Switched string iteration to using the github.com/rivo/uniseg package.

util.go

@@ -5,10 +5,10 @@ import (
 	"regexp"
 	"sort"
 	"strconv"
-	"unicode"
 
 	"github.com/gdamore/tcell"
 	runewidth "github.com/mattn/go-runewidth"
+	"github.com/rivo/uniseg"
 )
 
 // Text alignment within a box.
@@ -528,51 +528,24 @@ func Escape(text string) string {
 // returns true. This function returns true if the iteration was stopped before
 // the last character.
 func iterateString(text string, callback func(main rune, comb []rune, textPos, textWidth, screenPos, screenWidth int) bool) bool {
-	var (
-		runes               []rune
-		lastZeroWidthJoiner bool
-		startIndex          int
-		startPos            int
-		pos                 int
-	)
+	var screenPos int
 
-	// Helper function which invokes the callback.
-	flush := func(index int) bool {
+	gr := uniseg.NewGraphemes(text)
+	for gr.Next() {
+		r := gr.Runes()
+		from, to := gr.Positions()
+		width := runewidth.StringWidth(gr.Str())
 		var comb []rune
-		if len(runes) > 1 {
-			comb = runes[1:]
+		if len(r) > 1 {
+			comb = r[1:]
 		}
-		return callback(runes[0], comb, startIndex, index-startIndex, startPos, pos-startPos)
-	}
 
-	for index, r := range text {
-		if unicode.In(r, unicode.M) || r == '\u200d' {
-			lastZeroWidthJoiner = r == '\u200d'
-		} else {
-			// We have a rune that's not a modifier. It could be the beginning of a
-			// new character.
-			if !lastZeroWidthJoiner {
-				if len(runes) > 0 {
-					// It is. Invoke callback.
-					if flush(index) {
-						return true // We're done.
-					}
-					// Reset rune store.
-					runes = runes[:0]
-					startIndex = index
-					startPos = pos
-				}
-				pos += runewidth.RuneWidth(r)
-			} else {
-				lastZeroWidthJoiner = false
-			}
+		//		panic(fmt.Sprintf(`from=%d to=%d screenPos=%d width=%d`, from, to, screenPos, width))
+		if callback(r[0], comb, from, to-from, screenPos, width) {
+			return true
 		}
-		runes = append(runes, r)
-	}
 
-	// Flush any remaining runes.
-	if len(runes) > 0 {
-		flush(len(text))
+		screenPos += width
 	}
 
 	return false
@@ -587,50 +560,37 @@ func iterateString(text string, callback func(main rune, comb []rune, textPos, t
 // screen width of it. The iteration stops if the callback returns true. This
 // function returns true if the iteration was stopped before the last character.
 func iterateStringReverse(text string, callback func(main rune, comb []rune, textPos, textWidth, screenPos, screenWidth int) bool) bool {
-	type runePos struct {
-		r     rune
-		pos   int  // The byte position of the rune in the original string.
-		width int  // The screen width of the rune.
-		mod   bool // Modifier or zero-width-joiner.
+	type cluster struct {
+		main                                       rune
+		comb                                       []rune
+		textPos, textWidth, screenPos, screenWidth int
 	}
 
-	// We use the following:
-	// len(text) >= number of runes in text.
+	// Create the grapheme clusters.
+	var clusters []cluster
+	iterateString(text, func(main rune, comb []rune, textPos int, textWidth int, screenPos int, screenWidth int) bool {
+		clusters = append(clusters, cluster{
+			main:        main,
+			comb:        comb,
+			textPos:     textPos,
+			textWidth:   textWidth,
+			screenPos:   screenPos,
+			screenWidth: screenWidth,
+		})
+		return false
+	})
 
-	// Put all runes into a runePos slice in reverse.
-	runesReverse := make([]runePos, len(text))
-	index := len(text) - 1
-	for pos, ch := range text {
-		runesReverse[index].r = ch
-		runesReverse[index].pos = pos
-		runesReverse[index].width = runewidth.RuneWidth(ch)
-		runesReverse[index].mod = unicode.In(ch, unicode.Lm, unicode.M) || ch == '\u200d'
-		index--
-	}
-	runesReverse = runesReverse[index+1:]
-
-	// Parse reverse runes.
-	var screenWidth int
-	buffer := make([]rune, len(text)) // We fill this up from the back so it's forward again.
-	bufferPos := len(text)
-	stringWidth := runewidth.StringWidth(text)
-	for index, r := range runesReverse {
-		// Put this rune into the buffer.
-		bufferPos--
-		buffer[bufferPos] = r.r
-
-		// Do we need to flush the buffer?
-		if r.pos == 0 || !r.mod && runesReverse[index+1].r != '\u200d' {
-			// Yes, invoke callback.
-			var comb []rune
-			if len(text)-bufferPos > 1 {
-				comb = buffer[bufferPos+1:]
-			}
-			if callback(r.r, comb, r.pos, len(text)-r.pos, stringWidth-screenWidth, r.width) {
-				return true
-			}
-			screenWidth += r.width
-			bufferPos = len(text)
+	// Iterate in reverse.
+	for index := len(clusters) - 1; index >= 0; index-- {
+		if callback(
+			clusters[index].main,
+			clusters[index].comb,
+			clusters[index].textPos,
+			clusters[index].textWidth,
+			clusters[index].screenPos,
+			clusters[index].screenWidth,
+		) {
+			return true
 		}
 	}