defmodule Microwaveprop.Propagation.ScorerDiff do
  @moduledoc """
  Compare two weight sets on the same propagation scoring data.

  Loads the most recent HRRR frame's worth of grid points from
  `propagation_scores` (which stores the per-factor scores in JSONB),
  re-scores each with both weight sets, and returns diff statistics.
  """

  import Ecto.Query

  alias Microwaveprop.Propagation.GridScore
  alias Microwaveprop.Repo

  @regression_threshold 5
  @worst_count 10

  @doc """
  Compare old and new weights on recent propagation scores.

  Options:
    * `:limit` — max number of scores to process (default: all for the latest valid_time)

  Returns a map with:
    * `:summary` — aggregate stats (mean_diff, max_diff, regressions, improvements, total)
    * `:band_diffs` — per-band breakdown keyed by band_mhz
    * `:worst_regressions` — up to 10 rows with largest score decrease
  """
  @spec compare(map(), map(), keyword()) :: %{
          summary: map(),
          band_diffs: map(),
          worst_regressions: [map()]
        }
  def compare(old_weights, new_weights, opts \\ []) do
    scores = load_scores(opts)
    diffs = compute_diffs(scores, old_weights, new_weights)

    %{
      summary: summarize(diffs),
      band_diffs: band_breakdown(diffs),
      worst_regressions: worst_regressions(diffs)
    }
  end

  @doc """
  Compute a weighted sum from a factors map and a weights map.

  Both maps use string keys. Returns a rounded integer.
  """
  @spec weighted_sum(map(), map()) :: integer()
  def weighted_sum(factors, weights) do
    weights
    |> Enum.reduce(0.0, fn {factor, weight}, acc ->
      score = Map.get(factors, factor, 0)
      acc + score * weight
    end)
    |> round()
  end

  # ── Private ────────────────────────────────────────────────────────

  defp load_scores(opts) do
    limit = Keyword.get(opts, :limit)

    latest_time =
      GridScore
      |> select([g], max(g.valid_time))
      |> Repo.one()

    if is_nil(latest_time) do
      []
    else
      query =
        GridScore
        |> where([g], g.valid_time == ^latest_time)
        |> order_by([g], [g.band_mhz, g.lat, g.lon])

      query = if limit, do: limit(query, ^limit), else: query

      Repo.all(query)
    end
  end

  defp compute_diffs(scores, old_weights, new_weights) do
    Enum.map(scores, fn score ->
      factors = score.factors
      old_score = weighted_sum(factors, old_weights)
      new_score = weighted_sum(factors, new_weights)
      diff = new_score - old_score

      %{
        lat: score.lat,
        lon: score.lon,
        band_mhz: score.band_mhz,
        old_score: old_score,
        new_score: new_score,
        diff: diff
      }
    end)
  end

  defp summarize([]), do: %{mean_diff: 0.0, max_diff: 0, regressions: 0, improvements: 0, total: 0}

  defp summarize(diffs) do
    total = length(diffs)
    abs_diffs = Enum.map(diffs, fn d -> abs(d.diff) end)
    mean_diff = Float.round(Enum.sum(abs_diffs) / total, 2)
    max_diff = Enum.max(abs_diffs)
    regressions = Enum.count(diffs, fn d -> d.diff < -@regression_threshold end)
    improvements = Enum.count(diffs, fn d -> d.diff > @regression_threshold end)

    %{
      mean_diff: mean_diff,
      max_diff: max_diff,
      regressions: regressions,
      improvements: improvements,
      total: total
    }
  end

  defp band_breakdown(diffs) do
    diffs
    |> Enum.group_by(& &1.band_mhz)
    |> Map.new(fn {band_mhz, band_diffs} ->
      abs_diffs = Enum.map(band_diffs, fn d -> abs(d.diff) end)
      mean = Float.round(Enum.sum(abs_diffs) / length(abs_diffs), 2)

      regressions = Enum.filter(band_diffs, fn d -> d.diff < 0 end)

      max_regression =
        case regressions do
          [] -> 0
          list -> list |> Enum.map(fn d -> abs(d.diff) end) |> Enum.max()
        end

      {band_mhz, %{mean_diff: mean, max_regression: max_regression}}
    end)
  end

  defp worst_regressions(diffs) do
    diffs
    |> Enum.filter(fn d -> d.diff < 0 end)
    |> Enum.sort_by(fn d -> d.diff end)
    |> Enum.take(@worst_count)
    |> Enum.map(fn d ->
      %{
        lat: d.lat,
        lon: d.lon,
        band_mhz: d.band_mhz,
        old_score: d.old_score,
        new_score: d.new_score
      }
    end)
  end

  @doc """
  Format comparison results as a markdown report string.
  """
  @spec to_markdown(%{summary: map(), band_diffs: map(), worst_regressions: [map()]}) :: String.t()
  def to_markdown(result) do
    s = result.summary

    summary = """
    ## Scorer Weight Comparison

    | Metric | Value |
    |--------|-------|
    | Total grid points | #{s.total} |
    | Mean absolute diff | #{s.mean_diff} |
    | Max absolute diff | #{s.max_diff} |
    | Regressions (>#{@regression_threshold} pts) | #{s.regressions} |
    | Improvements (>#{@regression_threshold} pts) | #{s.improvements} |
    """

    band_header = """

    ## Per-Band Breakdown

    | Band (MHz) | Mean Diff | Max Regression |
    |-----------|-----------|----------------|
    """

    band_rows =
      result.band_diffs
      |> Enum.sort_by(fn {band, _} -> band end)
      |> Enum.map_join("\n", fn {band, stats} ->
        "| #{band} | #{stats.mean_diff} | #{stats.max_regression} |"
      end)

    worst_header = """

    ## Worst Regressions

    | Lat | Lon | Band | Old | New | Diff |
    |-----|-----|------|-----|-----|------|
    """

    worst_rows =
      Enum.map_join(result.worst_regressions, "\n", fn r ->
        diff = r.new_score - r.old_score
        "| #{r.lat} | #{r.lon} | #{r.band_mhz} | #{r.old_score} | #{r.new_score} | #{diff} |"
      end)

    summary <> band_header <> band_rows <> worst_header <> worst_rows
  end
end