defmodule SyntropyWeb.Api.Listing do
  @moduledoc """
  Shared helpers for paginated, filterable list endpoints.

  List endpoints accept `limit`, `cursor`, `q`, and resource-specific filter
  params. When persistence is enabled the controllers page through Postgres;
  otherwise these helpers paginate the bounded in-memory rings with the same
  opaque-cursor contract.
  """

  alias Syntropy.Persistence
  alias SyntropyWeb.Api.ControllerHelpers

  @type page :: Persistence.page()

  @filter_params %{status: "status", kind: "kind", q: "q", perspective: "perspective"}

  @doc """
  Parses shared list params (`limit`, `cursor`, plus the allowed filters).

  Returns `{:error, :invalid_cursor}` when a cursor is present but malformed.
  """
  @spec parse_opts(map(), [atom()]) :: {:ok, keyword()} | {:error, :invalid_cursor}
  def parse_opts(params, allowed_filters) do
    base = [limit: ControllerHelpers.limit(params)]

    filters =
      Enum.reduce(allowed_filters, base, fn filter, acc ->
        case Map.get(params, Map.fetch!(@filter_params, filter)) do
          value when is_binary(value) and value != "" -> Keyword.put(acc, filter, value)
          _other -> acc
        end
      end)

    case Map.get(params, "cursor") do
      nil ->
        {:ok, filters}

      cursor ->
        case Persistence.decode_cursor(cursor) do
          {:ok, decoded} -> {:ok, Keyword.put(filters, :cursor, decoded)}
          :error -> {:error, :invalid_cursor}
        end
    end
  end

  @doc """
  Applies cursor + limit pagination to an already-filtered in-memory list.

  Entries must be sorted newest-first. `timestamp_key` selects the field used
  for cursor ordering; the entry `:id` (a string) breaks ties.
  """
  @spec paginate(list(map()), keyword(), atom()) :: page()
  def paginate(entries, opts, timestamp_key) do
    limit = Keyword.get(opts, :limit, 100)

    remaining =
      case Keyword.get(opts, :cursor) do
        nil -> entries
        cursor -> Enum.filter(entries, &before_cursor?(&1, cursor, timestamp_key))
      end

    page_entries = Enum.take(remaining, limit)
    has_more = length(remaining) > limit

    next_cursor =
      case {has_more, List.last(page_entries)} do
        {true, %{} = last} ->
          Persistence.encode_cursor({sort_timestamp(last, timestamp_key), last.id})

        _other ->
          nil
      end

    %{entries: page_entries, has_more: has_more, next_cursor: next_cursor}
  end

  @doc """
  Filters in-memory run envelopes by `:status`, `:kind`, and `:q`.
  """
  @spec filter_runs(list(map()), keyword()) :: list(map())
  def filter_runs(runs, opts) do
    runs
    |> filter_equal(:status, opts[:status])
    |> filter_equal(:kind, opts[:kind])
    |> filter_query(opts[:q], fn run ->
      [Map.get(run, :id), Map.get(run, :prompt)]
    end)
  end

  @doc """
  Filters in-memory snapshots by `:kind` (trigger kind) and `:q`.
  """
  @spec filter_snapshots(list(map()), keyword()) :: list(map())
  def filter_snapshots(snapshots, opts) do
    snapshots
    |> filter_equal(:trigger_kind, opts[:kind])
    |> filter_query(opts[:q], fn snapshot ->
      [Map.get(snapshot, :id), Map.get(snapshot, :task_id)]
    end)
  end

  @doc """
  Filters in-memory recommendations by `:status`, `:kind`, and `:q`.
  """
  @spec filter_recommendations(list(map()), keyword()) :: list(map())
  def filter_recommendations(recommendations, opts) do
    recommendations
    |> filter_equal(:status, opts[:status])
    |> filter_equal(:kind, opts[:kind])
    |> filter_query(opts[:q], fn recommendation ->
      [
        Map.get(recommendation, :id)
        | Map.get(recommendation, :candidate_agent_ids) || []
      ]
    end)
  end

  @doc """
  Filters agent exports by `:perspective` and `:q` (id, name, or perspective).
  """
  @spec filter_agents(list(map()), keyword()) :: list(map())
  def filter_agents(agents, opts) do
    agents
    |> filter_equal(:perspective, opts[:perspective])
    |> filter_query(opts[:q], fn agent ->
      [
        Map.get(agent, :id),
        Map.get(agent, :runtime_id),
        Map.get(agent, :name),
        Map.get(agent, :perspective)
      ]
    end)
  end

  @doc """
  Builds the `meta` object appended to paginated list responses.
  """
  @spec meta(page()) :: map()
  def meta(page) do
    %{has_more: page.has_more, next_cursor: page.next_cursor}
  end

  defp filter_equal(entries, _key, nil), do: entries

  defp filter_equal(entries, key, value) do
    Enum.filter(entries, &(Map.get(&1, key) == value))
  end

  defp filter_query(entries, nil, _fields_fun), do: entries
  defp filter_query(entries, "", _fields_fun), do: entries

  defp filter_query(entries, q, fields_fun) do
    needle = String.downcase(q)

    Enum.filter(entries, fn entry ->
      entry
      |> fields_fun.()
      |> Enum.any?(fn
        value when is_binary(value) -> String.contains?(String.downcase(value), needle)
        _other -> false
      end)
    end)
  end

  defp before_cursor?(entry, {cursor_timestamp, cursor_id}, timestamp_key) do
    timestamp = sort_timestamp(entry, timestamp_key)

    case DateTime.compare(timestamp, cursor_timestamp) do
      :lt -> true
      :eq -> to_string(Map.get(entry, :id)) < cursor_id
      :gt -> false
    end
  end

  defp sort_timestamp(entry, timestamp_key) do
    case Map.get(entry, timestamp_key) do
      %DateTime{} = timestamp -> timestamp
      _other -> DateTime.from_unix!(0)
    end
  end
end