X-Git-Url: https://mattmccutchen.net/bigint/bigint.git/blobdiff_plain/3aaa5ce601b009373fa7464f6de044f7b2e017ea..5ff40cf5d6e822051da902b041ae7ae8f545123e:/BigUnsigned.cc

diff --git a/BigUnsigned.cc b/BigUnsigned.cc
index ad96026..f6a925c 100644
--- a/BigUnsigned.cc
+++ b/BigUnsigned.cc
@@ -218,11 +218,37 @@ BigUnsigned::CmpRes BigUnsigned::compareTo(const BigUnsigned &x) const {
 * See Section 4.3.1 of Knuth's ``The Art of Computer Programming''.
 */
 
+/*
+ * On most calls to put-here operations, it's safe to read the inputs little by
+ * little and write the outputs little by little.  However, if one of the
+ * inputs is coming from the same variable into which the output is to be
+ * stored (an "aliased" call), we risk overwriting the input before we read it.
+ * In this case, we first compute the result into a temporary BigUnsigned
+ * variable and then copy it into the requested output variable *this.
+ * Each put-here operation uses the DTRT_ALIASED macro (Do The Right Thing on
+ * aliased calls) to generate code for this check.
+ * 
+ * I adopted this approach on 2007.02.13 (see Assignment Operators in
+ * BigUnsigned.hh).  Before then, put-here operations rejected aliased calls
+ * with an exception.  I think doing the right thing is better.
+ * 
+ * Some of the put-here operations can probably handle aliased calls safely
+ * without the extra copy because (for example) they process blocks strictly
+ * right-to-left.  At some point I might determine which ones don't need the
+ * copy, but my reasoning would need to be verified very carefully.  For now
+ * I'll leave in the copy.
+ */
+#define DTRT_ALIASED(cond, op) \
+	if (cond) { \
+		BigUnsigned tmpThis; \
+		tmpThis.op; \
+		*this = tmpThis; \
+		return; \
+	}
+
 // Addition
 void BigUnsigned::add(const BigUnsigned &a, const BigUnsigned &b) {
-	// Block unsafe calls
-	if (this == &a || this == &b)
-		throw "BigUnsigned::add: One of the arguments is the invoked object";
+	DTRT_ALIASED(this == &a || this == &b, add(a, b));
 	// If one argument is zero, copy the other.
 	if (a.len == 0) {
 		operator =(b);
@@ -283,9 +309,7 @@ void BigUnsigned::add(const BigUnsigned &a, const BigUnsigned &b) {
 
 // Subtraction
 void BigUnsigned::subtract(const BigUnsigned &a, const BigUnsigned &b) {
-	// Block unsafe calls
-	if (this == &a || this == &b)
-		throw "BigUnsigned::subtract: One of the arguments is the invoked object";
+	DTRT_ALIASED(this == &a || this == &b, subtract(a, b));
 	// If b is zero, copy a.  If a is shorter than b, the result is negative.
 	if (b.len == 0) {
 		operator =(a);
@@ -397,9 +421,7 @@ inline BigUnsigned::Blk getShiftedBlock(const BigUnsigned &num,
 
 // Multiplication
 void BigUnsigned::multiply(const BigUnsigned &a, const BigUnsigned &b) {
-	// Block unsafe calls
-	if (this == &a || this == &b)
-		throw "BigUnsigned::multiply: One of the arguments is the invoked object";
+	DTRT_ALIASED(this == &a || this == &b, multiply(a, b));
 	// If either a or b is zero, set to zero.
 	if (a.len == 0 || b.len == 0) {
 		len = 0;
@@ -483,12 +505,29 @@ void BigUnsigned::multiply(const BigUnsigned &a, const BigUnsigned &b) {
 * and provide outputs in the most convenient places so that no value ever needs
 * to be copied in its entirety.  That way, the client can perform exactly the
 * copying it needs depending on where the inputs are and where it wants the output.
+* A better name for this function might be "modWithQuotient", but I would rather
+* not change the name now.
 */
 void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
-	// Block unsafe calls
-	if (this == &b || &q == &b || this == &q)
-		throw "BigUnsigned::divideWithRemainder: Some two objects involved are the same";
-	
+	/*
+	 * Defending against aliased calls is a bit tricky because we are
+	 * writing to both *this and q.
+	 * 
+	 * It would be silly to try to write quotient and remainder to the
+	 * same variable.  Rule that out right away.
+	 */
+	if (this == &q)
+		throw "BigUnsigned::divideWithRemainder: Cannot write quotient and remainder into the same variable";
+	/*
+	 * Now *this and q are separate, so the only concern is that b might be
+	 * aliased to one of them.  If so, use a temporary copy of b.
+	 */
+	if (this == &b || &q == &b) {
+		BigUnsigned tmpB(b);
+		divideWithRemainder(tmpB, q);
+		return;
+	}
+
 	/*
 	* Note that the mathematical definition of mod (I'm trusting Knuth) is somewhat
 	* different from the way the normal C++ % operator behaves in the case of division by 0.
@@ -502,7 +541,7 @@ void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
 		q.len = 0;
 		return;
 	}
-	
+
 	/*
 	* If *this.len < b.len, then *this < b, and we can be sure that b doesn't go into
 	* *this at all.  The quotient is 0 and *this is already the remainder (so leave it alone).
@@ -511,11 +550,11 @@ void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
 		q.len = 0;
 		return;
 	}
-	
+
 	/*
 	* At this point we know *this > b > 0.  (Whew!)
 	*/
-	
+
 	/*
 	* Overall method:
 	*
@@ -547,7 +586,7 @@ void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
 	unsigned int i2;
 	Blk temp;
 	bool borrowIn, borrowOut;
-	
+
 	/*
 	* Make sure we have an extra zero block just past the value.
 	*
@@ -566,17 +605,17 @@ void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
 	allocateAndCopy(len + 1); // Get the space.
 	len++; // Increase the length.
 	blk[origLen] = 0; // Zero the extra block.
-	
+
 	// work2 holds part of the result of a subtraction; see above.
 	Blk *work2 = new Blk[len];
-	
+
 	// Set preliminary length for quotient and make room
 	q.len = origLen - b.len + 1;
 	q.allocate(q.len);
 	// Zero out the quotient
 	for (i = 0; i < q.len; i++)
 		q.blk[i] = 0;
-	
+
 	// For each possible left-shift of b in blocks...
 	i = q.len;
 	while (i > 0) {
@@ -639,7 +678,7 @@ void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
 	// Deallocate temporary array.
 	// (Thanks to Brad Spencer for noticing my accidental omission of this!)
 	delete [] work2;
-	
+
 }
 /*
 * The out-of-bounds accesses story:
@@ -684,9 +723,7 @@ void BigUnsigned::divideWithRemainder(const BigUnsigned &b, BigUnsigned &q) {
 
 // Bitwise and
 void BigUnsigned::bitAnd(const BigUnsigned &a, const BigUnsigned &b) {
-	// Block unsafe calls
-	if (this == &a || this == &b)
-		throw "BigUnsigned::bitAnd: One of the arguments is the invoked object";
+	DTRT_ALIASED(this == &a || this == &b, bitAnd(a, b));
 	len = (a.len >= b.len) ? b.len : a.len;
 	allocate(len);
 	Index i;
@@ -697,9 +734,7 @@ void BigUnsigned::bitAnd(const BigUnsigned &a, const BigUnsigned &b) {
 
 // Bitwise or
 void BigUnsigned::bitOr(const BigUnsigned &a, const BigUnsigned &b) {
-	// Block unsafe calls
-	if (this == &a || this == &b)
-		throw "BigUnsigned::bitOr: One of the arguments is the invoked object";
+	DTRT_ALIASED(this == &a || this == &b, bitOr(a, b));
 	Index i;
 	const BigUnsigned *a2, *b2;
 	if (a.len >= b.len) {
@@ -719,9 +754,7 @@ void BigUnsigned::bitOr(const BigUnsigned &a, const BigUnsigned &b) {
 
 // Bitwise xor
 void BigUnsigned::bitXor(const BigUnsigned &a, const BigUnsigned &b) {
-	// Block unsafe calls
-	if (this == &a || this == &b)
-		throw "BigUnsigned::bitXor: One of the arguments is the invoked object";
+	DTRT_ALIASED(this == &a || this == &b, bitXor(a, b));
 	Index i;
 	const BigUnsigned *a2, *b2;
 	if (a.len >= b.len) {
@@ -740,6 +773,51 @@ void BigUnsigned::bitXor(const BigUnsigned &a, const BigUnsigned &b) {
 	zapLeadingZeros();
 }
 
+// Bitwise shift left
+void BigUnsigned::bitShiftLeft(const BigUnsigned &a, unsigned int b) {
+	DTRT_ALIASED(this == &a, bitShiftLeft(a, b));
+	Index shiftBlocks = b / N;
+	unsigned int shiftBits = b % N;
+	// + 1: room for high bits nudged left into another block
+	len = a.len + shiftBlocks + 1;
+	allocate(len);
+	Index i, j;
+	for (i = 0; i < shiftBlocks; i++)
+		blk[i] = 0;
+	for (j = 0, i = shiftBlocks; j <= a.len; j++, i++)
+		blk[i] = getShiftedBlock(a, j, shiftBits);
+	// Zap possible leading zero
+	if (blk[len - 1] == 0)
+		len--;
+}
+
+// Bitwise shift right
+void BigUnsigned::bitShiftRight(const BigUnsigned &a, unsigned int b) {
+	DTRT_ALIASED(this == &a, bitShiftRight(a, b));
+	// This calculation is wacky, but expressing the shift as a left bit shift
+	// within each block lets us use getShiftedBlock.
+	Index rightShiftBlocks = (b + N - 1) / N;
+	unsigned int leftShiftBits = N * rightShiftBlocks - b;
+	// Now (N * rightShiftBlocks - leftShiftBits) == b
+	// and 0 <= leftShiftBits < N.
+	if (rightShiftBlocks >= a.len + 1) {
+		// All of a is guaranteed to be shifted off, even considering the left
+		// bit shift.
+		len = 0;
+		return;
+	}
+	// Now we're allocating a positive amount.
+	// + 1: room for high bits nudged left into another block
+	len = a.len + 1 - rightShiftBlocks;
+	allocate(len);
+	Index i, j;
+	for (j = rightShiftBlocks, i = 0; j <= a.len; j++, i++)
+		blk[i] = getShiftedBlock(a, j, leftShiftBits);
+	// Zap possible leading zero
+	if (blk[len - 1] == 0)
+		len--;
+}
+
 // INCREMENT/DECREMENT OPERATORS
 
 // Prefix increment