Add --track option to track the current selection

Close #3186
Related #1890

6 files changed, 81 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index fe1bf3e0..c776f413 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,13 @@ CHANGELOG
 
 0.39.0
 ------
+- Added `--track` option that makes fzf track the current selection when the
+  result list is updated. This can be useful when browsing logs using fzf with
+  sorting disabled.
+  ```sh
+  git log --oneline --graph --color=always | nl |
+      fzf --ansi --track --no-sort --layout=reverse-list
+  ```
 - If you use `--listen` option without a port number fzf will automatically
   allocate an available port and export it as `$FZF_PORT` environment
   variable.
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index a538a91c..7d5ea73e 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -92,6 +92,16 @@ interface rather than a "fuzzy finder". You can later enable the search using
 .B "+s, --no-sort"
 Do not sort the result
 .TP
+.B "--track"
+Make fzf track the current selection when the result list is updated.
+This can be useful when browsing logs using fzf with sorting disabled.
+
+.RS
+e.g.
+     \fBgit log --oneline --graph --color=always | nl |
+         fzf --ansi --track --no-sort --layout=reverse-list\fR
+.RE
+.TP
 .B "--tac"
 Reverse the order of the input
 
diff --git a/src/merger.go b/src/merger.go
index 8e6a884c..cdf00acc 100644
--- a/src/merger.go
+++ b/src/merger.go
@@ -17,6 +17,7 @@ type Merger struct {
 	tac     bool
 	final   bool
 	count   int
+	pass    bool
 }
 
 // PassMerger returns a new Merger that simply returns the items in the
@@ -26,7 +27,8 @@ func PassMerger(chunks *[]*Chunk, tac bool) *Merger {
 		pattern: nil,
 		chunks:  chunks,
 		tac:     tac,
-		count:   0}
+		count:   0,
+		pass:    true}
 
 	for _, chunk := range *mg.chunks {
 		mg.count += chunk.count
@@ -58,6 +60,19 @@ func (mg *Merger) Length() int {
 	return mg.count
 }
 
+// FindIndex returns the index of the item with the given item index
+func (mg *Merger) FindIndex(itemIndex int32) int {
+	if mg.pass {
+		return int(itemIndex)
+	}
+	for i := 0; i < mg.count; i++ {
+		if mg.Get(i).item.Index() == itemIndex {
+			return i
+		}
+	}
+	return -1
+}
+
 // Get returns the pointer to the Result object indexed by the given integer
 func (mg *Merger) Get(idx int) Result {
 	if mg.chunks != nil {
diff --git a/src/options.go b/src/options.go
index 9f953bda..36ca7710 100644
--- a/src/options.go
+++ b/src/options.go
@@ -33,6 +33,7 @@ const usage = `usage: fzf [options]
                            field index expressions
     -d, --delimiter=STR    Field delimiter regex (default: AWK-style)
     +s, --no-sort          Do not sort the result
+    --track                Track the current selection when the result is updated
     --tac                  Reverse the order of the input
     --disabled             Do not perform search
     --tiebreak=CRI[,..]    Comma-separated list of sort criteria to apply
@@ -266,6 +267,7 @@ type Options struct {
 	WithNth      []Range
 	Delimiter    Delimiter
 	Sort         int
+	Track        bool
 	Tac          bool
 	Criteria     []criterion
 	Multi        int
@@ -338,6 +340,7 @@ func defaultOptions() *Options {
 		WithNth:      make([]Range, 0),
 		Delimiter:    Delimiter{},
 		Sort:         1000,
+		Track:        false,
 		Tac:          false,
 		Criteria:     []criterion{byScore, byLength},
 		Multi:        0,
@@ -1562,6 +1565,10 @@ func parseOptions(opts *Options, allArgs []string) {
 			opts.Sort = optionalNumeric(allArgs, &i, 1)
 		case "+s", "--no-sort":
 			opts.Sort = 0
+		case "--track":
+			opts.Track = true
+		case "--no-track":
+			opts.Track = false
 		case "--tac":
 			opts.Tac = true
 		case "--no-tac":
diff --git a/src/terminal.go b/src/terminal.go
index 468b90f8..57ff4f54 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -183,6 +183,7 @@ type Terminal struct {
 	multi              int
 	sort               bool
 	toggleSort         bool
+	track              bool
 	delimiter          Delimiter
 	expect             map[tui.Event]string
 	keymap             map[tui.Event][]*action
@@ -599,6 +600,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		multi:              opts.Multi,
 		sort:               opts.Sort > 0,
 		toggleSort:         opts.ToggleSort,
+		track:              opts.Track,
 		delimiter:          opts.Delimiter,
 		expect:             opts.Expect,
 		keymap:             opts.Keymap,
@@ -904,6 +906,10 @@ func (t *Terminal) UpdateProgress(progress float32) {
 // UpdateList updates Merger to display the list
 func (t *Terminal) UpdateList(merger *Merger, reset bool) {
 	t.mutex.Lock()
+	var prevIndex int32 = -1
+	if !reset && t.track && t.merger.Length() > 0 {
+		prevIndex = t.merger.Get(t.cy).item.Index()
+	}
 	t.progress = 100
 	t.merger = merger
 	if reset {
@@ -914,6 +920,18 @@ func (t *Terminal) UpdateList(merger *Merger, reset bool) {
 		t.triggerLoad = false
 		t.eventChan <- tui.Load.AsEvent()
 	}
+	if prevIndex >= 0 {
+		pos := t.cy - t.offset
+		count := t.merger.Length()
+		i := t.merger.FindIndex(prevIndex)
+		if i >= 0 {
+			t.cy = i
+			t.offset = t.cy - pos
+		} else if t.cy > count {
+			// Try to keep the vertical position when the list shrinks
+			t.cy = count - util.Min(count, t.maxItems()) + pos
+		}
+	}
 	t.mutex.Unlock()
 	t.reqBox.Set(reqInfo, nil)
 	t.reqBox.Set(reqList, nil)
diff --git a/test/test_go.rb b/test/test_go.rb
index 8da6cb5e..1556c73b 100755
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -2679,6 +2679,29 @@ class TestGoFZF < TestBase
     OUTPUT
     tmux.until { assert_block(expected, _1) }
   end
+
+  def test_track
+    tmux.send_keys "seq 1000 | #{FZF} --query 555 --track", :Enter
+    tmux.until do |lines|
+      assert_equal 1, lines.match_count
+      assert_includes lines, '> 555'
+    end
+    tmux.send_keys :BSpace
+    index = tmux.until do |lines|
+      assert_equal 28, lines.match_count
+      assert_includes lines, '> 555'
+    end.index('> 555')
+    tmux.send_keys :BSpace
+    tmux.until do |lines|
+      assert_equal 271, lines.match_count
+      assert_equal '> 555', lines[index]
+    end
+    tmux.send_keys :BSpace
+    tmux.until do |lines|
+      assert_equal 1000, lines.match_count
+      assert_equal '> 555', lines[index]
+    end
+  end
 end
 
 module TestShell