defmodule Mv.Membership.Import.ColumnResolver do
  @moduledoc """
  Read-only resolution of CSV import columns against the database.

  Given the `HeaderMapper.build_maps/2` result, the raw numbered rows, and an
  actor, `resolve/3` determines:

  - which group names in the groups column already exist (`groups_found`) and
    which would have to be created (`groups_to_create`);
  - a small set of preview rows for the mapping preview UI.

  No database writes happen here; the resolver only reads. Group creation and
  member-group assignment happen during processing via `create_or_find_group/3`.

  This module has no Phoenix or web dependencies.
  """

  require Logger

  alias Mv.Membership.Import.HeaderMapper

  @preview_row_limit 3

  @type numbered_row :: {pos_integer(), [String.t()]}

  @type resolution :: %{
          groups_found: [%{id: String.t(), name: String.t()}],
          groups_to_create: [String.t()],
          fee_type_map: %{String.t() => String.t()},
          fee_type_warnings: [String.t()],
          has_empty_fee_type_cells?: boolean(),
          preview_rows: [[String.t()]]
        }

  @doc """
  Resolves the group and fee-type columns of an import against the database and
  extracts preview rows.

  Returns a map with `:groups_found`, `:groups_to_create`, `:fee_type_map`,
  `:fee_type_warnings`, `:has_empty_fee_type_cells?`, and `:preview_rows`.
  """
  @spec resolve(map(), [numbered_row()], term()) :: resolution()
  def resolve(header_maps, rows, actor) do
    %{
      groups_found: groups_found,
      groups_to_create: groups_to_create
    } = resolve_groups(header_maps, rows, actor)

    %{
      fee_type_map: fee_type_map,
      fee_type_warnings: fee_type_warnings,
      has_empty_fee_type_cells?: has_empty_fee_type_cells?
    } = resolve_fee_types(header_maps, rows, actor)

    %{
      groups_found: groups_found,
      groups_to_create: groups_to_create,
      fee_type_map: fee_type_map,
      fee_type_warnings: fee_type_warnings,
      has_empty_fee_type_cells?: has_empty_fee_type_cells?,
      preview_rows: preview_rows(rows)
    }
  end

  defp resolve_groups(%{groups_column_index: nil}, _rows, _actor) do
    %{groups_found: [], groups_to_create: []}
  end

  defp resolve_groups(%{groups_column_index: index}, rows, actor) do
    existing_groups = list_groups(actor)
    lookup = build_group_lookup(existing_groups)

    names = unique_group_names(rows, index)

    {found, to_create} =
      Enum.reduce(names, {[], []}, fn name, {found, to_create} ->
        case Map.get(lookup, normalize_name(name)) do
          nil -> {found, [name | to_create]}
          group -> {[%{id: group.id, name: group.name} | found], to_create}
        end
      end)

    %{groups_found: Enum.reverse(found), groups_to_create: Enum.reverse(to_create)}
  end

  defp resolve_fee_types(%{fee_type_column_index: nil}, _rows, _actor) do
    %{fee_type_map: %{}, fee_type_warnings: [], has_empty_fee_type_cells?: false}
  end

  defp resolve_fee_types(%{fee_type_column_index: index}, rows, actor) do
    lookup = build_fee_type_lookup(actor)

    cells = Enum.map(rows, fn {_line, values} -> Enum.at(values, index) end)

    has_empty? = Enum.any?(cells, &blank?/1)

    {fee_type_map, warnings} =
      cells
      |> Enum.reject(&blank?/1)
      |> Enum.uniq_by(&normalize_fee_type_name/1)
      |> Enum.reduce({%{}, []}, fn name, {map, warnings} ->
        case Map.get(lookup, normalize_fee_type_name(name)) do
          nil -> {map, [String.trim(name) | warnings]}
          id -> {Map.put(map, normalize_fee_type_name(name), id), warnings}
        end
      end)

    %{
      fee_type_map: fee_type_map,
      fee_type_warnings: Enum.reverse(warnings),
      has_empty_fee_type_cells?: has_empty?
    }
  end

  @doc """
  Normalizes a fee-type name using the same rules as CSV header normalization
  (trim, lowercase, transliterate, drop hyphens and whitespace).
  """
  @spec normalize_fee_type_name(String.t() | nil) :: String.t()
  def normalize_fee_type_name(name) when is_binary(name), do: HeaderMapper.normalize_header(name)
  def normalize_fee_type_name(_), do: ""

  defp build_fee_type_lookup(actor) do
    actor
    |> list_fee_types()
    |> Enum.reduce(%{}, fn fee_type, acc ->
      normalized = normalize_fee_type_name(fee_type.name)

      if Map.has_key?(acc, normalized) do
        Logger.warning(
          "Multiple membership fee types normalize to #{inspect(normalized)}; using the first match for CSV import."
        )

        acc
      else
        Map.put(acc, normalized, fee_type.id)
      end
    end)
  end

  defp list_fee_types(actor) do
    Mv.MembershipFees.list_membership_fee_types!(actor: actor)
  end

  defp blank?(nil), do: true
  defp blank?(value) when is_binary(value), do: String.trim(value) == ""
  defp blank?(_), do: false

  @doc """
  Finds an existing group by name (case-insensitive) or creates it.

  Looks first in the pre-fetched `groups` list, then in the database (to catch
  groups created earlier in the same import), and only creates a new group when
  none is found. This keeps group resolution idempotent across re-imports.
  """
  @spec create_or_find_group(String.t(), [Mv.Membership.Group.t()], term()) ::
          {:ok, Mv.Membership.Group.t()} | {:error, term()}
  def create_or_find_group(name, groups, actor) when is_binary(name) do
    trimmed = String.trim(name)
    normalized = normalize_name(trimmed)

    case find_group_in_list(groups, normalized) do
      nil -> find_or_create_group(trimmed, normalized, actor)
      group -> {:ok, group}
    end
  end

  defp find_group_in_list(groups, normalized) do
    Enum.find(groups, fn group -> normalize_name(group.name) == normalized end)
  end

  defp find_or_create_group(trimmed, normalized, actor) do
    case fetch_group_by_normalized_name(normalized, actor) do
      nil -> create_group(trimmed, normalized, actor)
      group -> {:ok, group}
    end
  end

  # Normalizes the Ash code-interface return to a two-shape result.
  #
  # On a create failure the group may have been created concurrently by another
  # import session between our read and our write (the DB unique index is the
  # final arbiter, and the name validation is fail-open). Re-fetch by normalized
  # name and link to the existing group rather than failing the row.
  defp create_group(name, normalized, actor) do
    case Mv.Membership.create_group(%{name: name}, actor: actor) do
      {:ok, %Mv.Membership.Group{} = group} ->
        {:ok, group}

      {:error, reason} ->
        case fetch_group_by_normalized_name(normalized, actor) do
          nil -> {:error, reason}
          group -> {:ok, group}
        end
    end
  end

  # Fetches a single group by case-insensitive name using a name-filtered query
  # rather than reading the whole groups table. `normalized` is the trimmed,
  # lower-cased name; the DB comparison uses LOWER(name) consistent with the
  # Group resource's case-insensitive uniqueness constraint.
  defp fetch_group_by_normalized_name(normalized, actor) do
    require Ash.Query

    Mv.Membership.Group
    |> Ash.Query.filter(fragment("LOWER(?) = ?", name, ^normalized))
    |> Ash.read(actor: actor, domain: Mv.Membership)
    |> case do
      {:ok, [group | _]} -> group
      _ -> nil
    end
  end

  @doc """
  Splits a raw groups-cell value into trimmed, non-empty group names.
  """
  @spec split_group_names(String.t() | nil) :: [String.t()]
  def split_group_names(nil), do: []

  def split_group_names(cell) when is_binary(cell) do
    cell
    |> String.split(",")
    |> Enum.map(&String.trim/1)
    |> Enum.reject(&(&1 == ""))
  end

  defp unique_group_names(rows, index) do
    rows
    |> Enum.flat_map(fn {_line, values} ->
      values
      |> Enum.at(index)
      |> split_group_names()
    end)
    |> Enum.uniq_by(&normalize_name/1)
  end

  defp preview_rows(rows) do
    rows
    |> Enum.take(@preview_row_limit)
    |> Enum.map(fn {_line, values} -> values end)
  end

  defp list_groups(actor) do
    Mv.Membership.list_groups!(actor: actor)
  end

  defp build_group_lookup(groups) do
    Enum.reduce(groups, %{}, fn group, acc ->
      Map.put(acc, normalize_name(group.name), group)
    end)
  end

  # Case-insensitive comparison consistent with the Group resource's
  # case-insensitive name uniqueness.
  defp normalize_name(name) when is_binary(name) do
    name |> String.trim() |> String.downcase()
  end
end