Add --preview and --preview-window

Close #587

4 files changed, 467 insertions, 122 deletions

diff --git a/src/curses/curses.go b/src/curses/curses.go
index ca10fc7d..7a9ccd4a 100644
--- a/src/curses/curses.go
+++ b/src/curses/curses.go
@@ -113,7 +113,8 @@ const (
 	ColCursor
 	ColSelected
 	ColHeader
-	ColUser
+	ColBorder
+	ColUser // Should be the last entry
 )
 
 const (
@@ -136,6 +137,7 @@ type ColorTheme struct {
 	Cursor       int16
 	Selected     int16
 	Header       int16
+	Border       int16
 }
 
 type Event struct {
@@ -170,6 +172,31 @@ var (
 	DarkBG        int
 )
 
+type Window struct {
+	win    *C.WINDOW
+	Top    int
+	Left   int
+	Width  int
+	Height int
+}
+
+func NewWindow(top int, left int, width int, height int, border bool) *Window {
+	win := C.newwin(C.int(height), C.int(width), C.int(top), C.int(left))
+	if border {
+		attr := _color(ColBorder, false)
+		C.wattron(win, attr)
+		C.box(win, 0, 0)
+		C.wattroff(win, attr)
+	}
+	return &Window{
+		win:    win,
+		Top:    top,
+		Left:   left,
+		Width:  width,
+		Height: height,
+	}
+}
+
 func EmptyTheme() *ColorTheme {
 	return &ColorTheme{
 		UseDefault:   true,
@@ -184,7 +211,8 @@ func EmptyTheme() *ColorTheme {
 		Info:         colUndefined,
 		Cursor:       colUndefined,
 		Selected:     colUndefined,
-		Header:       colUndefined}
+		Header:       colUndefined,
+		Border:       colUndefined}
 }
 
 func init() {
@@ -204,7 +232,8 @@ func init() {
 		Info:         C.COLOR_WHITE,
 		Cursor:       C.COLOR_RED,
 		Selected:     C.COLOR_MAGENTA,
-		Header:       C.COLOR_CYAN}
+		Header:       C.COLOR_CYAN,
+		Border:       C.COLOR_BLACK}
 	Dark256 = &ColorTheme{
 		UseDefault:   true,
 		Fg:           15,
@@ -218,7 +247,8 @@ func init() {
 		Info:         144,
 		Cursor:       161,
 		Selected:     168,
-		Header:       109}
+		Header:       109,
+		Border:       59}
 	Light256 = &ColorTheme{
 		UseDefault:   true,
 		Fg:           15,
@@ -232,7 +262,8 @@ func init() {
 		Info:         101,
 		Cursor:       161,
 		Selected:     168,
-		Header:       31}
+		Header:       31,
+		Border:       145}
 }
 
 func attrColored(pair int, bold bool) C.int {
@@ -360,6 +391,7 @@ func initPairs(baseTheme *ColorTheme, theme *ColorTheme, black bool) {
 	C.init_pair(ColCursor, override(baseTheme.Cursor, theme.Cursor), darkBG)
 	C.init_pair(ColSelected, override(baseTheme.Selected, theme.Selected), darkBG)
 	C.init_pair(ColHeader, override(baseTheme.Header, theme.Header), bg)
+	C.init_pair(ColBorder, override(baseTheme.Border, theme.Border), bg)
 }
 
 func Close() {
@@ -415,7 +447,9 @@ func mouseSequence(sz *int) Event {
 		97, 101, 105, 113: // scroll-down / shift / cmd / ctrl
 		mod := _buf[3] >= 100
 		s := 1 - int(_buf[3]%2)*2
-		return Event{Mouse, 0, &MouseEvent{0, 0, s, false, false, mod}}
+		x := int(_buf[4] - 33)
+		y := int(_buf[5] - 33)
+		return Event{Mouse, 0, &MouseEvent{y, x, s, false, false, mod}}
 	}
 	return Event{Invalid, 0, nil}
 }
@@ -588,17 +622,25 @@ func GetChar() Event {
 	return Event{Rune, r, nil}
 }
 
-func Move(y int, x int) {
-	C.move(C.int(y), C.int(x))
+func (w *Window) Close() {
+	C.delwin(w.win)
+}
+
+func (w *Window) Enclose(y int, x int) bool {
+	return bool(C.wenclose(w.win, C.int(y), C.int(x)))
+}
+
+func (w *Window) Move(y int, x int) {
+	C.wmove(w.win, C.int(y), C.int(x))
 }
 
-func MoveAndClear(y int, x int) {
-	Move(y, x)
-	C.clrtoeol()
+func (w *Window) MoveAndClear(y int, x int) {
+	w.Move(y, x)
+	C.wclrtoeol(w.win)
 }
 
-func Print(text string) {
-	C.addstr(C.CString(strings.Map(func(r rune) rune {
+func (w *Window) Print(text string) {
+	C.waddstr(w.win, C.CString(strings.Map(func(r rune) rune {
 		if r < 32 {
 			return -1
 		}
@@ -606,11 +648,11 @@ func Print(text string) {
 	}, text)))
 }
 
-func CPrint(pair int, bold bool, text string) {
+func (w *Window) CPrint(pair int, bold bool, text string) {
 	attr := _color(pair, bold)
-	C.attron(attr)
-	Print(text)
-	C.attroff(attr)
+	C.wattron(w.win, attr)
+	w.Print(text)
+	C.wattroff(w.win, attr)
 }
 
 func Clear() {
@@ -625,6 +667,30 @@ func Refresh() {
 	C.refresh()
 }
 
+func (w *Window) Erase() {
+	C.werase(w.win)
+}
+
+func (w *Window) Fill(str string) bool {
+	return C.waddstr(w.win, C.CString(str)) == C.OK
+}
+
+func (w *Window) CFill(str string, fg int, bg int, bold bool) bool {
+	attr := _color(PairFor(fg, bg), bold)
+	C.wattron(w.win, attr)
+	ret := w.Fill(str)
+	C.wattroff(w.win, attr)
+	return ret
+}
+
+func (w *Window) Refresh() {
+	C.wnoutrefresh(w.win)
+}
+
+func DoUpdate() {
+	C.doupdate()
+}
+
 func PairFor(fg int, bg int) int {
 	key := (fg << 8) + bg
 	if found, prs := _colorMap[key]; prs {
diff --git a/src/options.go b/src/options.go
index ffd57e96..2a724274 100644
--- a/src/options.go
+++ b/src/options.go
@@ -1,6 +1,7 @@
 package fzf
 
 import (
+	"fmt"
 	"os"
 	"regexp"
 	"strconv"
@@ -23,36 +24,47 @@ const usage = `usage: fzf [options]
     -n, --nth=N[,..]      Comma-separated list of field index expressions
                           for limiting search scope. Each can be a non-zero
                           integer or a range expression ([BEGIN]..[END]).
-    --with-nth=N[,..]     Transform item using index expressions within finder
-    -d, --delimiter=STR   Field delimiter regex for --nth (default: AWK-style)
+    --with-nth=N[,..]     Transform the presentation of each line using
+                          field index expressions
+    -d, --delimiter=STR   Field delimiter regex (default: AWK-style)
     +s, --no-sort         Do not sort the result
     --tac                 Reverse the order of the input
     --tiebreak=CRI[,..]   Comma-separated list of sort criteria to apply
-                          when the scores are tied;
-                          [length|begin|end|index] (default: length)
+                          when the scores are tied [length|begin|end|index]
+                          (default: length)
 
   Interface
     -m, --multi           Enable multi-select with tab/shift-tab
-    --ansi                Enable processing of ANSI color codes
     --no-mouse            Disable mouse
-    --color=COLSPEC       Base scheme (dark|light|16|bw) and/or custom colors
-    --black               Use black background
-    --reverse             Reverse orientation
-    --margin=MARGIN       Screen margin (TRBL / TB,RL / T,RL,B / T,R,B,L)
-    --tabstop=SPACES      Number of spaces for a tab character (default: 8)
+    --bind=KEYBINDS       Custom key bindings. Refer to the man page.
     --cycle               Enable cyclic scroll
     --no-hscroll          Disable horizontal scroll
     --hscroll-off=COL     Number of screen columns to keep to the right of the
                           highlighted substring (default: 10)
-    --inline-info         Display finder info inline with the query
     --jump-labels=CHARS   Label characters for jump and jump-accept
+
+  Layout
+    --reverse             Reverse orientation
+    --margin=MARGIN       Screen margin (TRBL / TB,RL / T,RL,B / T,R,B,L)
+    --inline-info         Display finder info inline with the query
     --prompt=STR          Input prompt (default: '> ')
-    --bind=KEYBINDS       Custom key bindings. Refer to the man page.
-    --history=FILE        History file
-    --history-size=N      Maximum number of history entries (default: 1000)
     --header=STR          String to print as header
     --header-lines=N      The first N lines of the input are treated as header
 
+  Display
+    --ansi                Enable processing of ANSI color codes
+    --tabstop=SPACES      Number of spaces for a tab character (default: 8)
+    --color=COLSPEC       Base scheme (dark|light|16|bw) and/or custom colors
+
+  History
+    --history=FILE        History file
+    --history-size=N      Maximum number of history entries (default: 1000)
+
+  Preview
+    --preview=COMMAND     Command to preview highlighted line ({})
+    --preview-window=OPT  Preview window layout (default: right:50%)
+                          [up|down|left|right][:SIZE[%]][:hidden]
+
   Scripting
     -q, --query=STR       Start the finder with the given query
     -1, --select-1        Automatically select the only match
@@ -88,8 +100,29 @@ const (
 	byEnd
 )
 
-func defaultMargin() [4]string {
-	return [4]string{"0", "0", "0", "0"}
+type sizeSpec struct {
+	size    float64
+	percent bool
+}
+
+func defaultMargin() [4]sizeSpec {
+	return [4]sizeSpec{}
+}
+
+type windowPosition int
+
+const (
+	posUp windowPosition = iota
+	posDown
+	posLeft
+	posRight
+)
+
+type previewOpts struct {
+	command  string
+	position windowPosition
+	size     sizeSpec
+	hidden   bool
 }
 
 // Options stores the values of command-line options
@@ -123,13 +156,14 @@ type Options struct {
 	Expect      map[int]string
 	Keymap      map[int]actionType
 	Execmap     map[int]string
+	Preview     previewOpts
 	PrintQuery  bool
 	ReadZero    bool
 	Sync        bool
 	History     *History
 	Header      []string
 	HeaderLines int
-	Margin      [4]string
+	Margin      [4]sizeSpec
 	Tabstop     int
 	Version     bool
 }
@@ -165,6 +199,7 @@ func defaultOptions() *Options {
 		Expect:      make(map[int]string),
 		Keymap:      make(map[int]actionType),
 		Execmap:     make(map[int]string),
+		Preview:     previewOpts{"", posRight, sizeSpec{50, true}, false},
 		PrintQuery:  false,
 		ReadZero:    false,
 		Sync:        false,
@@ -458,6 +493,8 @@ func parseTheme(defaultTheme *curses.ColorTheme, str string) *curses.ColorTheme
 				theme.Match = ansi
 			case "hl+":
 				theme.CurrentMatch = ansi
+			case "border":
+				theme.Border = ansi
 			case "prompt":
 				theme.Prompt = ansi
 			case "spinner":
@@ -604,6 +641,8 @@ func parseKeymap(keymap map[int]actionType, execmap map[int]string, str string)
 			keymap[key] = actPreviousHistory
 		case "next-history":
 			keymap[key] = actNextHistory
+		case "toggle-preview":
+			keymap[key] = actTogglePreview
 		case "toggle-sort":
 			keymap[key] = actToggleSort
 		default:
@@ -659,40 +698,86 @@ func strLines(str string) []string {
 	return strings.Split(strings.TrimSuffix(str, "\n"), "\n")
 }
 
-func parseMargin(margin string) [4]string {
-	margins := strings.Split(margin, ",")
-	checked := func(str string) string {
-		if strings.HasSuffix(str, "%") {
-			val := atof(str[:len(str)-1])
-			if val < 0 {
-				errorExit("margin must be non-negative")
-			}
-			if val > 100 {
-				errorExit("margin too large")
-			}
-		} else {
-			val := atoi(str)
-			if val < 0 {
-				errorExit("margin must be non-negative")
-			}
+func parseSize(str string, maxPercent float64, label string) sizeSpec {
+	var val float64
+	percent := strings.HasSuffix(str, "%")
+	if percent {
+		val = atof(str[:len(str)-1])
+		if val < 0 {
+			errorExit(label + " must be non-negative")
 		}
-		return str
+		if val > maxPercent {
+			errorExit(fmt.Sprintf("%s too large (max: %d%%)", label, int(maxPercent)))
+		}
+	} else {
+		if strings.Contains(str, ".") {
+			errorExit(label + " (without %) must be a non-negative integer")
+		}
+
+		val = float64(atoi(str))
+		if val < 0 {
+			errorExit(label + " must be non-negative")
+		}
+	}
+	return sizeSpec{val, percent}
+}
+
+func parsePreviewWindow(opts *previewOpts, input string) {
+	layout := input
+	if strings.HasSuffix(layout, ":hidden") {
+		opts.hidden = true
+		layout = strings.TrimSuffix(layout, ":hidden")
+	}
+
+	tokens := strings.Split(layout, ":")
+	if len(tokens) == 0 || len(tokens) > 2 {
+		errorExit("invalid window layout: " + input)
+	}
+
+	if len(tokens) > 1 {
+		opts.size = parseSize(tokens[1], 99, "window size")
+	} else {
+		opts.size = sizeSpec{50, true}
+	}
+	if !opts.size.percent && opts.size.size > 0 {
+		// Adjust size for border
+		opts.size.size += 2
+	}
+
+	switch tokens[0] {
+	case "up":
+		opts.position = posUp
+	case "down":
+		opts.position = posDown
+	case "left":
+		opts.position = posLeft
+	case "right":
+		opts.position = posRight
+	default:
+		errorExit("invalid window position: " + input)
+	}
+}
+
+func parseMargin(margin string) [4]sizeSpec {
+	margins := strings.Split(margin, ",")
+	checked := func(str string) sizeSpec {
+		return parseSize(str, 49, "margin")
 	}
 	switch len(margins) {
 	case 1:
 		m := checked(margins[0])
-		return [4]string{m, m, m, m}
+		return [4]sizeSpec{m, m, m, m}
 	case 2:
 		tb := checked(margins[0])
 		rl := checked(margins[1])
-		return [4]string{tb, rl, tb, rl}
+		return [4]sizeSpec{tb, rl, tb, rl}
 	case 3:
 		t := checked(margins[0])
 		rl := checked(margins[1])
 		b := checked(margins[2])
-		return [4]string{t, rl, b, rl}
+		return [4]sizeSpec{t, rl, b, rl}
 	case 4:
-		return [4]string{
+		return [4]sizeSpec{
 			checked(margins[0]), checked(margins[1]),
 			checked(margins[2]), checked(margins[3])}
 	default:
@@ -858,6 +943,13 @@ func parseOptions(opts *Options, allArgs []string) {
 		case "--header-lines":
 			opts.HeaderLines = atoi(
 				nextString(allArgs, &i, "number of header lines required"))
+		case "--preview":
+			opts.Preview.command = nextString(allArgs, &i, "preview command required")
+		case "--no-preview":
+			opts.Preview.command = ""
+		case "--preview-window":
+			parsePreviewWindow(&opts.Preview,
+				nextString(allArgs, &i, "preview window layout required: [up|down|left|right][:SIZE[%]]"))
 		case "--no-margin":
 			opts.Margin = defaultMargin()
 		case "--margin":
@@ -900,6 +992,10 @@ func parseOptions(opts *Options, allArgs []string) {
 				opts.Header = strLines(value)
 			} else if match, value := optString(arg, "--header-lines="); match {
 				opts.HeaderLines = atoi(value)
+			} else if match, value := optString(arg, "--preview="); match {
+				opts.Preview.command = value
+			} else if match, value := optString(arg, "--preview-window="); match {
+				parsePreviewWindow(&opts.Preview, value)
 			} else if match, value := optString(arg, "--margin="); match {
 				opts.Margin = parseMargin(value)
 			} else if match, value := optString(arg, "--tabstop="); match {
diff --git a/src/terminal.go b/src/terminal.go
index b1455e8e..ce10ee75 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -7,7 +7,6 @@ import (
 	"os/signal"
 	"regexp"
 	"sort"
-	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -53,8 +52,10 @@ type Terminal struct {
 	header     []string
 	header0    []string
 	ansi       bool
-	margin     [4]string
-	marginInt  [4]int
+	margin     [4]sizeSpec
+	window     *C.Window
+	bwindow    *C.Window
+	pwindow    *C.Window
 	count      int
 	progress   int
 	reading    bool
@@ -63,6 +64,10 @@ type Terminal struct {
 	merger     *Merger
 	selected   map[int32]selectedItem
 	reqBox     *util.EventBox
+	preview    previewOpts
+	previewing bool
+	previewTxt string
+	previewBox *util.EventBox
 	eventBox   *util.EventBox
 	mutex      sync.Mutex
 	initFunc   func()
@@ -103,6 +108,8 @@ const (
 	reqRedraw
 	reqClose
 	reqPrintQuery
+	reqPreviewEnqueue
+	reqPreviewDisplay
 	reqQuit
 )
 
@@ -148,6 +155,7 @@ const (
 	actJumpAccept
 	actPrintQuery
 	actToggleSort
+	actTogglePreview
 	actPreviousHistory
 	actNextHistory
 	actExecute
@@ -220,6 +228,10 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 	} else {
 		delay = initialDelay
 	}
+	var previewBox *util.EventBox
+	if len(opts.Preview.command) > 0 {
+		previewBox = util.NewEventBox()
+	}
 	return &Terminal{
 		initDelay:  delay,
 		inlineInfo: opts.InlineInfo,
@@ -242,7 +254,6 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		printQuery: opts.PrintQuery,
 		history:    opts.History,
 		margin:     opts.Margin,
-		marginInt:  [4]int{0, 0, 0, 0},
 		cycle:      opts.Cycle,
 		header:     header,
 		header0:    header,
@@ -253,6 +264,10 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		merger:     EmptyMerger,
 		selected:   make(map[int32]selectedItem),
 		reqBox:     util.NewEventBox(),
+		preview:    opts.Preview,
+		previewing: previewBox != nil && !opts.Preview.hidden,
+		previewTxt: "",
+		previewBox: previewBox,
 		eventBox:   eventBox,
 		mutex:      sync.Mutex{},
 		suppress:   true,
@@ -332,7 +347,7 @@ func (t *Terminal) output() bool {
 	if !found {
 		cnt := t.merger.Length()
 		if cnt > 0 && cnt > t.cy {
-			fmt.Println(t.merger.Get(t.cy).AsString(t.ansi))
+			fmt.Println(t.current())
 			found = true
 		}
 	} else {
@@ -372,56 +387,113 @@ func displayWidth(runes []rune) int {
 	return l
 }
 
-const minWidth = 16
-const minHeight = 4
+const (
+	minWidth  = 16
+	minHeight = 4
+)
 
-func (t *Terminal) calculateMargins() {
+func calculateSize(base int, size sizeSpec, margin int, minSize int) int {
+	max := base - margin
+	if size.percent {
+		return util.Constrain(int(float64(base)*0.01*size.size), minSize, max)
+	}
+	return util.Constrain(int(size.size), minSize, max)
+}
+
+func (t *Terminal) resizeWindows() {
 	screenWidth := C.MaxX()
 	screenHeight := C.MaxY()
-	for idx, str := range t.margin {
-		if str == "0" {
-			t.marginInt[idx] = 0
-		} else if strings.HasSuffix(str, "%") {
-			num, _ := strconv.ParseFloat(str[:len(str)-1], 64)
-			var val float64
+	marginInt := [4]int{}
+	for idx, sizeSpec := range t.margin {
+		if sizeSpec.percent {
+			var max float64
 			if idx%2 == 0 {
-				val = float64(screenHeight)
+				max = float64(screenHeight)
 			} else {
-				val = float64(screenWidth)
+				max = float64(screenWidth)
 			}
-			t.marginInt[idx] = int(val * num * 0.01)
+			marginInt[idx] = int(max * sizeSpec.size * 0.01)
 		} else {
-			num, _ := strconv.Atoi(str)
-			t.marginInt[idx] = num
+			marginInt[idx] = int(sizeSpec.size)
 		}
 	}
 	adjust := func(idx1 int, idx2 int, max int, min int) {
 		if max >= min {
-			margin := t.marginInt[idx1] + t.marginInt[idx2]
+			margin := marginInt[idx1] + marginInt[idx2]
 			if max-margin < min {
 				desired := max - min
-				t.marginInt[idx1] = desired * t.marginInt[idx1] / margin
-				t.marginInt[idx2] = desired * t.marginInt[idx2] / margin
+				marginInt[idx1] = desired * marginInt[idx1] / margin
+				marginInt[idx2] = desired * marginInt[idx2] / margin
 			}
 		}
 	}
-	adjust(1, 3, screenWidth, minWidth)
-	adjust(0, 2, screenHeight, minHeight)
+	minAreaWidth := minWidth
+	minAreaHeight := minHeight
+	if t.isPreviewEnabled() {
+		switch t.preview.position {
+		case posUp, posDown:
+			minAreaHeight *= 2
+		case posLeft, posRight:
+			minAreaWidth *= 2
+		}
+	}
+	adjust(1, 3, screenWidth, minAreaWidth)
+	adjust(0, 2, screenHeight, minAreaHeight)
+	if t.window != nil {
+		t.window.Close()
+	}
+	if t.bwindow != nil {
+		t.bwindow.Close()
+		t.pwindow.Close()
+	}
+
+	width := screenWidth - marginInt[1] - marginInt[3]
+	height := screenHeight - marginInt[0] - marginInt[2]
+	if t.isPreviewEnabled() {
+		createPreviewWindow := func(y int, x int, w int, h int) {
+			t.bwindow = C.NewWindow(y, x, w, h, true)
+			t.pwindow = C.NewWindow(y+1, x+2, w-4, h-2, false)
+		}
+		switch t.preview.position {
+		case posUp:
+			pheight := calculateSize(height, t.preview.size, minHeight, 3)
+			t.window = C.NewWindow(
+				marginInt[0]+pheight, marginInt[3], width, height-pheight, false)
+			createPreviewWindow(marginInt[0], marginInt[3], width, pheight)
+		case posDown:
+			pheight := calculateSize(height, t.preview.size, minHeight, 3)
+			t.window = C.NewWindow(
+				marginInt[0], marginInt[3], width, height-pheight, false)
+			createPreviewWindow(marginInt[0]+height-pheight, marginInt[3], width, pheight)
+		case posLeft:
+			pwidth := calculateSize(width, t.preview.size, minWidth, 5)
+			t.window = C.NewWindow(
+				marginInt[0], marginInt[3]+pwidth, width-pwidth, height, false)
+			createPreviewWindow(marginInt[0], marginInt[3], pwidth, height)
+		case posRight:
+			pwidth := calculateSize(width, t.preview.size, minWidth, 5)
+			t.window = C.NewWindow(
+				marginInt[0], marginInt[3], width-pwidth, height, false)
+			createPreviewWindow(marginInt[0], marginInt[3]+width-pwidth, pwidth, height)
+		}
+	} else {
+		t.window = C.NewWindow(
+			marginInt[0],
+			marginInt[3],
+			width,
+			height, false)
+	}
 }
 
 func (t *Terminal) move(y int, x int, clear bool) {
-	x += t.marginInt[3]
-	maxy := C.MaxY()
 	if !t.reverse {
-		y = maxy - y - 1 - t.marginInt[2]
-	} else {
-		y += t.marginInt[0]
+		y = t.window.Height - y - 1
 	}
 
 	if clear {
-		C.MoveAndClear(y, x)
+		t.window.MoveAndClear(y, x)
 	} else {
-		C.Move(y, x)
+		t.window.Move(y, x)
 	}
 }
 
@@ -431,24 +503,24 @@ func (t *Terminal) placeCursor() {
 
 func (t *Terminal) printPrompt() {
 	t.move(0, 0, true)
-	C.CPrint(C.ColPrompt, true, t.prompt)
-	C.CPrint(C.ColNormal, true, string(t.input))
+	t.window.CPrint(C.ColPrompt, true, t.prompt)
+	t.window.CPrint(C.ColNormal, true, string(t.input))
 }
 
 func (t *Terminal) printInfo() {
 	if t.inlineInfo {
 		t.move(0, displayWidth([]rune(t.prompt))+displayWidth(t.input)+1, true)
 		if t.reading {
-			C.CPrint(C.ColSpinner, true, " < ")
+			t.window.CPrint(C.ColSpinner, true, " < ")
 		} else {
-			C.CPrint(C.ColPrompt, true, " < ")
+			t.window.CPrint(C.ColPrompt, true, " < ")
 		}
 	} else {
 		t.move(1, 0, true)
 		if t.reading {
 			duration := int64(spinnerDuration)
 			idx := (time.Now().UnixNano() % (duration * int64(len(_spinner)))) / duration
-			C.CPrint(C.ColSpinner, true, _spinner[idx])
+			t.window.CPrint(C.ColSpinner, true, _spinner[idx])
 		}
 		t.move(1, 2, false)
 	}
@@ -467,18 +539,14 @@ func (t *Terminal) printInfo() {
 	if t.progress > 0 && t.progress < 100 {
 		output += fmt.Sprintf(" (%d%%)", t.progress)
 	}
-	C.CPrint(C.ColInfo, false, output)
-}
-
-func (t *Terminal) maxHeight() int {
-	return C.MaxY() - t.marginInt[0] - t.marginInt[2]
+	t.window.CPrint(C.ColInfo, false, output)
 }
 
 func (t *Terminal) printHeader() {
 	if len(t.header) == 0 {
 		return
 	}
-	max := t.maxHeight()
+	max := t.window.Height
 	var state *ansiState
 	for idx, lineStr := range t.header {
 		line := idx + 2
@@ -529,19 +597,19 @@ func (t *Terminal) printItem(item *Item, i int, current bool) {
 	} else if current {
 		label = ">"
 	}
-	C.CPrint(C.ColCursor, true, label)
+	t.window.CPrint(C.ColCursor, true, label)
 	if current {
 		if selected {
-			C.CPrint(C.ColSelected, true, ">")
+			t.window.CPrint(C.ColSelected, true, ">")
 		} else {
-			C.CPrint(C.ColCurrent, true, " ")
+			t.window.CPrint(C.ColCurrent, true, " ")
 		}
 		t.printHighlighted(item, true, C.ColCurrent, C.ColCurrentMatch, true)
 	} else {
 		if selected {
-			C.CPrint(C.ColSelected, true, ">")
+			t.window.CPrint(C.ColSelected, true, ">")
 		} else {
-			C.Print(" ")
+			t.window.Print(" ")
 		}
 		t.printHighlighted(item, false, 0, C.ColMatch, false)
 	}
@@ -593,7 +661,7 @@ func (t *Terminal) printHighlighted(item *Item, bold bool, col1 int, col2 int, c
 	text := make([]rune, len(item.text))
 	copy(text, item.text)
 	offsets := item.colorOffsets(col2, bold, current)
-	maxWidth := C.MaxX() - 3 - t.marginInt[1] - t.marginInt[3]
+	maxWidth := t.window.Width - 3
 	maxe = util.Constrain(maxe+util.Min(maxWidth/2-2, t.hscrollOff), 0, len(text))
 	fullWidth := displayWidth(text)
 	if fullWidth > maxWidth {
@@ -643,11 +711,11 @@ func (t *Terminal) printHighlighted(item *Item, bold bool, col1 int, col2 int, c
 		e := util.Constrain32(offset.offset[1], index, maxOffset)
 
 		substr, prefixWidth = processTabs(text[index:b], prefixWidth)
-		C.CPrint(col1, bold, substr)
+		t.window.CPrint(col1, bold, substr)
 
 		if b < e {
 			substr, prefixWidth = processTabs(text[b:e], prefixWidth)
-			C.CPrint(offset.color, offset.bold, substr)
+			t.window.CPrint(offset.color, offset.bold, substr)
 		}
 
 		index = e
@@ -657,7 +725,29 @@ func (t *Terminal) printHighlighted(item *Item, bold bool, col1 int, col2 int, c
 	}
 	if index < maxOffset {
 		substr, _ = processTabs(text[index:], prefixWidth)
-		C.CPrint(col1, bold, substr)
+		t.window.CPrint(col1, bold, substr)
+	}
+}
+
+func (t *Terminal) printPreview() {
+	trimmed, ansiOffsets, _ := extractColor(t.previewTxt, nil)
+	var index int32
+	t.pwindow.Erase()
+	for _, o := range ansiOffsets {
+		b := o.offset[0]
+		e := o.offset[1]
+		if b > index {
+			if !t.pwindow.Fill(trimmed[index:b]) {
+				return
+			}
+		}
+		if !t.pwindow.CFill(trimmed[b:e], o.color.fg, o.color.bg, o.color.bold) {
+			return
+		}
+		index = e
+	}
+	if int(index) < len(trimmed) {
+		t.pwindow.Fill(trimmed[index:])
 	}
 }
 
@@ -677,16 +767,24 @@ func processTabs(runes []rune, prefixWidth int) (string, int) {
 }
 
 func (t *Terminal) printAll() {
-	t.calculateMargins()
+	t.resizeWindows()
 	t.printList()
 	t.printPrompt()
 	t.printInfo()
 	t.printHeader()
+	if t.isPreviewEnabled() {
+		t.printPreview()
+	}
 }
 
 func (t *Terminal) refresh() {
 	if !t.suppress {
-		C.Refresh()
+		if t.isPreviewEnabled() {
+			t.bwindow.Refresh()
+			t.pwindow.Refresh()
+		}
+		t.window.Refresh()
+		C.DoUpdate()
 	}
 }
 
@@ -746,7 +844,7 @@ func quoteEntry(entry string) string {
 	return "'" + strings.Replace(entry, "'", "'\\''", -1) + "'"
 }
 
-func executeCommand(template string, replacement string) {
+func (t *Terminal) executeCommand(template string, replacement string) {
 	command := strings.Replace(template, "{}", replacement, -1)
 	cmd := util.ExecCommand(command)
 	cmd.Stdin = os.Stdin
@@ -754,7 +852,19 @@ func executeCommand(template string, replacement string) {
 	cmd.Stderr = os.Stderr
 	C.Endwin()
 	cmd.Run()
-	C.Refresh()
+	t.refresh()
+}
+
+func (t *Terminal) hasPreviewWindow() bool {
+	return t.previewBox != nil
+}
+
+func (t *Terminal) isPreviewEnabled() bool {
+	return t.previewBox != nil && t.previewing
+}
+
+func (t *Terminal) current() string {
+	return t.merger.Get(t.cy).AsString(t.ansi)
 }
 
 // Loop is called to start Terminal I/O
@@ -779,10 +889,10 @@ func (t *Terminal) Loop() {
 
 		t.mutex.Lock()
 		t.initFunc()
-		t.calculateMargins()
+		t.resizeWindows()
 		t.printPrompt()
 		t.placeCursor()
-		C.Refresh()
+		t.refresh()
 		t.printInfo()
 		t.printHeader()
 		t.mutex.Unlock()
@@ -807,6 +917,29 @@ func (t *Terminal) Loop() {
 		}()
 	}
 
+	if t.hasPreviewWindow() {
+		go func() {
+			for {
+				focused := ""
+				t.previewBox.Wait(func(events *util.Events) {
+					for req, value := range *events {
+						switch req {
+						case reqPreviewEnqueue:
+							focused = value.(string)
+						}
+					}
+					events.Clear()
+				})
+				if len(focused) > 0 {
+					command := strings.Replace(t.preview.command, "{}", quoteEntry(focused), -1)
+					cmd := util.ExecCommand(command)
+					out, _ := cmd.CombinedOutput()
+					t.reqBox.Set(reqPreviewDisplay, string(out))
+				}
+			}
+		}()
+	}
+
 	exit := func(code int) {
 		if code <= exitNoMatch && t.history != nil {
 			t.history.append(string(t.input))
@@ -815,11 +948,12 @@ func (t *Terminal) Loop() {
 	}
 
 	go func() {
+		focused := ""
 		for {
 			t.reqBox.Wait(func(events *util.Events) {
 				defer events.Clear()
 				t.mutex.Lock()
-				for req := range *events {
+				for req, value := range *events {
 					switch req {
 					case reqPrompt:
 						t.printPrompt()
@@ -830,6 +964,21 @@ func (t *Terminal) Loop() {
 						t.printInfo()
 					case reqList:
 						t.printList()
+						cnt := t.merger.Length()
+						if cnt > 0 && cnt > t.cy {
+							currentFocus := t.current()
+							if currentFocus != focused {
+								focused = currentFocus
+								if t.isPreviewEnabled() {
+									t.previewBox.Set(reqPreviewEnqueue, focused)
+								}
+							}
+						} else {
+							if focused != "" && t.isPreviewEnabled() {
+								t.pwindow.Erase()
+							}
+							focused = ""
+						}
 					case reqJump:
 						if t.merger.Length() == 0 {
 							t.jumping = jumpDisabled
@@ -850,6 +999,9 @@ func (t *Terminal) Loop() {
 							exit(exitOk)
 						}
 						exit(exitNoMatch)
+					case reqPreviewDisplay:
+						t.previewTxt = value.(string)
+						t.printPreview()
 					case reqPrintQuery:
 						C.Close()
 						fmt.Println(string(t.input))
@@ -915,7 +1067,7 @@ func (t *Terminal) Loop() {
 			case actExecute:
 				if t.cy >= 0 && t.cy < t.merger.Length() {
 					item := t.merger.Get(t.cy)
-					executeCommand(t.execmap[mapkey], quoteEntry(item.AsString(t.ansi)))
+					t.executeCommand(t.execmap[mapkey], quoteEntry(item.AsString(t.ansi)))
 				}
 			case actExecuteMulti:
 				if len(t.selected) > 0 {
@@ -923,13 +1075,23 @@ func (t *Terminal) Loop() {
 					for i, sel := range t.sortSelected() {
 						sels[i] = quoteEntry(*sel.text)
 					}
-					executeCommand(t.execmap[mapkey], strings.Join(sels, " "))
+					t.executeCommand(t.execmap[mapkey], strings.Join(sels, " "))
 				} else {
 					return doAction(actExecute, mapkey)
 				}
 			case actInvalid:
 				t.mutex.Unlock()
 				return false
+			case actTogglePreview:
+				if t.hasPreviewWindow() {
+					t.previewing = !t.previewing
+					t.resizeWindows()
+					cnt := t.merger.Length()
+					if t.previewing && cnt > 0 && cnt > t.cy {
+						t.previewBox.Set(reqPreviewEnqueue, t.current())
+					}
+					req(reqList, reqInfo)
+				}
 			case actToggleSort:
 				t.sort = !t.sort
 				t.eventBox.Set(EvtSearchNew, t.sort)
@@ -1097,20 +1259,19 @@ func (t *Terminal) Loop() {
 				mx, my := me.X, me.Y
 				if me.S != 0 {
 					// Scroll
-					if t.merger.Length() > 0 {
+					if t.window.Enclose(my, mx) && t.merger.Length() > 0 {
 						if t.multi && me.Mod {
 							toggle()
 						}
 						t.vmove(me.S)
 						req(reqList)
 					}
-				} else if mx >= t.marginInt[3] && mx < C.MaxX()-t.marginInt[1] &&
-					my >= t.marginInt[0] && my < C.MaxY()-t.marginInt[2] {
-					mx -= t.marginInt[3]
-					my -= t.marginInt[0]
+				} else if t.window.Enclose(my, mx) {
+					mx -= t.window.Left
+					my -= t.window.Top
 					mx = util.Constrain(mx-displayWidth([]rune(t.prompt)), 0, len(t.input))
 					if !t.reverse {
-						my = t.maxHeight() - my - 1
+						my = t.window.Height - my - 1
 					}
 					min := 2 + len(t.header)
 					if t.inlineInfo {
@@ -1217,7 +1378,7 @@ func (t *Terminal) vset(o int) bool {
 }
 
 func (t *Terminal) maxItems() int {
-	max := t.maxHeight() - 2 - len(t.header)
+	max := t.window.Height - 2 - len(t.header)
 	if t.inlineInfo {
 		max++
 	}
diff --git a/test/test_go.rb b/test/test_go.rb
index bb35bbf0..a6d67190 100644
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -1228,6 +1228,28 @@ class TestGoFZF < TestBase
     assert_equal '3', readonce.chomp
   end
 
+  def test_preview
+    tmux.send_keys %[seq 1000 | #{FZF} --preview 'echo {{}-{}}' --bind ?:toggle-preview], :Enter
+    tmux.until { |lines| lines[1].include?(' {1-1}') }
+    tmux.send_keys '555'
+    tmux.until { |lines| lines[1].include?(' {555-555}') }
+    tmux.send_keys '?'
+    tmux.until { |lines| !lines[1].include?(' {555-555}') }
+    tmux.send_keys '?'
+    tmux.until { |lines| lines[1].include?(' {555-555}') }
+  end
+
+  def test_preview_hidden
+    tmux.send_keys %[seq 1000 | #{FZF} --preview 'echo {{}-{}}' --preview-window down:1:hidden --bind ?:toggle-preview], :Enter
+    tmux.until { |lines| lines[-1] == '>' }
+    tmux.send_keys '?'
+    tmux.until { |lines| lines[-2].include?(' {1-1}') }
+    tmux.send_keys '555'
+    tmux.until { |lines| lines[-2].include?(' {555-555}') }
+    tmux.send_keys '?'
+    tmux.until { |lines| lines[-1] == '> 555' }
+  end
+
 private
   def writelines path, lines
     File.unlink path while File.exists? path