213 lines
7.4 KiB
Java
213 lines
7.4 KiB
Java
/*
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* Copyright (c) 1999, 2007, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.math;
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/**
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* A simple bit sieve used for finding prime number candidates. Allows setting
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* and clearing of bits in a storage array. The size of the sieve is assumed to
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* be constant to reduce overhead. All the bits of a new bitSieve are zero, and
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* bits are removed from it by setting them.
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*
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* To reduce storage space and increase efficiency, no even numbers are
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* represented in the sieve (each bit in the sieve represents an odd number).
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* The relationship between the index of a bit and the number it represents is
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* given by
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* N = offset + (2*index + 1);
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* Where N is the integer represented by a bit in the sieve, offset is some
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* even integer offset indicating where the sieve begins, and index is the
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* index of a bit in the sieve array.
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*
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* @see BigInteger
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* @author Michael McCloskey
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* @since 1.3
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*/
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class BitSieve {
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/**
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* Stores the bits in this bitSieve.
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*/
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private long bits[];
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/**
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* Length is how many bits this sieve holds.
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*/
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private int length;
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/**
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* A small sieve used to filter out multiples of small primes in a search
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* sieve.
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*/
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private static BitSieve smallSieve = new BitSieve();
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/**
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* Construct a "small sieve" with a base of 0. This constructor is
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* used internally to generate the set of "small primes" whose multiples
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* are excluded from sieves generated by the main (package private)
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* constructor, BitSieve(BigInteger base, int searchLen). The length
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* of the sieve generated by this constructor was chosen for performance;
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* it controls a tradeoff between how much time is spent constructing
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* other sieves, and how much time is wasted testing composite candidates
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* for primality. The length was chosen experimentally to yield good
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* performance.
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*/
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private BitSieve() {
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length = 150 * 64;
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bits = new long[(unitIndex(length - 1) + 1)];
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// Mark 1 as composite
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set(0);
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int nextIndex = 1;
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int nextPrime = 3;
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// Find primes and remove their multiples from sieve
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do {
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sieveSingle(length, nextIndex + nextPrime, nextPrime);
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nextIndex = sieveSearch(length, nextIndex + 1);
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nextPrime = 2*nextIndex + 1;
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} while((nextIndex > 0) && (nextPrime < length));
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}
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/**
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* Construct a bit sieve of searchLen bits used for finding prime number
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* candidates. The new sieve begins at the specified base, which must
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* be even.
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*/
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BitSieve(BigInteger base, int searchLen) {
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/*
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* Candidates are indicated by clear bits in the sieve. As a candidates
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* nonprimality is calculated, a bit is set in the sieve to eliminate
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* it. To reduce storage space and increase efficiency, no even numbers
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* are represented in the sieve (each bit in the sieve represents an
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* odd number).
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*/
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bits = new long[(unitIndex(searchLen-1) + 1)];
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length = searchLen;
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int start = 0;
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int step = smallSieve.sieveSearch(smallSieve.length, start);
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int convertedStep = (step *2) + 1;
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// Construct the large sieve at an even offset specified by base
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MutableBigInteger b = new MutableBigInteger(base);
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MutableBigInteger q = new MutableBigInteger();
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do {
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// Calculate base mod convertedStep
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start = b.divideOneWord(convertedStep, q);
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// Take each multiple of step out of sieve
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start = convertedStep - start;
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if (start%2 == 0)
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start += convertedStep;
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sieveSingle(searchLen, (start-1)/2, convertedStep);
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// Find next prime from small sieve
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step = smallSieve.sieveSearch(smallSieve.length, step+1);
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convertedStep = (step *2) + 1;
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} while (step > 0);
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}
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/**
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* Given a bit index return unit index containing it.
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*/
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private static int unitIndex(int bitIndex) {
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return bitIndex >>> 6;
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}
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/**
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* Return a unit that masks the specified bit in its unit.
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*/
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private static long bit(int bitIndex) {
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return 1L << (bitIndex & ((1<<6) - 1));
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}
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/**
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* Get the value of the bit at the specified index.
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*/
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private boolean get(int bitIndex) {
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int unitIndex = unitIndex(bitIndex);
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return ((bits[unitIndex] & bit(bitIndex)) != 0);
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}
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/**
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* Set the bit at the specified index.
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*/
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private void set(int bitIndex) {
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int unitIndex = unitIndex(bitIndex);
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bits[unitIndex] |= bit(bitIndex);
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}
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/**
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* This method returns the index of the first clear bit in the search
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* array that occurs at or after start. It will not search past the
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* specified limit. It returns -1 if there is no such clear bit.
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*/
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private int sieveSearch(int limit, int start) {
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if (start >= limit)
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return -1;
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int index = start;
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do {
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if (!get(index))
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return index;
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index++;
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} while(index < limit-1);
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return -1;
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}
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/**
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* Sieve a single set of multiples out of the sieve. Begin to remove
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* multiples of the specified step starting at the specified start index,
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* up to the specified limit.
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*/
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private void sieveSingle(int limit, int start, int step) {
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while(start < limit) {
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set(start);
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start += step;
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}
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}
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/**
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* Test probable primes in the sieve and return successful candidates.
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*/
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BigInteger retrieve(BigInteger initValue, int certainty, java.util.Random random) {
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// Examine the sieve one long at a time to find possible primes
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int offset = 1;
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for (int i=0; i<bits.length; i++) {
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long nextLong = ~bits[i];
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for (int j=0; j<64; j++) {
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if ((nextLong & 1) == 1) {
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BigInteger candidate = initValue.add(
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BigInteger.valueOf(offset));
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if (candidate.primeToCertainty(certainty, random))
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return candidate;
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}
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nextLong >>>= 1;
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offset+=2;
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}
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}
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return null;
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}
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}
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