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8029501: BigInteger division algorithm selection heuristic is incorrect

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Change Burnikel-Ziegler division heuristic to require that the dividend int-length exceed that of the divisor by a minimum amount.

Reviewed-by: darcy
This commit is contained in:
Brian Burkhalter 2013-12-05 07:45:27 -08:00
parent 9fab2b1e9a
commit efe4e2425f
2 changed files with 19 additions and 11 deletions
jdk/src/share/classes/java/math
BigInteger.javaMutableBigInteger.java

26
jdk/src/share/classes/java/math/BigInteger.java

@ -244,12 +244,20 @@ public class BigInteger extends Number implements Comparable<BigInteger> {
/**
* The threshold value for using Burnikel-Ziegler division. If the number
* of ints in the number are larger than this value,
* Burnikel-Ziegler division will be used. This value is found
* experimentally to work well.
* of ints in the divisor are larger than this value, Burnikel-Ziegler
* division may be used. This value is found experimentally to work well.
*/
static final int BURNIKEL_ZIEGLER_THRESHOLD = 80;
/**
* The offset value for using Burnikel-Ziegler division. If the number
* of ints in the divisor exceeds the Burnikel-Ziegler threshold, and the
* number of ints in the dividend is greater than the number of ints in the
* divisor plus this value, Burnikel-Ziegler division will be used. This
* value is found experimentally to work well.
*/
static final int BURNIKEL_ZIEGLER_OFFSET = 40;
/**
* The threshold value for using Schoenhage recursive base conversion. If
* the number of ints in the number are larger than this value,
@ -2017,8 +2025,8 @@ public class BigInteger extends Number implements Comparable<BigInteger> {
* @throws ArithmeticException if {@code val} is zero.
*/
public BigInteger divide(BigInteger val) {
if (mag.length < BURNIKEL_ZIEGLER_THRESHOLD ||
val.mag.length < BURNIKEL_ZIEGLER_THRESHOLD) {
if (val.mag.length < BURNIKEL_ZIEGLER_THRESHOLD ||
mag.length - val.mag.length < BURNIKEL_ZIEGLER_OFFSET) {
return divideKnuth(val);
} else {
return divideBurnikelZiegler(val);
@ -2054,8 +2062,8 @@ public class BigInteger extends Number implements Comparable<BigInteger> {
* @throws ArithmeticException if {@code val} is zero.
*/
public BigInteger[] divideAndRemainder(BigInteger val) {
if (mag.length < BURNIKEL_ZIEGLER_THRESHOLD ||
val.mag.length < BURNIKEL_ZIEGLER_THRESHOLD) {
if (val.mag.length < BURNIKEL_ZIEGLER_THRESHOLD ||
mag.length - val.mag.length < BURNIKEL_ZIEGLER_OFFSET) {
return divideAndRemainderKnuth(val);
} else {
return divideAndRemainderBurnikelZiegler(val);
@ -2083,8 +2091,8 @@ public class BigInteger extends Number implements Comparable<BigInteger> {
* @throws ArithmeticException if {@code val} is zero.
*/
public BigInteger remainder(BigInteger val) {
if (mag.length < BURNIKEL_ZIEGLER_THRESHOLD ||
val.mag.length < BURNIKEL_ZIEGLER_THRESHOLD) {
if (val.mag.length < BURNIKEL_ZIEGLER_THRESHOLD ||
mag.length - val.mag.length < BURNIKEL_ZIEGLER_OFFSET) {
return remainderKnuth(val);
} else {
return remainderBurnikelZiegler(val);

4
jdk/src/share/classes/java/math/MutableBigInteger.java

@ -1148,8 +1148,8 @@ class MutableBigInteger {
}
MutableBigInteger divide(MutableBigInteger b, MutableBigInteger quotient, boolean needRemainder) {
if (intLen < BigInteger.BURNIKEL_ZIEGLER_THRESHOLD ||
b.intLen < BigInteger.BURNIKEL_ZIEGLER_THRESHOLD) {
if (b.intLen < BigInteger.BURNIKEL_ZIEGLER_THRESHOLD ||
intLen - b.intLen < BigInteger.BURNIKEL_ZIEGLER_OFFSET) {
return divideKnuth(b, quotient, needRemainder);
} else {
return divideAndRemainderBurnikelZiegler(b, quotient);