing"}
  end

  defp evaluate_segment_condition(condition, %Context{} = context) do
    %Context{logger: logger} = context

    case SegmentCondition.fetch_segment(condition) do
      {:error, :not_found} ->
        raise EvaluationError, "Segment reference is invalid."

      {:ok, segment} ->
        comparator = SegmentCondition.segment_comparator(condition)
        name = Segment.name(segment)

        EvaluationLogger.log_evaluating_segment_condition_start(logger, condition, name)

        segment
        |> Segment.conditions()
        |> Enum.with_index()
        |> Enum.reduce_while({:ok, true}, fn {condition, index}, acc ->
          EvaluationLogger.log_evaluating_condition_start(logger, index)

          result = evaluate_user_condition(condition, name, context)
          EvaluationLogger.log_evaluating_condition_result(logger, result)

          case result do
            {:ok, true} -> {:cont, acc}
            {:ok, false} -> {:halt, {:ok, false}}
            {:error, error} -> {:halt, {:error, error}}
          end
        end)
        |> case do
          {:ok, in_segment?} ->
            result = {:ok, SegmentComparator.compare(comparator, in_segment?)}
            EvaluationLogger.log_evaluating_segment_condition_result(logger, condition, in_segment?, result)
            result

          {:error, _error} = result ->
            EvaluationLogger.log_evaluating_segment_condition_result(logger, condition, false, result)
            result
        end
    end
  end

  defp evaluate_prerequisite_flag_condition(condition, %Context{} = context) do
    %Context{config: config, logger: logger, user: user, visited_keys: visited_keys} = context
    settings = Config.settings(config)
    prerequisite_key = PrerequisiteFlagCondition.prerequisite_flag_key(condition)
    comparator = PrerequisiteFlagCondition.comparator(condition)

    case Map.get(settings, prerequisite_key) do
      nil ->
        raise EvaluationError, "Prerequisite flag key is missing or invalid."

      setting ->
        setting_type = Setting.setting_type(setting)
        comparison_value_type = PrerequisiteFlagCondition.inferred_setting_type(condition)

        unless setting_type == comparison_value_type do
          value =
            unless is_nil(comparison_value_type) do
              PrerequisiteFlagCondition.comparison_value(condition, comparison_value_type)
            end

          raise EvaluationError,
                "Type mismatch between comparison value '#{value}' and prerequisite flag '#{prerequisite_key}'"
        end

        comparison_value = PrerequisiteFlagCondition.comparison_value(condition, setting_type)
        next_visited_keys = [context.key | visited_keys]

        if prerequisite_key in visited_keys do
          raise CircularDependencyError, prerequisite_key: prerequisite_key, visited_keys: next_visited_keys
        else
          EvaluationLogger.log_evaluating_prerequisite_condition_start(logger, condition, setting_type)

          %EvaluationDetails{value: prerequisite_value} =
            evaluate(prerequisite_key, user, nil, nil, config, logger, next_visited_keys)

          result = PrerequisiteFlagComparator.compare(comparator, prerequisite_value, comparison_value)

          EvaluationLogger.log_evaluating_prerequisite_condition_result(
            logger,
            condition,
            setting_type,
            prerequisite_value,
            result
          )

          {:ok, result}
        end
    end
  end

  defp evaluate_percentage_options([] = _percentage_options, _context), do: {:none, nil, nil}

  defp evaluate_percentage_options(_percentage_options, %Context{user: nil} = context) do
    EvaluationWarnings.warn_missing_user(context.warnings, context.key)
    EvaluationLogger.log_skipping_percentage_options_missing_user(context.logger)
    {:none, nil, nil}
  end

  defp evaluate_percentage_options(percentage_options, %Context{} = context) do
    case extract_user_key(context) do
      {:ok, user_key} ->
        hash_val = hash_user(user_key, context.key)
        Enum.reduce_while(percentage_options, {0, 1}, &evaluate_percentage_option(&1, &2, hash_val, context))

      {:error, :missing_user_key} ->
        attribute_name = context.percentage_option_attribute
        EvaluationWarnings.warn_missing_user_attribute(context.warnings, context.key, attribute_name)
        EvaluationLogger.log_skipping_percentage_options_missing_user_attribute(context.logger, attribute_name)
        {:none, nil, nil}
    end
  end

  defp evaluate_percentage_option(option, increment, hash_val, %Context{} = context) do
    percentage = PercentageOption.percentage(option)
    {last_bucket, index} = increment
    bucket = last_bucket + percentage

    if hash_val < bucket do
      value = PercentageOption.value(option, context.setting_type)
      variation_id = PercentageOption.variation_id(option, context.default_variation_id)
      attribute_name = context.percentage_option_attribute

      EvaluationLogger.log_matching_percentage_option(context.logger, attribute_name, hash_val, index, percentage, value)

      {:halt, {value, variation_id, option}}
    else
      {:cont, {bucket, index + 1}}
    end
  end

  defp extract_user_key(%Context{} = context) do
    attribute = context.percentage_option_attribute

    case User.get_attribute(context.user, attribute) do
      nil ->
        if attribute == @default_percentage_option_attribute do
          {:ok, nil}
        else
          {:error, :missing_user_key}
        end

      value ->
        UserComparator.user_value_to_string(value)
    end
  end

  defp hash_user(user_key, key) do
    hash_candidate = "#{key}#{user_key}"

    {hash_value, _} =
      :sha
      |> :crypto.hash(hash_candidate)
      |> Base.encode16()
      |> String.slice(0, 7)
      |> Integer.parse(16)

    rem(hash_value, 100)
  end

  defp handle_invalid_user_attribute(condition, message, %Context{} = context) do
    EvaluationWarnings.warn_type_mismatch(context.warnings, context.key, condition, message)

    attribute = UserCondition.comparison_attribute(condition)
    {:error, "cannot evaluate, the User.#{attribute} attribute is invalid (#{message})"}
  end
end