defmodule AshAge.Query do
  @moduledoc """
  Query structure for AGE graph queries.
  """

  defstruct [
    :resource,
    :graph,
    :label,
    :repo,
    :tenant,
    :expression,
    :limit,
    :offset,
    filters: [],
    sort: [],
    params: %{}
  ]

  @type t :: %__MODULE__{
          resource: module(),
          graph: atom() | String.t(),
          label: atom() | String.t(),
          repo: module(),
          tenant: term() | nil,
          expression: Ash.Filter.t() | nil,
          limit: non_neg_integer() | nil,
          offset: non_neg_integer() | nil,
          filters: [String.t()],
          sort: [{atom(), :asc | :desc}],
          params: map()
        }

  @doc """
  Converts a query to Cypher with parameters.

  Returns `{cypher_string, params_map}`.
  """
  @spec to_cypher(t()) :: {String.t(), map()}
  def to_cypher(%__MODULE__{} = query) do
    # Defense-in-depth: `label` feeds the cypher body and is only otherwise
    # validated at compile time — re-assert it here so a non-identifier can never
    # inject Cypher or break AGE dollar-quoting.
    label = AshAge.Migration.validate_identifier!(query.label)
    {where_parts, query} = build_where(query)

    parts =
      ["MATCH (n:#{label})"] ++
        build_where_clause(where_parts) ++
        ["RETURN n"] ++
        build_order_by(query.sort) ++
        build_skip(query.offset) ++
        build_limit(query.limit)

    {Enum.join(parts, " "), query.params}
  end

  @doc """
  Adds a parameter to the query, returning the updated query and a `$paramN` reference.
  """
  @spec add_param(t(), term()) :: {t(), String.t()}
  def add_param(%__MODULE__{params: params} = query, value) do
    key = next_param_key(params, map_size(params) + 1)
    {%{query | params: Map.put(params, key, value)}, "$#{key}"}
  end

  # Returns the next free `paramN` key, skipping any already taken. On the
  # update/destroy scoping path the params map is pre-seeded with SET-attribute
  # and `match_<pk>` keys; a resource attribute literally named `paramN` must
  # neither clobber nor be clobbered by a filter-scoping param.
  defp next_param_key(params, n) do
    key = "param#{n}"
    if Map.has_key?(params, key), do: next_param_key(params, n + 1), else: key
  end

  defp build_where(query) do
    filter_clauses = query.filters

    {expression_clauses, query} =
      if query.expression do
        case AshAge.Query.Filter.translate(query.expression, query) do
          {:ok, query, ""} -> {[], query}
          {:ok, query, clause} -> {[clause], query}
          _ -> {[], query}
        end
      else
        {[], query}
      end

    {filter_clauses ++ expression_clauses, query}
  end

  defp build_where_clause([]), do: []

  defp build_where_clause(parts) do
    ["WHERE " <> Enum.join(parts, " AND ")]
  end

  defp build_order_by([]), do: []

  defp build_order_by(sort_clauses) do
    order =
      Enum.map_join(sort_clauses, ", ", fn {field, direction} ->
        # Field names are interpolated into the cypher body — validate as identifiers.
        field = AshAge.Migration.validate_identifier!(field)
        dir = if direction == :desc, do: "DESC", else: "ASC"
        "n.#{field} #{dir}"
      end)

    ["ORDER BY " <> order]
  end

  defp build_skip(nil), do: []
  defp build_skip(offset) when is_integer(offset) and offset >= 0, do: ["SKIP #{offset}"]

  defp build_skip(offset) do
    raise ArgumentError, "invalid offset: #{inspect(offset)} (expected a non-negative integer)"
  end

  defp build_limit(nil), do: []
  defp build_limit(limit) when is_integer(limit) and limit >= 0, do: ["LIMIT #{limit}"]

  defp build_limit(limit) do
    raise ArgumentError, "invalid limit: #{inspect(limit)} (expected a non-negative integer)"
  end
end