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Ensuring that the hash is unpredictable and well distributed.

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We achieve this by using the same algorithm as the Perl version, but this implementation
is being written from scratch for license compliance.
This commit is contained in:
Inderjeet Singh 2012-03-15 22:27:55 +00:00
parent 14ebcc4ead
commit 5e3f5a6bbe
1 changed files with 19 additions and 6 deletions
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25
gson/src/main/java/com/google/gson/internal/StringMap.java
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@ -25,6 +25,7 @@ import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Random;
import java.util.Set;
/**
@ -481,12 +482,24 @@ public final class StringMap<V> extends AbstractMap<String, V> {
}
}
private static final int seed = new Random().nextInt();
private static int hash(String key) {
// TODO: use an unpredictable hash function
int h = 0;
for (int i = 0; i < key.length(); i++) {
h = 31 * h + key.charAt(i);
// Ensuring that the hash is unpredictable and well distributed.
//
// Finding unpredictable hash functions is a bit of a dark art as we need to balance
// good unpredictability (to avoid DoS) and good distribution (for performance).
//
// We achieve this by using the same algorithm as the Perl version, but this implementation
// is being written from scratch by inder who has never seen the
// Perl version (for license compliance).
//
// TODO: investigate http://code.google.com/p/cityhash/ and http://code.google.com/p/smhasher/
// both of which may have better distribution and/or unpredictability.
int h = seed;
for (int i = 0; i < key.length(); ++i) {
int h2 = h + key.charAt(i);
int h3 = h2 + h2 << 10; // h2 * 1024
h = h3 ^ (h3 >>> 6); // h3 / 64
}
/*
@ -496,4 +509,4 @@ public final class StringMap<V> extends AbstractMap<String, V> {
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
}
}