Highlighting trailing whitespaces

Added option `hltrailingws` for highlighting trailing whitespaces
at the end of lines. Note that it behaves in a "smart" way.
It doesn't highlight newly added (transient) trailing whitespaces
that naturally occur while typing text. It would be annoying to
see transient highlighting every time we enter a space at the end
of a line while typing.
So a newly added trailing whitespace starts being highlighting
only after the cursor moves to another line. Thus the highlighting
serves its purpose: it draws our attention to annoying sloppy
forgotten trailing whitespaces.

internal/buffer/cursor.go

@@ -30,6 +30,11 @@ type Cursor struct {
 	// to know what the original selection was
 	OrigSelection [2]Loc
 
+	// The line number where a new trailing whitespace has been added
+	// or -1 if there is no new trailing whitespace at this cursor.
+	// This is used for checking if a trailing whitespace should be highlighted
+	NewTrailingWsY int
+
 	// Which cursor index is this (for multiple cursors)
 	Num int
 }
@@ -38,6 +43,8 @@ func NewCursor(b *Buffer, l Loc) *Cursor {
 	c := &Cursor{
 		buf: b,
 		Loc: l,
+
+		NewTrailingWsY: -1,
 	}
 	c.StoreVisualX()
 	return c

internal/buffer/eventhandler.go

@@ -106,6 +106,8 @@ func (eh *EventHandler) DoTextEvent(t *TextEvent, useUndo bool) {
 		c.Relocate()
 		c.LastVisualX = c.GetVisualX()
 	}
+
+	eh.updateTrailingWs(t)
 }
 
 // ExecuteTextEvent runs a text event
@@ -342,3 +344,52 @@ func (eh *EventHandler) RedoOneEvent() {
 
 	eh.UndoStack.Push(t)
 }
+
+// updateTrailingWs updates the cursor's trailing whitespace status after a text event
+func (eh *EventHandler) updateTrailingWs(t *TextEvent) {
+	if len(t.Deltas) != 1 {
+		return
+	}
+	text := t.Deltas[0].Text
+	start := t.Deltas[0].Start
+	end := t.Deltas[0].End
+
+	c := eh.cursors[eh.active]
+	isEol := func(loc Loc) bool {
+		return loc.X == util.CharacterCount(eh.buf.LineBytes(loc.Y))
+	}
+	if t.EventType == TextEventInsert && c.Loc == end && isEol(end) {
+		var addedTrailingWs bool
+		addedAfterWs := false
+		addedWsOnly := false
+		if start.Y == end.Y {
+			addedTrailingWs = util.HasTrailingWhitespace(text)
+			addedWsOnly = util.IsBytesWhitespace(text)
+			addedAfterWs = start.X > 0 && util.IsWhitespace(c.buf.RuneAt(Loc{start.X - 1, start.Y}))
+		} else {
+			lastnl := bytes.LastIndex(text, []byte{'\n'})
+			addedTrailingWs = util.HasTrailingWhitespace(text[lastnl+1:])
+		}
+
+		if addedTrailingWs && !(addedAfterWs && addedWsOnly) {
+			c.NewTrailingWsY = c.Y
+		} else if !addedTrailingWs {
+			c.NewTrailingWsY = -1
+		}
+	} else if t.EventType == TextEventRemove && c.Loc == start && isEol(start) {
+		removedAfterWs := util.HasTrailingWhitespace(eh.buf.LineBytes(start.Y))
+		var removedWsOnly bool
+		if start.Y == end.Y {
+			removedWsOnly = util.IsBytesWhitespace(text)
+		} else {
+			firstnl := bytes.Index(text, []byte{'\n'})
+			removedWsOnly = util.IsBytesWhitespace(text[:firstnl])
+		}
+
+		if removedAfterWs && !removedWsOnly {
+			c.NewTrailingWsY = c.Y
+		} else if !removedAfterWs {
+			c.NewTrailingWsY = -1
+		}
+	}
+}