defmodule ImageBriefValidator do
  @moduledoc """
  Validates the structure of an image-generation brief before it is passed to
  a rendering workflow.

  The validator is provider-independent and performs no network requests.
  """

  @valid_styles ~w(photographic illustration flat_vector product_shot poster)
  @required_keys [:prompt, :width, :height]

  @doc """
  Validates a brief and returns either the original brief or all detected
  errors.

  ## Examples

      iex> ImageBriefValidator.validate(%{prompt: "A studio product photo", width: 1200, height: 800})
      {:ok, %{prompt: "A studio product photo", width: 1200, height: 800}}

      iex> ImageBriefValidator.validate(%{prompt: "", width: 0, height: 800})
      {:error, [prompt: :must_be_present, width: :must_be_a_positive_integer]}
  """
  @spec validate(map()) :: {:ok, map()} | {:error, keyword()}
  def validate(%{} = brief) do
    case validate_all(brief) do
      [] -> {:ok, brief}
      errors -> {:error, errors}
    end
  end

  def validate(_brief), do: {:error, brief: :must_be_a_map}

  @doc """
  Returns all structural errors as a keyword list.

  String and atom keys are both accepted. Optional `style` values are limited
  to the documented presets, and `references` must be a list of non-empty
  strings.
  """
  @spec validate_all(map()) :: keyword()
  def validate_all(%{} = brief) do
    normalized = normalize_keys(brief)

    []
    |> check_required(normalized)
    |> check_prompt(normalized)
    |> check_dimension(:width, normalized)
    |> check_dimension(:height, normalized)
    |> check_style(normalized)
    |> check_references(normalized)
    |> Enum.reverse()
  end

  @doc """
  Returns the reduced aspect ratio for valid positive integer dimensions.

      iex> ImageBriefValidator.aspect_ratio(1920, 1080)
      {:ok, "16:9"}
  """
  @spec aspect_ratio(integer(), integer()) :: {:ok, String.t()} | {:error, atom()}
  def aspect_ratio(width, height)
      when is_integer(width) and width > 0 and is_integer(height) and height > 0 do
    divisor = Integer.gcd(width, height)
    {:ok, "#{div(width, divisor)}:#{div(height, divisor)}"}
  end

  def aspect_ratio(_width, _height), do: {:error, :invalid_dimensions}

  defp normalize_keys(brief) do
    Map.new(brief, fn
      {key, value} when is_binary(key) ->
        normalized_key =
          case key do
            "prompt" -> :prompt
            "width" -> :width
            "height" -> :height
            "style" -> :style
            "references" -> :references
            other -> other
          end

        {normalized_key, value}

      pair ->
        pair
    end)
  end

  defp check_required(errors, brief) do
    Enum.reduce(@required_keys, errors, fn key, acc ->
      if Map.has_key?(brief, key), do: acc, else: [{key, :is_required} | acc]
    end)
  end

  defp check_prompt(errors, %{prompt: prompt}) when is_binary(prompt) do
    if String.trim(prompt) == "", do: [{:prompt, :must_be_present} | errors], else: errors
  end

  defp check_prompt(errors, %{prompt: _}), do: [{:prompt, :must_be_a_string} | errors]
  defp check_prompt(errors, _brief), do: errors

  defp check_dimension(errors, key, brief) do
    case Map.fetch(brief, key) do
      {:ok, value} when is_integer(value) and value > 0 -> errors
      {:ok, _value} -> [{key, :must_be_a_positive_integer} | errors]
      :error -> errors
    end
  end

  defp check_style(errors, %{style: style}) when style in @valid_styles, do: errors
  defp check_style(errors, %{style: nil}), do: errors
  defp check_style(errors, %{style: _style}), do: [{:style, :is_not_supported} | errors]
  defp check_style(errors, _brief), do: errors

  defp check_references(errors, %{references: references}) when is_list(references) do
    valid? =
      Enum.all?(references, fn reference ->
        is_binary(reference) and String.trim(reference) != ""
      end)

    if valid?, do: errors, else: [{:references, :must_contain_non_empty_strings} | errors]
  end

  defp check_references(errors, %{references: _references}),
    do: [{:references, :must_be_a_list} | errors]

  defp check_references(errors, _brief), do: errors
end