From e7b2caee7e9a513cbae01a0826de2250d4120383 Mon Sep 17 00:00:00 2001
From: Takashi Sawanaka <sdottaka@users.sourceforge.net>
Date: Sat, 11 Nov 2023 18:43:32 +0900
Subject: [PATCH] Fix an issue where "Copy to Left/Right and Advance" would
 sometimes skip differences when comparing 3 files. refs #1234

---
 Src/MergeEditView.cpp | 65 +++++++++++++++++++++++++++++++++++++------
 Src/MergeEditView.h   |  1 +
 2 files changed, 58 insertions(+), 8 deletions(-)

diff --git a/Src/MergeEditView.cpp b/Src/MergeEditView.cpp
index 7cb374d9c06..f97d3ba2eda 100644
--- a/Src/MergeEditView.cpp
+++ b/Src/MergeEditView.cpp
@@ -2952,12 +2952,63 @@ void CMergeEditView::OnUpdateConvertEolTo(CCmdUI* pCmdUI)
 /**
  * @brief Copy diff from left to right and advance to next diff
  */
-void CMergeEditView::OnL2RNext()
+void CMergeEditView::OnX2YNext(int srcPane, int dstPane)
 {
-	OnL2r();
-	if (GetDocument()->m_nBuffers > 2 && IsCursorInDiff()) // for 3-way file compare
+	CMergeDoc *pDoc = GetDocument();
+	if (pDoc->m_nBuffers < 3)
+	{
+		OnX2Y(srcPane, dstPane);
 		OnNextdiff();
-	OnNextdiff();
+	}
+	else
+	{
+		int currentDiff = pDoc->GetCurrentDiff();
+		if (currentDiff == -1 && m_bCurrentLineIsDiff)
+			currentDiff = pDoc->m_diffList.LineToDiff(GetCursorPos().y);
+		if (currentDiff != -1)
+		{
+			int nNextDiff = -1;
+			DIFFRANGE di, diNext;
+			pDoc->m_diffList.GetDiff(currentDiff, di);
+			pDoc->m_diffList.GetNextDiff(di.dend + 1, nNextDiff);
+			if (nNextDiff != -1)
+			{
+				pDoc->m_diffList.GetDiff(nNextDiff, diNext);
+				int nRealLine;
+				int nPane = srcPane;
+				if (diNext.end[nPane] - diNext.begin[nPane] < 0)
+				{
+					for (nPane = 0; nPane < pDoc->m_nBuffers; ++nPane)
+						if (diNext.end[nPane] - diNext.begin[nPane] >= 0)
+							break;
+				}
+				nRealLine = diNext.begin[nPane];
+				if (nPane == dstPane)
+					nRealLine += (di.end[srcPane] - di.begin[srcPane]) - (di.end[dstPane] - di.begin[dstPane]);
+
+				OnX2Y(srcPane, dstPane);
+
+				nNextDiff = pDoc->m_diffList.LineToDiff(pDoc->m_ptBuf[nPane]->ComputeApparentLine(nRealLine));
+				if (nNextDiff != -1)
+					SelectDiff(nNextDiff, true, false);
+				else
+					OnNextdiff();
+			}
+			else
+			{
+				OnX2Y(srcPane, dstPane);
+			}
+		}
+	}
+}
+
+/**
+ * @brief Copy diff from left to right and advance to next diff
+ */
+void CMergeEditView::OnL2RNext()
+{
+	auto [srcPane, dstPane] = CMergeFrameCommon::MenuIDtoXY(ID_L2R, m_nThisPane, GetDocument()->m_nBuffers);
+	OnX2YNext(srcPane, dstPane);
 }
 
 /**
@@ -2973,10 +3024,8 @@ void CMergeEditView::OnUpdateL2RNext(CCmdUI* pCmdUI)
  */
 void CMergeEditView::OnR2LNext()
 {
-	OnR2l();
-	if (GetDocument()->m_nBuffers > 2 && IsCursorInDiff()) // for 3-way file compare
-		OnNextdiff();
-	OnNextdiff();
+	auto [srcPane, dstPane] = CMergeFrameCommon::MenuIDtoXY(ID_R2L, m_nThisPane, GetDocument()->m_nBuffers);
+	OnX2YNext(srcPane, dstPane);
 }
 
 /**
diff --git a/Src/MergeEditView.h b/Src/MergeEditView.h
index 94898dc1804..2ae45774f44 100644
--- a/Src/MergeEditView.h
+++ b/Src/MergeEditView.h
@@ -284,6 +284,7 @@ class CMergeEditView : public CGhostTextView
 	afx_msg void OnConvertEolTo(UINT nID );
 	afx_msg void OnUpdateConvertEolTo(CCmdUI* pCmdUI);
 	afx_msg void OnUpdateStatusEOL(CCmdUI* pCmdUI);
+	afx_msg void OnX2YNext(int srcPane, int dstPane);
 	afx_msg void OnL2RNext();
 	afx_msg void OnUpdateL2RNext(CCmdUI* pCmdUI);
 	afx_msg void OnR2LNext();