kitten diff: add support for sticky header

kittens/diff/main.py

@@ -89,6 +89,11 @@ If an invalid expression is provided, diff will fail with an error.
 '''
     )
 
+opt('sticky_header', 'no', option_type='to_bool', long_text='''
+When scrolled past the header of a file, keep the file's name and separator
+pinned to the top of the screen.
+''')
+
 egr()  # }}}
 
 # colors {{{

kittens/diff/render.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"math"
 	"os"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -318,11 +319,24 @@ func title_lines(left_path, right_path string, columns, margin_size int, ans []*
 
 type LogicalLines struct {
 	lines                []*LogicalLine
+	title_line_indices   []int // sorted indices of TITLE_LINEs in lines -- used for sticky_header
 	margin_size, columns int
 }
 
 func (self *LogicalLines) At(i int) *LogicalLine { return self.lines[i] }
 
+func (self *LogicalLines) TitleLineIdxFor(line_idx int) int {
+	if len(self.title_line_indices) == 0 || line_idx < 0 {
+		return -1
+	}
+	// first title index > line_idx, so the one before it is our answer
+	i := sort.SearchInts(self.title_line_indices, line_idx+1)
+	if i == 0 {
+		return -1
+	}
+	return self.title_line_indices[i-1]
+}
+
 func (self *LogicalLines) ScreenLineAt(pos ScrollPos) *ScreenLine {
 	if pos.logical_line < len(self.lines) && pos.logical_line >= 0 {
 		line := self.lines[pos.logical_line]
@@ -793,8 +807,13 @@ func rename_lines(path, other_path string, columns, margin_size int, ans []*Logi
 func render(collection *Collection, diff_map map[string]*Patch, screen_size screen_size, largest_line_number int, image_size graphics.Size) (result *LogicalLines, err error) {
 	margin_size := utils.Max(3, len(strconv.Itoa(largest_line_number))+1)
 	ans := make([]*LogicalLine, 0, 1024)
+	var title_line_indices []int
+	track_titles := conf != nil && conf.Sticky_header
 	columns := screen_size.columns
 	err = collection.Apply(func(path, item_type, changed_path string) error {
+		if track_titles {
+			title_line_indices = append(title_line_indices, len(ans))
+		}
 		ans = title_lines(path, changed_path, columns, margin_size, ans)
 		defer func() {
 			ans = append(ans, &LogicalLine{line_type: EMPTY_LINE, screen_lines: []*ScreenLine{{}}})
@@ -863,5 +882,5 @@ func render(collection *Collection, diff_map map[string]*Patch, screen_size scre
 		// Having am empty list of lines causes panics later on
 		ll = []*LogicalLine{{line_type: EMPTY_LINE, screen_lines: []*ScreenLine{{}}}}
 	}
-	return &LogicalLines{lines: ll, margin_size: margin_size, columns: columns}, err
+	return &LogicalLines{lines: ll, title_line_indices: title_line_indices, margin_size: margin_size, columns: columns}, err
 }

kittens/diff/ui.go

@@ -414,7 +414,11 @@ func (self *Handler) draw_screen() {
 	}
 	pos := self.scroll_pos
 	seen_images := utils.NewSet[int]()
-	for num_written := 0; num_written < self.screen_size.num_lines; num_written++ {
+	num_written := 0
+	if conf != nil && conf.Sticky_header {
+		num_written = self.draw_sticky_header(&pos)
+	}
+	for ; num_written < self.screen_size.num_lines; num_written++ {
 		ll := self.logical_lines.At(pos.logical_line)
 		if ll == nil || self.logical_lines.ScreenLineAt(pos) == nil {
 			num_written--
@@ -443,6 +447,26 @@ func (self *Handler) draw_screen() {
 	self.draw_status_line()
 }
 
+func (self *Handler) draw_sticky_header(pos *ScrollPos) int {
+	title_idx := self.logical_lines.TitleLineIdxFor(pos.logical_line)
+	if title_idx < 0 {
+		return 0
+	}
+	// file title
+	self.logical_lines.At(title_idx).render_screen_line(0, self.lp, self.logical_lines.margin_size, self.logical_lines.columns)
+	self.lp.MoveCursorVertically(1)
+	self.lp.QueueWriteString("\x1b[m\r")
+	self.logical_lines.IncrementScrollPosBy(pos, 1)
+
+	// separator
+	self.logical_lines.At(title_idx+1).render_screen_line(0, self.lp, self.logical_lines.margin_size, self.logical_lines.columns)
+	self.lp.MoveCursorVertically(1)
+	self.lp.QueueWriteString("\x1b[m\r")
+	self.logical_lines.IncrementScrollPosBy(pos, 1)
+
+	return 2
+}
+
 func (self *Handler) draw_status_line() {
 	if self.logical_lines == nil || self.diff_map == nil {
 		return