defmodule PhoenixKitWeb.Components.MultilangForm do
  @moduledoc """
  Shared multilang form components and helpers for PhoenixKit modules.

  Provides the language tab switcher UI, skeleton loading placeholders,
  translatable field components, and Elixir-side helpers for merging
  multilang data in LiveView forms.

  Designed for two main use cases:

  1. **Whole-form translation** — wrap all translatable fields in a card with
     language tabs. The tab bar, skeleton placeholders, and field wrappers
     are handled automatically.

  2. **Single-field translation** — drop a `<.translatable_field>` into any
     form to make one field translatable, with no tab UI required (the caller
     manages `current_lang` however they like).

  ## Usage in a LiveView

  ### Mount

      import PhoenixKitWeb.Components.MultilangForm

      def mount(params, session, socket) do
        # ... load your record and changeset ...
        {:ok, mount_multilang(socket)}
      end

  ### Events

      def handle_event("switch_language", %{"lang" => lang_code}, socket) do
        {:noreply, handle_switch_language(socket, lang_code)}
      end

      def handle_event("validate", %{"record" => params}, socket) do
        params = merge_translatable_params(params, socket, ["name", "description"],
          changeset: socket.assigns.changeset)
        changeset = MySchema.changeset(socket.assigns.record, params)
        {:noreply, assign(socket, :changeset, changeset)}
      end

  ### Template — whole-form translation

      <.multilang_tabs
        multilang_enabled={@multilang_enabled}
        language_tabs={@language_tabs}
        current_lang={@current_lang}
      />

      <.multilang_fields_wrapper
        multilang_enabled={@multilang_enabled}
        current_lang={@current_lang}
      >
        <.translatable_field
          field_name="name"
          form_prefix="catalogue"
          changeset={@changeset}
          schema_field={:name}
          multilang_enabled={@multilang_enabled}
          current_lang={@current_lang}
          primary_language={@primary_language}
          lang_data={@lang_data}
          label={gettext("Name")}
          required
        />
      </.multilang_fields_wrapper>

  ### Template — single-field translation (no tabs needed)

      <.translatable_field
        field_name="description"
        form_prefix="product"
        changeset={@changeset}
        schema_field={:description}
        multilang_enabled={@multilang_enabled}
        current_lang={@current_lang}
        primary_language={@primary_language}
        lang_data={@lang_data}
        label={gettext("Description")}
        type="textarea"
        rows={5}
      />
  """

  use Phoenix.Component
  use Gettext, backend: PhoenixKitWeb.Gettext

  import PhoenixKitWeb.Components.Core.Icon
  import PhoenixKitWeb.Components.Core.FormFieldError, only: [error: 1]
  import PhoenixKitWeb.Components.Core.Input, only: [translate_error: 1]
  import PhoenixKitWeb.Components.LanguageSwitcher, only: [language_switcher: 1]

  alias Phoenix.LiveView.JS
  # PhoenixKit.Utils.Multilang is in an external package — referenced by full name
  # with Code.ensure_loaded?/rescue guards throughout this module.

  # ═══════════════════════════════════════════════════════════════════
  # Mount & Event Helpers
  # ═══════════════════════════════════════════════════════════════════

  @doc """
  Adds multilang assigns to the socket. Call from `mount/3`.

  Adds: `:multilang_enabled`, `:primary_language`, `:current_lang`,
  `:language_tabs`, `:show_multilang_tabs`, `:switching_lang` (no-op
  compat assign — kept because consumer templates pass it through to
  the wrapper). Also attaches an internal `:handle_info` hook that
  receives the debounced language-switch timer message.
  """
  def mount_multilang(socket) do
    multilang_enabled = multilang_enabled?()
    primary_language = if multilang_enabled, do: safe_primary_language(), else: nil

    language_tabs = if(multilang_enabled, do: safe_build_language_tabs(), else: [])

    socket
    |> Phoenix.Component.assign(
      multilang_enabled: multilang_enabled,
      primary_language: primary_language,
      current_lang: primary_language,
      language_tabs: language_tabs,
      show_multilang_tabs: multilang_enabled and length(language_tabs) > 1,
      # Kept for backwards compat with consumer templates that still pass
      # `switching_lang={@switching_lang}` to `<.multilang_fields_wrapper>`.
      # The wrapper no longer reads it — visibility is client-side JS.
      switching_lang: false
    )
    |> attach_multilang_hook()
  end

  # Attaches a `:handle_info` hook that intercepts the internal
  # `{:__multilang_apply_lang__, lang}` message. The hook lives on the
  # consumer's socket but is invisible to them — no `handle_info/2`
  # clause needed. Returning `{:halt, socket}` prevents the message
  # from reaching the consumer's own handle_info (and suppresses the
  # "unhandled message" warning). Other messages pass through with
  # `{:cont, socket}`.
  #
  # `attach_hook/4` is idempotent-by-name within a process — re-running
  # `mount_multilang/1` (e.g. across reconnects) with the same hook id
  # simply replaces the prior callback, so we don't need a guard.
  defp attach_multilang_hook(socket) do
    Phoenix.LiveView.attach_hook(
      socket,
      :__phoenix_kit_multilang_apply_lang__,
      :handle_info,
      fn
        {:__multilang_apply_lang__, lang_code}, socket ->
          {:halt, handle_multilang_apply_lang(socket, lang_code)}

        _msg, socket ->
          {:cont, socket}
      end
    )
  rescue
    # `attach_hook/4` only works on LiveView sockets, not LiveComponent
    # sockets. Multilang consumers are all `Phoenix.LiveView` today, but
    # if someone wires it into a component in the future, fall back
    # silently — they'll need to add the `handle_info` themselves.
    ArgumentError -> socket
  end

  @doc """
  Refreshes multilang assigns after external changes (e.g. entity schema update).

  Unlike `mount_multilang/1`, this preserves `current_lang` when it's still valid,
  and resets it to the primary language if it was removed.
  """
  def refresh_multilang(socket) do
    multilang_enabled = multilang_enabled?()
    primary_language = if multilang_enabled, do: safe_primary_language(), else: nil
    language_tabs = if multilang_enabled, do: safe_build_language_tabs(), else: []

    current_lang = socket.assigns[:current_lang]
    enabled_langs = if multilang_enabled, do: safe_enabled_languages(), else: []

    current_lang =
      cond do
        not multilang_enabled -> nil
        current_lang not in enabled_langs -> primary_language
        true -> current_lang
      end

    Phoenix.Component.assign(socket,
      multilang_enabled: multilang_enabled,
      primary_language: primary_language,
      current_lang: current_lang,
      language_tabs: language_tabs,
      show_multilang_tabs: multilang_enabled and length(language_tabs) > 1
    )
  end

  @doc """
  Handles the `"switch_language"` event. Call from `handle_event/3`.

  Returns a socket that **defers** applying `:current_lang` via a short
  trailing debounce (150 ms). Rapid click-through (EN → JA → FR → DE)
  keeps rescheduling the timer; only the last click actually updates
  `:current_lang` and triggers a content re-render. Without this, every
  intermediate event caused its own server render and the client
  briefly flashed each language's content before landing on the final
  one.

  Nothing to do on the consumer's end — `mount_multilang/1` attaches a
  `:handle_info` hook via `Phoenix.LiveView.attach_hook/4` that
  intercepts the internal `{:__multilang_apply_lang__, lang}` message
  and applies the language transparently. Calling code never sees the
  message.

  Ignores unknown language codes.
  """
  @multilang_debounce_ms 150
  # The timer ref lives in `socket.private` (not `socket.assigns`) so
  # that storing it doesn't trigger a render+diff cycle that would fight
  # the client-side skeleton/fields visibility toggles. Private state
  # is per-socket — visible only inside the LV process — and survives
  # the LV's lifecycle the same way assigns do, but with zero diff cost.
  @multilang_timer_private_key :__phoenix_kit_multilang_timer__

  def handle_switch_language(socket, lang_code) do
    if lang_code in safe_enabled_languages() do
      socket = cancel_multilang_timer(socket)

      timer_ref =
        Process.send_after(
          self(),
          {:__multilang_apply_lang__, lang_code},
          @multilang_debounce_ms
        )

      # Skeleton/fields visibility is driven entirely by
      # `switch_lang_js/2`'s client-side class toggles. The server just
      # holds off applying the new `current_lang` until the debounce
      # fires, which is what causes the final morphdom swap that
      # restores the fields (with new-lang content) and hides the
      # skeleton again.
      Phoenix.LiveView.put_private(socket, @multilang_timer_private_key, timer_ref)
    else
      socket
    end
  end

  @doc """
  Applies a debounced language change. Call from `handle_info/2` when
  `{:__multilang_apply_lang__, lang_code}` is received.
  """
  def handle_multilang_apply_lang(socket, lang_code) do
    socket
    |> Phoenix.LiveView.put_private(@multilang_timer_private_key, nil)
    |> Phoenix.Component.assign(:current_lang, lang_code)
  end

  defp cancel_multilang_timer(socket) do
    case Map.get(socket.private, @multilang_timer_private_key) do
      ref when is_reference(ref) ->
        Process.cancel_timer(ref)
        Phoenix.LiveView.put_private(socket, @multilang_timer_private_key, nil)

      _ ->
        socket
    end
  end

  @doc """
  Merges translatable field params into the multilang `data` JSONB structure.

  Takes the raw form params, the socket, and a list of translatable field names
  (the DB column names, e.g. `["name", "description"]`). Returns updated params
  with the `"data"` key set to the merged multilang structure.

  On primary language tabs, reads from `params["name"]`.
  On secondary language tabs, reads from `params["lang_name"]`.

  Also preserves primary language values for non-translatable fields when on
  secondary tabs via the `preserve_fields` option.

  ## Options

    * `:changeset` — the current changeset (required)
    * `:preserve_fields` — map of `%{"field_name" => :schema_field}` for fields
      that should keep their primary language DB column value on secondary tabs.
      Defaults to `%{}`.
  """
  def merge_translatable_params(params, socket, translatable_fields, opts \\ []) do
    changeset = Keyword.fetch!(opts, :changeset)
    preserve_fields = Keyword.get(opts, :preserve_fields, %{})
    assigns = socket.assigns
    current_lang = assigns[:current_lang]
    primary = assigns[:primary_language]

    params =
      if assigns[:multilang_enabled] do
        form_data = extract_translatable_data(params, translatable_fields, current_lang, primary)
        final_data = do_merge_multilang_data(changeset, current_lang, form_data, assigns)
        Map.put(params, "data", final_data)
      else
        params
      end

    do_preserve_primary_fields(params, changeset, assigns, preserve_fields)
  end

  @doc """
  Injects a DB column value into the JSONB `data` field for multilang storage.

  This handles the common pattern where a field exists both as a top-level DB
  column (for queries/sorting) and inside the JSONB `data` (for translations).

  On the primary language tab, reads from `params[field_name]` (the DB column input).
  On secondary language tabs, reads from `params["lang_" <> field_name]` (the translation input).

  The value is stored in the data map under `"_" <> field_name` (e.g. `"_title"`).

  If no value is submitted (field not in form), preserves the existing value from
  the changeset's JSONB data.

  ## Requirements

  `assigns` must contain:
  - `:multilang_enabled` — boolean
  - `:primary_language` — the primary language code
  - `:changeset` — an `Ecto.Changeset` with a `:data` field (JSONB)

  ## Examples

      # In handle_event("validate", ...)
      form_data =
        form_data
        |> inject_db_field_into_data("title", data_params, current_lang, socket.assigns)
        |> inject_db_field_into_data("slug", data_params, current_lang, socket.assigns)
  """
  def inject_db_field_into_data(form_data, field_name, params, current_lang, assigns) do
    if assigns[:multilang_enabled] == true do
      primary = assigns[:primary_language]

      value =
        if current_lang == primary,
          do: params[field_name],
          else: params["lang_#{field_name}"]

      data_key = "_#{field_name}"

      if is_binary(value) do
        Map.put(form_data, data_key, value)
      else
        # No value submitted — preserve existing from JSONB data
        existing_data = safe_get_changeset_data(assigns.changeset)

        case PhoenixKit.Utils.Multilang.get_raw_language_data(existing_data, current_lang) do
          %{^data_key => existing} -> Map.put(form_data, data_key, existing)
          _ -> form_data
        end
      end
    else
      form_data
    end
  end

  @doc """
  Merges language-specific validated data into the full multilang JSONB structure.

  Reads existing data from the changeset's `:data` field, then merges the new
  `validated_data` for the given `lang_code`.

  Handles three cases:
  - Multilang enabled: uses `PhoenixKit.Utils.Multilang.put_language_data/3`
  - Multilang disabled but data has multilang structure: preserves translations
  - Flat data, no multilang: passes through as-is

  The `changeset` must be an `Ecto.Changeset` with a `:data` field.
  `assigns` must contain `:multilang_enabled`.
  """
  def merge_multilang_data(changeset, lang_code, validated_data, assigns) do
    do_merge_multilang_data(changeset, lang_code, validated_data, assigns)
  end

  @doc """
  Gets the raw language data for the current language from a changeset.

  Use this in templates to read override-only values for secondary language tabs.
  Returns `%{}` when multilang is disabled.
  """
  def get_lang_data(changeset, current_lang, multilang_enabled) do
    if multilang_enabled && changeset do
      PhoenixKit.Utils.Multilang.get_raw_language_data(
        Ecto.Changeset.get_field(changeset, :data),
        current_lang
      )
    else
      %{}
    end
  end

  @doc """
  Returns true when on the primary language tab (or multilang is disabled).
  """
  def primary_tab?(assigns) do
    !assigns[:multilang_enabled] || assigns[:current_lang] == assigns[:primary_language]
  end

  @doc """
  Preserves primary-language DB field values when on a secondary language tab.

  On secondary tabs, some fields (like title, slug) are absent from form params
  because they're replaced by `lang_*` inputs. This function fills in the missing
  values from the changeset so the DB columns keep their primary-language values.

  `preserve_fields` is a map of `%{"field_name" => :schema_field}`.

  No-ops when multilang is disabled or on the primary tab.
  """
  def preserve_primary_fields(params, changeset, assigns, preserve_fields) do
    do_preserve_primary_fields(params, changeset, assigns, preserve_fields)
  end

  @doc "Returns true when the Languages module is enabled with 2+ languages."
  def multilang_enabled? do
    Code.ensure_loaded?(PhoenixKit.Utils.Multilang) and PhoenixKit.Utils.Multilang.enabled?()
  rescue
    _ -> false
  end

  defp safe_primary_language do
    PhoenixKit.Utils.Multilang.primary_language()
  rescue
    _ -> "en-US"
  end

  defp safe_enabled_languages do
    PhoenixKit.Utils.Multilang.enabled_languages()
  rescue
    _ -> []
  end

  defp safe_build_language_tabs do
    PhoenixKit.Utils.Multilang.build_language_tabs()
  rescue
    _ -> []
  end

  # Safely reads a field from a changeset, returning "" on any error.
  defp safe_get_field(%Ecto.Changeset{} = changeset, field) when is_atom(field) do
    Ecto.Changeset.get_field(changeset, field) || ""
  rescue
    _ -> ""
  end

  defp safe_get_field(_changeset, _field), do: ""

  # Safely reads the :data field from a changeset, returning %{} on any error.
  defp safe_get_changeset_data(%Ecto.Changeset{} = changeset) do
    Ecto.Changeset.get_field(changeset, :data) || %{}
  rescue
    _ -> %{}
  end

  defp safe_get_changeset_data(_), do: %{}

  # ── Private helpers ────────────────────────────────────────────

  defp extract_translatable_data(params, fields, current_lang, primary) do
    Enum.reduce(fields, %{}, fn field, acc ->
      value =
        if current_lang == primary,
          do: params[field],
          else: params["lang_#{field}"]

      if is_binary(value), do: Map.put(acc, "_#{field}", value), else: acc
    end)
  end

  defp do_merge_multilang_data(changeset, lang_code, validated_data, assigns) do
    existing_data = safe_get_changeset_data(changeset)

    cond do
      assigns[:multilang_enabled] == true ->
        PhoenixKit.Utils.Multilang.put_language_data(existing_data, lang_code, validated_data)

      PhoenixKit.Utils.Multilang.multilang_data?(existing_data) ->
        PhoenixKit.Utils.Multilang.put_language_data(existing_data, lang_code, validated_data)

      true ->
        validated_data
    end
  end

  defp do_preserve_primary_fields(params, _changeset, assigns, preserve_fields)
       when map_size(preserve_fields) == 0 or
              not is_map_key(assigns, :multilang_enabled) do
    params
  end

  defp do_preserve_primary_fields(params, changeset, assigns, preserve_fields) do
    if assigns[:multilang_enabled] && assigns[:current_lang] != assigns[:primary_language] do
      Enum.reduce(preserve_fields, params, fn {str_key, atom_key}, acc ->
        preserve_field_value(acc, changeset, str_key, atom_key)
      end)
    else
      params
    end
  end

  defp preserve_field_value(params, changeset, str_key, atom_key) do
    if Map.has_key?(params, str_key) do
      params
    else
      case Ecto.Changeset.get_field(changeset, atom_key) do
        nil -> params
        value -> Map.put(params, str_key, value)
      end
    end
  end

  # ═══════════════════════════════════════════════════════════════════
  # Components
  # ═══════════════════════════════════════════════════════════════════

  @doc """
  Renders the language tab bar with compact/full mode.

  Shows a header with language icon, an info alert explaining the translation
  workflow, and the shared `<.language_switcher>` in `:tabs` variant with flags,
  names, and primary star indicator.

  Display mode:
  - `compact: nil` (default) — auto: full names when ≤ 5 languages, short codes when more
  - `compact: true` — always short codes
  - `compact: false` — always full names

  Delegates to `PhoenixKitWeb.Components.LanguageSwitcher.language_switcher/1`
  for the tab bar rendering.

  ## Attributes

    * `multilang_enabled` — boolean, whether multilang is active
    * `language_tabs` — list of tab maps from `PhoenixKit.Utils.Multilang.build_language_tabs/0`
    * `current_lang` — the currently selected language code
    * `compact` — force compact mode (short codes). Default: nil (auto)
    * `show_header` — show the "Content Language" header. Default: true
    * `show_info` — show the info alert. Default: true
  """
  attr :multilang_enabled, :boolean, required: true
  attr :language_tabs, :list, required: true
  attr :current_lang, :string, required: true
  attr :compact, :boolean, default: nil
  attr :show_header, :boolean, default: true
  attr :show_info, :boolean, default: true
  attr :class, :string, default: "card-body pb-0"

  def multilang_tabs(assigns) do
    display =
      cond do
        assigns.compact == true -> :compact
        assigns.compact == false -> :full
        true -> :auto
      end

    assigns = assign(assigns, :display, display)

    ~H"""
    <div :if={@multilang_enabled && match?([_, _ | _], @language_tabs)} class={@class}>
      <div :if={@show_header} class="flex items-center justify-between mb-3">
        <div class="flex items-center gap-2">
          <.icon name="hero-language" class="w-5 h-5 text-primary" />
          <h2 class="card-title text-lg m-0">{gettext("Content Language")}</h2>
        </div>
        <% primary_tab = Enum.find(@language_tabs, fn t -> t.is_primary end) %>
        <div :if={primary_tab} class="flex items-center gap-1.5 text-xs text-base-content/60">
          <.icon name="hero-star-solid" class="w-3.5 h-3.5 text-primary" />
          <span>{gettext("Primary: %{lang}", lang: primary_tab.name)}</span>
        </div>
      </div>

      <div :if={@show_info} class="alert alert-info py-2 text-xs mb-4">
        <.icon name="hero-information-circle" class="w-4 h-4" />
        <span>
          {gettext(
            "Use the language tabs below to translate this record's content. The primary language (marked with a star) is required. Other languages are optional — any empty fields will fall back to the primary language value."
          )}
        </span>
      </div>

      <div class="mb-4">
        <.language_switcher
          languages={@language_tabs}
          current_language={@current_lang}
          on_click_js={&switch_lang_js(&1, @current_lang)}
          display={@display}
          auto_threshold={5}
          show_flags={true}
          show_primary={true}
          primary_divider={true}
          variant={:tabs}
          size={:sm}
        />
      </div>
    </div>
    """
  end

  @doc """
  Renders skeleton loading placeholders and a content wrapper for translatable fields.

  The skeleton is shown instantly on tab switch via JS, then hidden when LiveView
  re-renders with the new language data.

  Wrap your translatable form fields inside this component's inner block.

  ## Customizing skeletons

  Use the `:skeleton` slot to provide custom skeleton markup that matches
  your form layout. If omitted, a default two-field skeleton is rendered.

  ## Attributes

    * `multilang_enabled` — boolean
    * `current_lang` — current language code (used in element IDs for morphdom)
    * `skeleton_class` — CSS class for the skeleton container. Default: `"card-body pt-4"`
    * `fields_class` — CSS class for the fields container. Default: nil

  ## Example — default skeleton (card context)

      <.multilang_fields_wrapper multilang_enabled={@multilang_enabled} current_lang={@current_lang}>
        <%!-- translatable fields here --%>
      </.multilang_fields_wrapper>

  ## Example — custom skeleton and classes (non-card context)

      <.multilang_fields_wrapper
        multilang_enabled={@multilang_enabled}
        current_lang={@current_lang}
        skeleton_class="space-y-6"
        fields_class="space-y-6"
      >
        <:skeleton>
          <div class="grid grid-cols-2 gap-6">
            <div class="skeleton h-12 w-full"></div>
            <div class="skeleton h-12 w-full"></div>
          </div>
        </:skeleton>
        <%!-- translatable fields here --%>
      </.multilang_fields_wrapper>
  """
  attr :multilang_enabled, :boolean, required: true
  attr :current_lang, :string, required: true

  attr :switching_lang, :boolean,
    default: false,
    doc:
      "accepted for backwards compatibility but no longer used — skeleton/fields visibility is client-side via `switch_lang_js/2`."

  attr :skeleton_class, :string, default: "card-body pt-4"
  attr :fields_class, :string, default: nil
  slot :skeleton
  slot :inner_block, required: true

  def multilang_fields_wrapper(assigns) do
    ~H"""
    <%!-- Skeleton placeholders (shown instantly on tab click via JS).
         IDs include current_lang so that when the debounce fires and
         current_lang changes, morphdom treats these as new elements
         and replaces them — which resets the skeleton back to `hidden`
         and inserts the fields with new-language content. --%>
    <div
      :if={@multilang_enabled}
      id={"translatable-skeletons-#{@current_lang}"}
      data-translatable="skeletons"
      class={["hidden", @skeleton_class]}
      aria-busy="true"
      aria-label={gettext("Loading language content")}
    >
      <%= if @skeleton != [] do %>
        {render_slot(@skeleton)}
      <% else %>
        <div class="space-y-4">
          <div class="space-y-2">
            <div class="skeleton h-4 w-24"></div>
            <div class="skeleton h-12 w-full"></div>
          </div>
          <div class="space-y-2">
            <div class="skeleton h-4 w-32"></div>
            <div class="skeleton h-24 w-full"></div>
          </div>
        </div>
      <% end %>
    </div>

    <div
      id={if @multilang_enabled, do: "translatable-fields-#{@current_lang}", else: "form-fields"}
      data-translatable="fields"
      class={@fields_class}
    >
      {render_slot(@inner_block)}
    </div>
    """
  end

  @doc """
  Renders a translatable text input or textarea field.

  On the primary language tab, renders a standard input reading from the changeset.
  On secondary language tabs, renders with a language-specific name and uses the
  primary language value as placeholder text.

  Works both inside a `<.multilang_fields_wrapper>` (whole-form translation) and
  standalone (single-field translation).

  ## Translation models

  Supports two naming patterns for secondary language inputs:

  **Default (JSONB data column)** — used by entity data records:
  - Secondary name: `form_prefix[lang_field_name]`
  - Lang data key: `"_field_name"`

  **Settings translations** — used by entity definitions and other models
  that store translations in a `settings["translations"]` map. Set
  `secondary_name` and `lang_data_key` to override the defaults:

      <.translatable_field
        field_name="display_name"
        form_prefix="entities"
        secondary_name={"entities[translations][\#{@current_lang}][display_name]"}
        lang_data_key="display_name"
        ...
      />

  ## Attributes

    * `field_name` — the DB column name (e.g., "name")
    * `form_prefix` — the form name prefix (e.g., "catalogue")
    * `changeset` — the Ecto changeset
    * `schema_field` — the schema field atom (e.g., `:name`)
    * `multilang_enabled` — boolean
    * `current_lang` — current language code
    * `primary_language` — primary language code
    * `lang_data` — raw language data map for the current language
    * `label` — field label text
    * `placeholder` — placeholder for primary language (optional)
    * `type` — "input" or "textarea". Default: "input"
    * `rows` — textarea rows (only for type="textarea"). Default: 3
    * `required` — marks the primary language field as required. Default: false
    * `disabled` — disables the field. Default: false
    * `class` — additional CSS class(es) for the input element. Default: nil
    * `pattern` — HTML pattern attribute for input validation. Default: nil
    * `title` — HTML title attribute (for pattern validation message). Default: nil
    * `hint` — hint text shown below the field. Default: nil
    * `secondary_hint` — hint text shown only on secondary language tabs. Default: nil
    * `secondary_name` — override the secondary tab input name. Default: `"form_prefix[lang_field_name]"`
    * `lang_data_key` — key to look up in `lang_data` for secondary value.
      Default: `"_field_name"`. Set to `"field_name"` for settings translations.
  """
  attr :field_name, :string, required: true
  attr :form_prefix, :string, required: true
  attr :changeset, :any, required: true
  attr :schema_field, :atom, required: true
  attr :multilang_enabled, :boolean, required: true
  attr :current_lang, :string, required: true
  attr :primary_language, :string, required: true
  attr :lang_data, :map, required: true
  attr :label, :string, required: true
  attr :placeholder, :string, default: nil
  attr :type, :string, default: "input"
  attr :rows, :integer, default: 3
  attr :required, :boolean, default: false
  attr :disabled, :boolean, default: false
  attr :class, :string, default: nil
  attr :pattern, :string, default: nil
  attr :title, :string, default: nil
  attr :hint, :string, default: nil
  attr :secondary_hint, :string, default: nil
  attr :secondary_name, :string, default: nil
  attr :lang_data_key, :string, default: nil
  slot :label_extra

  def translatable_field(assigns) do
    # Coerce lang_data to map if nil/invalid
    assigns =
      if is_map(assigns.lang_data), do: assigns, else: assign(assigns, :lang_data, %{})

    is_primary = !assigns.multilang_enabled || assigns.current_lang == assigns.primary_language

    # Resolve the lang_data lookup key: custom or default "_field_name"
    data_key = assigns.lang_data_key || "_#{assigns.field_name}"

    # Resolve the secondary input name: custom or default "prefix[lang_field]"
    sec_name =
      assigns.secondary_name || "#{assigns.form_prefix}[lang_#{assigns.field_name}]"

    # Extract changeset errors for this field (primary tab only)
    errors = field_errors(assigns.changeset, assigns.schema_field, is_primary)

    assigns =
      assigns
      |> assign(:is_primary, is_primary)
      |> assign(:errors, errors)
      |> assign_new(:primary_value, fn ->
        safe_get_field(assigns.changeset, assigns.schema_field)
      end)
      |> assign_new(:lang_value, fn ->
        Map.get(assigns.lang_data, data_key)
      end)
      |> assign(:secondary_input_name, sec_name)
      |> assign_new(:input_id, fn ->
        if is_primary do
          "#{assigns.form_prefix}_#{assigns.field_name}"
        else
          "#{assigns.form_prefix}_#{assigns.field_name}_#{assigns.current_lang}"
        end
      end)
      |> assign_new(:input_class, fn ->
        base =
          if assigns.type == "textarea",
            do: "textarea textarea-bordered",
            else: "input input-bordered"

        base = if assigns.class, do: "#{base} #{assigns.class}", else: base
        if errors != [], do: "#{base} input-error", else: base
      end)

    ~H"""
    <div class="form-control" phx-feedback-for={if @is_primary, do: "#{@form_prefix}[#{@field_name}]"}>
      <label for={@input_id} class="label">
        <span class="label-text font-semibold">
          {@label}
          <%= if @required && @is_primary do %>
            *
          <% end %>
        </span>
        {render_slot(@label_extra)}
      </label>
      <%= if @type == "textarea" do %>
        <%= if @is_primary do %>
          <textarea
            name={"#{@form_prefix}[#{@field_name}]"}
            id={@input_id}
            class={@input_class}
            rows={@rows}
            phx-debounce="300"
            required={@required}
            disabled={@disabled}
          >{@primary_value}</textarea>
        <% else %>
          <textarea
            name={@secondary_input_name}
            id={@input_id}
            class={@input_class}
            rows={@rows}
            placeholder={@primary_value}
            phx-debounce="300"
            disabled={@disabled}
          >{@lang_value || ""}</textarea>
        <% end %>
      <% else %>
        <%= if @is_primary do %>
          <input
            type="text"
            name={"#{@form_prefix}[#{@field_name}]"}
            id={@input_id}
            value={@primary_value}
            class={@input_class}
            placeholder={@placeholder}
            phx-debounce="300"
            required={@required}
            disabled={@disabled}
            pattern={@pattern}
            title={@title}
          />
        <% else %>
          <input
            type="text"
            name={@secondary_input_name}
            id={@input_id}
            value={@lang_value}
            placeholder={@primary_value}
            class={@input_class}
            phx-debounce="300"
            disabled={@disabled}
            pattern={@pattern}
            title={@title}
          />
        <% end %>
      <% end %>
      <.error :for={msg <- @errors}>{msg}</.error>
      <label :if={@hint && @is_primary && @errors == []} class="label">
        <span class="label-text-alt">{@hint}</span>
      </label>
      <label :if={@secondary_hint && !@is_primary} class="label">
        <span class="label-text-alt">{@secondary_hint}</span>
      </label>
    </div>
    """
  end

  # Extracts translated error messages for a field from the changeset.
  # Only returns errors on the primary tab — secondary tabs don't validate DB columns.
  defp field_errors(%Ecto.Changeset{action: action, errors: errors}, field, true = _is_primary)
       when not is_nil(action) do
    errors
    |> Keyword.get_values(field)
    |> Enum.map(&translate_error/1)
  end

  defp field_errors(_changeset, _field, _is_primary), do: []

  # ── JS helper ──────────────────────────────────────────────────

  @doc """
  Returns a `Phoenix.LiveView.JS` command that switches languages.

  Toggles skeleton/fields visibility **client-side, instantly** (hides
  `[data-translatable=fields]`, reveals `[data-translatable=skeletons]`)
  and pushes `"switch_language"` to the server. The server side holds
  the class state untouched — `handle_switch_language/2` just schedules
  a 150 ms debounced timer that eventually updates `:current_lang`,
  which changes the wrapper div ids and makes morphdom replace both
  divs with their new-language versions (skeleton back to `hidden`,
  fields visible with new content). Nothing on the server renders
  `hidden` on the fields or removes `hidden` from the skeleton, so the
  JS toggles never fight a server diff.

  Returns a no-op when `lang_code == current_lang` — clicking the
  already-active tab doesn't push and doesn't flash the skeleton.
  """
  def switch_lang_js(lang_code, current_lang) do
    if lang_code == current_lang do
      # Same-lang click during an in-flight debounce: we need to cancel
      # the pending server timer (which would otherwise still fire and
      # flip to the wrong lang) AND revert the skeleton/fields toggles
      # on the client (so the UI doesn't sit stuck in skeleton-visible
      # state after the no-op server render). Pushing with the current
      # lang hits `handle_switch_language/2`'s `cancel_multilang_timer`
      # and reschedules a no-op `current_lang` set.
      JS.push("switch_language", value: %{lang: lang_code})
      |> JS.remove_class("hidden", to: "[data-translatable=fields]")
      |> JS.add_class("hidden", to: "[data-translatable=skeletons]")
    else
      JS.push("switch_language", value: %{lang: lang_code})
      |> JS.add_class("hidden", to: "[data-translatable=fields]")
      |> JS.remove_class("hidden", to: "[data-translatable=skeletons]")
    end
  end
end