defmodule Microwaveprop.Weather.Era5BatchClient do
  @moduledoc """
  Fetches ERA5 reanalysis data from Copernicus Climate Data Store (CDS) one
  month-tile at a time and bulk-inserts the resulting profiles into
  `era5_profiles`.

  The per-point `Era5Client.fetch_profile/3` path is tragically slow because
  every point-hour triggers its own asynchronous CDS job (submit → poll →
  assemble → download). For backfill this balloons into thousands of
  independent CDS jobs. This module instead groups requests by month and a
  small lat/lon tile (2° × 2° by default) so one submit/poll/download cycle
  produces many thousands of profile rows.

  Idempotency: `fetch_month_into_db/1` is a no-op when the requested
  (year, month, tile) already has profile rows in `era5_profiles`. Combined
  with Oban uniqueness on `Era5MonthBatchWorker`, redundant fetches are
  collapsed.
  """

  import Ecto.Query

  alias Microwaveprop.Repo
  alias Microwaveprop.Weather.Era5Client
  alias Microwaveprop.Weather.Era5Profile
  alias Microwaveprop.Weather.Grib2.Wgrib2
  alias Microwaveprop.Weather.SoundingParams

  require Logger

  # ERA5 hourly reanalysis runs on a 0.25° global grid. Tiles are aligned
  # to an even-degree lattice (2° × 2°), producing 9 × 9 = 81 grid points
  # per tile per hour. A month-tile expands to ~60k profiles.
  @tile_degrees 2
  @era5_step 0.25

  @doc """
  Returns the integer tile coordinate `{tile_lat, tile_lon}` for a point,
  where `tile_lat` is the south edge and `tile_lon` is the west edge.
  """
  @spec tile_for_point(float(), float()) :: {integer(), integer()}
  def tile_for_point(lat, lon) do
    {floor_to_tile(lat), floor_to_tile(lon)}
  end

  defp floor_to_tile(value) do
    value
    |> Kernel./(@tile_degrees)
    |> Float.floor()
    |> trunc()
    |> Kernel.*(@tile_degrees)
  end

  @doc false
  @spec tile_degrees() :: pos_integer()
  def tile_degrees, do: @tile_degrees

  @doc """
  Fetches one month of ERA5 data covering the given tile and stores every
  resulting hourly profile in `era5_profiles`. Returns
  `{:ok, inserted_count}` or `{:error, reason}`.
  """
  @spec fetch_month_into_db(%{
          year: pos_integer(),
          month: pos_integer(),
          tile_lat: integer(),
          tile_lon: integer()
        }) :: {:ok, non_neg_integer()} | {:error, term()}
  def fetch_month_into_db(%{year: year, month: month, tile_lat: tile_lat, tile_lon: tile_lon}) do
    if month_tile_cached?(year, month, tile_lat, tile_lon) do
      Logger.info("ERA5Batch: #{year}-#{pad(month)} tile #{tile_lat},#{tile_lon} already cached")
      {:ok, 0}
    else
      do_fetch_month(year, month, tile_lat, tile_lon)
    end
  end

  defp do_fetch_month(year, month, tile_lat, tile_lon) do
    area = tile_area(tile_lat, tile_lon)
    days = days_in_month(year, month)

    Logger.info("ERA5Batch: fetching #{year}-#{pad(month)} tile #{tile_lat},#{tile_lon} (#{days} days)")

    with {:ok, single_grib} <- fetch_single_level_month(year, month, days, area),
         {:ok, pressure_grib} <- fetch_pressure_level_month(year, month, days, area),
         {:ok, profiles} <- build_month_profiles(tile_lat, tile_lon, single_grib, pressure_grib) do
      inserted = bulk_insert_profiles(profiles)

      Logger.info("ERA5Batch: stored #{inserted} profiles for #{year}-#{pad(month)} tile #{tile_lat},#{tile_lon}")

      {:ok, inserted}
    end
  end

  defp month_tile_cached?(year, month, tile_lat, tile_lon) do
    {first, last} = month_bounds(year, month)

    Era5Profile
    |> where(
      [p],
      p.valid_time >= ^first and p.valid_time < ^last and
        p.lat >= ^(tile_lat * 1.0) and p.lat <= ^(tile_lat * 1.0 + @tile_degrees) and
        p.lon >= ^(tile_lon * 1.0) and p.lon <= ^(tile_lon * 1.0 + @tile_degrees)
    )
    |> Repo.exists?()
  end

  defp month_bounds(year, month) do
    {:ok, first} = Date.new(year, month, 1)
    {:ok, first_dt} = DateTime.new(first, ~T[00:00:00], "Etc/UTC")
    last_dt = DateTime.add(first_dt, days_in_month(year, month) * 86_400, :second)
    {first_dt, last_dt}
  end

  defp days_in_month(year, month), do: Calendar.ISO.days_in_month(year, month)

  defp tile_area(tile_lat, tile_lon) do
    # CDS area is [north, west, south, east]
    Enum.map([tile_lat + @tile_degrees, tile_lon, tile_lat, tile_lon + @tile_degrees], &(&1 * 1.0))
  end

  defp pad(n), do: String.pad_leading(Integer.to_string(n), 2, "0")

  # -- CDS requests ----------------------------------------------------------

  defp fetch_single_level_month(year, month, days, area) do
    request = %{
      "product_type" => ["reanalysis"],
      "variable" => Era5Client.single_level_vars(),
      "year" => [Integer.to_string(year)],
      "month" => [pad(month)],
      "day" => Enum.map(1..days, &pad/1),
      "time" => Enum.map(0..23, &"#{pad(&1)}:00"),
      "area" => area,
      "data_format" => "grib"
    }

    Era5Client.submit_and_download("reanalysis-era5-single-levels", request)
  end

  defp fetch_pressure_level_month(year, month, days, area) do
    request = %{
      "product_type" => ["reanalysis"],
      "variable" => Era5Client.pressure_level_vars(),
      "pressure_level" => Era5Client.pressure_levels(),
      "year" => [Integer.to_string(year)],
      "month" => [pad(month)],
      "day" => Enum.map(1..days, &pad/1),
      "time" => Enum.map(0..23, &"#{pad(&1)}:00"),
      "area" => area,
      "data_format" => "grib"
    }

    Era5Client.submit_and_download("reanalysis-era5-pressure-levels", request)
  end

  # -- GRIB extraction -------------------------------------------------------

  defp grid_spec_for_tile(tile_lat, tile_lon) do
    count = round(@tile_degrees / @era5_step) + 1

    %{
      lon_start: tile_lon * 1.0,
      lon_count: count,
      lon_step: @era5_step,
      lat_start: tile_lat * 1.0,
      lat_count: count,
      lat_step: @era5_step
    }
  end

  defp build_month_profiles(tile_lat, tile_lon, single_grib, pressure_grib) do
    grid_spec = grid_spec_for_tile(tile_lat, tile_lon)

    with {:ok, single_msgs} <-
           Wgrib2.extract_grid_messages(single_grib, ":(TMP|DPT|PRES|UGRD|VGRD|PWAT|HPBL):", grid_spec),
         {:ok, pressure_msgs} <- Wgrib2.extract_grid_messages(pressure_grib, ":(TMP|DPT|HGT):", grid_spec) do
      # Index both message sets by (datetime, lat, lon) → var:level map.
      points_single = index_messages(single_msgs)
      points_pressure = index_messages(pressure_msgs)

      all_keys =
        MapSet.union(
          MapSet.new(Map.keys(points_single)),
          MapSet.new(Map.keys(points_pressure))
        )

      profiles =
        all_keys
        |> Enum.map(fn {dt, lat, lon} ->
          single_fields = Map.get(points_single, {dt, lat, lon}, %{})
          pressure_fields = Map.get(points_pressure, {dt, lat, lon}, %{})
          build_profile_attrs(lat, lon, dt, single_fields, pressure_fields)
        end)
        |> Enum.reject(&is_nil/1)

      {:ok, profiles}
    end
  end

  defp index_messages(messages) do
    Enum.reduce(messages, %{}, fn msg, acc ->
      key_name = "#{msg.var}:#{msg.level}"

      Enum.reduce(msg.values, acc, fn {{lat, lon}, value}, inner ->
        Map.update(
          inner,
          {msg.datetime, lat, lon},
          %{key_name => value},
          &Map.put(&1, key_name, value)
        )
      end)
    end)
  end

  defp build_profile_attrs(_lat, _lon, nil, _single, _pressure), do: nil

  defp build_profile_attrs(lat, lon, datetime, single_fields, pressure_fields) do
    temp_c = kelvin_to_celsius(single_fields["TMP:2 m above ground"])
    dewpoint_c = kelvin_to_celsius(single_fields["DPT:2 m above ground"])
    pressure_mb = pa_to_mb(single_fields["PRES:surface"])
    hpbl_m = single_fields["HPBL:surface"]
    pwat_mm = single_fields["PWAT:entire atmosphere"]

    profile =
      Era5Client.pressure_levels()
      |> Enum.map(fn level_str ->
        level = String.to_integer(level_str)
        pres_key = "#{level} mb"

        %{
          "pres" => level * 1.0,
          "tmpc" => kelvin_to_celsius(pressure_fields["TMP:#{pres_key}"]),
          "dwpc" => kelvin_to_celsius(pressure_fields["DPT:#{pres_key}"]),
          "hght" => geopotential_to_height(pressure_fields["HGT:#{pres_key}"])
        }
      end)
      |> Enum.reject(&is_nil(&1["tmpc"]))

    if profile == [] and is_nil(temp_c) do
      nil
    else
      params = SoundingParams.derive(profile)

      base = %{
        valid_time: datetime,
        lat: lat,
        lon: lon,
        profile: profile,
        hpbl_m: hpbl_m,
        pwat_mm: pwat_mm,
        surface_temp_c: temp_c,
        surface_dewpoint_c: dewpoint_c,
        surface_pressure_mb: pressure_mb
      }

      if params do
        Map.merge(base, %{
          surface_refractivity: params.surface_refractivity,
          min_refractivity_gradient: params.min_refractivity_gradient,
          ducting_detected: params.ducting_detected || false,
          duct_characteristics: params.duct_characteristics
        })
      else
        base
      end
    end
  end

  defp kelvin_to_celsius(nil), do: nil
  defp kelvin_to_celsius(k), do: k - 273.15

  defp pa_to_mb(nil), do: nil
  defp pa_to_mb(pa), do: pa / 100.0

  defp geopotential_to_height(nil), do: nil
  defp geopotential_to_height(gp), do: gp / 9.80665

  # -- Bulk insert -----------------------------------------------------------

  # Insert in chunks to keep each query under Postgres' parameter limit
  # (65,535 params / ~15 fields ≈ 4,000 rows per statement).
  @chunk_size 2_000

  defp bulk_insert_profiles(profiles) do
    now = DateTime.truncate(DateTime.utc_now(), :second)

    profiles
    |> Enum.chunk_every(@chunk_size)
    |> Enum.reduce(0, fn chunk, acc ->
      rows =
        Enum.map(chunk, fn attrs ->
          attrs
          |> Map.put(:inserted_at, now)
          |> Map.put(:updated_at, now)
        end)

      {count, _} =
        Repo.insert_all(Era5Profile, rows,
          on_conflict: :nothing,
          conflict_target: [:lat, :lon, :valid_time]
        )

      acc + count
    end)
  end
end