import type { CollectionBeforeChangeHook } from 'payload'

/**
 * When a document's slug changes, appends the OLD slug to a `slugHistory` array
 * field. The project reads slugHistory to serve a 301 redirect from old URLs to
 * the current one — so changing a slug doesn't 404 the old address (a real SEO
 * loss / audit finding).
 *
 * Requires a `slugHistory` field on the collection:
 *   { name: 'slugHistory', type: 'array', fields: [{ name: 'slug', type: 'text' }],
 *     admin: { readOnly: true } }
 *
 *   hooks: { beforeChange: [trackSlugHistoryHook] }
 *
 * The project then, in resolveRoute or a redirect check, looks up slugHistory and
 * 301s to the current slug. See docs/hooks.md.
 */
export const trackSlugHistoryHook: CollectionBeforeChangeHook = ({ data, originalDoc }) => {
  const oldSlug = originalDoc?.slug
  const newSlug = data.slug
  if (
    typeof oldSlug === 'string' &&
    typeof newSlug === 'string' &&
    oldSlug !== newSlug &&
    oldSlug.length > 0
  ) {
    const history: Array<{ slug: string }> = Array.isArray(data.slugHistory)
      ? data.slugHistory
      : Array.isArray(originalDoc?.slugHistory)
        ? originalDoc.slugHistory
        : []
    // Avoid duplicates; don't record the new slug itself.
    if (!history.some((h) => h?.slug === oldSlug)) {
      data.slugHistory = [...history, { slug: oldSlug }]
    }
  }
  return data
}