import type { I18nConfig, LocalizedSlugs } from '../i18n/index.js'
import type { TitleOrder } from './composeTitle.js'
import type { SeoMeta } from './types.js'

import { buildLocalizedPath } from '../i18n/index.js'
import { composeTitle } from './composeTitle.js'
import { buildHreflangAlternates } from './hreflang.js'

/**
 * Subset of Next.js `Metadata` this helper produces. Kept local so the plugin
 * doesn't depend on `next` types; the shape is assignable to Next's Metadata.
 */
export type PageMetadata = {
  alternates?: {
    canonical?: string
    languages?: Record<string, string>
  }
  description?: string
  openGraph?: {
    description?: string
    images?: { url: string }[]
    locale?: string
    title: string
  }
  robots?: {
    follow: boolean
    index: boolean
  }
  title: string
}

type BuildMetadataArgs = {
  /** Absolute site origin, e.g. 'https://example.com'. */
  baseUrl?: string
  config: I18nConfig
  /** Home slug that collapses to the locale root. Defaults to 'home'. */
  homeSlug?: string
  /** Resolved OG image URL (page image or site defaultShareImage). */
  imageUrl?: null | string
  /** Current locale being rendered. */
  locale: string
  /** SEO meta from the document (plugin-seo group). */
  meta?: null | SeoMeta
  /** Page title or site name first. Defaults to 'page-first'. */
  order?: TitleOrder
  /**
   * Localized segment the document lives under (an archive page's slugs).
   * Feeds both canonical and hreflang, so /pl/artykuly/moj-post and
   * /en/articles/my-post point at each other correctly.
   */
  prefix?: LocalizedSlugs
  /**
   * Query string appended to canonical and every hreflang, e.g. '?page=2'.
   *
   * A paginated listing must be canonical to itself — pointing page 2 at page 1
   * tells Google the entries on it don't exist. Alternates carry the same page,
   * since /pl/artykuly?page=2 corresponds to /en/articles?page=2.
   */
  query?: string
  /** Separator between page title and site name. Defaults to ' | '. */
  separator?: string
  /** Site name for title composition and OG. */
  siteName?: null | string
  /** slug per locale for this document — drives canonical + hreflang. */
  slugs: LocalizedSlugs
}

/**
 * Assembles a Next.js-compatible Metadata object from document SEO fields and
 * site-level data. Locale-aware: canonical points at the current locale's
 * path, and hreflang alternates cover every locale the document exists in.
 *
 * Designed for use inside Next.js `generateMetadata`. The caller resolves the
 * pieces (meta group, site name, image URL, localized slugs) and passes them
 * in — the plugin composes, it doesn't fetch.
 */
export function buildMetadata({
  baseUrl,
  config,
  homeSlug = 'home',
  imageUrl,
  locale,
  meta,
  order,
  prefix,
  query,
  separator,
  siteName,
  slugs,
}: BuildMetadataArgs): PageMetadata {
  // titleOverride wins outright: an editor who filled it in wants that exact
  // string in the tab, not a composition.
  const override = meta?.titleOverride?.trim()
  const title = override || composeTitle({ order, pageTitle: meta?.title, separator, siteName })
  const description = meta?.description?.trim() || undefined
  const origin = baseUrl?.replace(/\/$/, '') ?? ''

  const suffix = query ?? ''

  const currentPath = buildLocalizedPath({ config, homeSlug, locale, prefix, slugs })
  const canonical = currentPath ? `${origin}${currentPath}${suffix}` : undefined

  const languages = buildHreflangAlternates({ baseUrl, config, homeSlug, prefix, slugs })
  if (suffix) {
    for (const code of Object.keys(languages)) {
      languages[code] = `${languages[code]}${suffix}`
    }
  }

  const images = imageUrl ? [{ url: imageUrl }] : undefined

  return {
    title,
    ...(description && { description }),
    alternates: {
      ...(canonical && { canonical }),
      ...(Object.keys(languages).length > 0 && { languages }),
    },
    ...(meta?.noindex ? { robots: { follow: true, index: false } } : {}),
    openGraph: {
      title,
      ...(description && { description }),
      ...(images && { images }),
      locale,
    },
  }
}