import type { CollectionBeforeChangeHook } from 'payload'

/**
 * Maps document fields to SEO meta fields for auto-fill.
 * Defaults match the Payload website template (title → meta.title).
 */
export type AutoFillMapping = {
  /** Document field used to fill meta.description when empty. */
  description?: string
  /** Document field used to fill meta.title when empty. Default: 'title'. */
  title?: string
}

/**
 * Reads a possibly-localized field value into a plain string.
 * With localized fields at this hook stage the value is the active-locale
 * string, so we just coerce defensively.
 */
function readString(value: unknown): string | undefined {
  if (typeof value === 'string' && value.trim()) {return value.trim()}
  return undefined
}

/**
 * Builds a beforeChange hook that fills empty SEO meta fields from document
 * content. Only fills when the meta field is blank — never overwrites what an
 * editor typed. This runs server-side on every create/update, so editors get
 * sensible meta without clicking "auto-generate".
 *
 * The plugin doesn't know the client's collection shape, so the field mapping
 * is configurable; defaults follow the website template.
 */
export function buildAutoFillMetaHook(mapping: AutoFillMapping = {}): CollectionBeforeChangeHook {
  const titleField = mapping.title ?? 'title'
  const descriptionField = mapping.description

  return ({ data }) => {
    if (!data) {return data}

    // plugin-seo stores meta as a group under `meta`
    const meta = (data.meta as Record<string, unknown> | undefined) ?? {}

    // Fill meta.title from the document title when empty
    if (!readString(meta.title)) {
      const sourceTitle = readString(data[titleField])
      if (sourceTitle) {meta.title = sourceTitle}
    }

    // Fill meta.description from a configured field when empty
    if (descriptionField && !readString(meta.description)) {
      const sourceDescription = readString(data[descriptionField])
      if (sourceDescription) {meta.description = sourceDescription}
    }

    data.meta = meta
    return data
  }
}