From 5ad2dc6896e36aae02d14965f9a98c9778c2e2d1 Mon Sep 17 00:00:00 2001
From: Zufu Liu <unknown>
Date: Sun, 1 Dec 2024 09:04:00 +1100
Subject: Feature [feature-requests:#1535]. Improve performance of DBCS text by
 avoiding calling LineStartPosition.

---
 doc/ScintillaHistory.html |  4 ++++
 src/Document.cxx          | 23 +++++++----------------
 2 files changed, 11 insertions(+), 16 deletions(-)

diff --git a/doc/ScintillaHistory.html b/doc/ScintillaHistory.html
index b8fe778a7..84cbab7ad 100644
--- a/doc/ScintillaHistory.html
+++ b/doc/ScintillaHistory.html
@@ -603,6 +603,10 @@
 	<a href="https://sourceforge.net/p/scintilla/feature-requests/1533/">Feature #1533</a>.
 	</li>
 	<li>
+	Improve performance of DBCS text.
+	<a href="https://sourceforge.net/p/scintilla/feature-requests/1535/">Feature #1535</a>.
+	</li>
+	<li>
 	Fix wrapping removed lines.
 	<a href="https://sourceforge.net/p/scintilla/bugs/2456/">Bug #2456</a>.
 	</li>
diff --git a/src/Document.cxx b/src/Document.cxx
index 188ae658f..a601d48ce 100644
--- a/src/Document.cxx
+++ b/src/Document.cxx
@@ -833,15 +833,9 @@ Sci::Position Document::MovePositionOutsideChar(Sci::Position pos, Sci::Position
 				// Else invalid UTF-8 so return position of isolated trail byte
 			}
 		} else {
-			// Anchor DBCS calculations at start of line because start of line can
-			// not be a DBCS trail byte.
-			const Sci::Position posStartLine = LineStartPosition(pos);
-			if (pos == posStartLine)
-				return pos;
-
 			// Step back until a non-lead-byte is found.
 			Sci::Position posCheck = pos;
-			while ((posCheck > posStartLine) && IsDBCSLeadByteNoExcept(cb.CharAt(posCheck-1)))
+			while ((posCheck > 0) && IsDBCSLeadByteNoExcept(cb.CharAt(posCheck-1)))
 				posCheck--;
 
 			// Check from known start of character.
@@ -916,14 +910,11 @@ Sci::Position Document::NextPosition(Sci::Position pos, int moveDir) const noexc
 				if (pos > cb.Length())
 					pos = cb.Length();
 			} else {
-				// Anchor DBCS calculations at start of line because start of line can
-				// not be a DBCS trail byte.
-				const Sci::Position posStartLine = LineStartPosition(pos);
-				// See http://msdn.microsoft.com/en-us/library/cc194792%28v=MSDN.10%29.aspx
-				// http://msdn.microsoft.com/en-us/library/cc194790.aspx
-				if ((pos - 1) <= posStartLine) {
-					return pos - 1;
-				} else if (IsDBCSLeadByteNoExcept(cb.CharAt(pos - 1))) {
+				// How to Go Backward in a DBCS String
+				// https://msdn.microsoft.com/en-us/library/cc194792.aspx
+				// DBCS-Enabled Programs vs. Non-DBCS-Enabled Programs
+				// https://msdn.microsoft.com/en-us/library/cc194790.aspx
+				if (IsDBCSLeadByteNoExcept(cb.CharAt(pos - 1))) {
 					// Should actually be trail byte
 					if (IsDBCSDualByteAt(pos - 2)) {
 						return pos - 2;
@@ -934,7 +925,7 @@ Sci::Position Document::NextPosition(Sci::Position pos, int moveDir) const noexc
 				} else {
 					// Otherwise, step back until a non-lead-byte is found.
 					Sci::Position posTemp = pos - 1;
-					while (posStartLine <= --posTemp && IsDBCSLeadByteNoExcept(cb.CharAt(posTemp)))
+					while (--posTemp >= 0 && IsDBCSLeadByteNoExcept(cb.CharAt(posTemp)))
 						;
 					// Now posTemp+1 must point to the beginning of a character,
 					// so figure out whether we went back an even or an odd
-- 
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