/* Copyright (C) 2004 Ariya Hidayat <ariya@kde.org>
   2005-2006 Johan Thelin <e8johan@gmail.com>
   2007 Helder Correia <helder.pereira.correia@gmail.com>

   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License
   as published by the Free Software Foundation; either version 2
   of the License, or (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; see the file COPYING.  If not, write to
   the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
   Boston, MA 02110-1301, USA.
*/

#include "evaluator.h"
#include "functions.h"
#include "opcode.h"

#include <float.h>
#include <limits.h>
#include <math.h>

#include <QCoreApplication>
#include <QLocale>
#include <QMap>
#include <QString>
#include <QStringList>
#include <QVector>
#include <QStack>

#define qApp QCoreApplication::instance()

class EvaluatorPrivate
{
public:
    Evaluator *evaluator;
    bool dirty;
    bool valid;
    QString expression;

    QString error;
    QMap<QString,Variable> variables;
    Evaluator::AngleMode angleMode;
    QString decimalPoint;

    QString assignId;
    QVector<Opcode> codes;
    QStringList identifiers;
    QVector<HNumber> constants;
};

class TokenStack : public QVector<Token>
{
public:
    TokenStack();
    bool isEmpty() const;
    unsigned itemCount() const;
    void push(const Token& token);
    Token pop();
    const Token& top();
    const Token& top(unsigned index);

private:
    void ensureSpace();
    int topIndex;
};

// for null token
const Token Token::null;

// helper function: return operator of given token text
// e.g. "*" yields Operator::Asterisk, and so on
static Token::Op matchOperator(const QString& text)
{
    Token::Op result = Token::InvalidOp;

    if (text.length() == 1) {
        QChar p = text[0];
        switch (p.unicode())
        {
        case '+': result = Token::Plus       ; break;
        case '-': result = Token::Minus      ; break;
        case '*': result = Token::Asterisk   ; break;
        case '/': result = Token::Slash      ; break;
        case '^': result = Token::Caret      ; break;
        case ';': result = Token::Semicolon  ; break;
        case '(': result = Token::LeftPar    ; break;
        case ')': result = Token::RightPar   ; break;
        case '%': result = Token::Percent    ; break;
        case '!': result = Token::Exclamation; break;
        case '=': result = Token::Equal      ; break;
        default : result = Token::InvalidOp  ; break;
        }
    }

    else if (text.length() == 2)
    {
        if (text == "**") result = Token::Caret;
    }
#if 0
    else if (text.length() == 3)
    {
        if (text == "mod") result = Token::Modulo;
        else if (text == "div") result = Token::Div;
    }
#endif

    return result;
}

// helper function: give operator precedence
// e.g. "+" is 1 while "*" is 3
static int opPrecedence(Token::Op op)
{
    int prec = -1;
    switch (op)
    {
    case Token::Exclamation : prec =  8; break;
    case Token::Percent     : prec =  8; break;
    case Token::Caret       : prec =  7; break;
    case Token::Asterisk    : prec =  5; break;
    case Token::Slash       : prec =  6; break;
    case Token::Modulo      : prec =  6; break;
    case Token::Div         : prec =  6; break;
    case Token::Plus        : prec =  3; break;
    case Token::Minus       : prec =  3; break;
    case Token::RightPar    : prec =  0; break;
    case Token::LeftPar     : prec = -1; break;
    case Token::Semicolon   : prec =  0; break;
    default                 : prec = -1; break;
    }
    return prec;
}

// creates a token
Token::Token(Type type, const QString& text, int pos)
{
    m_type = type;
    m_text = text;
    m_pos = pos;
}

// copy constructor
Token::Token(const Token& token)
{
    m_type = token.m_type;
    m_text = token.m_text;
    m_pos  = token.m_pos;
}

// assignment operator
Token& Token::operator=(const Token& token)
{
    m_type = token.m_type;
    m_text = token.m_text;
    m_pos  = token.m_pos;
    return *this;
}

HNumber Token::asNumber() const
{
    if (isNumber())
        return HNumber((const char*) m_text.toLatin1());
    else
        return HNumber(0);
}

Token::Op Token::asOperator() const
{
    if (isOperator())
        return matchOperator(m_text);
    else
        return InvalidOp;
}

QString Token::description() const
{
    QString desc;

    switch (m_type)
    {
    case Number     : desc = "Number"    ; break;
    case Identifier : desc = "Identifier"; break;
    case Operator   : desc = "Operator"  ; break;
    default         : desc = "Unknown"   ; break;
    }

    while (desc.length() < 10)
        desc.prepend(' ');
    desc.prepend("  ");
    desc.prepend(QString::number(m_pos));
    desc.append(" : ").append(m_text);

    return desc;
}


TokenStack::TokenStack(): QVector<Token>()
{
    topIndex = 0;
    ensureSpace();
}

bool TokenStack::isEmpty() const
{
    return topIndex == 0;
}

unsigned TokenStack::itemCount() const
{
    return topIndex;
}

void TokenStack::push(const Token& token)
{
    ensureSpace();
    (*this)[ topIndex++ ] = token;
}

Token TokenStack::pop()
{
    return (topIndex > 0) ? Token(at(--topIndex)) : Token();
}

const Token& TokenStack::top()
{
    return top(0);
}

const Token& TokenStack::top(unsigned index)
{
    if (topIndex > (int)index)
        return at(topIndex-index-1);
    return Token::null;
}

void TokenStack::ensureSpace()
{
    while (topIndex >= size())
        resize(size() + 10);
}


// helper function: return true for valid identifier character
static bool isIdentifier(QChar ch)
{
    return (ch.unicode() == '_') || (ch.unicode() == '$') || (ch.isLetter());
}


// Constructor

Evaluator::Evaluator()
{
    d = new EvaluatorPrivate;
    clear();
}

// Destructor

Evaluator::~Evaluator()
{
    delete d;
}

// Sets a new expression
// note that both the real lex and parse processes will happen later on
// when needed (i.e. "lazy parse"), for example during evaluation.

void Evaluator::setExpression(const QString& expr)
{
    d->expression = expr;
    d->dirty      = true;
    d->valid      = false;
    d->error      = QString();
}

// Returns the expression

QString Evaluator::expression() const
{
    return d->expression;
}

// Returns the validity
// note: empty expression is always invalid.

bool Evaluator::isValid() const
{
    if (d->dirty)
    {
        Tokens tokens = scan(d->expression, d->decimalPoint);
        if (!tokens.valid())
            compile(tokens);
        else
            d->valid = false;
    }
    return d->valid;
}

// Clears everything, also mark it as invalid.

void Evaluator::clear()
{
    d->expression = QString();
    d->dirty      = true;
    d->valid      = false;

    d->error        = QString();
    d->angleMode    = Degree;
    d->decimalPoint = QString();

    d->constants.clear();
    d->codes.clear();
    d->assignId = QString();

    clearVariables();
}

// Returns error message

QString Evaluator::error() const
{
    return d->error;
}

// Returns list of token for the expression.
// this triggers again the lexical analysis step. it is however preferable
// (even when there's small performance penalty) because otherwise we need to
// store parsed tokens all the time which serves no good purpose.

Tokens Evaluator::tokens() const
{
    return scan(d->expression, d->decimalPoint);
}

Tokens Evaluator::scan(const QString& expr, const QString& settingsDecimal)
{
    // to hold the result
    Tokens tokens;

    // auto-detect, always dot, or always comma

    QChar decimalPoint;
    QChar wrongDecimalPoint;

    if (settingsDecimal.length() == 1)
        decimalPoint = settingsDecimal[0];
    else
        decimalPoint = QLocale().decimalPoint();

    // sanity check for wrong decimal separator usage

    if (decimalPoint == ',')
        wrongDecimalPoint = '.';
    else
        wrongDecimalPoint = ',';

    int size = expr.size();
    for (int i = 0; i < size; i++)
        if (expr[i] == wrongDecimalPoint)
            return tokens;

    // parsing state
    enum { Start, Finish, Bad, InNumber, InHexa, InOctal, InBinary, InDecimal, InExpIndicator,
           InExponent, InIdentifier } state;

    // initialize variables
    state = Start;
    int i = 0;
    QString ex = expr;
    QString tokenText;
    int tokenStart = 0;

    // force a terminator
    ex.append(QChar());

    // main loop
    while ((state != Bad) && (state != Finish) && (i < ex.length()))
    {
        QChar ch = ex.at(i);

        switch (state)
        {

        case Start:

            tokenStart = i;

            // skip any whitespaces
            if (ch.isSpace()) i++;

            // check for number
            else if (ch.isDigit())
            {
                state = InNumber;
            }
            else if (ch == '#') // simple hexadec notation
            {
                tokenText.append("0x");
                state = InHexa;
                i++;
            }

            // decimal dot ?
            else if (ch == decimalPoint)
            {
                tokenText.append(ex.at(i++));
                state = InDecimal;
            }

            // terminator character
            else if (ch.isNull())
                state = Finish;

            // look for operator match
            else
            {
                int op;
                QString s;

#if 0
                // check for three-chars operator
                s.append(ch).append(ex.at(i+1)).append(ex.at(i+2));
                op = matchOperator(s);

                // check for two-chars operator
                if (op == Token::InvalidOp)
#endif
                {

                    s = QString(ch).append(ex.at(i+1));
                    op = matchOperator(s);
                }

                // check for one-char operator
                if (op == Token::InvalidOp)
                {
                    s = QString(ch);
                    op = matchOperator(s);
                }

                // any matched operator ?
                if (op != Token::InvalidOp)
                {
                    int len = s.length();
                    i += len;
                    tokens.append(Token(Token::Operator, s.left(len), tokenStart));
                }
                else state = Bad;
            }

            // beginning with unknown alphanumeric ?
            // could be identifier, or function...
            if (state == Bad && isIdentifier(ch))
            {
                state = InIdentifier;
            }
            break;

        case InIdentifier:

            // consume as long as alpha, dollar sign, underscore, or digit
            if (isIdentifier(ch)  || ch.isDigit())
                tokenText.append(ex.at(i++));

            // we're done with identifier
            else
            {
                tokens.append(Token(Token::Identifier, tokenText, tokenStart));
                tokenStart = i;
                tokenText = "";
                state = Start;
            }
            break;


        case InNumber:

            // consume as long as it's digit
            if (ch.isDigit()) tokenText.append(ex.at(i++));

            // convert decimal separator to '.'
            else if (ch == decimalPoint)
            {
                tokenText.append('.');
                i++;
                state = InDecimal;
            }

            // exponent ?
            else if (ch.toUpper() == 'E')
            {
                tokenText.append('E');
                i++;
                state = InExpIndicator;
            }
            else if (ch.toUpper() == 'X' && tokenText == "0") // normal hexadec notation
            {
                tokenText.append('x'); i++; state = InHexa;
            }
            else if (ch.toUpper() == 'B' && tokenText == "0") // binary notation
            {
                tokenText.append('b'); i++; state = InBinary;
            }
            else if (ch.toUpper() == 'O' && tokenText == "0") // octal notation
            {
                tokenText.append('o'); i++; state = InOctal;
            }
            else if (ch.toUpper() == 'D' && tokenText == "0") // explicit decimal notation
            {
                // we also need to get rid of the leading zero
                tokenText = ""; i++;
            }

            // we're done with integer number
            else
            {
                tokens.append(Token(Token::Number, tokenText, tokenStart));
                tokenText = ""; state = Start;
            }
            break;

        case InHexa:
            if (ch.isDigit() || (ch >= 'A' && ch < 'G') || (ch >= 'a' && ch < 'g'))
                tokenText.append(ex.at(i++).toUpper());
            else // we're done with hexa number
            {
                tokens.append(Token(Token::Number, tokenText, tokenStart));
                tokenText = ""; state = Start;
            }
            break;
        case InBinary:
            if (ch == '0' || ch == '1') // very strict rule ;)
                tokenText.append(ex.at(i++));
            else // we're done with binary number
            {
                tokens.append(Token(Token::Number, tokenText, tokenStart));
                tokenText = "";
                state = Start;
            }
            break;
        case InOctal:
            if (ch >= '0' && ch < '8') // octal has only 8 digits, 8 & 9 are invalid
                tokenText.append(ex.at(i++));
            else // we're done with octal number
            {
                tokens.append(Token(Token::Number, tokenText, tokenStart));
                tokenText = ""; state = Start;
            }
            break;

        case InDecimal:

            // consume as long as it's digit
            if (ch.isDigit()) tokenText.append(ex.at(i++));

            // exponent ?
            else if (ch.toUpper() == 'E')
            {
                tokenText.append('E');
                i++;
                state = InExpIndicator;
            }

            // we're done with floating-point number
            else
            {
                tokens.append(Token(Token::Number, tokenText, tokenStart));
                tokenText = "";
                state = Start;
            };
            break;

        case InExpIndicator:

            // possible + or - right after E, e.g 1.23E+12 or 4.67E-8
            if ((ch == '+') || (ch == '-')) tokenText.append(ex.at(i++));

            // consume as long as it's digit
            else if (ch.isDigit()) state = InExponent;

            // invalid thing here
            else state = Bad;

            break;

        case InExponent:

            // consume as long as it's digit
            if (ch.isDigit()) tokenText.append(ex.at(i++));

            // we're done with floating-point number
            else
            {
                tokens.append(Token(Token::Number, tokenText, tokenStart));
                tokenText = "";
                state = Start;
            };
            break;

        case Bad: // bad bad bad
            tokens.setValid(false);
            break;

        default:
            break;
        };
    };

    return tokens;
}


void Evaluator::compile(const Tokens& tokens) const
{
    // initialize variables
    d->dirty = false;
    d->valid = false;
    d->codes.clear();
    d->constants.clear();
    d->identifiers.clear();
    d->error = QString();

    // sanity check
    if (tokens.count() == 0) return;

    TokenStack syntaxStack;
    QStack<int> argStack;
    unsigned argCount = 1;

    for (int i = 0; i <= tokens.count(); i++)
    {
        // helper token: InvalidOp is end-of-expression
        Token token =  (i < tokens.count()) ? tokens[i] : Token(Token::Operator);
        Token::Type tokenType = token.type();

        // unknown token is invalid
        if (tokenType == Token::Unknown) break;

        // for constants, push immediately to stack
        // generate code to load from a constant
        if (tokenType == Token::Number)
        {
            syntaxStack.push(token);
            d->constants.append(token.asNumber());
            d->codes.append(Opcode(Opcode::Load, d->constants.count()-1));
        }

        // for identifier, push immediately to stack
        // generate code to load from reference
        if (tokenType == Token::Identifier)
        {
            syntaxStack.push(token);
            d->identifiers.append(token.text());
            d->codes.append(Opcode(Opcode::Ref, d->identifiers.count()-1));
        }

        // special case for percentage
        if (tokenType == Token::Operator)
            if (token.asOperator() == Token::Percent)
                if (syntaxStack.itemCount() >= 1)
                    if (!syntaxStack.top().isOperator())
                    {
                        d->constants.append(HNumber("0.01"));
                        d->codes.append(Opcode(Opcode::Load, d->constants.count()-1));
                        d->codes.append(Opcode(Opcode::Mul));
                    }

        // for any other operator, try to apply all parsing rules
        if (tokenType == Token::Operator)
            if (token.asOperator() != Token::Percent)
            {
                // repeat until no more rule applies
                bool argHandled = false;
                for (; ;)
                {
                    bool ruleFound = false;

                    // rule for function last argument:
                    //  id (arg) -> arg
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 4)
                        {
                            Token par2 = syntaxStack.top();
                            Token arg = syntaxStack.top(1);
                            Token par1 = syntaxStack.top(2);
                            Token id = syntaxStack.top(3);
                            if (par2.asOperator() == Token::RightPar)
                                if (!arg.isOperator())
                                    if (par1.asOperator() == Token::LeftPar)
                                        if (id.isIdentifier())
                                        {
                                            ruleFound = true;
                                            syntaxStack.pop();
                                            syntaxStack.pop();
                                            syntaxStack.pop();
                                            syntaxStack.pop();
                                            syntaxStack.push(arg);
                                            d->codes.append(Opcode(Opcode::Function, argCount));
                                            argCount = argStack.empty() ? 0 : argStack.pop();
                                        }
                        }

                    // are we entering a function ?
                    // if token is operator, and stack already has: id (arg
                    if (!ruleFound)
                        if (!argHandled)
                            if (tokenType == Token::Operator)
                                if (syntaxStack.itemCount() >= 3)
                                {
                                    Token arg = syntaxStack.top();
                                    Token par = syntaxStack.top(1);
                                    Token id = syntaxStack.top(2);
                                    if (!arg.isOperator())
                                        if (par.asOperator() == Token::LeftPar)
                                            if (id.isIdentifier())
                                            {
                                                ruleFound = true;
                                                argStack.push(argCount);
                                                argCount = 1;
                                                break;
                                            }
                                }


                    // rule for simplified syntax for function e.g. "sin pi" or "cos 1.2"
                    // i.e no need for parentheses like "sin(pi)" or "cos(1.2)"
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 2)
                        {
                            Token arg = syntaxStack.top();
                            Token id = syntaxStack.top(1);
                            if (!arg.isOperator())
                                if (id.isIdentifier())
                                    if (FunctionRepository::self()->function(id.text()))
                                    {
                                        ruleFound = true;
                                        d->codes.append(Opcode(Opcode::Function, 1));
                                        syntaxStack.pop();
                                        syntaxStack.pop();
                                        syntaxStack.push(arg);
                                    }
                        }

                    // rule for unary operator in simplified function syntax
                    // this handles case like "sin -90"
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 3)
                        {
                            Token x = syntaxStack.top();
                            Token op = syntaxStack.top(1);
                            Token id = syntaxStack.top(2);
                            if (!x.isOperator())
                                if (op.isOperator())
                                    if (id.isIdentifier())
                                        if (FunctionRepository::self()->function(id.text()))
                                            if ((op.asOperator() == Token::Plus) ||
                                                (op.asOperator() == Token::Minus))
                                            {
                                                ruleFound = true;
                                                syntaxStack.pop();
                                                syntaxStack.pop();
                                                syntaxStack.push(x);
                                                if (op.asOperator() == Token::Minus)
                                                    d->codes.append(Opcode(Opcode::Neg));
                                            }
                        }

                    // rule for unary postfix operator in simplified function syntax
                    // this handles case like "sin 90!"
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 3)
                        {
                            Token op = syntaxStack.top();
                            Token x = syntaxStack.top(1);
                            Token id = syntaxStack.top(2);
                            if (id.isIdentifier() && FunctionRepository::self()->function(id.text()))
                            {
                                if (!x.isOperator() && op.isOperator() &&
                                    op.asOperator() == Token::Exclamation)
                                {
                                    ruleFound = true;
                                    syntaxStack.pop();
                                    syntaxStack.pop();
                                    syntaxStack.push(x);
                                    d->codes.append(Opcode(Opcode::Fact));
                                }
                            }
                        }


                    // rule for function arguments, if token is , or)
                    // id (arg1 ; arg2 -> id (arg
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 5)
                            if ((token.asOperator() == Token::RightPar) ||
                                (token.asOperator() == Token::Semicolon))
                            {
                                Token arg2 = syntaxStack.top();
                                Token sep = syntaxStack.top(1);
                                Token arg1 = syntaxStack.top(2);
                                Token par = syntaxStack.top(3);
                                Token id = syntaxStack.top(4);
                                if (!arg2.isOperator())
                                    if (sep.asOperator() == Token::Semicolon)
                                        if (!arg1.isOperator())
                                            if (par.asOperator() == Token::LeftPar)
                                                if (id.isIdentifier())
                                                {
                                                    ruleFound = true;
                                                    argHandled = true;
                                                    syntaxStack.pop();
                                                    syntaxStack.pop();
                                                    argCount++;
                                                }
                            }

                    // rule for function call with parentheses, but without argument
                    // e.g. "2*PI()"
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 3)
                        {
                            Token par2 = syntaxStack.top();
                            Token par1 = syntaxStack.top(1);
                            Token id = syntaxStack.top(2);
                            if (par2.asOperator() == Token::RightPar)
                                if (par1.asOperator() == Token::LeftPar)
                                    if (id.isIdentifier())
                                    {
                                        ruleFound = true;
                                        syntaxStack.pop();
                                        syntaxStack.pop();
                                        syntaxStack.pop();
                                        syntaxStack.push(Token(Token::Number));
                                        d->codes.append(Opcode(Opcode::Function, 0));
                                    }
                        }

                    // rule for parenthesis:  (Y) -> Y
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 3)
                        {
                            Token right = syntaxStack.top();
                            Token y = syntaxStack.top(1);
                            Token left = syntaxStack.top(2);
                            if (right.isOperator())
                                if (!y.isOperator())
                                    if (left.isOperator())
                                        if (right.asOperator() == Token::RightPar)
                                            if (left.asOperator() == Token::LeftPar)
                                            {
                                                ruleFound = true;
                                                syntaxStack.pop();
                                                syntaxStack.pop();
                                                syntaxStack.pop();
                                                syntaxStack.push(y);
                                            }
                        }

                    // rule for binary operator:  A (op) B -> A
                    // conditions: precedence of op >= precedence of token
                    // action: push (op) to result
                    // e.g. "A * B" becomes "A" if token is operator "+"
                    // exception: for caret (power operator), if op is another caret
                    // then the rule doesn't apply, e.g. "2^3^2" is evaluated as "2^(3^2)"
                    if (!ruleFound)
                        if (syntaxStack.itemCount() >= 3)
                        {
                            Token b = syntaxStack.top();
                            Token op = syntaxStack.top(1);
                            Token a = syntaxStack.top(2);
                            if (!a.isOperator())
                                if (!b.isOperator())
                                    if (op.isOperator())
                                        if (token.asOperator() != Token::LeftPar)
                                            if (token.asOperator() != Token::Caret)
                                                if (opPrecedence(op.asOperator()) >= opPrecedence(token.asOperator()))
                                                {
                                                    ruleFound = true;
                                                    syntaxStack.pop();
                                                    syntaxStack.pop();
                                                    syntaxStack.pop();
                                                    syntaxStack.push(b);
                                                    switch (op.asOperator())
                                                    {
                                                        // simple binary operations
                                                    case Token::Plus:         d->codes.append(Opcode::Add); break;
                                                    case Token::Minus:        d->codes.append(Opcode::Sub); break;
                                                    case Token::Asterisk:     d->codes.append(Opcode::Mul); break;
                                                    case Token::Slash:        d->codes.append(Opcode::Div); break;
                                                    case Token::Caret:        d->codes.append(Opcode::Pow); break;
                                                    case Token::Modulo:       d->codes.append(Opcode::Modulo); break;
                                                    case Token::Div:          d->codes.append(Opcode::IntDiv); break;
                                                    default: break;
                                                    };
                                                }
                        }

                    // rule for unary operator:  (op1) (op2) X -> (op1) X
                    // conditions: op2 is unary, token is not '('
                    // action: push (op2) to result
                    // e.g.  "* - 2" becomes "*"
                    if (!ruleFound)
                        if (token.asOperator() != Token::LeftPar)
                            if (syntaxStack.itemCount() >= 3)
                            {
                                Token x = syntaxStack.top();
                                Token op2 = syntaxStack.top(1);
                                Token op1 = syntaxStack.top(2);
                                if (!x.isOperator() && op1.isOperator() && op2.isOperator() &&
                                    (op2.asOperator() == Token::Plus || op2.asOperator() == Token::Minus))
                                {
                                    ruleFound = true;
                                    if (op2.asOperator() == Token::Minus)
                                        d->codes.append(Opcode(Opcode::Neg));
                                }
                                else // maybe postfix
                                {
                                    x = op2; op2 = syntaxStack.top();
                                    if (!x.isOperator() && op1.isOperator() && op2.isOperator() &&
                                        op2.asOperator() == Token::Exclamation)
                                    {
                                        ruleFound = true;
                                        d->codes.append(Opcode(Opcode::Fact));
                                    }
                                }
                                if (ruleFound)
                                {
                                    syntaxStack.pop();
                                    syntaxStack.pop();
                                    syntaxStack.push(x);
                                }
                            }

                    // auxiliary rule for unary operator:  (op) X -> X
                    // conditions: op is unary, op is first in syntax stack, token is not '(' or '^' or '!'
                    // action: push (op) to result
                    if (!ruleFound)
                        if (token.asOperator() != Token::LeftPar)
                            if (token.asOperator() != Token::Caret)
                                if (token.asOperator() != Token::Exclamation)
                                    if (syntaxStack.itemCount() == 2)
                                    {
                                        Token x = syntaxStack.top();
                                        Token op = syntaxStack.top(1);
                                        if (!x.isOperator() && op.isOperator() &&
                                            (op.asOperator() == Token::Plus || op.asOperator() == Token::Minus))
                                        {
                                            ruleFound = true;
                                            if (op.asOperator() == Token::Minus)
                                                d->codes.append(Opcode(Opcode::Neg));
                                        }
                                        else
                                        {
                                            x = op; op = syntaxStack.top();
                                            if (!x.isOperator() && op.isOperator() &&
                                                (op.asOperator() == Token::Exclamation))
                                            {
                                                ruleFound = true;
                                                d->codes.append(Opcode(Opcode::Fact));
                                            }
                                        }
                                        if (ruleFound)
                                        {
                                            syntaxStack.pop();
                                            syntaxStack.pop();
                                            syntaxStack.push(x);
                                        }
                                    }

                    if (!ruleFound) break;
                }

                // can't apply rules anymore, push the token
                if (token.asOperator() != Token::Percent)
                    syntaxStack.push(token);
            }
    }

    // syntaxStack must left only one operand and end-of-expression (i.e. InvalidOp)
    d->valid = false;
    if (syntaxStack.itemCount() == 2)
        if (syntaxStack.top().isOperator())
            if (syntaxStack.top().asOperator() == Token::InvalidOp)
                if (!syntaxStack.top(1).isOperator())
                    d->valid = true;

    // bad parsing ? clean-up everything
    if (!d->valid)
    {
        d->constants.clear();
        d->codes.clear();
        d->identifiers.clear();
    }
}

Evaluator::AngleMode Evaluator::angleMode() const
{
    return d->angleMode;
}

void Evaluator::setAngleMode(AngleMode am)
{
    d->angleMode = am;
}

QString Evaluator::decimalPoint() const
{
    return d->decimalPoint;
}

void Evaluator::setDecimalPoint(const QString& dp)
{
    d->decimalPoint = dp;
}

HNumber Evaluator::eval()
{
    QStack<HNumber> stack;
    QStack<QString> refs;
    int index;
    HNumber val1, val2;
    QVector<HNumber> args;
    QString fname;
    Function* function;

    if (d->dirty)
    {
        Tokens tokens = scan(d->expression, d->decimalPoint);

        // invalid expression?
        if (!tokens.valid())
        {
            d->error = QObject::tr("Invalid expression.");
            return HNumber(0);
        }

        // variable assignment?
        d->assignId = QString();
        if (tokens.count() > 2)
            if (tokens[0].isIdentifier())
                if (tokens[1].asOperator() == Token::Equal)
                {
                    d->assignId = tokens[0].text();
                    tokens.erase(tokens.begin());
                    tokens.erase(tokens.begin());
                }

        compile(tokens);
    }

    // can not overwrite pi
    if (d->assignId == QString("pi"))
    {
        d->error = d->assignId + ": " + QObject::tr("Variable cannot be overwritten.");
        return HNumber(0);
    }
    // can not overwrite phi
    if (d->assignId == QString("phi"))
    {
        d->error = d->assignId + ": " + QObject::tr("Variable cannot be overwritten.");
        return HNumber(0);
    }
    // can not overwrite ans
    if (d->assignId == QString("ans"))
    {
        d->error = d->assignId + ": " + QObject::tr("Variable cannot be overwritten.");
        return HNumber(0);
    }

    // variable can't have the same name as function
    if (FunctionRepository::self()->function(d->assignId))
    {
        d->error = d->assignId + ": " + QObject::tr("Identifier matches an existing function name.");
        return HNumber(0);
    }

    // magic: always set here to avoid being overwritten by user
    set(QString("pi"),  HMath::pi());
    set(QString("phi"), HMath::phi());

    for (int pc = 0; pc < d->codes.count(); pc++)
    {
        Opcode& opcode = d->codes[pc];
        index = opcode.index;
        switch (opcode.type)
        {
            // no operation
        case Opcode::Nop:
            break;

            // load a constant, push to stack
        case Opcode::Load:
            val1 = d->constants[index];
            stack.push(val1);
            break;

            // unary operation
        case Opcode::Neg:
            if (stack.count() < 1)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val1 = HMath::negate(val1);
            stack.push(val1);
            break;

            // binary operation: take two values from stack, do the operation,
            // push the result to stack
        case Opcode::Add:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            val2 += val1;
            stack.push(val2);
            break;

        case Opcode::Sub:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            val2 -= val1;
            stack.push(val2);
            break;

        case Opcode::Mul:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            val2 *= val1;
            stack.push(val2);
            break;

        case Opcode::Div:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            if (val1.isZero())
            {
                d->error = QObject::tr("Division by zero.");
                return HNumber(0);
            }
            val2 /= val1;
            stack.push(val2);
            break;

        case Opcode::Pow:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            val2 = HMath::raise(val2, val1);
            stack.push(val2);
            break;

        case Opcode::Fact:
            if (stack.count() < 1)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val1 = HMath::factorial(val1);
            stack.push(val1);
            break;

        case Opcode::Modulo:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            if (val1.isZero())
            {
                d->error = QObject::tr("Division by zero.");
                return HNumber(0);
            }
            val2 = val2 % val1;
            stack.push(val2);
            break;

        case Opcode::IntDiv:
            if (stack.count() < 2)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }
            val1 = stack.pop();
            val2 = stack.pop();
            if (val1.isZero())
            {
                d->error = QObject::tr("Division by zero.");
                return HNumber(0);
            }
            val2 /= val1;
            stack.push(HMath::integer(val2));
            break;

            // reference
        case Opcode::Ref:
            fname = d->identifiers[index];
            if (has(fname))
            {
                // variable
                stack.push(get(fname));
            }
            else
            {
                // function
                function = FunctionRepository::self()->function(fname);
                if (function)
                    refs.push(fname);
                else
                {
                    d->error = fname + ": " + QObject::tr("Unknown function or variable.");
                    return HNumber(0);
                }
            }
            break;

            // calling function
        case Opcode::Function:
            // must do this first to avoid crash when using vars like functions
            if (refs.isEmpty())
                break;

            fname = refs.pop();
            function = FunctionRepository::self()->function(fname);
            if (!function)
            {
                d->error = fname + ": " + QObject::tr("Unknown function or variable.");
                return HNumber(0);
            }

            if (stack.count() < index)
            {
                d->error = QObject::tr("Invalid expression.");
                return HNumber(0);
            }

            args.clear();
            for (; index; index--)
                args.insert(args.begin(), stack.pop());

            stack.push(function->exec(this, args));
            if (!function->error().isEmpty())
            {
                d->error = function->error();
                return HNumber(0);
            }

            break;

        default:
            break;
        }
    }

    // more than one value in stack ? unsuccesfull execution...
    if (stack.count() != 1)
    {
        d->error = QObject::tr("Invalid expression.");
        return HNumber(0);
    }

    HNumber result = stack.pop();

    // handle variable assignment, e.g. "x=2*4"
    if (!d->assignId.isEmpty())
        set(d->assignId, result);

    // "ans" is default variable to hold calculation result
    set(QString("ans"), result);

    return result;
}

void Evaluator::set(const QString& id, HNumber value)
{
    if (!id.isEmpty())
    {
        d->variables[ id ].name = id;
        d->variables[ id ].value = value;
    }
}

HNumber Evaluator::get(const QString& id)
{
    if (id.isEmpty()) return HNumber(0);

    if (!d->variables.contains(id))
        set(id, HNumber(0));
    return d->variables[ id ].value;
}

bool Evaluator::has(const QString& id)
{
    return id.isEmpty() ? false : d->variables.contains(id);
}

void Evaluator::remove(const QString& id)
{
    d->variables.remove(id);
}

QVector<Variable> Evaluator::variables() const
{
    QVector<Variable> result;

    QMap<QString,Variable>::Iterator it;
    for (it = d->variables.begin(); it != d->variables.end(); ++it)
    {
        Variable var;
        var.name = it.value().name;
        var.value = it.value().value;
        result.append(var);
    }

    return result;
}

void Evaluator::clearVariables()
{
    d->variables.clear();
    set(QString("pi"),  HMath::pi() );
    set(QString("phi"), HMath::phi());
    set(QString("ans"), HNumber(0)  );
}

QString Evaluator::autoFix(const QString& expr, const QString& decimalPoint)
{
    int par = 0;
    QString result;

    // strip off all funny characters
    for (int c = 0; c < expr.length(); c++)
        if (expr[c] >= QChar(32))
            result.append(expr[c]);

    // no extra whitespaces at the beginning and at the end
    result = result.trimmed();

    // strip trailing equal sign (=)
    while (result.endsWith("="))
        result = result.left(result.length()-1);

    // automagically close all parenthesis
    Tokens tokens = Evaluator::scan(result, decimalPoint);
    if (tokens.count())
    {
        for (int i=0; i<tokens.count(); i++)
            if (tokens[i].asOperator() == Token::LeftPar) par++;
            else if (tokens[i].asOperator() == Token::RightPar) par--;

        if (par < 0)
            par = 0;

        // if the scanner stops in the middle, do not bother to apply fix
        const Token& lastToken = tokens[tokens.count()-1];
        if (lastToken.pos()+lastToken.text().length() >= result.length())
            while (par--)
                result.append(')');
    }

    // special treatment for simple function
    // e.g. "cos" is regarded as "cos(ans)"
    if (!result.isEmpty())
    {
        Tokens tokens = Evaluator::scan(result, decimalPoint);
        if (tokens.count() == 1)
        {
            if (tokens[0].isIdentifier())
            {
                Function* f = FunctionRepository::self()->function(tokens[0].text());
                if (f) result.append("(ans)");
            }
        }
    }

    return result;
}

// Debugging aid

QString Evaluator::dump() const
{
    QString result;
    int c;

    if (d->dirty)
    {
        Tokens tokens = scan(d->expression, d->decimalPoint);
        compile(tokens);
    }

    result = QString("Expression: [%1]\n").arg(d->expression);

    result.append("  Constants:\n");
    for (c = 0; c < d->constants.count(); c++)
    {
        QString vtext;
        HNumber val = d->constants[c];
        char* ss = HMath::formatFixed(val);
        result += QString("    #%1 = %2\n").arg(c).arg(ss);
        free(ss);
    }

    result.append("\n");
    result.append("  Identifiers:\n");
    for (c = 0; c < d->identifiers.count(); c++)
    {
        QString vtext;
        result += QString("    #%1 = %2\n").arg(c).arg(d->identifiers[c]);
    }

    result.append("\n");
    result.append("  Code:\n");
    for (int i = 0; i < d->codes.count(); i++)
    {
        QString ctext;
        switch (d->codes[i].type)
        {
        case Opcode::Load    : ctext = QString("Load #%1"     ).arg(d->codes[i].index); break;
        case Opcode::Ref     : ctext = QString("Ref #%1"      ).arg(d->codes[i].index); break;
        case Opcode::Function: ctext = QString("Function (%1)").arg(d->codes[i].index); break;
        case Opcode::Add     : ctext = "Add"    ; break;
        case Opcode::Sub     : ctext = "Sub"    ; break;
        case Opcode::Mul     : ctext = "Mul"    ; break;
        case Opcode::Div     : ctext = "Div"    ; break;
        case Opcode::Neg     : ctext = "Neg"    ; break;
        case Opcode::Pow     : ctext = "Pow"    ; break;
        case Opcode::Fact    : ctext = "Fact"   ; break;
        default              : ctext = "Unknown"; break;
        }
        result.append("   ").append(ctext).append("\n");
    }

    return result;
}