// Page URLs in the CMS-chosen pagination form (D-40, UI-SPEC L2): query
// (/blog?page=3) or path (/blog/strona/3, the segment per locale). Page 1
// is always the unnumbered URL; other query parameters stay. Built with
// URLSearchParams, never by string concatenation of request input, and
// rendered through Vue bindings (T-33-35). Pure, so `node --test` drives
// it; names start with `seo` because app/utils is auto-imported.

export type SeoPaginationForm = 'query' | 'path'

const PAGE_PATH = /^(.*?)\/([^/]+)\/(\d+)$/

/**
 * The URL of page `page` for the listing at `url` (a path with an optional
 * query; a page number already in it, in either form, is replaced). `param`
 * is the query parameter, `segment` the path segment of the page's locale
 * (the param when empty).
 */
export function seoPageHref(url: string, page: number, form: SeoPaginationForm = 'query', segment = '', param = 'page'): string {
  const [rawPath = '/', rawQuery = ''] = (url.split('#')[0] ?? '').split('?')
  const query = new URLSearchParams(rawQuery)
  query.delete(param)
  const seg = segment || param
  let path = rawPath || '/'
  const numbered = PAGE_PATH.exec(path)
  if (form === 'path' && numbered && numbered[2] === seg) path = numbered[1] || '/'
  if (form === 'path' && page > 1) path = `${path.replace(/\/+$/, '')}/${encodeURIComponent(seg)}/${page}`
  if (form === 'query' && page > 1) query.set(param, String(page))
  const qs = query.toString()
  return qs ? `${path}?${qs}` : path
}

/**
 * Where an explicit page 1 (/mapa-strony/posty/strona/1, /blog?page=1)
 * redirects: the unnumbered URL, built with the locale's segment, or null
 * when that target is the URL itself (a segment the URL does not carry), so
 * a page never answers a 301 to its own address (L4 WR-01).
 */
export function seoFirstPageRedirect(fullPath: string, form: SeoPaginationForm, segment = '', param = 'page'): string | null {
  const target = seoPageHref(fullPath, 1, form, segment, param)
  return target === fullPath ? null : target
}

/**
 * The page number a URL carries in the given form: 1 without one, the
 * number for a positive integer, null for anything else (a 404).
 */
export function seoPageNumber(url: string, form: SeoPaginationForm = 'query', segment = '', param = 'page'): number | null {
  const [rawPath = '/', rawQuery = ''] = (url.split('#')[0] ?? '').split('?')
  let raw: string | null = null
  if (form === 'path') {
    const m = PAGE_PATH.exec(rawPath)
    if (m && m[2] === (segment || param)) raw = m[3] ?? null
  } else {
    const values = new URLSearchParams(rawQuery).getAll(param)
    if (values.length > 1) return null
    raw = values[0] ?? null
  }
  if (raw === null) return 1
  if (!/^\d+$/.test(raw)) return null
  const n = Number(raw)
  return Number.isSafeInteger(n) && n >= 1 ? n : null
}