Patch: BigInteger fully implemented
Warren Levy
warrenl@cygnus.com
Mon Feb 14 02:22:00 GMT 2000
Folks,
I'm checking in the following patch. It includes implementations for the
remainder of java.math.BigInteger and an improved way of dealing with the
BigInteger.modPow method. Enjoy!
--warrenl
2000-02-14 Warren Levy <warrenl@cygnus.com>
* gnu/gcj/math/MPN.java(findLowestBit): Made methods public.
* java/math/BigInteger.java(BigInteger(int,int,java.util.Random):
New constructor.
(min): Implemented.
(max): Implemented.
(modPow): Rewritten to not use the naive, slow, brute force approach.
(isProbablePrime): Implemented.
(testBit): Implemented.
(flipBit): Implemented.
(getLowestSetBit): Implemented.
Index: gnu/gcj/math/MPN.java
===================================================================
RCS file: /cvs/java/libgcj/libjava/gnu/gcj/math/MPN.java,v
retrieving revision 1.1
diff -u -p -r1.1 MPN.java
--- MPN.java 2000/02/04 22:00:36 1.1
+++ MPN.java 2000/02/14 10:04:23
@@ -571,7 +571,7 @@ public class MPN
/** Return least i such that word&(1<<i). Assumes word!=0. */
- static int findLowestBit (int word)
+ public static int findLowestBit (int word)
{
int i = 0;
while ((word & 0xF) == 0)
@@ -591,7 +591,7 @@ public class MPN
/** Return least i such that words & (1<<i). Assumes there is such an i. */
- static int findLowestBit (int[] words)
+ public static int findLowestBit (int[] words)
{
for (int i = 0; ; i++)
{
Index: java/math/BigInteger.java
===================================================================
RCS file: /cvs/java/libgcj/libjava/java/math/BigInteger.java,v
retrieving revision 1.2
diff -u -p -r1.2 BigInteger.java
--- BigInteger.java 2000/02/11 19:09:03 1.2
+++ BigInteger.java 2000/02/14 10:04:25
@@ -19,7 +19,9 @@ import java.util.Random;
/**
* Written using on-line Java Platform 1.2 API Specification, as well
- * as "The Java Class Libraries", 2nd edition (Addison-Wesley, 1998).
+ * as "The Java Class Libraries", 2nd edition (Addison-Wesley, 1998) and
+ * "Applied Cryptography, Second Edition" by Bruce Schneier (Wiley, 1996).
+
*
* Based primarily on IntNum.java BitOps.java by Per Bothner <per@bothner.com>
* (found in Kawa 1.6.62).
@@ -60,6 +62,15 @@ public class BigInteger extends Number i
private static final int TRUNCATE = 3;
private static final int ROUND = 4;
+ /** When checking the probability of primes, it is most efficient to
+ * first check the factoring of small primes, so we'll use this array.
+ */
+ private static final int[] primes =
+ { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
+ 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107,
+ 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181,
+ 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251 };
+
private BigInteger()
{
}
@@ -138,6 +149,21 @@ public class BigInteger extends Number i
this.words = result.words;
}
+ public BigInteger(int bitLength, int certainty, Random rnd)
+ {
+ this(bitLength, rnd);
+
+ // Keep going until we find a probable prime.
+ while (true)
+ {
+ if (isProbablePrime(certainty))
+ return;
+
+ BigInteger next = new BigInteger(bitLength, rnd);
+ this.ival = next.ival;
+ this.words = next.words;
+ }
+ }
/** Return a (possibly-shared) BigInteger with a given long value. */
private static BigInteger make(long value)
@@ -290,6 +316,16 @@ public class BigInteger extends Number i
return compareTo(this, val);
}
+ public BigInteger min(BigInteger val)
+ {
+ return compareTo(this, val) < 0 ? this : val;
+ }
+
+ public BigInteger max(BigInteger val)
+ {
+ return compareTo(this, val) > 0 ? this : val;
+ }
+
private final boolean isOdd()
{
int low = words == null ? ival : words[0];
@@ -1120,8 +1156,30 @@ public class BigInteger extends Number i
return modInverse(m);
if (exponent.isOne())
return mod(m);
+
+ // To do this naively by first raising this to the power of exponent
+ // and then performing modulo m would be extremely expensive, especially
+ // for very large numbers. The solution is found in Number Theory
+ // where a combination of partial powers and modulos can be done easily.
+ //
+ // We'll use the algorithm for Additive Chaining which can be found on
+ // p. 244 of "Applied Cryptography, Second Edition" by Bruce Schneier.
+ BigInteger s, t, u;
+ int i;
- return pow(exponent).mod(m);
+ s = ONE;
+ t = this;
+ u = exponent;
+
+ while (!u.isZero())
+ {
+ if (u.and(ONE).isOne())
+ s = times(s, t).mod(m);
+ u = u.shiftRight(1);
+ t = times(t, t).mod(m);
+ }
+
+ return s;
}
/** Calculate Greatest Common Divisor for non-negative ints. */
@@ -1184,6 +1242,72 @@ public class BigInteger extends Number i
return result.canonicalize();
}
+ public boolean isProbablePrime(int certainty)
+ {
+ /** We'll use the Rabin-Miller algorithm for doing a probabilistic
+ * primality test. It is fast, easy and has faster decreasing odds of a
+ * composite passing than with other tests. This means that this
+ * method will actually have a probability much greater than the
+ * 1 - .5^certainty specified in the JCL (p. 117), but I don't think
+ * anyone will complain about better performance with greater certainty.
+ *
+ * The Rabin-Miller algorithm can be found on pp. 259-261 of "Applied
+ * Cryptography, Second Edition" by Bruce Schneier.
+ */
+
+ // First rule out small prime factors and assure the number is odd.
+ for (int i = 0; i < primes.length; i++)
+ {
+ if (words == null && ival == primes[i])
+ return true;
+ if (remainder(make(primes[i])).isZero())
+ return false;
+ }
+
+ // Now perform the Rabin-Miller test.
+ // NB: I know that this can be simplified programatically, but
+ // I have tried to keep it as close as possible to the algorithm
+ // as written in the Schneier book for reference purposes.
+
+ // Set b to the number of times 2 evenly divides (this - 1).
+ // I.e. 2^b is the largest power of 2 that divides (this - 1).
+ BigInteger pMinus1 = add(this, -1);
+ int b = pMinus1.getLowestSetBit();
+
+ // Set m such that this = 1 + 2^b * m.
+ BigInteger m = pMinus1.divide(make(2L << b - 1));
+
+ Random rand = new Random();
+ while (certainty-- > 0)
+ {
+ // Pick a random number greater than 1 and less than this.
+ // The algorithm says to pick a small number to make the calculations
+ // go faster, but it doesn't say how small; we'll use 2 to 1024.
+ int a = rand.nextInt();
+ a = (a < 0 ? -a : a) % 1023 + 2;
+
+ BigInteger z = make(a).modPow(m, this);
+ if (z.isOne() || z.equals(pMinus1))
+ continue; // Passes the test; may be prime.
+
+ int i;
+ for (i = 0; i < b; )
+ {
+ if (z.isOne())
+ return false;
+ i++;
+ if (z.equals(pMinus1))
+ break; // Passes the test; may be prime.
+
+ z = z.modPow(make(2), this);
+ }
+
+ if (i == b && !z.equals(pMinus1))
+ return false;
+ }
+ return true;
+ }
+
private void setInvert()
{
if (words == null)
@@ -1727,7 +1851,7 @@ public class BigInteger extends Number i
case 1: return x.and(y);
case 3: return x;
case 5: return y;
- case 15: return smallFixNums[-1 - minFixNum]; // Returns -1.
+ case 15: return make(-1);
}
BigInteger result = new BigInteger();
setBitOp(result, op, x, y);
@@ -1998,6 +2122,33 @@ public class BigInteger extends Number i
return or(ONE.shiftLeft(n));
}
+ public boolean testBit(int n)
+ {
+ if (n < 0)
+ throw new ArithmeticException();
+
+ return !and(ONE.shiftLeft(n)).isZero();
+ }
+
+ public BigInteger flipBit(int n)
+ {
+ if (n < 0)
+ throw new ArithmeticException();
+
+ return xor(ONE.shiftLeft(n));
+ }
+
+ public int getLowestSetBit()
+ {
+ if (isZero())
+ return -1;
+
+ if (words == null)
+ return MPN.findLowestBit(ival);
+ else
+ return MPN.findLowestBit(words);
+ }
+
// bit4count[I] is number of '1' bits in I.
private static final byte[] bit4_count = { 0, 1, 1, 2, 1, 2, 2, 3,
1, 2, 2, 3, 2, 3, 3, 4};
@@ -2039,21 +2190,4 @@ public class BigInteger extends Number i
}
return isNegative() ? x_len * 32 - i : i;
}
-
-/* TODO:
-
- public BigInteger(int bitLength, int certainty, Random rnd)
-
- public boolean testBit(int n)
-
- public BigInteger flipBit(int n)
-
- public int getLowestSetBit()
-
- public boolean isProbablePrime(int certainty)
-
- public BigInteger min(BigInteger val)
-
- public BigInteger max(BigInteger val)
-*/
}
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