import type { I18nConfig, LocalizedSlugs } from '../i18n/index.js'

import { buildLocalizedPath, getDefaultLocale, getLocaleCodes } from '../i18n/index.js'

type BuildHreflangArgs = {
  /** Absolute site origin, e.g. 'https://example.com'. Omit for relative paths. */
  baseUrl?: string
  config: I18nConfig
  /** Home slug that collapses to the locale root. Defaults to 'home'. */
  homeSlug?: string
  /**
   * Localized segment the document lives under (an archive page's slugs),
   * e.g. { pl: 'artykuly', en: 'articles' }. Locales missing from the prefix
   * are omitted — an entry with no archive in that language has no URL there.
   */
  prefix?: LocalizedSlugs
  /** slug per locale for the current document, e.g. { pl: 'o-nas', en: 'about' } */
  slugs: LocalizedSlugs
}

/**
 * Next.js Metadata `alternates.languages`.
 *
 * Bridges SEO and i18n: for each configured locale that the document has a
 * slug in, it produces the locale-aware path (via buildLocalizedPath),
 * optionally prefixed with an absolute origin.
 *
 * Also emits `x-default` pointing at the default locale — the version Google
 * serves when the user's language/region matches no hreflang, and the fallback
 * when the root ('/') redirect is ambiguous (Googlebot with no/foreign
 * Accept-Language).
 *
 * @example
 * buildHreflangAlternates({
 *   slugs: { pl: 'o-nas', en: 'about' },
 *   config,
 *   baseUrl: 'https://example.com',
 * })
 * // → {
 * //   pl: 'https://example.com/pl/o-nas',
 * //   en: 'https://example.com/en/about',
 * //   'x-default': 'https://example.com/pl/o-nas',
 * // }
 */
export function buildHreflangAlternates({
  baseUrl,
  config,
  homeSlug = 'home',
  prefix,
  slugs,
}: BuildHreflangArgs): Record<string, string> {
  const origin = baseUrl?.replace(/\/$/, '') ?? ''
  const alternates: Record<string, string> = {}

  for (const locale of getLocaleCodes(config)) {
    const path = buildLocalizedPath({ config, homeSlug, locale, prefix, slugs })
    if (path) {
      alternates[locale] = `${origin}${path}`
    }
  }

  // x-default: the version Google serves when the user's language/region doesn't
  // match any hreflang — and, crucially here, the fallback when the root ('/')
  // redirect is ambiguous (Googlebot with no/foreign Accept-Language). Point it
  // at the default locale (the primary market) so search shows that version by
  // default instead of guessing. Only set when the default locale has a URL.
  const defaultLocalePath = alternates[getDefaultLocale(config)]
  if (defaultLocalePath) {
    alternates['x-default'] = defaultLocalePath
  }

  return alternates
}