Support setting preview window's size to achieve a specific items list size

man/man1/fzf.1

@@ -878,6 +878,9 @@ 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.
 
+* If size is -N (negative and % is not used), the preview window will occupy the
+amount of space for which the entry list's size is N.
+
 * Long lines are truncated by default. Line wrap can be enabled with
 \fBwrap\fR flag.
 

src/options.go

@@ -340,7 +340,7 @@ type previewOpts struct {
 }
 
 func (o *previewOpts) Visible() bool {
-	return o.size.size > 0 || o.alternative != nil && o.alternative.size.size > 0
+	return o.size.size != 0 || o.alternative != nil && o.alternative.size.size > 0
 }
 
 func (o *previewOpts) Toggle() {
@@ -468,7 +468,7 @@ func parseLabelPosition(opts *labelOpts, arg string) error {
 }
 
 func (a previewOpts) aboveOrBelow() bool {
-	return a.size.size > 0 && (a.position == posUp || a.position == posDown)
+	return a.size.size != 0 && (a.position == posUp || a.position == posDown)
 }
 
 type previewOptsCompare int
@@ -1878,24 +1878,21 @@ func parseSize(str string, maxPercent float64, label string) (sizeSpec, error) {
 		}
 
 		if val < 0 {
-			return spec, errors.New(label + " must be non-negative")
+			return spec, errors.New(label + " (with %) must be non-negative")
 		}
 		if val > maxPercent {
 			return spec, fmt.Errorf("%s too large (max: %d%%)", label, int(maxPercent))
 		}
 	} else {
 		if strings.Contains(str, ".") {
-			return spec, errors.New(label + " (without %) must be a non-negative integer")
+			return spec, errors.New(label + " (without %) must be an integer")
 		}
 
 		i, err := atoi(str)
 		if err != nil {
 			return spec, err
 		}
 		val = float64(i)
-		if val < 0 {
-			return spec, errors.New(label + " must be non-negative")
-		}
 	}
 	return sizeSpec{val, percent}, nil
 }
@@ -1969,7 +1966,7 @@ func parsePreviewWindow(opts *previewOpts, input string) error {
 func parsePreviewWindowImpl(opts *previewOpts, input string) error {
 	var err error
 	tokenRegex := regexp.MustCompile(`[:,]*(<([1-9][0-9]*)\(([^)<]+)\)|[^,:]+)`)
-	sizeRegex := regexp.MustCompile("^[0-9]+%?$")
+	sizeRegex := regexp.MustCompile("^-?[0-9]+%?$")
 	offsetRegex := regexp.MustCompile(`^(\+{(-?[0-9]+|n)})?([+-][0-9]+)*(-?/[1-9][0-9]*)?$`)
 	headerRegex := regexp.MustCompile("^~(0|[1-9][0-9]*)$")
 	tokens := tokenRegex.FindAllStringSubmatch(input, -1)

src/options_test.go

@@ -434,6 +434,15 @@ func TestPreviewOpts(t *testing.T) {
 		opts.Preview.size.size == 15) {
 		t.Error(opts.Preview)
 	}
+	opts = optsFor("--preview-window=up:-15:wrap:hidden")
+	if !(opts.Preview.command == "" &&
+		opts.Preview.hidden == true &&
+		opts.Preview.wrap == true &&
+		opts.Preview.position == posUp &&
+		opts.Preview.size.percent == false &&
+		opts.Preview.size.size == -15) {
+		t.Error(opts.Preview)
+	}
 	opts = optsFor("--preview=foo", "--preview-window=up", "--preview-window=default:70%")
 	if !(opts.Preview.command == "foo" &&
 		opts.Preview.position == posRight &&

src/terminal.go

@@ -1622,7 +1622,13 @@ func calculateSize(base int, size sizeSpec, occupied int, minSize int) int {
 	if size.percent {
 		return util.Constrain(int(float64(base)*0.01*size.size), minSize, max)
 	}
-	return util.Constrain(int(size.size)+minSize-1, minSize, max)
+	var unconstrained int
+	if size.size < 0 {
+		unconstrained = int(float64(base) + size.size) - occupied + 1
+	} else {
+		unconstrained = int(size.size) + minSize - 1
+	}
+	return util.Constrain(unconstrained, minSize, max)
 }
 
 func (t *Terminal) minPreviewSize(opts *previewOpts) (int, int) {