Lesson 27 · Backspacing - Build a Fuzzy Finder

Backspacing

Deleting a character can bring matches back, so unlike growing the query, shrinking it has to reconsider the whole corpus. Today you handle query editing correctly in both directions.

The goal

Handle backspacing by re-ranking the shortened query against the full corpus, so matches removed by a now-deleted character reappear.

Start here - the target

TO DO

Scenario: Re-widening on backspace

Givena Finder over ["abc", "abx", "ab", "zzz"] whose query is "abc" (results ["abc"])

Whena character is deleted so the query becomes "ab"

Thenthe results widen back to ["ab", "abc", "abx"] (all three re-ranked from the full corpus, not just the prior single result), with the selection reset to 0

Background

Narrowing had a shortcut: a longer query only removes matches, so you can filter the previous results. Deleting a character breaks that shortcut completely. A shorter query can match more candidates - the ones the deleted character had ruled out - and those are not in the current result set anymore. So backspace has to go back to the full corpus and re-rank from scratch.

That is exactly what SetQuery already does, so backspace is a thin wrapper that trims the last character and re-ranks everything, resetting the selection like any query change. The contrast is the lesson: growing the query can narrow from the previous results, but shrinking it must widen from the whole list. Pinning the widen case - a candidate that had disappeared reappearing after a backspace - is what proves the finder reconsiders the corpus instead of only ever shrinking its view.

Make it work

// Backspace shortens the query; the match set can GROW, so you must
// re-rank the FULL corpus, not the current results. Reuse SetQuery.
func (f *Finder) Backspace() {
  if len(f.Query) == 0 { return }
  f.SetQuery(f.Query[:len(f.Query)-1])  // ranks over all candidates
}

CheckpointDONE

Editing the query works in both directions - narrowing and widening. Commit and stop here.