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

  Populated lazily by `Microwaveprop.Propagation.scores_at/3` on the first
  request for a given `{band_mhz, valid_time}`; subsequent requests hit ETS
  until the entry is evicted. The cache is bounded to `max_entries/0` total
  entries via LRU-by-`valid_time` eviction; on overflow the oldest
  `valid_time` is dropped first so the freshly-arrived hour stays warm.

  `broadcast_put/3` and the matching `"propagation:cache"` PubSub topic are
  retained for the rare cluster-wide pre-warm path; the hot pipeline (Rust
  Phase 2 cutover) does not call them — `NotifyListener` only emits the
  lighter-weight `"propagation:updated"` PubSub so LiveView clients refresh
  while the actual scores load on demand from `/data/scores/.../*.prop`.

  Each cache entry stores a `%{{lat, lon} => score}` map so per-point lookups
  (used by `point_forecast/3` and `point_detail/4`) stay O(1) on hits.
  """
  use GenServer

  alias Phoenix.PubSub

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

  # Hard cap on cached `{band_mhz, valid_time}` entries. Each value is a
  # 92k-cell `Map.new` weighing ~1.94 MiB; without a cap the eager warm
  # path materialises 23 bands × 19 forecast hours = 437 entries per pod
  # (~850 MiB) and OOMs the BEAM. The cap keeps the LiveView hot path
  # warm (one band × the hour or two the user is actively scrubbing)
  # and lets the rest fall back to direct `.prop` reads via
  # `read_from_disk_and_cache/3`, which is sub-100 ms per file.
  @max_entries 32

  @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
      [{_, grid}] -> {:ok, grid_to_list(grid)}
      [] -> :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 :ets.lookup(@table, {band_mhz, valid_time}) do
      [{_, grid}] -> {:ok, grid_to_filtered_list(grid, bounds)}
      [] -> :miss
    end
  end

  @doc """
  Look up the score for a single `{lat, lon}` point at `{band_mhz, valid_time}`.
  Returns `{:ok, score}` on hit or `:miss` on cache miss. The coordinates are
  used as-is (no snap-to-grid) — callers that need the nearest grid cell should
  snap first.
  """
  @spec fetch_point(non_neg_integer(), DateTime.t(), float(), float()) ::
          {:ok, non_neg_integer()} | :miss
  def fetch_point(band_mhz, valid_time, lat, lon) do
    case :ets.lookup(@table, {band_mhz, valid_time}) do
      [{_, grid}] ->
        case Map.get(grid, {lat, lon}) do
          nil -> :miss
          score -> {:ok, score}
        end

      [] ->
        :miss
    end
  end

  @doc "Maximum number of `{band_mhz, valid_time}` entries kept in ETS."
  @spec max_entries() :: pos_integer()
  def max_entries, do: @max_entries

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

  # When the table is over the cap, evict whichever entry has the oldest
  # `valid_time`. Picking the oldest matches the LiveView access pattern:
  # the timeline scrubs forward, the hourly chain replaces the trailing
  # edge, and the freshest hours are by far the most-requested. Loops
  # until the table is back at or below the cap so that any historical
  # over-shoot self-heals after this commit deploys.
  defp enforce_capacity do
    if :ets.info(@table, :size) > @max_entries do
      case oldest_key() do
        nil -> :ok
        key -> :ets.delete(@table, key)
      end

      enforce_capacity()
    end
  end

  defp oldest_key do
    fun = fn
      {{band, vt}, _}, nil ->
        {band, vt}

      {{band, vt}, _}, {_, oldest} = acc ->
        if DateTime.before?(vt, oldest), do: {band, vt}, else: acc
    end

    :ets.foldl(fun, nil, @table)
  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

  @doc "Returns the sorted list of valid_times cached for `band_mhz`."
  @spec valid_times(non_neg_integer()) :: [DateTime.t()]
  def valid_times(band_mhz) do
    match_spec = [{{{band_mhz, :"$1"}, :_}, [], [:"$1"]}]

    @table
    |> :ets.select(match_spec)
    |> Enum.sort({:asc, DateTime})
  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

  @doc """
  Delete every entry whose `valid_time` falls outside the inclusive
  window `[past_cutoff, future_cutoff]`. Used to bound the cache to
  the active forecast horizon (past ~1 h + HRRR's 18 h forward) so
  long-horizon GEFS valid_times never accumulate in ETS.
  """
  @spec prune_outside_window(DateTime.t(), DateTime.t()) :: non_neg_integer()
  def prune_outside_window(past_cutoff, future_cutoff) do
    match_spec = [
      {{{:_, :"$1"}, :_},
       [
         {:orelse, {:<, :"$1", {:const, past_cutoff}}, {:>, :"$1", {:const, future_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, :compressed, read_concurrency: true])
    :ok = 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 list_to_grid(scores) do
    Map.new(scores, fn %{lat: lat, lon: lon, score: score} -> {{lat, lon}, score} end)
  end

  defp grid_to_list(grid) do
    Enum.map(grid, fn {{lat, lon}, score} -> %{lat: lat, lon: lon, score: score} end)
  end

  defp grid_to_filtered_list(grid, nil), do: grid_to_list(grid)

  defp grid_to_filtered_list(grid, %{"south" => s, "north" => n, "west" => w, "east" => e}) do
    # Single pass over 92k cells: the former Enum.filter |> Enum.map
    # walked the map twice and allocated an intermediate list of
    # {key, value} tuples between them. Enum.reduce emits only the
    # in-bounds result maps directly.
    Enum.reduce(grid, [], fn {{lat, lon}, score}, acc ->
      if lat >= s and lat <= n and lon >= w and lon <= e do
        [%{lat: lat, lon: lon, score: score} | acc]
      else
        acc
      end
    end)
  end
end