1 files changed, 58 insertions, 31 deletions

diff --git a/src/Document.cxx b/src/Document.cxx
index 3ebd357df..0d8b00d09 100644
--- a/src/Document.cxx
+++ b/src/Document.cxx
@@ -1127,47 +1127,74 @@ bool Document::IsDBCSDualByteAt(Sci::Position pos) const noexcept {
 		&& IsDBCSTrailByteNoExcept(cb.CharAt(pos + 1));
 }
 
-// Need to break text into segments near lengthSegment but taking into
-// account the encoding to not break inside a UTF-8 or DBCS character
-// and also trying to avoid breaking inside a pair of combining characters.
+// Need to break text into segments near end but taking into account the
+// encoding to not break inside a UTF-8 or DBCS character and also trying
+// to avoid breaking inside a pair of combining characters, or inside
+// ligatures.
+// TODO: implement grapheme cluster boundaries,
+// see https://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries.
+//
 // The segment length must always be long enough (more than 4 bytes)
 // so that there will be at least one whole character to make a segment.
 // For UTF-8, text must consist only of valid whole characters.
 // In preference order from best to worst:
-//   1) Break after space
-//   2) Break before punctuation
-//   3) Break after whole character
-
-int Document::SafeSegment(const char *text, int lengthSegment) const noexcept {
-	int lastSpaceBreak = -1;
-	int lastPunctuationBreak = -1;
-	int lastEncodingAllowedBreak = 0;
-	for (int j=0; j < lengthSegment;) {
-		const unsigned char ch = text[j];
-		if (j > 0) {
-			if (IsSpaceOrTab(text[j - 1]) && !IsSpaceOrTab(text[j])) {
-				lastSpaceBreak = j;
+//   1) Break before or after spaces or controls
+//   2) Break at word and punctuation boundary for better kerning and ligature support
+//   3) Break after whole character, this may break combining characters
+
+size_t Document::SafeSegment(std::string_view text) const noexcept {
+	// check space first as most written language use spaces.
+	for (std::string_view::iterator it = text.end() - 1; it != text.begin(); --it) {
+		if (IsBreakSpace(*it)) {
+			return it - text.begin();
+		}
+	}
+
+	if (!dbcsCodePage || dbcsCodePage == CpUtf8) {
+		// backward iterate for UTF-8 and single byte encoding to find word and punctuation boundary.
+		std::string_view::iterator it = text.end() - 1;
+		const bool punctuation = IsPunctuation(*it);
+		do {
+			--it;
+			if (punctuation != IsPunctuation(*it)) {
+				return it - text.begin() + 1;
 			}
-			if (ch < 'A') {
-				lastPunctuationBreak = j;
+		} while (it != text.begin());
+
+		it = text.end() - 1;
+		if (dbcsCodePage) {
+			// for UTF-8 go back the start of last character.
+			for (int trail = 0; trail < UTF8MaxBytes - 1 && UTF8IsTrailByte(*it); trail++) {
+				--it;
 			}
 		}
-		lastEncodingAllowedBreak = j;
+		return it - text.begin();
+	}
 
-		if (dbcsCodePage == CpUtf8) {
-			j += UTF8BytesOfLead[ch];
-		} else if (dbcsCodePage) {
-			j += IsDBCSLeadByteNoExcept(ch) ? 2 : 1;
-		} else {
-			j++;
+	{
+		// forward iterate for DBCS to find word and punctuation boundary.
+		size_t lastPunctuationBreak = 0;
+		size_t lastEncodingAllowedBreak = 0;
+		CharacterClass ccPrev = CharacterClass::space;
+		for (size_t j = 0; j < text.length();) {
+			const unsigned char ch = text[j];
+			lastEncodingAllowedBreak = j++;
+
+			CharacterClass cc = CharacterClass::word;
+			if (UTF8IsAscii(ch)) {
+				if (IsPunctuation(ch)) {
+					cc = CharacterClass::punctuation;
+				}
+			} else {
+				j += IsDBCSLeadByteNoExcept(ch);
+			}
+			if (cc != ccPrev) {
+				ccPrev = cc;
+				lastPunctuationBreak = lastEncodingAllowedBreak;
+			}
 		}
+		return lastPunctuationBreak ? lastPunctuationBreak : lastEncodingAllowedBreak;
 	}
-	if (lastSpaceBreak >= 0) {
-		return lastSpaceBreak;
-	} else if (lastPunctuationBreak >= 0) {
-		return lastPunctuationBreak;
-	}
-	return lastEncodingAllowedBreak;
 }
 
 EncodingFamily Document::CodePageFamily() const noexcept {