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

type BuildHreflangArgs = {
  /** slug per locale for the current document, e.g. { pl: 'o-nas', en: 'about' } */
  slugs: LocalizedSlugs
  config: I18nConfig
  /** Absolute site origin, e.g. 'https://example.com'. Omit for relative paths. */
  baseUrl?: string
  /** Home slug that collapses to the locale root. Defaults to 'home'. */
  homeSlug?: string | Record<string, 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
}

/**
 * Builds a map of locale → URL for hreflang alternate links, suitable for
 * 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.
 *
 * @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' }
 */
export function buildHreflangAlternates({
  slugs,
  config,
  baseUrl,
  homeSlug = 'home',
  prefix,
}: BuildHreflangArgs): Record<string, string> {
  const origin = baseUrl?.replace(/\/$/, '') ?? ''
  const alternates: Record<string, string> = {}

  // Single-locale sites have no language alternatives — hreflang describes
  // relationships BETWEEN language versions, and there's only one. Emitting
  // hreflang (or x-default) here would be wrong, so return empty: the page keeps
  // its canonical, but no alternate-language links.
  if (config.locales.length === 1) {
    return alternates
  }

  for (const locale of getLocaleCodes(config)) {
    const path = buildLocalizedPath({ slugs, locale, config, homeSlug, prefix })
    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[config.defaultLocale]
  if (defaultLocalePath) {
    alternates['x-default'] = defaultLocalePath
  }

  return alternates
}