// What saving the machine form should do about its controlling-PC link.
//
// Pulled out of MachineForm.vue because the component decided this inline and
// got it wrong in a way nothing could catch: it treated "I know the id of the
// existing link" as "there IS an existing link". The id was read from
// `rel.assetrelationshipid`, a key the API has never emitted - the column is
// `relationshipid` and to_dict serialises column names - so it was undefined on
// every load, and the two questions gave opposite answers on a machine that
// plainly had a controlling PC.
//
// Both failures followed from that single conflation:
//   - Saving ANY change, a map position included, re-POSTed the link that was
//     already there. The server answered 409 "this relationship already
//     exists", correctly, and the form surfaced it as a failed save.
//   - Clearing the controlling PC did nothing at all, because "no selection and
//     no known id" reads as "nothing to do".
//
// So THE LOADED PC IS THE AUTHORITY on whether a link exists, and the
// relationship id is only ever used to delete a row. If the id goes missing
// again the worst case is a link that cannot be removed, never a duplicate
// create.

/**
 * Decide what to do with the controlling-PC link on save.
 *
 * @param {object} state
 * @param {number|string|null} state.selected - PC id chosen in the form now.
 * @param {number|string|null} state.loaded - PC id the link named on load.
 * @param {number|null} state.existingRelationshipId - row id to delete, if known.
 * @returns {{action: 'none'|'unlink'|'link'|'relink', deleteId: number|null}}
 */
export function planControllingPcChange({ selected, loaded, existingRelationshipId } = {}) {
  const now = normalize(selected)
  const before = normalize(loaded)
  const deleteId = existingRelationshipId ?? null

  // Unchanged is not a change. Somebody moving a marker on the map is not
  // asking to be re-linked to the same PC, and delete-then-create on an
  // untouched link opens a window where the machine has no controlling PC -
  // which resolve_asset_position walks across, so a failure between the two
  // calls would lose the machine's inherited position along with the link.
  if (now === before) {
    return { action: 'none', deleteId: null }
  }

  if (now === null) {
    // Cleared. Nothing to delete means nothing to do, rather than an error:
    // the link is already absent, which is the state being asked for.
    return deleteId === null
      ? { action: 'none', deleteId: null }
      : { action: 'unlink', deleteId }
  }

  if (before === null) {
    return { action: 'link', deleteId: null }
  }

  // Swapped to a different PC. The old row goes first: 'controls' is singular
  // per source, not per target, so leaving it would leave the machine with two.
  return { action: 'relink', deleteId }
}

// Ids arrive as numbers from the API and strings from a <select>, and '' means
// "nothing chosen". Comparing those raw is how an unchanged PC looks changed.
function normalize(value) {
  if (value === null || value === undefined || value === '') return null
  return String(value)
}