Add --preview-window follow option

5 files changed, 78 insertions, 36 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7609316f..9d2bf95b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,15 @@ CHANGELOG
 
 0.24.4
 ------
+- Added `--preview-window` option `follow`
+  ```sh
+  # Preview window will automatically scroll to the bottom
+  fzf --preview-window follow --preview 'for i in $(seq 100000); do
+    echo "$i"
+    sleep 0.01
+    (( i % 300 == 0 )) && printf "\033[2J"
+  done'
+  ```
 - Added `change-prompt` action
   ```sh
   fzf --prompt 'foo> ' --bind $'a:change-prompt:\x1b[31mbar> '
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index f779ad5e..eeb83945 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -439,7 +439,7 @@ e.g.
       done'\fR
 .RE
 .TP
-.BI "--preview-window=" "[POSITION][:SIZE[%]][:rounded|sharp|noborder][:[no]wrap][:[no]cycle][:[no]hidden][:+SCROLL[-OFFSET]][:default]"
+.BI "--preview-window=" "[POSITION][:SIZE[%]][:rounded|sharp|noborder][:[no]wrap][:[no]follow][:[no]cycle][:[no]hidden][:+SCROLL[-OFFSET]][:default]"
 
 .RS
 .B POSITION: (default: right)
@@ -448,27 +448,43 @@ e.g.
     \fBleft
     \fBright
 
-\fRDetermines the layout of the preview window. If the argument contains
-\fB:hidden\fR, the preview window will be hidden by default until
-\fBtoggle-preview\fR action is triggered. Long lines are truncated by default.
-Line wrap can be enabled with \fB:wrap\fR flag. Cyclic scrolling is enabled
-with \fB:cycle\fR flag.
+\fRDetermines the layout of the preview window.
 
-If size is given as 0, preview window will not be visible, but fzf will still
+* If the argument contains \fB:hidden\fR, the preview window will be hidden by
+default until \fBtoggle-preview\fR action is triggered.
+
+* If size is given as 0, preview window will not be visible, but fzf will still
 execute the command in the background.
 
-To change the style of the border of the preview window, specify one of
+* Long lines are truncated by default. Line wrap can be enabled with
+\fB:wrap\fR flag.
+
+* Preview window will automatically scroll to the bottom when \fB:follow\fR
+flag is set, similarly to how \fBtail -f\fR works.
+
+.RS
+e.g.
+      \fBfzf --preview-window follow --preview 'for i in $(seq 100000); do
+        echo "$i"
+        sleep 0.01
+        (( i % 300 == 0 )) && printf "\\033[2J"
+      done'\fR
+.RE
+
+* Cyclic scrolling is enabled with \fB:cycle\fR flag.
+
+* To change the style of the border of the preview window, specify one of
 \fBrounded\fR (border with rounded edges, default), \fBsharp\fR (border with
 sharp edges), or \fBnoborder\fR (no border).
 
-\fB+SCROLL[-OFFSET]\fR determines the initial scroll offset of the preview
+* \fB+SCROLL[-OFFSET]\fR determines the initial scroll offset of the preview
 window. \fBSCROLL\fR can be either a numeric integer or a single-field index
 expression that refers to a numeric integer. The optional \fB-OFFSET\fR part is
 for adjusting the base offset so that you can see the text above it. It should
 be given as a numeric integer (\fB-INTEGER\fR), or as a denominator form
 (\fB-/INTEGER\fR) for specifying a fraction of the preview window height.
 
-\fBdefault\fR resets all options previously set to the default.
+* \fBdefault\fR resets all options previously set to the default.
 
 .RS
 e.g.
diff --git a/src/options.go b/src/options.go
index b3fb78cf..0607e5dc 100644
--- a/src/options.go
+++ b/src/options.go
@@ -83,7 +83,7 @@ const usage = `usage: fzf [options]
     --preview=COMMAND     Command to preview highlighted line ({})
     --preview-window=OPT  Preview window layout (default: right:50%)
                           [up|down|left|right][:SIZE[%]]
-                          [:[no]wrap][:[no]cycle][:[no]hidden]
+                          [:[no]wrap][:[no]cycle][:[no]follow][:[no]hidden]
                           [:rounded|sharp|noborder]
                           [:+SCROLL[-OFFSET]]
                           [:default]
@@ -169,6 +169,7 @@ type previewOpts struct {
 	hidden   bool
 	wrap     bool
 	cycle    bool
+	follow   bool
 	border   tui.BorderShape
 }
 
@@ -231,7 +232,7 @@ type Options struct {
 }
 
 func defaultPreviewOpts(command string) previewOpts {
-	return previewOpts{command, posRight, sizeSpec{50, true}, "", false, false, false, tui.BorderRounded}
+	return previewOpts{command, posRight, sizeSpec{50, true}, "", false, false, false, false, tui.BorderRounded}
 }
 
 func defaultOptions() *Options {
@@ -1081,6 +1082,10 @@ func parsePreviewWindow(opts *previewOpts, input string) {
 			opts.border = tui.BorderSharp
 		case "noborder":
 			opts.border = tui.BorderNone
+		case "follow":
+			opts.follow = true
+		case "nofollow":
+			opts.follow = false
 		default:
 			if sizeRegex.MatchString(token) {
 				opts.size = parseSize(token, 99, "window size")
diff --git a/src/terminal.go b/src/terminal.go
index 08535484..83b3a7b5 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -51,6 +51,7 @@ type previewer struct {
 	enabled    bool
 	scrollable bool
 	final      bool
+	following  bool
 	spinner    string
 }
 
@@ -140,7 +141,7 @@ type Terminal struct {
 	selected     map[int32]selectedItem
 	version      int64
 	reqBox       *util.EventBox
-	preview      previewOpts
+	previewOpts  previewOpts
 	previewer    previewer
 	previewed    previewed
 	previewBox   *util.EventBox
@@ -493,8 +494,8 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		merger:      EmptyMerger,
 		selected:    make(map[int32]selectedItem),
 		reqBox:      util.NewEventBox(),
-		preview:     opts.Preview,
-		previewer:   previewer{0, []string{}, 0, previewBox != nil && !opts.Preview.hidden, false, true, ""},
+		previewOpts: opts.Preview,
+		previewer:   previewer{0, []string{}, 0, previewBox != nil && !opts.Preview.hidden, false, true, false, ""},
 		previewed:   previewed{0, 0, 0, false},
 		previewBox:  previewBox,
 		eventBox:    eventBox,
@@ -732,11 +733,11 @@ func (t *Terminal) resizeWindows() {
 		}
 	}
 
-	previewVisible := t.isPreviewEnabled() && t.preview.size.size > 0
+	previewVisible := t.isPreviewEnabled() && t.previewOpts.size.size > 0
 	minAreaWidth := minWidth
 	minAreaHeight := minHeight
 	if previewVisible {
-		switch t.preview.position {
+		switch t.previewOpts.position {
 		case posUp, posDown:
 			minAreaHeight *= 2
 		case posLeft, posRight:
@@ -805,8 +806,8 @@ func (t *Terminal) resizeWindows() {
 		createPreviewWindow := func(y int, x int, w int, h int) {
 			pwidth := w
 			pheight := h
-			if t.preview.border != tui.BorderNone {
-				previewBorder := tui.MakeBorderStyle(t.preview.border, t.unicode)
+			if t.previewOpts.border != tui.BorderNone {
+				previewBorder := tui.MakeBorderStyle(t.previewOpts.border, t.unicode)
 				t.pborder = t.tui.NewWindow(y, x, w, h, true, previewBorder)
 				pwidth -= 4
 				pheight -= 2
@@ -822,28 +823,28 @@ func (t *Terminal) resizeWindows() {
 		}
 		verticalPad := 2
 		minPreviewHeight := 3
-		if t.preview.border == tui.BorderNone {
+		if t.previewOpts.border == tui.BorderNone {
 			verticalPad = 0
 			minPreviewHeight = 1
 		}
-		switch t.preview.position {
+		switch t.previewOpts.position {
 		case posUp:
-			pheight := calculateSize(height, t.preview.size, minHeight, minPreviewHeight, verticalPad)
+			pheight := calculateSize(height, t.previewOpts.size, minHeight, minPreviewHeight, verticalPad)
 			t.window = t.tui.NewWindow(
 				marginInt[0]+pheight, marginInt[3], width, height-pheight, false, noBorder)
 			createPreviewWindow(marginInt[0], marginInt[3], width, pheight)
 		case posDown:
-			pheight := calculateSize(height, t.preview.size, minHeight, minPreviewHeight, verticalPad)
+			pheight := calculateSize(height, t.previewOpts.size, minHeight, minPreviewHeight, verticalPad)
 			t.window = t.tui.NewWindow(
 				marginInt[0], marginInt[3], width, height-pheight, false, noBorder)
 			createPreviewWindow(marginInt[0]+height-pheight, marginInt[3], width, pheight)
 		case posLeft:
-			pwidth := calculateSize(width, t.preview.size, minWidth, 5, 4)
+			pwidth := calculateSize(width, t.previewOpts.size, minWidth, 5, 4)
 			t.window = t.tui.NewWindow(
 				marginInt[0], marginInt[3]+pwidth, width-pwidth, height, false, noBorder)
 			createPreviewWindow(marginInt[0], marginInt[3], pwidth, height)
 		case posRight:
-			pwidth := calculateSize(width, t.preview.size, minWidth, 5, 4)
+			pwidth := calculateSize(width, t.previewOpts.size, minWidth, 5, 4)
 			t.window = t.tui.NewWindow(
 				marginInt[0], marginInt[3], width-pwidth, height, false, noBorder)
 			createPreviewWindow(marginInt[0], marginInt[3]+width-pwidth, pwidth, height)
@@ -1291,7 +1292,7 @@ func (t *Terminal) renderPreviewText(unchanged bool) {
 			prefixWidth := 0
 			_, _, ansi = extractColor(line, ansi, func(str string, ansi *ansiState) bool {
 				trimmed := []rune(str)
-				if !t.preview.wrap {
+				if !t.previewOpts.wrap {
 					trimmed, _ = t.trimRight(trimmed, maxWidth-t.pwindow.X())
 				}
 				str, width := t.processTabs(trimmed, prefixWidth)
@@ -1559,7 +1560,7 @@ func atopi(s string) int {
 }
 
 func (t *Terminal) evaluateScrollOffset(list []*Item, height int) int {
-	offsetExpr := t.replacePlaceholder(t.preview.scroll, false, "", list)
+	offsetExpr := t.replacePlaceholder(t.previewOpts.scroll, false, "", list)
 	nums := strings.Split(offsetExpr, "-")
 	switch len(nums) {
 	case 0:
@@ -2041,7 +2042,7 @@ func (t *Terminal) Loop() {
 						if focusedIndex != currentIndex || version != t.version {
 							version = t.version
 							focusedIndex = currentIndex
-							refreshPreview(t.preview.command)
+							refreshPreview(t.previewOpts.command)
 						}
 					case reqJump:
 						if t.merger.Length() == 0 {
@@ -2066,10 +2067,15 @@ func (t *Terminal) Loop() {
 						})
 					case reqPreviewDisplay:
 						result := value.(previewResult)
-						t.previewer.version = result.version
+						if t.previewer.version != result.version {
+							t.previewer.version = result.version
+							t.previewer.following = t.previewOpts.follow
+						}
 						t.previewer.lines = result.lines
 						t.previewer.spinner = result.spinner
-						if result.offset >= 0 {
+						if t.previewer.following {
+							t.previewer.offset = len(t.previewer.lines) - t.pwindow.Height()
+						} else if result.offset >= 0 {
 							t.previewer.offset = util.Constrain(result.offset, 0, len(t.previewer.lines)-1)
 						}
 						t.printPreview()
@@ -2133,9 +2139,10 @@ func (t *Terminal) Loop() {
 			if !t.previewer.scrollable {
 				return
 			}
+			t.previewer.following = false
 			newOffset := t.previewer.offset + amount
 			numLines := len(t.previewer.lines)
-			if t.preview.cycle {
+			if t.previewOpts.cycle {
 				newOffset = (newOffset + numLines) % numLines
 			}
 			newOffset = util.Constrain(newOffset, 0, numLines-1)
@@ -2176,17 +2183,17 @@ func (t *Terminal) Loop() {
 				if t.hasPreviewer() {
 					togglePreview(!t.previewer.enabled)
 					if t.previewer.enabled {
-						valid, list := t.buildPlusList(t.preview.command, false)
+						valid, list := t.buildPlusList(t.previewOpts.command, false)
 						if valid {
 							t.cancelPreview()
 							t.previewBox.Set(reqPreviewEnqueue,
-								previewRequest{t.preview.command, t.pwindow, list})
+								previewRequest{t.previewOpts.command, t.pwindow, list})
 						}
 					}
 				}
 			case actTogglePreviewWrap:
 				if t.hasPreviewWindow() {
-					t.preview.wrap = !t.preview.wrap
+					t.previewOpts.wrap = !t.previewOpts.wrap
 					req(reqPreviewRefresh)
 				}
 			case actToggleSort:
@@ -2231,7 +2238,7 @@ func (t *Terminal) Loop() {
 				togglePreview(true)
 				refreshPreview(a.a)
 			case actRefreshPreview:
-				refreshPreview(t.preview.command)
+				refreshPreview(t.previewOpts.command)
 			case actReplaceQuery:
 				if t.cy >= 0 && t.cy < t.merger.Length() {
 					t.input = t.merger.Get(t.cy).item.text.ToRunes()
@@ -2554,7 +2561,7 @@ func (t *Terminal) Loop() {
 
 		if queryChanged {
 			if t.isPreviewEnabled() {
-				_, _, q := hasPreviewFlags(t.preview.command)
+				_, _, q := hasPreviewFlags(t.previewOpts.command)
 				if q {
 					t.version++
 				}
diff --git a/test/test_go.rb b/test/test_go.rb
index d9d1e1bc..416692e2 100755
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -1832,6 +1832,11 @@ class TestGoFZF < TestBase
     tmux.send_keys 'b'
     tmux.until { |lines| assert_equal 'b> foo', lines[-1] }
   end
+
+  def test_preview_window_follow
+    tmux.send_keys "#{FZF} --preview 'seq 1000 | nl' --preview-window down:noborder:follow", :Enter
+    tmux.until { |lines| assert_equal '1000  1000', lines[-1].strip }
+  end
 end
 
 module TestShell