defmodule Localize.Unit.Preference do
  @moduledoc """
  Returns the preferred units for a given unit, territory, and usage.

  In many cultures, the common unit of measure for some unit categories
  varies based upon the usage. For example, when describing length in
  the US, the units vary by context:

  * road distance — miles for larger distances, feet for smaller
  * person height — feet and inches
  * snowfall — inches

  This module provides functions to look up CLDR preference data for
  different use cases in different territories.

  """

  alias Localize.Unit
  alias Localize.Unit.{Conversion, Data, Parser, BaseUnit}

  @unit_preferences (
                      geq_to_base = fn unit_name, geq_string ->
                        case geq_string do
                          nil ->
                            0.0

                          "" ->
                            0.0

                          s when is_binary(s) ->
                            case Float.parse(s) do
                              {+0.0, _} ->
                                +0.0

                              {geq_value, _} ->
                                primary = unit_name |> String.split("-and-") |> hd()

                                case Parser.parse(primary) do
                                  {:ok, parsed} ->
                                    case BaseUnit.base_unit(parsed) do
                                      {:ok, base} ->
                                        case Conversion.convert(geq_value, primary, base) do
                                          {:ok, v} -> v
                                          _ -> 0.0
                                        end

                                      _ ->
                                        0.0
                                    end

                                  _ ->
                                    0.0
                                end

                              :error ->
                                0.0
                            end
                        end
                      end

                      Data.unit_preferences()
                      |> Enum.group_by(& &1.category)
                      |> Map.new(fn {category, entries} ->
                        by_usage =
                          entries
                          |> Enum.group_by(& &1.usage)
                          |> Map.new(fn {usage, usage_entries} ->
                            prefs =
                              usage_entries
                              |> Enum.flat_map(fn entry ->
                                Enum.map(entry.preferences, fn p ->
                                  %{
                                    unit: p.unit,
                                    geq: geq_to_base.(p.unit, p[:geq]),
                                    regions: String.split(p.regions, " ", trim: true),
                                    skeleton: p[:skeleton] || ""
                                  }
                                end)
                              end)

                            {usage, prefs}
                          end)

                        {category, by_usage}
                      end)
                    )

  @doc """
  Returns the preferred units for a given unit, territory, and usage.

  The unit's value is converted to the base unit for its category and
  compared against the `geq` (greater-than-or-equal) thresholds in the
  preference data to select the appropriate unit set.

  ### Arguments

  * `unit` is a `t:Localize.Unit.t/0` struct.

  * `options` is a keyword list of options.

  ### Options

  * `:usage` is the unit usage context as an atom (e.g., `:person_height`,
    `:road`). The default is `:default`.

  * `:territory` is a territory code atom (e.g., `:US`, `:GB`). The
    default is derived from the current locale.

  * `:locale` is a locale identifier. Used to derive the territory when
    `:territory` is not provided.

  ### Returns

  * `{:ok, units, options}` where `units` is a list of unit name atoms
    and `options` is a keyword list of formatting hints (e.g.,
    `[round_nearest: 1]`).

  * `{:error, exception}` if preferences cannot be resolved.

  ### Examples

      iex> meter = Localize.Unit.new!(1, "meter")
      iex> {:ok, units, _opts} = Localize.Unit.Preference.preferred_units(meter, usage: :person, territory: :US)
      iex> units
      [:inch]

      iex> many_meters = Localize.Unit.new!(10_000, "meter")
      iex> {:ok, units, _opts} = Localize.Unit.Preference.preferred_units(many_meters, usage: :road, territory: :US)
      iex> units
      [:mile]

  """
  @spec preferred_units(Unit.t(), Keyword.t()) ::
          {:ok, [atom()], Keyword.t()} | {:error, Exception.t()}
  def preferred_units(%Unit{} = unit, options \\ []) do
    usage = Keyword.get(options, :usage, :default)
    territory = resolve_territory(options)

    with {:ok, category} <- unit_category(unit),
         {:ok, base_value} <- base_unit_value(unit),
         {:ok, territory_chain} <- Localize.Territory.territory_chain(territory) do
      usage_chain = build_usage_chain(usage)
      find_preference(category, usage_chain, territory_chain, abs(base_value))
    end
  end

  @doc """
  Same as `preferred_units/2` but raises on error.

  """
  @spec preferred_units!(Unit.t(), Keyword.t()) :: [atom()] | no_return()
  def preferred_units!(%Unit{} = unit, options \\ []) do
    case preferred_units(unit, options) do
      {:ok, units, _opts} -> units
      {:error, exception} -> raise exception
    end
  end

  # ── Category resolution ─────────────────────────────────────

  defp unit_category(%Unit{name: name}) do
    with {:ok, parsed} <- Parser.parse(name),
         {:ok, base} <- BaseUnit.base_unit(parsed) do
      btq = Data.base_unit_to_quantity()

      # Try the original unit name first (handles compound units like
      # cubic-meter-per-meter → consumption), then fall back to base unit
      case Map.get(btq, name) || Map.get(btq, base) do
        nil ->
          {:error,
           Localize.InvalidValueError.exception(
             value: name,
             expected: "a unit with a known category"
           )}

        category ->
          {:ok, category}
      end
    end
  end

  # ── Base unit value ─────────────────────────────────────────

  defp base_unit_value(%Unit{value: value, name: name}) do
    with {:ok, parsed} <- Parser.parse(name),
         {:ok, base} <- BaseUnit.base_unit(parsed) do
      Conversion.convert(to_float(value), name, base)
    end
  end

  defp to_float(%Decimal{} = d), do: Decimal.to_float(d)
  defp to_float(n) when is_number(n), do: n * 1.0
  defp to_float(nil), do: 0.0

  # ── Territory resolution ────────────────────────────────────

  defp resolve_territory(options) do
    case Keyword.get(options, :territory) do
      nil ->
        locale = Keyword.get(options, :locale, Localize.get_locale())

        case Localize.Territory.territory_from_locale(locale) do
          {:ok, territory} -> territory
          _ -> :US
        end

      territory ->
        territory
    end
  end

  # ── Usage chain ─────────────────────────────────────────────

  # Build a chain of usages to try, from most specific to least.
  # For example, :person_height → [:person_height, :person, :default]
  defp build_usage_chain(:default), do: [:default]

  defp build_usage_chain(usage) when is_atom(usage) do
    parts = usage |> Atom.to_string() |> String.split("_")

    chain =
      parts
      |> Enum.scan(fn part, acc -> acc <> "_" <> part end)
      |> Enum.reverse()
      |> Enum.map(&String.to_atom/1)

    chain ++ [:default]
  end

  # ── Preference lookup ───────────────────────────────────────

  defp find_preference(category, usage_chain, territory_chain, base_value) do
    Enum.find_value(usage_chain, fn usage ->
      find_for_usage(category, usage, territory_chain, base_value)
    end) ||
      {:error,
       Localize.InvalidValueError.exception(
         value: {category, hd(usage_chain)},
         expected: "a known unit preference"
       )}
  end

  defp find_for_usage(category, usage, territory_chain, base_value) do
    usage_str = Atom.to_string(usage) |> String.replace("_", "-")

    case get_in(@unit_preferences, [category, usage_str]) do
      nil -> nil
      preferences -> find_for_territory(preferences, territory_chain, base_value)
    end
  end

  defp find_for_territory(preferences, territory_chain, base_value) do
    Enum.find_value(territory_chain, fn territory ->
      territory_str = Atom.to_string(territory)
      matching = Enum.filter(preferences, fn p -> territory_str in p.regions end)

      case find_by_geq(matching, base_value) do
        nil -> nil
        result -> result
      end
    end)
  end

  defp find_by_geq([], _value), do: nil

  defp find_by_geq(preferences, base_value) do
    # Process preferences in order. For each:
    # - If geq > 0: check if base_value >= geq (threshold match)
    # - If geq == 0: convert to this unit, check if value >= 1 (size cascade)
    # First match wins. If none match, use the last preference.
    match =
      Enum.find(preferences, fn pref ->
        if pref.geq > 0.0 do
          # Use a small epsilon for floating-point comparison to handle
          # IEEE 754 rounding (e.g., 3.0 feet = 0.9144000000000001 meters)
          base_value >= pref.geq * (1.0 - 1.0e-12)
        else
          # Size cascade: convert base value to this unit, check >= 1
          primary_unit = pref.unit |> String.split("-and-") |> hd()
          converted_value_gte_1?(base_value, primary_unit)
        end
      end)

    case match do
      nil ->
        # Fall back to the last (smallest) preference
        case List.last(preferences) do
          nil -> nil
          pref -> format_result(pref)
        end

      pref ->
        format_result(pref)
    end
  end

  @dialyzer {:nowarn_function, converted_value_gte_1?: 2}
  defp converted_value_gte_1?(base_value, unit_name) do
    case Parser.parse(unit_name) do
      {:ok, parsed} ->
        case BaseUnit.base_unit(parsed) do
          {:ok, base} ->
            case Conversion.convert(base_value, base, unit_name) do
              {:ok, converted} -> abs(converted) >= 1.0 - 1.0e-8
              _ -> false
            end

          _ ->
            false
        end

      _ ->
        false
    end
  end

  defp format_result(pref) do
    units =
      pref.unit
      |> String.split("-and-")
      |> Enum.map(fn u -> u |> String.replace("-", "_") |> String.to_atom() end)

    skeleton = parse_skeleton(pref)
    {:ok, units, skeleton}
  end

  defp parse_skeleton(%{skeleton: skeleton}) when is_binary(skeleton) and skeleton != "" do
    skeleton
    |> String.split("/")
    |> Enum.flat_map(fn part ->
      case part do
        "1" -> [round_nearest: 1]
        "5" -> [round_nearest: 5]
        "10" -> [round_nearest: 10]
        "50" -> [round_nearest: 50]
        "100" -> [round_nearest: 100]
        _ -> []
      end
    end)
  end

  defp parse_skeleton(_), do: []
end