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Merged
merged 2 commits into from
May 10, 2019
Merged

Added FastPower #731

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7216f28
Added FastPower
DDullahan Apr 8, 2019
c85caf5
fixed problems of code review from @havanagrawal
DDullahan May 3, 2019
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37 changes: 37 additions & 0 deletions src/main/java/com/Others/FastPower.java
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Original file line number Diff line number Diff line change
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package src.main.java.com.Others;

import java.math.BigInteger;

/**
* We may calculate power with loops, but what if the index is too large ?
* FastPower aims to calculate quickly in this circumstances with time complexity O(log k),
* where k is the index.
*
* @author DDullahan
*/
public class FastPower {
public static BigInteger calculate(BigInteger n, BigInteger k, BigInteger mod) {
BigInteger ans = BigInteger.ONE;
while (!k.equals(BigInteger.ZERO)) {
int odd = k.mod(BigInteger.valueOf(2)).compareTo(BigInteger.ZERO);
if(odd > 0){
ans = ans.multiply(n).mod(mod);
}
k = k.divide(BigInteger.valueOf(2));
n = n.multiply(n).mod(mod);
}
return ans.mod(mod);
}

public static long calculate(long n, long k, long mod) {
long ans = 1;
while (k != 0) {
if (k % 2 == 1) {
ans = (ans * n) % mod;
}
k >>= 1;
n = (n * n) % mod;
}
return ans % mod;
}
}
32 changes: 32 additions & 0 deletions src/main/java/com/Others/FastPowerTest.java
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package src.main.java.com.Others;

import org.junit.*;

import java.math.BigInteger;

import static org.junit.Assert.*;

public class FastPowerTest {

void testLong(long n, long k, long m){
long result = FastPower.calculate(n,k,m);
assertEquals(result, BigInteger.valueOf(n).modPow(BigInteger.valueOf(k), BigInteger.valueOf(m)).longValue());
}

void testBigInteger(BigInteger n, BigInteger k, BigInteger m){
BigInteger result = FastPower.calculate(n,k,m);
assertEquals(result, n.modPow(k,m));
}

@Test
public void test() {
testLong(2,2,10);
testLong(100,1000,20);
testLong(123456,123456789,234);

testBigInteger(BigInteger.TEN,BigInteger.TEN, BigInteger.valueOf(4));
testBigInteger(new BigInteger("123456"), new BigInteger("123456789"), new BigInteger("234"));
testBigInteger(new BigInteger("123456789101112"), new BigInteger("12345678910111213"), new BigInteger("567890"));

}
}