Advanced preview scroll offset expression to better support fixed header

6 files changed, 68 insertions, 52 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 82422907..78fed7d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,21 @@ CHANGELOG
   # Display top 3 lines as the fixed header
   fzf --preview 'bat --style=header,grid --color=always {}' --preview-window '~3'
   ```
+- More advanced preview offset expression to better support the fixed header
+  ```sh
+  # Preview with bat, matching line in the middle of the window below
+  # the fixed header of the top 3 lines
+  #
+  #   ~3    Top 3 lines as the fixed header
+  #   +{2}  Base scroll offset extracted from the second field
+  #   +3    Extra offset to compensate for the 3-line header
+  #   /2    Put in the middle of the preview area
+  #
+  git grep --line-number '' |
+    fzf --delimiter : \
+        --preview 'bat --style=full --color=always --highlight-line {2} {1}' \
+        --preview-window '~3:+{2}+3/2'
+  ```
 - Added `select` and `deselect` action for unconditionally selecting or
   deselecting a single item in `--multi` mode. Complements `toggle` action.
 - Sigificant performance improvement in ANSI code processing
diff --git a/man/man1/fzf-tmux.1 b/man/man1/fzf-tmux.1
index 21cd0135..7aae95a8 100644
--- a/man/man1/fzf-tmux.1
+++ b/man/man1/fzf-tmux.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-tmux 1 "Feb 2021" "fzf 0.25.1" "fzf-tmux - open fzf in tmux split pane"
+.TH fzf-tmux 1 "Mar 2021" "fzf 0.26.0" "fzf-tmux - open fzf in tmux split pane"
 
 .SH NAME
 fzf-tmux - open fzf in tmux split pane
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index d4388940..54f9fe0c 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 "Feb 2021" "fzf 0.25.1" "fzf - a command-line fuzzy finder"
+.TH fzf 1 "Mar 2021" "fzf 0.26.0" "fzf - a command-line fuzzy finder"
 
 .SH NAME
 fzf - a command-line fuzzy finder
@@ -442,7 +442,7 @@ e.g.
       done'\fR
 .RE
 .TP
-.BI "--preview-window=" "[POSITION][:SIZE[%]][:rounded|sharp|noborder][:[no]wrap][:[no]follow][:[no]cycle][:[no]hidden][:+SCROLL[-OFFSET]][:~HEADER_LINES][:default]"
+.BI "--preview-window=" "[POSITION][:SIZE[%]][:rounded|sharp|noborder][:[no]wrap][:[no]follow][:[no]cycle][:[no]hidden][:+SCROLL[OFFSETS][/DENOM]][:~HEADER_LINES][:default]"
 
 .RS
 .B POSITION: (default: right)
@@ -480,12 +480,14 @@ e.g.
 \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
-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.
+* \fB[:+SCROLL[OFFSETS][/DENOM]]\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 \fBOFFSETS\fR part is for adjusting the base offset. It should be given as a series of signed integers (\fB-INTEGER\fR or \fB+INTEGER\fR).
+
+  - The final \fB/DENOM\fR part is for specifying a fraction of the preview window height.
 
 * \fB~HEADER_LINES\fR keeps the top N lines as the fixed header so that they
 are always visible.
@@ -501,16 +503,23 @@ e.g.
      # Initial scroll offset is set to the line number of each line of
      # git grep output *minus* 5 lines (-5)
      git grep --line-number '' |
-       fzf --delimiter : --preview 'nl {1}' --preview-window +{2}-5
-
-     # Preview with bat, matching line in the middle of the window (-/2)
+       fzf --delimiter : --preview 'nl {1}' --preview-window '+{2}-5'
+
+     # Preview with bat, matching line in the middle of the window below
+     # the fixed header of the top 3 lines
+     #
+     #   ~3    Top 3 lines as the fixed header
+     #   +{2}  Base scroll offset extracted from the second field
+     #   +3    Extra offset to compensate for the 3-line header
+     #   /2    Put in the middle of the preview area
+     #
      git grep --line-number '' |
        fzf --delimiter : \\
-           --preview 'bat --style=numbers --color=always --highlight-line {2} {1}' \\
-           --preview-window +{2}-/2\fR
+           --preview 'bat --style=full --color=always --highlight-line {2} {1}' \\
+           --preview-window '~3:+{2}+3/2'
 
      # Display top 3 lines as the fixed header
-     fzf --preview 'bat --style=header,grid --color=always {}' --preview-window '~3'
+     fzf --preview 'bat --style=full --color=always {}' --preview-window '~3'\fR
 .RE
 
 .SS Scripting
diff --git a/src/options.go b/src/options.go
index 29c90373..8d0afe06 100644
--- a/src/options.go
+++ b/src/options.go
@@ -84,7 +84,7 @@ const usage = `usage: fzf [options]
                           [up|down|left|right][:SIZE[%]]
                           [:[no]wrap][:[no]cycle][:[no]follow][:[no]hidden]
                           [:rounded|sharp|noborder]
-                          [:+SCROLL[-OFFSET]][:~HEADER_LINES]
+                          [:+SCROLL[OFFSETS][/DENOM]][:~HEADER_LINES]
                           [:default]
 
   Scripting
@@ -1078,7 +1078,7 @@ func parseInfoStyle(str string) infoStyle {
 func parsePreviewWindow(opts *previewOpts, input string) {
 	tokens := strings.Split(input, ":")
 	sizeRegex := regexp.MustCompile("^[0-9]+%?$")
-	offsetRegex := regexp.MustCompile("^\\+([0-9]+|{-?[0-9]+})(-[0-9]+|-/[1-9][0-9]*)?$")
+	offsetRegex := regexp.MustCompile(`^(\+{-?[0-9]+})?([+-][0-9]+)*(-?/[1-9][0-9]*)?$`)
 	headerRegex := regexp.MustCompile("^~(0|[1-9][0-9]*)$")
 	for _, token := range tokens {
 		switch token {
@@ -1121,7 +1121,7 @@ func parsePreviewWindow(opts *previewOpts, input string) {
 			} else if sizeRegex.MatchString(token) {
 				opts.size = parseSize(token, 99, "window size")
 			} else if offsetRegex.MatchString(token) {
-				opts.scroll = token[1:]
+				opts.scroll = token
 			} else {
 				errorExit("invalid preview window option: " + token)
 			}
@@ -1368,7 +1368,7 @@ func parseOptions(opts *Options, allArgs []string) {
 			opts.Preview.command = ""
 		case "--preview-window":
 			parsePreviewWindow(&opts.Preview,
-				nextString(allArgs, &i, "preview window layout required: [up|down|left|right][:SIZE[%]][:rounded|sharp|noborder][:wrap][:cycle][:hidden][:+SCROLL[-OFFSET]][:~HEADER_LINES][:default]"))
+				nextString(allArgs, &i, "preview window layout required: [up|down|left|right][:SIZE[%]][:rounded|sharp|noborder][:wrap][:cycle][:hidden][:+SCROLL[OFFSETS][/DENOM]][:~HEADER_LINES][:default]"))
 		case "--height":
 			opts.Height = parseHeight(nextString(allArgs, &i, "height required: HEIGHT[%]"))
 		case "--min-height":
diff --git a/src/options_test.go b/src/options_test.go
index 791b55d4..ce135413 100644
--- a/src/options_test.go
+++ b/src/options_test.go
@@ -389,18 +389,18 @@ func TestPreviewOpts(t *testing.T) {
 		opts.Preview.hidden == true &&
 		opts.Preview.wrap == true &&
 		opts.Preview.position == posLeft &&
-		opts.Preview.scroll == "{1}-/2" &&
+		opts.Preview.scroll == "+{1}-/2" &&
 		opts.Preview.size.percent == false &&
 		opts.Preview.size.size == 15) {
 		t.Error(opts.Preview)
 	}
-	opts = optsFor("--preview-window=up:15:wrap:hidden:+{1}-/2", "--preview-window=down", "--preview-window=cycle")
+	opts = optsFor("--preview-window=up:15:wrap:hidden:+{1}+3-1-2/2", "--preview-window=down", "--preview-window=cycle")
 	if !(opts.Preview.command == "" &&
 		opts.Preview.hidden == true &&
 		opts.Preview.wrap == true &&
 		opts.Preview.cycle == true &&
 		opts.Preview.position == posDown &&
-		opts.Preview.scroll == "{1}-/2" &&
+		opts.Preview.scroll == "+{1}+3-1-2/2" &&
 		opts.Preview.size.percent == false &&
 		opts.Preview.size.size == 15) {
 		t.Error(opts.Preview.size.size)
diff --git a/src/terminal.go b/src/terminal.go
index 9fa11212..53a32769 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -22,8 +22,9 @@ import (
 // import "github.com/pkg/profile"
 
 var placeholder *regexp.Regexp
-var numericPrefix *regexp.Regexp
 var whiteSuffix *regexp.Regexp
+var offsetComponentRegex *regexp.Regexp
+var offsetTrimCharsRegex *regexp.Regexp
 var activeTempFiles []string
 
 const ellipsis string = ".."
@@ -31,8 +32,9 @@ const clearCode string = "\x1b[2J"
 
 func init() {
 	placeholder = regexp.MustCompile(`\\?(?:{[+sf]*[0-9,-.]*}|{q}|{\+?f?nf?})`)
-	numericPrefix = regexp.MustCompile(`^[[:punct:]]*([0-9]+)`)
 	whiteSuffix = regexp.MustCompile(`\s*$`)
+	offsetComponentRegex = regexp.MustCompile(`([+-][0-9]+)|(-?/[1-9][0-9]*)`)
+	offsetTrimCharsRegex = regexp.MustCompile(`[^0-9/+-]`)
 	activeTempFiles = []string{}
 }
 
@@ -1591,43 +1593,33 @@ func (t *Terminal) replacePlaceholder(template string, forcePlus bool, input str
 		template, t.ansi, t.delimiter, t.printsep, forcePlus, input, list)
 }
 
-// Ascii to positive integer
-func atopi(s string) int {
-	matches := numericPrefix.FindStringSubmatch(s)
-	if len(matches) < 2 {
-		return 0
-	}
-	n, e := strconv.Atoi(matches[1])
-	if e != nil || n < 1 {
-		return 0
-	}
-	return n
-}
-
 func (t *Terminal) evaluateScrollOffset(list []*Item, height int) int {
-	offsetExpr := t.replacePlaceholder(t.previewOpts.scroll, false, "", list)
-	nums := strings.Split(offsetExpr, "-")
-	switch len(nums) {
-	case 0:
-		return 0
-	case 1, 2:
-		base := atopi(nums[0])
-		if base == 0 {
+	offsetExpr := offsetTrimCharsRegex.ReplaceAllString(
+		t.replacePlaceholder(t.previewOpts.scroll, false, "", list), "")
+
+	atoi := func(s string) int {
+		n, e := strconv.Atoi(s)
+		if e != nil {
 			return 0
-		} else if len(nums) == 1 {
-			return base - 1
 		}
-		if nums[1][0] == '/' {
-			denom := atopi(nums[1][1:])
+		return n
+	}
+
+	base := -1
+	for _, component := range offsetComponentRegex.FindAllString(offsetExpr, -1) {
+		if strings.HasPrefix(component, "-/") {
+			component = component[1:]
+		}
+		if component[0] == '/' {
+			denom := atoi(component[1:])
 			if denom == 0 {
 				return base
 			}
 			return base - height/denom
 		}
-		return base - atopi(nums[1]) - 1
-	default:
-		return 0
+		base += atoi(component)
 	}
+	return base
 }
 
 func replacePlaceholder(template string, stripAnsi bool, delimiter Delimiter, printsep string, forcePlus bool, query string, allItems []*Item) string {