defmodule <%= module %> do
  @moduledoc """
  Subscriber-side session state model for a conversation's agent.

  This module owns the **shape** of the subscriber's session state and the
  state transitions in response to agent events. It is host-agnostic — every
  function takes a plain state map (or just the inputs it needs) and returns
  a changes map (or a tagged tuple for operations that need orchestration,
  such as HITL decisions that may trigger an `AgentServer.resume/2` call).

  Hosts apply the changes in their native idiom:

  - LiveView: `socket = assign(socket, changes)` (each key is properly
    tracked in `__changed__` for re-render diffing).
  - GenServer / plain process: `state = Map.merge(state, changes)`.

  ## Why this module exists

  A LiveView is one consumer of agent events; a GraphQL bridge GenServer
  (or any other host that proxies agent events to an external transport)
  is another. Both want the same state-transition logic. Putting it here
  lets the LiveView helpers stay focused on LiveView-specific concerns
  (streams, flash, `connected?`) and lets non-LiveView hosts call this
  module directly without dragging in any LiveView dependency.

  ## Tagged-tuple operations

  A few handlers need to signal "the host should now perform a side effect"
  in addition to producing a state change. They return tagged tuples:

  - `{:resume, resume_data, changes}` — caller should
    `AgentServer.resume(agent_id, resume_data)`. On success, also apply
    `hitl_resume_running_changes/0` (or `question_resume_running_changes/0`)
    on top of `changes`.
  - `{:more, changes}` — no side effect required; just merge changes.

  See `handle_hitl_decision/3` and `handle_question_response/2`.
  """

  alias <%= conversations_module %>
  alias <%= coordinator_module %>
  alias Sagents.AgentUtils
  alias Sagents.StreamingSession
  alias Sagents.Subscriber
  alias LangChain.MessageDelta

  require Logger

  # ===========================================================================
  # Default state
  # ===========================================================================

  @doc """
  Default empty session state. Caller merges this into its own state map
  (or passes to `assign/2` for a LiveView).
  """
  def init_session_state do
    %{
      conversation: nil,
      conversation_id: nil,
      agent_id: nil,
      agent_status: :not_running,
      todos: [],
      has_messages: false,
      streaming_delta: nil,
      loading: false,
      pending_tools: [],
      pending_question: nil,
      pending_halt: nil,
      remaining_questions: [],
      question_responses: [],
      interrupt_data: nil,
      hitl_decisions: [],
      sagents_subs: %{}
    }
  end

  # ===========================================================================
  # Status handlers
  # ===========================================================================

  # Every non-interrupted status merges `AgentUtils.cleared_interrupt_changes/0`
  # to enforce the invariant: any status other than `:interrupted` means there
  # is no pending interrupt UI. This stops a dismissed/superseded interrupt's
  # derived keys (e.g. `:pending_halt`) from re-appearing on the next
  # transition back into `:interrupted`.

  @doc "Status changed to :running."
  def handle_status_running,
    do: Map.merge(AgentUtils.cleared_interrupt_changes(), %{agent_status: :running})

  @doc "Status changed to :idle (execution completed)."
  def handle_status_idle,
    do:
      Map.merge(
        AgentUtils.cleared_interrupt_changes(),
        %{loading: false, agent_status: :idle, streaming_delta: nil}
      )

  @doc "Status changed to :cancelled (user cancelled execution)."
  def handle_status_cancelled,
    do:
      Map.merge(
        AgentUtils.cleared_interrupt_changes(),
        %{loading: false, agent_status: :cancelled, streaming_delta: nil}
      )

  @doc """
  Status changed to :error.

  Returns only the state changes. The caller is responsible for surfacing
  the error to the user (e.g. `put_flash` in LiveView). Use
  `format_error_message/1` for a consistent message.
  """
  def handle_status_error(_reason),
    do:
      Map.merge(
        AgentUtils.cleared_interrupt_changes(),
        %{loading: false, agent_status: :error, streaming_delta: nil}
      )

  @doc """
  Status changed to :interrupted (waiting for human input).

  Returns an empty changes map when `interrupt_data` is `nil` (e.g. an agent
  whose status is `:interrupted` but whose interrupt payload hasn't been
  populated yet). Callers can apply the result unconditionally.
  """
  def handle_status_interrupted(nil), do: %{}

  def handle_status_interrupted(interrupt_data) do
    Map.merge(
      %{loading: false, agent_status: :interrupted, interrupt_data: interrupt_data},
      AgentUtils.interrupt_session_changes(interrupt_data)
    )
  end

  # ===========================================================================
  # Message handlers
  # ===========================================================================

  @doc "Streaming LLM deltas (incremental response chunks)."
  def handle_llm_deltas(state, deltas) do
    current = Map.get(state, :streaming_delta)
    %{streaming_delta: MessageDelta.merge_deltas(current, deltas)}
  end

  @doc "Complete LLM message received."
  def handle_llm_message_complete, do: %{streaming_delta: nil, loading: false}

  # ===========================================================================
  # Tool execution handlers
  # ===========================================================================
  #
  # The tool-call lifecycle logic lives in `Sagents.StreamingSession` so every
  # downstream host (this LiveView, GraphQL bridges, plain GenServers) gets
  # the same call_id-based behaviour and the same "keep the delta until every
  # sibling tool call has terminated" semantics. We just delegate.

  @doc "Tool call identified — see `Sagents.StreamingSession.handle_tool_call_identified/2`."
  defdelegate handle_tool_call_identified(state, tool_info), to: StreamingSession

  @doc "Tool execution lifecycle update — see `Sagents.StreamingSession.handle_tool_execution_update/3`."
  defdelegate handle_tool_execution_update(state, status, tool_info), to: StreamingSession

  # ===========================================================================
  # Recovery handlers (DOWN, presence_diff, agent shutdown)
  # ===========================================================================

  @doc """
  Route a `{:DOWN, ref, ...}` from a producer crash to
  `Sagents.Subscriber.handle_publisher_down/3`. Returns changes to apply
  if the ref matched a tracked subscription, or an empty map.
  """
  def handle_publisher_down(state, ref, reason \\ :noproc) do
    subs = Map.get(state, :sagents_subs, %{})

    case Subscriber.handle_publisher_down(subs, ref, reason) do
      {:matched, new_subs} -> %{sagents_subs: new_subs}
      :no_match -> %{}
    end
  end

  @doc """
  Route a Phoenix presence diff for the agent presence topic to
  `Sagents.Subscriber.handle_presence_diff/3`.
  """
  def handle_presence_diff(state, payload) do
    subs = Map.get(state, :sagents_subs, %{})
    new_subs = Subscriber.handle_presence_diff(subs, Subscriber.presence_topic(), payload)
    %{sagents_subs: new_subs}
  end

  @doc """
  Agent shutdown event — clear `agent_id` and unsubscribe locally so the
  subs map doesn't carry a stale entry.
  """
  def handle_agent_shutdown(state, _shutdown_data) do
    base =
      Map.merge(AgentUtils.cleared_interrupt_changes(), %{
        agent_id: nil,
        agent_status: :not_running,
        loading: false,
        streaming_delta: nil
      })

    case Map.get(state, :agent_id) do
      nil ->
        base

      agent_id ->
        subs = Map.get(state, :sagents_subs, %{})
        new_subs = Subscriber.unsubscribe_from_agent(subs, agent_id)
        Map.put(base, :sagents_subs, new_subs)
    end
  end

  # ===========================================================================
  # Lifecycle
  # ===========================================================================

  @doc """
  Conversation title generated — write the new title to the DB and return
  a change with the updated conversation. No-op (returns `%{}`) if the
  event isn't for our agent or there's no conversation in state.
  """
  def handle_conversation_title_generated(state, new_title, target_agent_id) do
    cond do
      target_agent_id != Map.get(state, :agent_id) ->
        %{}

      is_nil(Map.get(state, :conversation)) ->
        %{}

      true ->
        case <%= conversations_alias %>.update_conversation(state.conversation, %{title: new_title}) do
          {:ok, updated_conversation} ->
            %{conversation: updated_conversation}

          {:error, reason} ->
            Logger.error("Failed to update conversation title: #{inspect(reason)}")
            %{}
        end
    end
  end

  # ===========================================================================
  # HITL decisions
  # ===========================================================================

  @doc """
  Compute the state transition for a single HITL approve/reject decision.

  Returns `{:resume, accumulated, changes}` when this decision settles all
  pending tools (the host should call `AgentServer.resume(agent_id, accumulated)`
  and then merge `changes` plus `hitl_resume_running_changes/0` on success),
  or `{:more, changes}` when there are still tools to decide.
  """
  def handle_hitl_decision(state, index, decision_type) do
    AgentUtils.advance_hitl_decisions(state, index, decision_type)
  end

  @doc """
  Changes to apply on top of `handle_hitl_decision/3`'s `:resume` changes
  after `AgentServer.resume/2` succeeds.
  """
  def hitl_resume_running_changes, do: %{agent_status: :running, loading: true}

  @doc """
  Persist a HITL decision label on the matching tool call display message.
  No-op when there's no scope or the call_id can't be resolved.
  """
  def persist_hitl_decision(state, pending_tools, index, decision) do
    interrupt_data = Map.get(state, :interrupt_data)
    tool = Enum.at(pending_tools, index)

    call_id =
      case interrupt_data do
        %{type: :subagent_hitl, tool_call_id: parent_call_id} -> parent_call_id
        _other -> tool[:tool_call_id]
      end

    if call_id && Map.get(state, :current_scope) do
      <%= conversations_alias %>.record_hitl_decision(state.current_scope, call_id, decision)
    end
  end

  # ===========================================================================
  # Question response
  # ===========================================================================

  @doc """
  Compute the state transition for a single answer to a pending
  AskUserQuestion interrupt.

  Returns `{:resume, resume_data, changes}` when the answered question was
  the last one, or `{:more, changes}` when more questions remain.
  """
  def handle_question_response(state, response) do
    current_question = Map.fetch!(state, :pending_question)
    response = Map.put(response, :tool_call_id, current_question.tool_call_id)

    accumulated = (Map.get(state, :question_responses, []) || []) ++ [response]
    remaining = Map.get(state, :remaining_questions, []) || []

    case remaining do
      [] ->
        resume_data = if length(accumulated) == 1, do: hd(accumulated), else: accumulated

        {:resume, resume_data,
         %{
           pending_question: nil,
           remaining_questions: [],
           question_responses: [],
           interrupt_data: nil
         }}

      [next | rest] ->
        {:more,
         %{
           pending_question: next,
           remaining_questions: rest,
           question_responses: accumulated
         }}
    end
  end

  @doc """
  Changes to apply on top of `handle_question_response/2`'s `:resume`
  changes after `AgentServer.resume/2` succeeds.
  """
  def question_resume_running_changes, do: %{agent_status: :running, loading: true}

  # ===========================================================================
  # Presence tracking
  # ===========================================================================

  @doc """
  Track the viewer in Phoenix.Presence so the agent can use smart shutdown.
  No-op if `user_id` is nil.
  """
  def maybe_track_viewer(_conversation_id, nil), do: :ok

  def maybe_track_viewer(conversation_id, user_id) do
    case <%= coordinator_alias %>.track_conversation_viewer(conversation_id, user_id) do
      {:ok, _ref} ->
        :ok

      {:error, {:already_tracked, _topic, _key, _meta}} ->
        :ok

      {:error, reason} ->
        Logger.warning("Failed to track presence: #{inspect(reason)}")
        :ok
    end
  end

  # ===========================================================================
  # Error formatting
  # ===========================================================================

  @doc """
  Render an agent error reason as a user-facing message. Hosts use this
  before `put_flash`-ing or otherwise surfacing the error.
  """
  def format_error_message(reason) do
    error_display =
      case reason do
        %LangChain.LangChainError{} = error -> error.message
        other -> inspect(other)
      end

    "Sorry, I encountered an error: #{error_display}"
  end
end