Add --threads option to control matcher concurrency

By default, fzf uses 8 * NumCPU goroutines (capped at 32) for
parallel matching. --threads N overrides this to use exactly N
goroutines, which is useful for benchmarking and profiling.

src/core.go

@@ -235,7 +235,7 @@ func Run(opts *Options) (int, error) {
 			opts.Fuzzy, opts.FuzzyAlgo, opts.Extended, opts.Case, opts.Normalize, forward, withPos,
 			opts.Filter == nil, nth, opts.Delimiter, inputRevision, runes, denylistCopy, headerLines)
 	}
-	matcher := NewMatcher(cache, patternBuilder, sort, opts.Tac, eventBox, inputRevision)
+	matcher := NewMatcher(cache, patternBuilder, sort, opts.Tac, eventBox, inputRevision, opts.Threads)
 
 	// Filtering mode
 	if opts.Filter != nil {

src/matcher.go

@@ -54,8 +54,11 @@ const (
 
 // NewMatcher returns a new Matcher
 func NewMatcher(cache *ChunkCache, patternBuilder func([]rune) *Pattern,
-	sort bool, tac bool, eventBox *util.EventBox, revision revision) *Matcher {
+	sort bool, tac bool, eventBox *util.EventBox, revision revision, threads int) *Matcher {
 	partitions := min(numPartitionsMultiplier*runtime.NumCPU(), maxPartitions)
+	if threads > 0 {
+		partitions = threads
+	}
 	return &Matcher{
 		cache:          cache,
 		patternBuilder: patternBuilder,

src/options.go

@@ -678,6 +678,7 @@ type Options struct {
 	WalkerSkip        []string
 	Version           bool
 	Help              bool
+	Threads           int
 	Bench             time.Duration
 	CPUProfile        string
 	MEMProfile        string
@@ -3375,6 +3376,13 @@ func parseOptions(index *int, opts *Options, allArgs []string) error {
 				return err
 			}
 			opts.WalkerSkip = filterNonEmpty(strings.Split(str, ","))
+		case "--threads":
+			if opts.Threads, err = nextInt("number of threads required"); err != nil {
+				return err
+			}
+			if opts.Threads < 0 {
+				return errors.New("--threads must be a positive integer")
+			}
 		case "--bench":
 			str, err := nextString("duration required (e.g. 3s, 500ms)")
 			if err != nil {