44 lines
1.4 KiB
Java
44 lines
1.4 KiB
Java
private static final BigInteger TWO = BigInteger.valueOf(2);
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private static final BigInteger THREE = BigInteger.valueOf(3);
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private static final BigInteger FIVE = BigInteger.valueOf(5);
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public static ArrayList<BigInteger> primeDecomp(BigInteger n){
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if(n.compareTo(TWO) < 0) return null;
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ArrayList<BigInteger> factors = new ArrayList<BigInteger>();
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// handle even values
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while(n.and(BigInteger.ONE).equals(BigInteger.ZERO)){
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n = n.shiftRight(1);
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factors.add(TWO);
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}
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// handle values divisible by three
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while(n.mod(THREE).equals(BigInteger.ZERO)){
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factors.add(THREE);
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n = n.divide(THREE);
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}
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// handle values divisible by five
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while(n.mod(FIVE).equals(BigInteger.ZERO)){
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factors.add(FIVE);
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n = n.divide(FIVE);
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}
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// much like how we can skip multiples of two, we can also skip
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// multiples of three and multiples of five. This increment array
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// helps us to accomplish that
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int[] pattern = {4,2,4,2,4,6,2,6};
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int pattern_index = 0;
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BigInteger current_test = BigInteger.valueOf(7);
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while(!n.equals(BigInteger.ONE)){
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while(n.mod(current_test).equals(BigInteger.ZERO)){
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factors.add(current_test);
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n = n.divide(current_test);
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}
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current_test = current_test.add(BigInteger.valueOf(pattern[pattern_index]));
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pattern_index = (pattern_index + 1) & 7;
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}
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return factors;
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}
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