defmodule Towerops.Agents do
  @moduledoc """
  The Agents context for managing remote SNMP polling agents.

  This context provides functions for creating and managing agent authentication tokens,
  assigning equipment to agents, and verifying agent credentials.
  """

  import Ecto.Query, warn: false

  alias Towerops.Agents.AgentAssignment
  alias Towerops.Agents.AgentToken
  alias Towerops.Equipment.Equipment
  alias Towerops.Repo

  ## Token management

  @doc """
  Creates a new agent token for an organization.

  Returns {:ok, agent_token, token_string} where token_string is the plain token
  that should be shown to the user ONCE and never stored.

  ## Examples

      iex> create_agent_token(org_id, "Datacenter Agent")
      {:ok, %AgentToken{}, "abc123..."}

      iex> create_agent_token(nil, "")
      {:error, %Ecto.Changeset{}}

  """
  def create_agent_token(organization_id, name) do
    {token, changeset} = AgentToken.build_token(organization_id, name)

    case Repo.insert(changeset) do
      {:ok, agent_token} -> {:ok, agent_token, token}
      {:error, changeset} -> {:error, changeset}
    end
  end

  @doc """
  Lists all agent tokens for an organization.

  Returns a list of agent tokens ordered by creation date (newest first).

  ## Examples

      iex> list_organization_agent_tokens(org_id)
      [%AgentToken{}, ...]

  """
  def list_organization_agent_tokens(organization_id) do
    AgentToken
    |> where([t], t.organization_id == ^organization_id)
    |> order_by([t], desc: t.inserted_at)
    |> Repo.all()
  end

  @doc """
  Counts the number of equipment directly assigned to an agent.

  This only counts equipment with explicit assignments, not equipment
  that inherit the agent from site or organization defaults.

  ## Examples

      iex> count_assigned_equipment(agent_token_id)
      5

  """
  def count_assigned_equipment(agent_token_id) do
    AgentAssignment
    |> where([a], a.agent_token_id == ^agent_token_id and a.enabled == true)
    |> Repo.aggregate(:count)
  end

  @doc """
  Gets a single agent token by ID.

  Raises `Ecto.NoResultsError` if the token does not exist.

  ## Examples

      iex> get_agent_token!(id)
      %AgentToken{}

      iex> get_agent_token!("nonexistent")
      ** (Ecto.NoResultsError)

  """
  def get_agent_token!(id), do: Repo.get!(AgentToken, id)

  @doc """
  Updates the heartbeat timestamp and metadata for an agent token.

  This is called automatically when an agent makes an API request.

  ## Examples

      iex> update_agent_token_heartbeat(token_id, "192.168.1.1", %{version: "0.1.0"})
      {1, nil}

  """
  def update_agent_token_heartbeat(agent_token_id, ip_address, metadata \\ nil) do
    now = DateTime.truncate(DateTime.utc_now(), :second)

    # Only update metadata if explicitly provided (not nil and not empty)
    updates =
      case metadata do
        nil -> [last_seen_at: now, last_ip: ip_address]
        meta when map_size(meta) > 0 -> [last_seen_at: now, last_ip: ip_address, metadata: meta]
        _ -> [last_seen_at: now, last_ip: ip_address]
      end

    AgentToken
    |> where([t], t.id == ^agent_token_id)
    |> Repo.update_all(set: updates)
  end

  @doc """
  Verifies an agent token and returns the agent token record if valid.

  Returns {:ok, agent_token} if the token is valid and enabled,
  otherwise {:error, :invalid_token}.

  ## Examples

      iex> verify_agent_token("valid_token")
      {:ok, %AgentToken{}}

      iex> verify_agent_token("invalid_token")
      {:error, :invalid_token}

  """
  def verify_agent_token(token) do
    with {:ok, token_string} <- AgentToken.verify_token(token),
         %AgentToken{} = agent_token <-
           Repo.get_by(AgentToken, token: token_string, enabled: true) do
      {:ok, agent_token}
    else
      _ -> {:error, :invalid_token}
    end
  end

  @doc """
  Revokes an agent token by setting enabled to false.

  ## Examples

      iex> revoke_agent_token(id)
      {:ok, %AgentToken{enabled: false}}

  """
  def revoke_agent_token(id) do
    AgentToken
    |> Repo.get!(id)
    |> Ecto.Changeset.change(enabled: false)
    |> Repo.update()
  end

  ## Assignment management

  @doc """
  Assigns equipment to an agent.

  Creates an agent assignment that links equipment to an agent token.
  Equipment can only be assigned to one agent at a time due to unique constraint.

  ## Examples

      iex> assign_equipment_to_agent(agent_id, equipment_id)
      {:ok, %AgentAssignment{}}

      iex> assign_equipment_to_agent(agent_id, already_assigned_equipment_id)
      {:error, %Ecto.Changeset{}}

  """
  def assign_equipment_to_agent(agent_token_id, equipment_id) do
    %AgentAssignment{}
    |> AgentAssignment.changeset(%{
      agent_token_id: agent_token_id,
      equipment_id: equipment_id
    })
    |> Repo.insert()
  end

  @doc """
  Removes the agent assignment for a piece of equipment.

  ## Examples

      iex> unassign_equipment(equipment_id)
      {1, nil}

  """
  def unassign_equipment(equipment_id) do
    AgentAssignment
    |> where([a], a.equipment_id == ^equipment_id)
    |> Repo.delete_all()
  end

  @doc """
  Lists all equipment assigned to an agent.

  Returns equipment records with preloaded site and SNMP device associations.

  ## Examples

      iex> list_agent_equipment(agent_token_id)
      [%Equipment{site: %Site{}, snmp_device: %Device{sensors: [...], interfaces: [...]}}]

  """
  def list_agent_equipment(agent_token_id) do
    Repo.all(
      from(e in Equipment,
        join: a in AgentAssignment,
        on: a.equipment_id == e.id,
        where: a.agent_token_id == ^agent_token_id and a.enabled == true,
        preload: [:site, snmp_device: [:sensors, :interfaces]]
      )
    )
  end

  @doc """
  Lists all equipment that should be polled by an agent.

  This includes equipment that is:
  1. Directly assigned to this agent
  2. At a site that has this agent as default
  3. In an organization that has this agent as default (and not overridden by site or equipment)

  Returns equipment records with preloaded associations, filtered to only SNMP-enabled equipment.

  ## Examples

      iex> list_agent_polling_targets(agent_token_id)
      [%Equipment{snmp_enabled: true, site: %Site{}, snmp_device: %Device{sensors: [...], interfaces: [...]}}]

  """
  def list_agent_polling_targets(agent_token_id) do
    # Get all equipment with necessary preloads
    equipment_query =
      from(e in Equipment,
        where: e.snmp_enabled == true,
        preload: [
          :site,
          site: :organization,
          snmp_device: [:sensors, :interfaces]
        ]
      )

    equipment_list = Repo.all(equipment_query)

    # Filter to only equipment that resolves to this agent
    Enum.filter(equipment_list, fn equipment ->
      equipment_matches_agent?(equipment, agent_token_id)
    end)
  end

  defp equipment_matches_agent?(equipment, agent_token_id) do
    # Check direct assignment first
    case get_equipment_assignment(equipment.id) do
      %AgentAssignment{agent_token_id: ^agent_token_id, enabled: true} ->
        true

      %AgentAssignment{} ->
        # Has a different direct assignment
        false

      nil ->
        # No direct assignment, check site then org
        site = equipment.site
        site_matches_agent?(site, agent_token_id)
    end
  end

  defp site_matches_agent?(site, agent_token_id) do
    cond do
      # Check site agent
      site.agent_token_id == agent_token_id ->
        true

      # Check org default (only if site doesn't have an agent set)
      is_nil(site.agent_token_id) and site.organization.default_agent_token_id == agent_token_id ->
        true

      true ->
        false
    end
  end

  @doc """
  Gets the agent assignment for a piece of equipment.

  Returns the assignment record if it exists, nil otherwise.

  ## Examples

      iex> get_equipment_assignment(equipment_id)
      %AgentAssignment{}

      iex> get_equipment_assignment(unassigned_equipment_id)
      nil

  """
  def get_equipment_assignment(equipment_id) do
    Repo.get_by(AgentAssignment, equipment_id: equipment_id)
  end

  @doc """
  Updates or creates an equipment assignment.

  If agent_token_id is nil, removes any existing assignment.
  Otherwise, creates or updates the assignment.

  ## Examples

      iex> update_equipment_assignment(equipment_id, agent_token_id)
      {:ok, %AgentAssignment{}}

      iex> update_equipment_assignment(equipment_id, nil)
      {:ok, nil}

  """
  def update_equipment_assignment(equipment_id, nil) do
    # Remove assignment if setting to nil
    unassign_equipment(equipment_id)
    {:ok, nil}
  end

  def update_equipment_assignment(equipment_id, agent_token_id) do
    case get_equipment_assignment(equipment_id) do
      nil ->
        # Create new assignment
        assign_equipment_to_agent(agent_token_id, equipment_id)

      existing ->
        # Update existing assignment
        existing
        |> AgentAssignment.changeset(%{agent_token_id: agent_token_id})
        |> Repo.update()
    end
  end

  @doc """
  Gets the effective agent token for a piece of equipment.

  Resolves the agent token using the following hierarchy:
  1. Equipment's direct agent assignment (highest priority)
  2. Site's agent_token_id
  3. Organization's default_agent_token_id (lowest priority)

  Returns the agent_token_id if found, nil otherwise.

  The equipment must be preloaded with site: [organization: :default_agent_token].

  ## Examples

      iex> equipment = Repo.preload(equipment, site: [organization: :default_agent_token])
      iex> get_effective_agent_token(equipment)
      "agent-token-uuid"

      iex> get_effective_agent_token(equipment_without_any_assignment)
      nil

  """
  def get_effective_agent_token(%Equipment{} = equipment) do
    # 1. Check for direct equipment assignment
    case get_equipment_assignment(equipment.id) do
      %AgentAssignment{agent_token_id: agent_token_id} when not is_nil(agent_token_id) ->
        agent_token_id

      _ ->
        # 2. Check site's agent token
        site = equipment.site

        case site do
          %{agent_token_id: agent_token_id} when not is_nil(agent_token_id) ->
            agent_token_id

          %{organization: %{default_agent_token_id: agent_token_id}}
          when not is_nil(agent_token_id) ->
            # 3. Use organization's default agent token
            agent_token_id

          _ ->
            nil
        end
    end
  end

  @doc """
  Gets information about where the effective agent token comes from.

  Returns a tuple of {agent_token_id, source} where source is one of:
  - :equipment - Direct equipment assignment
  - :site - Inherited from site
  - :organization - Inherited from organization
  - :none - No agent assigned

  ## Examples

      iex> get_effective_agent_token_with_source(equipment)
      {"token-id", :equipment}

      iex> get_effective_agent_token_with_source(equipment_no_assignment)
      {nil, :none}

  """
  def get_effective_agent_token_with_source(%Equipment{} = equipment) do
    # 1. Check for direct equipment assignment
    case get_equipment_assignment(equipment.id) do
      %AgentAssignment{agent_token_id: agent_token_id} when not is_nil(agent_token_id) ->
        {agent_token_id, :equipment}

      _ ->
        # 2. Check site's agent token
        site = equipment.site

        case site do
          %{agent_token_id: agent_token_id} when not is_nil(agent_token_id) ->
            {agent_token_id, :site}

          %{organization: %{default_agent_token_id: agent_token_id}}
          when not is_nil(agent_token_id) ->
            # 3. Use organization's default agent token
            {agent_token_id, :organization}

          _ ->
            {nil, :none}
        end
    end
  end
end