Lesson 06 · Highlighting the match

Highlighting the match

A finder that shows you why a line matched is far easier to trust. Today you turn match positions into a highlighted string and wire it into the filter output, closing the first chapter with a visible result.

The goal

Render a candidate with its matched characters marked, given the candidate and its list of match positions.

Start here - the target

TO DO

Scenario: Marking matched characters

Giventhe candidate "src/main.go" and the match positions [0, 4, 9]

Whenhighlight is called

Thenit returns "[s]rc/[m]ain.[g]o" - each matched character wrapped in square brackets, every other character unchanged

Andan empty position list returns the candidate unchanged, and adjacent positions each get their own brackets

Background

Highlighting is the finder’s feedback loop: it shows the user why a candidate matched, which characters the query landed on. With positions already in hand from the last lesson, rendering is a straight walk over the candidate - wrap the characters whose index is in the match set, leave the rest alone. Using a simple [x] marker keeps this testable as an exact string; a real terminal build swaps the brackets for reverse-video or color later without changing the logic.

Wiring this into filter mode closes the loop for chapter one: pipe in a list, pass a query, and see each surviving line with its match lit up. The positions are still the greedy ones, so the brackets sit on the leftmost occurrence of each query character - occasionally not the prettiest choice. That is exactly the gap the scoring chapter opens next: once you can score an alignment, you can find the one that both reads best and highlights best.

Make it work

// Walk the candidate; when the index is in the position set, wrap it.
// A set (map[int]bool) or a sorted-positions cursor both work.
func highlight(candidate string, pos []int) string {
  inMatch := indexSet(pos)
  var b strings.Builder
  for i, ch := range candidate {
    if inMatch[i] { b.WriteString("[" + string(ch) + "]") } else {
      b.WriteRune(ch)
    }
  }
  return b.String()
}

CheckpointDONE

The filter can now show which characters matched. Commit and stop here.