Hive的内置HASH()函数使用了什么样的散列算法 [英] What kind of hash algorithm is used for Hive's built-in HASH() Function
问题描述
理想情况下,我正在寻找SHA512 / SHA256哈希,类似于什么样的哈希算法sha()函数在linkedin中为Pig提供了datafu UDF。
HASH
函数(自Hive 0.11开始)使用类似于 $ b
其代码如下所示:
int hashCode = 0; // Hive HASH使用0作为种子,List#hashCode使用1.我不知道为什么。
for(Object item:items){
hashCode = hashCode * 31 +(item == null?0:item.hashCode());
$ b基本上它是一本经典的散列算法,正如Effective Java一书中推荐的那样。
引用一个伟大的人(和一个伟大的书):
选择值31是因为它是一个奇素数。如果它甚至是
并且乘法溢出,则信息将会丢失,因为
乘以2相当于移位。使用
作为素数的优点不太清楚,但它是传统的。 31的一个不错的属性是
,为了获得更好的性能,可以用移位和减法
来代替乘法:31 * i ==(i <5) - i。现代虚拟机会自动执行这种
的优化。
我离题了。您可以查看 HASH
来源这里。
如果您想在Hive中使用SHAxxx,那么您可以使用Apache DigestUtils 类和Hive内置反映
函数(我希望这会起作用):
SELECT反射('org.apache.commons.codec.digest.DigestUtils','sha256Hex','your_string')
What kind of hashing algorithm is used in the built-in HASH() function?
I'm ideally looking for a SHA512/SHA256 hash, similar to what the SHA() function offers within the linkedin datafu UDFs for Pig.
解决方案 HASH
function (as of Hive 0.11) uses algorithm similar to java.util.List#hashCode.
Its code looks like this:
int hashCode = 0; // Hive HASH uses 0 as the seed, List#hashCode uses 1. I don't know why.
for (Object item: items) {
hashCode = hashCode * 31 + (item == null ? 0 : item.hashCode());
}
Basically it's a classic hash algorithm as recommended in the book Effective Java.
To quote a great man (and a great book):
The value 31 was chosen because it is an odd prime. If it were even
and the multiplication overflowed, information would be lost, as
multiplication by 2 is equivalent to shifting. The advantage of using
a prime is less clear, but it is traditional. A nice property of 31 is
that the multiplication can be replaced by a shift and a subtraction
for better performance: 31 * i == (i << 5) - i. Modern VMs do this
sort of optimization automatically.
I digress. You can look at the HASH
source here.
If you want to use SHAxxx in Hive then you can use Apache DigestUtils class and Hive built-in reflect
function (I hope that'll work):
SELECT reflect('org.apache.commons.codec.digest.DigestUtils', 'sha256Hex', 'your_string')
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