defmodule PhoenixKit.Emails.Template do
  @moduledoc """
  Email template schema for managing reusable email templates.

  This module defines the structure and validations for email templates that can be
  used throughout the application. Templates support variable substitution and
  categorization for better organization.

  ## Template Variables

  Templates support variable substitution using the `{{variable_name}}` syntax.
  Common variables include:

  - `{{email}}` - User's email address
  - `{{url}}` - Action URL (magic link, confirmation, etc.)
  - `{{timestamp}}` - Current timestamp
  - `{{user_name}}` - User's display name

  ## Categories

  - **system** - Core authentication and system emails (protected)
  - **marketing** - Promotional and marketing communications
  - **transactional** - Order confirmations, notifications, etc.

  ## Status

  - **active** - Template is live and can be used
  - **draft** - Template is being edited
  - **archived** - Template is no longer active but preserved

  ## Examples

      # Create a new template
      %EmailTemplate{}
      |> EmailTemplate.changeset(%{
        name: "welcome_email",
        slug: "welcome-email",
        display_name: "Welcome Email",
        subject: "Welcome to {{app_name}}!",
        html_body: "<h1>Welcome {{user_name}}!</h1>",
        text_body: "Welcome {{user_name}}!",
        category: "transactional",
        status: "active"
      })

  """
  use Ecto.Schema
  import Ecto.Changeset

  @type t :: %__MODULE__{
          id: integer() | nil,
          name: String.t(),
          slug: String.t(),
          display_name: String.t(),
          description: String.t() | nil,
          subject: String.t(),
          html_body: String.t(),
          text_body: String.t(),
          category: String.t(),
          status: String.t(),
          variables: map(),
          metadata: map(),
          usage_count: integer(),
          last_used_at: DateTime.t() | nil,
          version: integer(),
          is_system: boolean(),
          created_by_user_id: integer() | nil,
          updated_by_user_id: integer() | nil,
          inserted_at: DateTime.t() | nil,
          updated_at: DateTime.t() | nil
        }

  # Valid categories for email templates
  @valid_categories ["system", "marketing", "transactional"]

  # Valid statuses for email templates
  @valid_statuses ["active", "draft", "archived"]

  # Common template variables that can be used
  @common_variables [
    "email",
    "user_name",
    "url",
    "timestamp",
    "app_name",
    "support_email",
    "company_name"
  ]

  schema "phoenix_kit_email_templates" do
    field :name, :string
    field :slug, :string
    field :display_name, :string
    field :description, :string
    field :subject, :string
    field :html_body, :string
    field :text_body, :string
    field :category, :string, default: "transactional"
    field :status, :string, default: "draft"
    field :variables, :map, default: %{}
    field :metadata, :map, default: %{}
    field :usage_count, :integer, default: 0
    field :last_used_at, :utc_datetime_usec
    field :version, :integer, default: 1
    field :is_system, :boolean, default: false
    field :created_by_user_id, :integer
    field :updated_by_user_id, :integer

    timestamps(type: :utc_datetime_usec)
  end

  @doc """
  Returns the list of valid categories for email templates.
  """
  def valid_categories, do: @valid_categories

  @doc """
  Returns the list of valid statuses for email templates.
  """
  def valid_statuses, do: @valid_statuses

  @doc """
  Returns the list of common template variables.
  """
  def common_variables, do: @common_variables

  @doc """
  Creates a changeset for email template creation and updates.

  ## Parameters

  - `template` - The email template struct (new or existing)
  - `attrs` - Map of attributes to change

  ## Required Fields

  - `:name` - Unique template identifier
  - `:slug` - URL-friendly identifier
  - `:display_name` - Human-readable name
  - `:subject` - Email subject line
  - `:html_body` - HTML version of email
  - `:text_body` - Plain text version of email

  ## Validations

  - Name must be unique and follow snake_case format
  - Slug must be unique and URL-friendly
  - Category must be one of the valid categories
  - Status must be one of the valid statuses
  - Subject and body fields cannot be empty
  - Variables must be a valid map
  """
  def changeset(template, attrs) do
    template
    |> cast(attrs, [
      :name,
      :slug,
      :display_name,
      :description,
      :subject,
      :html_body,
      :text_body,
      :category,
      :status,
      :variables,
      :metadata,
      :is_system,
      :created_by_user_id,
      :updated_by_user_id
    ])
    |> auto_generate_slug()
    |> validate_required([
      :name,
      :slug,
      :display_name,
      :subject,
      :html_body,
      :text_body,
      :category,
      :status
    ])
    |> validate_length(:name, min: 2, max: 100)
    |> validate_length(:slug, min: 2, max: 100)
    |> validate_length(:display_name, min: 2, max: 200)
    |> validate_length(:subject, min: 1, max: 300)
    |> validate_length(:html_body, min: 1)
    |> validate_length(:text_body, min: 1)
    |> validate_inclusion(:category, @valid_categories)
    |> validate_inclusion(:status, @valid_statuses)
    |> validate_format(:name, ~r/^[a-z][a-z0-9_]*$/,
      message:
        "must start with a letter and contain only lowercase letters, numbers, and underscores"
    )
    |> validate_format(:slug, ~r/^[a-z][a-z0-9-]*$/,
      message: "must start with a letter and contain only lowercase letters, numbers, and hyphens"
    )
    |> unique_constraint(:name)
    |> unique_constraint(:slug)
    |> validate_template_variables()
  end

  @doc """
  Creates a changeset for updating template usage statistics.
  """
  def usage_changeset(template, attrs \\ %{}) do
    template
    |> cast(attrs, [:usage_count, :last_used_at])
    |> validate_number(:usage_count, greater_than_or_equal_to: 0)
  end

  @doc """
  Creates a changeset for updating template version.
  """
  def version_changeset(template, attrs \\ %{}) do
    template
    |> cast(attrs, [:version, :updated_by_user_id])
    |> validate_number(:version, greater_than: 0)
  end

  @doc """
  Extracts variables from template content (subject, html_body, text_body).

  Returns a list of unique variable names found in the template.

  ## Examples

      iex> template = %EmailTemplate{
      ...>   subject: "Welcome {{user_name}}!",
      ...>   html_body: "<p>Hi {{user_name}}, click {{url}}</p>",
      ...>   text_body: "Hi {{user_name}}, visit {{url}}"
      ...> }
      iex> EmailTemplate.extract_variables(template)
      ["user_name", "url"]

  """
  def extract_variables(%__MODULE__{} = template) do
    content = "#{template.subject} #{template.html_body} #{template.text_body}"

    Regex.scan(~r/\{\{([^}]+)\}\}/, content)
    |> Enum.map(fn [_, var] -> String.trim(var) end)
    |> Enum.uniq()
    |> Enum.sort()
  end

  @doc """
  Substitutes variables in template content with provided values.

  ## Parameters

  - `template` - The email template
  - `variables` - Map of variable names to values

  Returns a new template struct with variables substituted.

  ## Examples

      iex> template = %EmailTemplate{
      ...>   subject: "Welcome {{user_name}}!",
      ...>   html_body: "<p>Hi {{user_name}}</p>"
      ...> }
      iex> result = EmailTemplate.substitute_variables(template, %{"user_name" => "John"})
      iex> result.subject
      "Welcome John!"

  """
  def substitute_variables(%__MODULE__{} = template, variables) when is_map(variables) do
    %{
      template
      | subject: substitute_string(template.subject, variables),
        html_body: substitute_string(template.html_body, variables),
        text_body: substitute_string(template.text_body, variables)
    }
  end

  # Private helper functions

  # Automatically generate slug from name if not provided
  defp auto_generate_slug(changeset) do
    slug = get_change(changeset, :slug) || get_field(changeset, :slug)

    case slug do
      s when s in [nil, ""] ->
        name = get_change(changeset, :name) || get_field(changeset, :name)

        case name do
          n when is_binary(n) and n != "" ->
            put_change(changeset, :slug, String.replace(n, "_", "-"))

          _ ->
            changeset
        end

      _ ->
        changeset
    end
  end

  # Validate that template variables are correctly formatted
  defp validate_template_variables(changeset) do
    case get_field(changeset, :variables) do
      nil ->
        changeset

      variables when is_map(variables) ->
        # Extract variables from template content and validate against declared variables
        case {get_field(changeset, :subject), get_field(changeset, :html_body),
              get_field(changeset, :text_body)} do
          {subject, html_body, text_body}
          when is_binary(subject) and is_binary(html_body) and is_binary(text_body) ->
            template = %__MODULE__{
              subject: subject,
              html_body: html_body,
              text_body: text_body
            }

            extracted_vars = extract_variables(template)
            declared_vars = Map.keys(variables)

            # Check for undefined variables in template
            undefined_vars = extracted_vars -- declared_vars

            if length(undefined_vars) > 0 do
              add_error(
                changeset,
                :variables,
                "Template uses undefined variables: #{Enum.join(undefined_vars, ", ")}"
              )
            else
              changeset
            end

          _ ->
            changeset
        end

      _ ->
        add_error(changeset, :variables, "must be a valid map")
    end
  end

  # Substitute variables in a string
  defp substitute_string(content, variables) when is_binary(content) and is_map(variables) do
    Enum.reduce(variables, content, fn {key, value}, acc ->
      String.replace(acc, "{{#{key}}}", to_string(value))
    end)
  end

  defp substitute_string(content, _variables), do: content
end