diff --git a/RedPandaIDE/qsynedit/SynEdit.cpp b/RedPandaIDE/qsynedit/SynEdit.cpp
index f9d469fa..5c2a3623 100644
--- a/RedPandaIDE/qsynedit/SynEdit.cpp
+++ b/RedPandaIDE/qsynedit/SynEdit.cpp
@@ -1371,8 +1371,8 @@ int SynEdit::calcIndentSpaces(int line, const QString& lineText, bool addIndent)
     line = std::min(line, mLines->count()+1);
     if (line<=1)
         return 0;
+    // find the first non-empty preceeding line
     int startLine = line-1;
-    int indentSpaces = 0;
     QString s;
     while (startLine>=1) {
         s = mLines->getString(startLine-1);
@@ -1381,25 +1381,37 @@ int SynEdit::calcIndentSpaces(int line, const QString& lineText, bool addIndent)
         }
         startLine -- ;
     }
+    int indentSpaces = 0;
     if (startLine>=1) {
         indentSpaces = leftSpaces(s);
         if (addIndent) {
-            SynRangeState range = mLines->ranges(startLine-1);
-            if ((!range.indentStartLines.isEmpty()
-                    && range.indentStartLines.back() == startLine-1)
+            SynRangeState rangePreceeding = mLines->ranges(startLine-1);
+            if (!rangePreceeding.lastMatchingIndent.isNull()) {
+                // find the indent's start line, and use it's indent as the default indent;
+                int l = startLine-1;
+                while (l>=1) {
+                    SynRangeState range = mLines->ranges(l-1);
+                    if (range.indents.mid(range.firstIndentThisLine).contains(rangePreceeding.lastMatchingIndent)) {
+                        indentSpaces = leftSpaces(mLines->getString(l-1));
+                        break;
+                    }
+                    l--;
+                }
+            }
+            if ((rangePreceeding.firstIndentThisLine < rangePreceeding.indents.length()) // there are indents added at this (preceeding) line
                 || (s.trimmed().endsWith(':'))
                     ) {
                 indentSpaces += mTabWidth;
             }
-            mHighlighter->setState(range);
-            mHighlighter->setLine(lineText,line-1);
-            mHighlighter->nextToEol();
-            SynRangeState newRange = mHighlighter->getRangeState();
-            while (!newRange.indentStartLines.isEmpty() && newRange.indentStartLines.back()==line-1) {
-                newRange.indentStartLines.pop_back();
+            mHighlighter->setState(rangePreceeding);
+            mHighlighter->setLine(lineText.trimmed(),line-1);
+            SynRangeState rangeAfterFirstToken = mHighlighter->getRangeState();
+            if (rangeAfterFirstToken.indents.length() < rangePreceeding.indents.length()) {
+                indentSpaces -= mTabWidth;
+            } else if (rangeAfterFirstToken.getLastIndent() == BraceIndentType
+                       && rangePreceeding.getLastIndent() == StatementIndentType) {
+                indentSpaces -= mTabWidth;
             }
-            if (newRange.indentStartLines.length() < range.indentStartLines.length())
-                indentSpaces-=mTabWidth;
         }
     }
     return std::max(0,indentSpaces);
diff --git a/RedPandaIDE/qsynedit/highlighter/base.cpp b/RedPandaIDE/qsynedit/highlighter/base.cpp
index ca92d70b..a57b370c 100644
--- a/RedPandaIDE/qsynedit/highlighter/base.cpp
+++ b/RedPandaIDE/qsynedit/highlighter/base.cpp
@@ -211,17 +211,16 @@ SynHighlighterAttribute::SynHighlighterAttribute(const QString &name):
 
 bool SynRangeState::operator==(const SynRangeState &s2)
 {
+    // indents contains the information of brace/parenthesis/brackets embedded levels
     return (state == s2.state)
             && (spaceState == s2.spaceState)
-            && (braceLevel == s2.braceLevel)
-            && (bracketLevel == s2.bracketLevel)
-            && (parenthesisLevel == s2.parenthesisLevel)
-            && (leftBraces == s2.leftBraces)
-            && (leftParenthesis = s2.leftParenthesis)
-            && (leftBrackets == s2.leftBrackets)
-            && (rightBraces == s2.rightBraces)
-            && (rightParenthesis = s2.rightParenthesis)
-            && (rightBrackets == s2.rightBrackets)
             && (indents == s2.indents)
             ;
 }
+
+QChar SynRangeState::getLastIndent()
+{
+    if (indents.isEmpty())
+        return QChar();
+    return indents.back();
+}
diff --git a/RedPandaIDE/qsynedit/highlighter/base.h b/RedPandaIDE/qsynedit/highlighter/base.h
index b55351ab..b778281f 100644
--- a/RedPandaIDE/qsynedit/highlighter/base.h
+++ b/RedPandaIDE/qsynedit/highlighter/base.h
@@ -13,23 +13,24 @@
 constexpr QChar BraceIndentType('{');
 constexpr QChar ParenthesisIndentType('(');
 constexpr QChar BracketIndentType('[');
-constexpr QChar StatementIndentType('K');
+constexpr QChar StatementIndentType('S');
 
 struct SynRangeState {
-    int state;
-    int spaceState;
-    int braceLevel;
-    int bracketLevel;
-    int parenthesisLevel;
-    int leftBraces;
-    int rightBraces;
-    int leftBrackets;
-    int rightBrackets;
-    int leftParenthesis;
-    int rightParenthesis;
-    QString indents;
-    QVector<int> indentStartLines;
+    int state;  // current syntax parsing state
+    int spaceState; // the last syntax parsing state before meeting space
+    int braceLevel; // current braces embedding level (needed by rainbow color)
+    int bracketLevel; // current brackets embedding level (needed by rainbow color)
+    int parenthesisLevel; // current parenthesis embedding level (needed by rainbow color)
+    int leftBraces; // unpairing left braces in the current line ( needed by block folding)
+    int rightBraces; // unparing right braces in the current line (needed by block folding)
+    QString indents; // indents stack (each char represents an indent) (needed by auto indent)
+    int firstIndentThisLine; /* index of first indent that appended to the indents
+                              *  stack at this line ( need by auto indent) */
+    QChar lastMatchingIndent; /* the last indent matched ( and removed )
+                              but not started at this line
+                                (need by auto indent) */
     bool operator==(const SynRangeState& s2);
+    QChar getLastIndent();
 };
 
 typedef int SynTokenKind;
diff --git a/RedPandaIDE/qsynedit/highlighter/cpp.cpp b/RedPandaIDE/qsynedit/highlighter/cpp.cpp
index 3a6adcf6..1e5cd80f 100644
--- a/RedPandaIDE/qsynedit/highlighter/cpp.cpp
+++ b/RedPandaIDE/qsynedit/highlighter/cpp.cpp
@@ -375,7 +375,7 @@ void SynEditCppHighlighter::braceCloseProc()
     } else {
         mRange.rightBraces++ ;
     }
-    popIndentsByType(BraceIndentType);
+    popIndents(BraceIndentType);
 }
 
 void SynEditCppHighlighter::braceOpenProc()
@@ -392,11 +392,15 @@ void SynEditCppHighlighter::braceOpenProc()
     if (!mRange.indents.isEmpty() && mRange.indents.back() == StatementIndentType) {
         // if last indent is started by 'if' 'for' etc
         // just replace it
-        mRange.indents.replace(mRange.indents.length()-1,1,BraceIndentType);
-        mRange.indentStartLines.replace(mRange.indents.length()-1,mLineNumber);
+        popIndents(StatementIndentType);
+        pushIndents(BraceIndentType);
+//        int idx = mRange.indents.length()-1;
+//        if (idx < mRange.firstIndentThisLine) {
+//            mRange.firstIndentThisLine = idx;
+//        }
+//        mRange.indents.replace(idx,1,BraceIndentType);
     } else {
-        mRange.indents.append(BraceIndentType);
-        mRange.indentStartLines.append(mLineNumber);
+        pushIndents(BraceIndentType);
     }
 }
 
@@ -540,8 +544,7 @@ void SynEditCppHighlighter::identProc()
     if (isKeyword(word)) {
         mTokenId = TokenKind::Key;
         if (StatementKeyWords.contains(word)) {
-            mRange.indents.append(StatementIndentType);
-            mRange.indentStartLines.append(mLineNumber);
+            pushIndents(StatementIndentType);
         }
     } else {
         mTokenId = TokenKind::Identifier;
@@ -903,12 +906,7 @@ void SynEditCppHighlighter::roundCloseProc()
     mTokenId = TokenKind::Symbol;
     mExtTokenId = ExtTokenKind::RoundClose;
     mRange.parenthesisLevel--;
-    if (mRange.leftParenthesis>0) {
-        mRange.leftParenthesis--;
-    } else {
-        mRange.rightParenthesis++ ;
-    }
-    popIndentsByType(ParenthesisIndentType);
+    popIndents(ParenthesisIndentType);
 }
 
 void SynEditCppHighlighter::roundOpenProc()
@@ -917,9 +915,7 @@ void SynEditCppHighlighter::roundOpenProc()
     mTokenId = TokenKind::Symbol;
     mExtTokenId = ExtTokenKind::RoundOpen;
     mRange.parenthesisLevel++;
-    mRange.leftParenthesis++;
-    mRange.indents.append(ParenthesisIndentType);
-    mRange.indentStartLines.append(mLineNumber);
+    pushIndents(ParenthesisIndentType);
 }
 
 void SynEditCppHighlighter::semiColonProc()
@@ -930,8 +926,7 @@ void SynEditCppHighlighter::semiColonProc()
     if (mRange.state == RangeState::rsAsm)
         mRange.state = RangeState::rsUnknown;
     if (mRange.indents.back() == StatementIndentType) {
-        mRange.indents.remove(mRange.indents.length()-1,1);
-        mRange.indentStartLines.pop_back();
+        popIndents(StatementIndentType);
     }
 }
 
@@ -986,12 +981,7 @@ void SynEditCppHighlighter::squareCloseProc()
     mTokenId = TokenKind::Symbol;
     mExtTokenId = ExtTokenKind::SquareClose;
     mRange.bracketLevel--;
-    if (mRange.leftBrackets>0) {
-        mRange.leftBrackets--;
-    } else {
-        mRange.rightBrackets++ ;
-    }
-    popIndentsByType(BracketIndentType);
+    popIndents(BracketIndentType);
 }
 
 void SynEditCppHighlighter::squareOpenProc()
@@ -1000,9 +990,7 @@ void SynEditCppHighlighter::squareOpenProc()
     mTokenId = TokenKind::Symbol;
     mExtTokenId = ExtTokenKind::SquareOpen;
     mRange.bracketLevel++;
-    mRange.leftBrackets++;
-    mRange.indents.append(BracketIndentType);
-    mRange.indentStartLines.append(mLineNumber);
+    pushIndents(BracketIndentType);
 }
 
 void SynEditCppHighlighter::starProc()
@@ -1366,17 +1354,26 @@ void SynEditCppHighlighter::processChar()
     }
 }
 
-void SynEditCppHighlighter::popIndentsByType(QChar indentType)
+void SynEditCppHighlighter::popIndents(QChar indentType)
 {
     while (!mRange.indents.isEmpty() && mRange.indents.back()!=indentType) {
         mRange.indents.remove(mRange.indents.length()-1,1);
-        mRange.indentStartLines.pop_back();
     }
     if (!mRange.indents.isEmpty()) {
-        mRange.indents.remove(mRange.indents.length()-1,1);
-        mRange.indentStartLines.pop_back();
+        int idx = mRange.indents.length()-1;
+        if (idx < mRange.firstIndentThisLine) {
+            mRange.lastMatchingIndent = mRange.indents[idx];
+        }
+        mRange.indents.remove(idx,1);
     }
+}
 
+void SynEditCppHighlighter::pushIndents(QChar indentType)
+{
+    int idx = mRange.indents.length();
+    if (idx<mRange.firstIndentThisLine)
+        mRange.firstIndentThisLine = idx;
+    mRange.indents.append(indentType);
 }
 
 bool SynEditCppHighlighter::getTokenFinished() const
@@ -1532,11 +1529,9 @@ void SynEditCppHighlighter::setLine(const QString &newLine, int lineNumber)
     mLineNumber = lineNumber;
     mRun = 0;
     mRange.leftBraces = 0;
-    mRange.leftBrackets = 0;
-    mRange.leftParenthesis = 0;
     mRange.rightBraces = 0;
-    mRange.rightBrackets = 0;
-    mRange.rightParenthesis = 0;
+    mRange.firstIndentThisLine = mRange.indents.length();
+    mRange.lastMatchingIndent = QChar();
     next();
 }
 
@@ -1602,11 +1597,9 @@ void SynEditCppHighlighter::setState(const SynRangeState& rangeState)
     mRange = rangeState;
     // current line's left / right parenthesis count should be reset before parsing each line
     mRange.leftBraces = 0;
-    mRange.leftBrackets = 0;
-    mRange.leftParenthesis = 0;
     mRange.rightBraces = 0;
-    mRange.rightBrackets = 0;
-    mRange.rightParenthesis = 0;
+    mRange.firstIndentThisLine = mRange.indents.length();
+    mRange.lastMatchingIndent = QChar();
 }
 
 void SynEditCppHighlighter::resetState()
@@ -1617,13 +1610,10 @@ void SynEditCppHighlighter::resetState()
     mRange.bracketLevel = 0;
     mRange.parenthesisLevel = 0;
     mRange.leftBraces = 0;
-    mRange.leftBrackets = 0;
-    mRange.leftParenthesis = 0;
     mRange.rightBraces = 0;
-    mRange.rightBrackets = 0;
-    mRange.rightParenthesis = 0;
     mRange.indents = "";
-    mRange.indentStartLines.clear();
+    mRange.firstIndentThisLine = 0;
+    mRange.lastMatchingIndent = QChar();
     mAsmStart = false;
 }
 
diff --git a/RedPandaIDE/qsynedit/highlighter/cpp.h b/RedPandaIDE/qsynedit/highlighter/cpp.h
index 8fa45a8f..159be3cc 100644
--- a/RedPandaIDE/qsynedit/highlighter/cpp.h
+++ b/RedPandaIDE/qsynedit/highlighter/cpp.h
@@ -125,7 +125,8 @@ private:
     void unknownProc();
     void xorSymbolProc();
     void processChar();
-    void popIndentsByType(QChar indentType);
+    void popIndents(QChar indentType);
+    void pushIndents(QChar indentType);
 
 private:
     bool mAsmStart;