added: IEEE 754 half-to-even rounding (bankers' rounding) to the following

floating point algorithms:
         Big::Add
         Big::Sub
         Big::Mul
         Big::Div



git-svn-id: svn://ttmath.org/publicrep/ttmath/trunk@258 e52654a7-88a9-db11-a3e9-0013d4bc506e

CHANGELOG

@@ -1,7 +1,13 @@
-Version 0.9.1 prerelease (2009.12.05):
+Version 0.9.1 prerelease (2009.12.06):
     * fixed:   the parser didn't use characters for changing the base (# and &)
                those characters were skipped
                (this bug was introduced in 0.9.0)
+    * added:   IEEE 754 half-to-even rounding (bankers' rounding) to the following
+               floating point algorithms:
+               Big::Add
+               Big::Sub
+               Big::Mul
+               Big::Div
     * added:   to Big::ToString() - additional rounding when conv.base_round is used
                if the value is not an integer we calculate how many valid digits there are
                after the comma operator (in conv.base radix) and then we skipped the rest

ttmath/ttmathbig.h

@@ -714,7 +714,36 @@ public:
 
 
 
+private:
 
+	/*!
+		this method does the half-to-even rounding (banker's rounding)
+
+		if is_half is:
+		  true  - that means the rest was equal the half (0.5 decimal)
+		  false - that means the rest was greater than a half (greater than 0.5 decimal)
+
+	    if the rest was less than a half then don't call this method
+		(the rounding should does nothing then)
+	*/
+	uint RoundHalfToEven(bool is_half)
+	{
+	uint c = 0;
+
+		if( !is_half || mantissa.IsTheLowestBitSet() )
+		{
+			if( mantissa.AddOne() )
+			{
+				mantissa.Rcr(1, 1);
+				c = exponent.AddOne();
+			}
+		}
+
+	return c;
+	}
+
+
+public:
 
 	/*!
 	*
@@ -772,8 +801,11 @@ public:
 			// values have the same signs
 			if( mantissa.Add(ss2.mantissa) )
 			{
-				mantissa.Rcr(1,1);
-				c = exponent.AddOne();
+				bool is_exact_half = mantissa.Rcr(1,1);
+				c += exponent.AddOne();
+
+				if( is_exact_half )
+					c += RoundHalfToEven(is_exact_half);
 			}
 		}
 		else
@@ -1078,6 +1110,41 @@ public:
 	}
 
 
+private:
+
+
+	/*!
+		this method checks whether a table pointed by 'tab' and 'len'
+		has the value 0.5 decimal
+		(it is treated as the comma operator would be before the highest bit)
+		call this method only if the highest bit is set - you have to test it beforehand
+
+		return:
+		  true  - tab was equal the half (0.5 decimal)
+		  false - tab was greater than a half (greater than 0.5 decimal)
+
+	*/
+	bool CheckGreaterOrEqualHalf(uint * tab, uint len)
+	{
+	uint i;
+
+		TTMATH_ASSERT( len>0 && (tab[len-1] & TTMATH_UINT_HIGHEST_BIT)!=0 )
+
+		for(i=0 ; i<len-1 ; ++i)
+			if( tab[i] != 0 )
+				return false;
+
+		if( tab[i] != TTMATH_UINT_HIGHEST_BIT )
+			return false;
+
+	return true;
+	}
+
+
+
+public:
+
+
 	/*!
 		multiplication this = this * ss2
 		this method returns a carry
@@ -1087,7 +1154,8 @@ public:
 	TTMATH_REFERENCE_ASSERT( ss2 )
 
 	UInt<man*2> man_result;
-	uint i,c;
+	uint c = 0;
+	uint i;
 
 		if( IsNan() || ss2.IsNan() )
 			return CheckCarry(1);
@@ -1110,13 +1178,22 @@ public:
 		// if there is a zero value in man_result the method CompensationToLeft()
 		// returns 0 but we'll correct this at the end in Standardizing() method)
 		i = man_result.CompensationToLeft();
+		uint exp_add = man * TTMATH_BITS_PER_UINT - i;
+
+		if( exp_add )
+			c += exponent.Add( exp_add );
 
-		c  = exponent.Add( man * TTMATH_BITS_PER_UINT - i );
 		c += exponent.Add( ss2.exponent );
 
 		for(i=0 ; i<man ; ++i)
 			mantissa.table[i] = man_result.table[i+man];
 
+		if( (man_result.table[man-1] & TTMATH_UINT_HIGHEST_BIT) != 0 )
+		{
+			bool is_half = CheckGreaterOrEqualHalf(man_result.table, man);
+			c += RoundHalfToEven(is_half);		
+		}
+
 		if( IsSign() == ss2.IsSign() )
 		{
 			// the signs are the same, the result is positive
@@ -1187,6 +1264,12 @@ public:
 		for(i=0 ; i<man ; ++i)
 			mantissa.table[i] = man1.table[i+man];
 
+		if( (man1.table[man-1] & TTMATH_UINT_HIGHEST_BIT) != 0 )
+		{
+			bool is_half = CheckGreaterOrEqualHalf(man1.table, man);
+			c += RoundHalfToEven(is_half);		
+		}
+
 		if( IsSign() == ss2.IsSign() )
 			Abs();
 		else
@@ -4374,11 +4457,6 @@ private:
 
 		for( ; true ; ++source, ++index )
 		{
-			if( index == 98 )
-			{
-				index = 98;
-			}
-
 			if( conv.group!=0 && *source==static_cast<char>(conv.group) )
 				continue;