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42 | daniel-mar | 1 | package de.viathinksoft.immortal.bases; |
2 | /** |
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3 | * Powmod.java |
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4 | * |
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5 | * Implementation von a^b mod m durch iteriertes Quadrieren. Zum Testen: Der |
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6 | * Aufruf java powmod a b m gibt a^b mod m aus. |
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7 | * |
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8 | * @author Thorsten Altenkirch |
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9 | * @version Modified by Daniel Marschall |
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10 | * |
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11 | */ |
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12 | |||
13 | public class PowMod { |
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14 | |||
15 | /** |
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16 | * Ermittelt das nte Bit von l |
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17 | */ |
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18 | private static boolean bit(long l, int n) { |
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19 | return (((l >> n) & 1) != 0); |
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20 | } |
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21 | |||
22 | /** |
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23 | * Berechnet a^b mod m durch iteriertes Quadrieren. |
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24 | */ |
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25 | public static long powmod(long a, long m, long n) { |
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26 | long y = 1; |
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27 | for (int i = 62; i >= 0; i--) { |
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28 | y = y * y % n; |
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29 | if (bit(m, i)) { |
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30 | y = a * y % n; |
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31 | } |
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32 | } |
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33 | return y; |
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34 | } |
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35 | |||
36 | private PowMod() { |
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37 | } |
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38 | } |