Add support for an alternative preview window layout

Close #2804
Close #2844

Related #2277

4 files changed, 136 insertions, 76 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 96a9945a..b5a8519f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,16 @@ CHANGELOG
 
 0.31.0
 ------
+- Added support for an alternative preview window layout that is activated
+  when the size of the preview window is smaller than a certain threshold.
+  ```sh
+  # If the width of the preview window is smaller than 50 columns,
+  # it will be displayed above the search window.
+  fzf --preview 'cat {}' --preview-window 'right,50%,border-left,<50(up,30%,border-bottom)'
+
+  # Or you can just hide it like so
+  fzf --preview 'cat {}' --preview-window '<50(hidden)'
+  ```
 - Use SGR mouse mode to support larger terminals
 - Bug fixes and improvements
 
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index d4c0de16..098c3743 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -21,7 +21,7 @@ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 ..
-.TH fzf 1 "Apr 2022" "fzf 0.30.0" "fzf - a command-line fuzzy finder"
+.TH fzf 1 "Jul 2022" "fzf 0.31.0" "fzf - a command-line fuzzy finder"
 
 .SH NAME
 fzf - a command-line fuzzy finder
@@ -470,7 +470,7 @@ e.g.
       done'\fR
 .RE
 .TP
-.BI "--preview-window=" "[POSITION][,SIZE[%]][,border-BORDER_OPT][,[no]wrap][,[no]follow][,[no]cycle][,[no]hidden][,+SCROLL[OFFSETS][/DENOM]][,~HEADER_LINES][,default]"
+.BI "--preview-window=" "[POSITION][,SIZE[%]][,border-BORDER_OPT][,[no]wrap][,[no]follow][,[no]cycle][,[no]hidden][,+SCROLL[OFFSETS][/DENOM]][,~HEADER_LINES][,default][,<SIZE_THRESHOLD(ALTERNATIVE_LAYOUT)]"
 
 .RS
 .B POSITION: (default: right)
@@ -488,9 +488,9 @@ default until \fBtoggle-preview\fR action is triggered.
 execute the command in the background.
 
 * Long lines are truncated by default. Line wrap can be enabled with
-\fB:wrap\fR flag.
+\fBwrap\fR flag.
 
-* Preview window will automatically scroll to the bottom when \fB:follow\fR
+* Preview window will automatically scroll to the bottom when \fBfollow\fR
 flag is set, similarly to how \fBtail -f\fR works.
 
 .RS
@@ -502,7 +502,7 @@ e.g.
       done'\fR
 .RE
 
-* Cyclic scrolling is enabled with \fB:cycle\fR flag.
+* Cyclic scrolling is enabled with \fBcycle\fR flag.
 
 * To change the style of the border of the preview window, specify one of
 the options for \fB--border\fR with \fBborder-\fR prefix.
@@ -552,6 +552,15 @@ e.g.
      fzf --preview 'bat --style=full --color=always {}' --preview-window '~3'\fR
 .RE
 
+* You can specify an alternative set of options that are used only when the size
+  of the preview window is below a certain threshold. Note that only one
+  alternative layout is allowed.
+
+.RS
+e.g.
+      \fBfzf --preview 'cat {}' --preview-window 'right,border-left,<30(up,30%,border-bottom)'\fR
+.RE
+
 .SS Scripting
 .TP
 .BI "-q, --query=" "STR"
diff --git a/src/options.go b/src/options.go
index b0afacda..93178df5 100644
--- a/src/options.go
+++ b/src/options.go
@@ -89,7 +89,7 @@ const usage = `usage: fzf [options]
                           [,[no]wrap][,[no]cycle][,[no]follow][,[no]hidden]
                           [,border-BORDER_OPT]
                           [,+SCROLL[OFFSETS][/DENOM]][,~HEADER_LINES]
-                          [,default]
+                          [,default][,<SIZE_THRESHOLD(ALTERNATIVE_LAYOUT)]
 
   Scripting
     -q, --query=STR       Start the finder with the given query
@@ -175,10 +175,14 @@ type previewOpts struct {
 	follow      bool
 	border      tui.BorderShape
 	headerLines int
+	threshold   int
+	alternative *previewOpts
 }
 
 func (a previewOpts) sameLayout(b previewOpts) bool {
-	return a.size == b.size && a.position == b.position && a.border == b.border && a.hidden == b.hidden
+	return a.size == b.size && a.position == b.position && a.border == b.border && a.hidden == b.hidden && a.threshold == b.threshold &&
+		(a.alternative != nil && b.alternative != nil && a.alternative.sameLayout(*b.alternative) ||
+			a.alternative == nil && b.alternative == nil)
 }
 
 func (a previewOpts) sameContentLayout(b previewOpts) bool {
@@ -247,7 +251,7 @@ type Options struct {
 }
 
 func defaultPreviewOpts(command string) previewOpts {
-	return previewOpts{command, posRight, sizeSpec{50, true}, "", false, false, false, false, tui.BorderRounded, 0}
+	return previewOpts{command, posRight, sizeSpec{50, true}, "", false, false, false, false, tui.BorderRounded, 0, 0, nil}
 }
 
 func defaultOptions() *Options {
@@ -1169,12 +1173,19 @@ func parseInfoStyle(str string) infoStyle {
 }
 
 func parsePreviewWindow(opts *previewOpts, input string) {
-	delimRegex := regexp.MustCompile("[:,]") // : for backward compatibility
+	tokenRegex := regexp.MustCompile(`[:,]*(<([1-9][0-9]*)\(([^)<]+)\)|[^,:]+)`)
 	sizeRegex := regexp.MustCompile("^[0-9]+%?$")
 	offsetRegex := regexp.MustCompile(`^(\+{-?[0-9]+})?([+-][0-9]+)*(-?/[1-9][0-9]*)?$`)
 	headerRegex := regexp.MustCompile("^~(0|[1-9][0-9]*)$")
-	tokens := delimRegex.Split(input, -1)
-	for _, token := range tokens {
+	tokens := tokenRegex.FindAllStringSubmatch(input, -1)
+	var alternative string
+	for _, match := range tokens {
+		if len(match[2]) > 0 {
+			opts.threshold = atoi(match[2])
+			alternative = match[3]
+			continue
+		}
+		token := match[1]
 		switch token {
 		case "":
 		case "default":
@@ -1233,6 +1244,12 @@ func parsePreviewWindow(opts *previewOpts, input string) {
 			}
 		}
 	}
+	if len(alternative) > 0 {
+		alternativeOpts := *opts
+		opts.alternative = &alternativeOpts
+		opts.alternative.alternative = nil
+		parsePreviewWindow(opts.alternative, alternative)
+	}
 }
 
 func parseMargin(opt string, margin string) [4]sizeSpec {
diff --git a/src/terminal.go b/src/terminal.go
index 3c13b373..96bb6411 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -819,12 +819,15 @@ func (t *Terminal) resizeWindows() {
 	}
 	if t.window != nil {
 		t.window.Close()
+		t.window = nil
 	}
 	if t.pborder != nil {
 		t.pborder.Close()
+		t.pborder = nil
 	}
 	if t.pwindow != nil {
 		t.pwindow.Close()
+		t.pwindow = nil
 	}
 	// Reset preview version so that full redraw occurs
 	t.previewed.version = 0
@@ -869,76 +872,97 @@ func (t *Terminal) resizeWindows() {
 	width = screenWidth - marginInt[1] - marginInt[3]
 	height = screenHeight - marginInt[0] - marginInt[2]
 
+	// Set up preview window
 	noBorder := tui.MakeBorderStyle(tui.BorderNone, t.unicode)
 	if previewVisible {
-		createPreviewWindow := func(y int, x int, w int, h int) {
-			pwidth := w
-			pheight := h
-			var previewBorder tui.BorderStyle
-			if t.previewOpts.border == tui.BorderNone {
-				previewBorder = tui.MakeTransparentBorder()
-			} else {
-				previewBorder = tui.MakeBorderStyle(t.previewOpts.border, t.unicode)
+		var resizePreviewWindows func(previewOpts previewOpts)
+		resizePreviewWindows = func(previewOpts previewOpts) {
+			if previewOpts.hidden {
+				return
+			}
+			hasThreshold := previewOpts.threshold > 0 && previewOpts.alternative != nil
+			createPreviewWindow := func(y int, x int, w int, h int) {
+				pwidth := w
+				pheight := h
+				var previewBorder tui.BorderStyle
+				if previewOpts.border == tui.BorderNone {
+					previewBorder = tui.MakeTransparentBorder()
+				} else {
+					previewBorder = tui.MakeBorderStyle(previewOpts.border, t.unicode)
+				}
+				t.pborder = t.tui.NewWindow(y, x, w, h, true, previewBorder)
+				switch previewOpts.border {
+				case tui.BorderSharp, tui.BorderRounded:
+					pwidth -= 4
+					pheight -= 2
+					x += 2
+					y += 1
+				case tui.BorderLeft:
+					pwidth -= 2
+					x += 2
+				case tui.BorderRight:
+					pwidth -= 2
+				case tui.BorderTop:
+					pheight -= 1
+					y += 1
+				case tui.BorderBottom:
+					pheight -= 1
+				case tui.BorderHorizontal:
+					pheight -= 2
+					y += 1
+				case tui.BorderVertical:
+					pwidth -= 4
+					x += 2
+				}
+				t.pwindow = t.tui.NewWindow(y, x, pwidth, pheight, true, noBorder)
 			}
-			t.pborder = t.tui.NewWindow(y, x, w, h, true, previewBorder)
-			switch t.previewOpts.border {
-			case tui.BorderSharp, tui.BorderRounded:
-				pwidth -= 4
-				pheight -= 2
-				x += 2
-				y += 1
-			case tui.BorderLeft:
-				pwidth -= 2
-				x += 2
-			case tui.BorderRight:
-				pwidth -= 2
-			case tui.BorderTop:
-				pheight -= 1
-				y += 1
-			case tui.BorderBottom:
-				pheight -= 1
-			case tui.BorderHorizontal:
-				pheight -= 2
-				y += 1
-			case tui.BorderVertical:
-				pwidth -= 4
-				x += 2
+			verticalPad := 2
+			minPreviewHeight := 3
+			switch previewOpts.border {
+			case tui.BorderNone, tui.BorderVertical, tui.BorderLeft, tui.BorderRight:
+				verticalPad = 0
+				minPreviewHeight = 1
+			case tui.BorderTop, tui.BorderBottom:
+				verticalPad = 1
+				minPreviewHeight = 2
+			}
+			switch previewOpts.position {
+			case posUp, posDown:
+				pheight := calculateSize(height, previewOpts.size, minHeight, minPreviewHeight, verticalPad)
+				if hasThreshold && pheight < previewOpts.threshold {
+					resizePreviewWindows(*previewOpts.alternative)
+					return
+				}
+				if previewOpts.position == posUp {
+					t.window = t.tui.NewWindow(
+						marginInt[0]+pheight, marginInt[3], width, height-pheight, false, noBorder)
+					createPreviewWindow(marginInt[0], marginInt[3], width, pheight)
+				} else {
+					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, posRight:
+				pwidth := calculateSize(width, previewOpts.size, minWidth, 5, 4)
+				if hasThreshold && pwidth < previewOpts.threshold {
+					fmt.Println("Alternative", (*previewOpts.alternative).position == posDown)
+					resizePreviewWindows(*previewOpts.alternative)
+					return
+				}
+				if previewOpts.position == posLeft {
+					t.window = t.tui.NewWindow(
+						marginInt[0], marginInt[3]+pwidth, width-pwidth, height, false, noBorder)
+					createPreviewWindow(marginInt[0], marginInt[3], pwidth, height)
+				} else {
+					t.window = t.tui.NewWindow(
+						marginInt[0], marginInt[3], width-pwidth, height, false, noBorder)
+					createPreviewWindow(marginInt[0], marginInt[3]+width-pwidth, pwidth, height)
+				}
 			}
-			t.pwindow = t.tui.NewWindow(y, x, pwidth, pheight, true, noBorder)
-		}
-		verticalPad := 2
-		minPreviewHeight := 3
-		switch t.previewOpts.border {
-		case tui.BorderNone, tui.BorderVertical, tui.BorderLeft, tui.BorderRight:
-			verticalPad = 0
-			minPreviewHeight = 1
-		case tui.BorderTop, tui.BorderBottom:
-			verticalPad = 1
-			minPreviewHeight = 2
-		}
-		switch t.previewOpts.position {
-		case posUp:
-			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.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.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.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)
 		}
-	} else {
+		resizePreviewWindows(t.previewOpts)
+	}
+	if t.window == nil {
 		t.window = t.tui.NewWindow(
 			marginInt[0],
 			marginInt[3],