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/action/bufpane.go

@@ -509,6 +509,13 @@ func (h *BufPane) HandleEvent(event tcell.Event) {
 			InfoBar.ClearGutter()
 		}
 	}
+
+	cursors := h.Buf.GetCursors()
+	for _, c := range cursors {
+		if c.NewTrailingWsY != c.Y {
+			c.NewTrailingWsY = -1
+		}
+	}
 }
 
 // Bindings returns the current bindings tree for this buffer.

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
+		}
+	}
+}

internal/config/settings.go

@@ -290,6 +290,7 @@ var defaultCommonSettings = map[string]interface{}{
 	"filetype":        "unknown",
 	"hlsearch":        false,
 	"hltaberrors":     false,
+	"hltrailingws":    false,
 	"incsearch":       true,
 	"ignorecase":      true,
 	"indentchar":      " ",

internal/display/bufwindow.go

@@ -495,7 +495,11 @@ func (w *BufWindow) displayBuffer() {
 			vloc.X = w.gutterOffset
 		}
 
-		leadingwsEnd := len(util.GetLeadingWhitespace(b.LineBytes(bloc.Y)))
+		bline := b.LineBytes(bloc.Y)
+		blineLen := util.CharacterCount(bline)
+
+		leadingwsEnd := len(util.GetLeadingWhitespace(bline))
+		trailingwsStart := blineLen - util.CharacterCount(util.GetTrailingWhitespace(bline))
 
 		line, nColsBeforeStart, bslice, startStyle := w.getStartInfo(w.StartCol, bloc.Y)
 		if startStyle != nil {
@@ -532,6 +536,25 @@ func (w *BufWindow) displayBuffer() {
 						}
 					}
 
+					if b.Settings["hltrailingws"].(bool) {
+						if s, ok := config.Colorscheme["trailingws"]; ok {
+							if bloc.X >= trailingwsStart && bloc.X < blineLen {
+								hl := true
+								for _, c := range cursors {
+									if c.NewTrailingWsY == bloc.Y {
+										hl = false
+										break
+									}
+								}
+								if hl {
+									fg, _, _ := s.Decompose()
+									style = style.Background(fg)
+									dontOverrideBackground = true
+								}
+							}
+						}
+					}
+
 					for _, c := range cursors {
 						if c.HasSelection() &&
 							(bloc.GreaterEqual(c.CurSelection[0]) && bloc.LessThan(c.CurSelection[1]) ||

internal/util/util.go

@@ -16,6 +16,7 @@ import (
 	"strings"
 	"time"
 	"unicode"
+	"unicode/utf8"
 
 	"github.com/blang/semver"
 	runewidth "github.com/mattn/go-runewidth"
@@ -363,6 +364,28 @@ func GetLeadingWhitespace(b []byte) []byte {
 	return ws
 }
 
+// GetTrailingWhitespace returns the trailing whitespace of the given byte array
+func GetTrailingWhitespace(b []byte) []byte {
+	ws := []byte{}
+	for len(b) > 0 {
+		r, size := utf8.DecodeLastRune(b)
+		if IsWhitespace(r) {
+			ws = append([]byte(string(r)), ws...)
+		} else {
+			break
+		}
+
+		b = b[:len(b)-size]
+	}
+	return ws
+}
+
+// HasTrailingWhitespace returns true if the given byte array ends with a whitespace
+func HasTrailingWhitespace(b []byte) bool {
+	r, _ := utf8.DecodeLastRune(b)
+	return IsWhitespace(r)
+}
+
 // IntOpt turns a float64 setting to an int
 func IntOpt(opt interface{}) int {
 	return int(opt.(float64))

runtime/help/options.md

@@ -181,6 +181,12 @@ Here are the available options:
 
 	default value: `false`
 
+* `hltrailingws`: highlight trailing whitespaces at ends of lines. Note that
+   it doesn't highlight newly added trailing whitespaces that naturally occur
+   while typing text. It highlights only nasty forgotten trailing whitespaces.
+
+	default value: `false`
+
 * `incsearch`: enable incremental search in "Find" prompt (matching as you type).
 
 	default value: `true`