Add --tail=NUM to limit the number of items to keep in memory

src/core.go

@@ -18,6 +18,28 @@ Matcher  -> EvtSearchFin      -> Terminal (update list)
 Matcher  -> EvtHeader         -> Terminal (update header)
 */
 
+type revision struct {
+	major int
+	minor int
+}
+
+func (r *revision) bumpMajor() {
+	r.major++
+	r.minor = 0
+}
+
+func (r *revision) bumpMinor() {
+	r.minor++
+}
+
+func (r revision) equals(other revision) bool {
+	return r.major == other.major && r.minor == other.minor
+}
+
+func (r revision) compatible(other revision) bool {
+	return r.major == other.major
+}
+
 // Run starts fzf
 func Run(opts *Options) (int, error) {
 	if opts.Tmux != nil && len(os.Getenv("TMUX")) > 0 && opts.Tmux.index >= opts.Height.index {
@@ -155,8 +177,8 @@ func Run(opts *Options) (int, error) {
 			opts.Fuzzy, opts.FuzzyAlgo, opts.Extended, opts.Case, opts.Normalize, forward, withPos,
 			opts.Filter == nil, opts.Nth, opts.Delimiter, runes)
 	}
-	inputRevision := 0
-	snapshotRevision := 0
+	inputRevision := revision{}
+	snapshotRevision := revision{}
 	matcher := NewMatcher(cache, patternBuilder, sort, opts.Tac, eventBox, inputRevision)
 
 	// Filtering mode
@@ -190,7 +212,8 @@ func Run(opts *Options) (int, error) {
 			eventBox.Unwatch(EvtReadNew)
 			eventBox.WaitFor(EvtReadFin)
 
-			snapshot, _ := chunkList.Snapshot()
+			// NOTE: Streaming filter is inherently not compatible with --tail
+			snapshot, _, _ := chunkList.Snapshot(opts.Tail)
 			merger, _ := matcher.scan(MatchRequest{
 				chunks:  snapshot,
 				pattern: pattern})
@@ -261,7 +284,7 @@ func Run(opts *Options) (int, error) {
 		reading = true
 		chunkList.Clear()
 		itemIndex = 0
-		inputRevision++
+		inputRevision.bumpMajor()
 		header = make([]string, 0, opts.HeaderLines)
 		go reader.restart(command, environ)
 	}
@@ -306,10 +329,14 @@ func Run(opts *Options) (int, error) {
 						useSnapshot = false
 					}
 					if !useSnapshot {
-						if snapshotRevision != inputRevision {
+						if !snapshotRevision.compatible(inputRevision) {
 							query = []rune{}
 						}
-						snapshot, count = chunkList.Snapshot()
+						var changed bool
+						snapshot, count, changed = chunkList.Snapshot(opts.Tail)
+						if changed {
+							inputRevision.bumpMinor()
+						}
 						snapshotRevision = inputRevision
 					}
 					total = count
@@ -349,7 +376,10 @@ func Run(opts *Options) (int, error) {
 						break
 					}
 					if !useSnapshot {
-						newSnapshot, newCount := chunkList.Snapshot()
+						newSnapshot, newCount, changed := chunkList.Snapshot(opts.Tail)
+						if changed {
+							inputRevision.bumpMinor()
+						}
 						// We want to avoid showing empty list when reload is triggered
 						// and the query string is changed at the same time i.e. command != nil && changed
 						if command == nil || newCount > 0 {