defmodule Microwaveprop.Propagation.ScoreCache do
  @moduledoc """
  Node-local ETS cache of propagation scores keyed by `{band_mhz, valid_time}`.

  Populated eagerly by `Microwaveprop.Workers.PropagationGridWorker` after each
  hourly compute (via `broadcast_put/3`) and lazily by `Microwaveprop.Propagation.scores_at/3`
  on a cache miss. `broadcast_put/3` fans the payload out across the cluster on
  the `"propagation:cache"` PubSub topic so every BEAM node stays in sync with
  a single compute.
  """
  use GenServer

  alias Phoenix.PubSub

  @table :propagation_score_cache
  @topic "propagation:cache"
  @pubsub Microwaveprop.PubSub

  @type score :: %{lat: float(), lon: float(), score: non_neg_integer()}
  @type bounds :: %{optional(String.t()) => float()}

  # ---------- Public API ----------

  @spec start_link(keyword()) :: GenServer.on_start()
  def start_link(opts) do
    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
  end

  @spec fetch(non_neg_integer(), DateTime.t()) :: {:ok, [score()]} | :miss
  def fetch(band_mhz, valid_time) do
    case :ets.lookup(@table, {band_mhz, valid_time}) do
      [{_, scores}] -> {:ok, scores}
      [] -> :miss
    end
  end

  @spec fetch_bounds(non_neg_integer(), DateTime.t(), bounds() | nil) ::
          {:ok, [score()]} | :miss
  def fetch_bounds(band_mhz, valid_time, bounds) do
    case fetch(band_mhz, valid_time) do
      {:ok, scores} -> {:ok, filter_bounds(scores, bounds)}
      :miss -> :miss
    end
  end

  @spec put(non_neg_integer(), DateTime.t(), [score()]) :: :ok
  def put(band_mhz, valid_time, scores) do
    :ets.insert(@table, {{band_mhz, valid_time}, scores})
    :ok
  end

  @doc """
  Insert locally AND broadcast to peer nodes via PubSub.

  Call this from the single pod that computed the scores; every cluster member
  (including the caller) will receive the PubSub message and populate its local
  ETS. Use `put/3` directly if you already have a cluster-wide fan-out.
  """
  @spec broadcast_put(non_neg_integer(), DateTime.t(), [score()]) :: :ok
  def broadcast_put(band_mhz, valid_time, scores) do
    PubSub.broadcast(@pubsub, @topic, {:cache_refresh, band_mhz, valid_time, scores})
    :ok
  end

  @spec prune_older_than(DateTime.t()) :: non_neg_integer()
  def prune_older_than(cutoff) do
    match_spec = [
      {{{:_, :"$1"}, :_}, [{:<, :"$1", {:const, cutoff}}], [true]}
    ]

    :ets.select_delete(@table, match_spec)
  end

  @spec clear() :: :ok
  def clear do
    :ets.delete_all_objects(@table)
    :ok
  end

  @doc "Flush the GenServer mailbox so any in-flight cache_refresh messages are applied."
  @spec sync() :: :ok
  def sync do
    GenServer.call(__MODULE__, :sync)
  end

  # ---------- GenServer callbacks ----------

  @impl true
  def init(_opts) do
    :ets.new(@table, [:set, :named_table, :public, read_concurrency: true])
    PubSub.subscribe(@pubsub, @topic)
    {:ok, %{}}
  end

  @impl true
  def handle_call(:sync, _from, state), do: {:reply, :ok, state}

  @impl true
  def handle_info({:cache_refresh, band_mhz, valid_time, scores}, state) do
    put(band_mhz, valid_time, scores)
    {:noreply, state}
  end

  def handle_info(_msg, state), do: {:noreply, state}

  # ---------- Internal ----------

  defp filter_bounds(scores, nil), do: scores

  defp filter_bounds(scores, %{"south" => s, "north" => n, "west" => w, "east" => e}) do
    Enum.filter(scores, fn %{lat: lat, lon: lon} ->
      lat >= s and lat <= n and lon >= w and lon <= e
    end)
  end
end