Implementation of JAVA-compatible hashCode algorithm code sharing in javascript_Basic knowledge

In Java, a hashCode algorithm can be used to calculate the hash value of a string. Today, a friend suddenly asked me if I can calculate hashCode in js. The calculation result is required to be the same as Java's hashCode calculation.

As for Java’s hashCode, I have never understood its algorithm before, but I guess it shouldn’t be too difficult, so now I wrote this code in Java for testing:
Running result: 899755

Press the Ctrl key and click on the hashCode method name to follow up and take a look at the algorithm. I found that it is a very simple code, as shown below:

Copy code The code is as follows:

public int hashCode() {
int h = hash;
if (h == 0) {
int off = offset;
char val[] = value;
int len ​​= count;

for (int i = 0; i < len; i ) {
h = 31*h val[off];
}
hash = h;
}
return h;
}

Okay now, simply transplant it into js and it should be ok. So I wrote the following JS code:

Copy code The code is as follows:

Running result: 899755

OK, the result is the same as the java calculation. I thought that was done, and then I thought about finding a random string to test:

"Shenyang Shenyang", the running result in JAVA is: 1062711668, but in js it becomes: 26832515444.

I’m so dizzy, there’s something wrong with just trying this! After thinking for a while, I suddenly thought that the length of int in Java seems to be about 2.1 billion, but there is no such limit in js. The problem should be here, so I made a little modification to the previous method:

Copy code The code is as follows:

<script><br> function hashCode(str){<br> var h = 0, off = 0;<br>         var len = str.length;<br> for(var i = 0; i < len; i ){<br />                h = 31 * h str.charCodeAt(off);<br />          }<br /> var t=-2147483648*2;<br /> While(h>2147483647){<br> h =t<br> }<br>           return h;<br> }<br> alert(hashCode('Shenyang Shenyang'));</script>

Test again! OK! You're done. There is no technical content, just a brief summary
Updated on 2013-02-19, the above one is relatively inefficient and will crash when the content is very long. The following code is the optimized code:

Copy code The code is as follows:

<script><br> Function hashCode(str) {<br> var h = 0;<br> var len = str.length;<br> var t = 2147483648;<br> for (var i = 0; i < len; i ) {<br />               h = 31 * h str.charCodeAt(i);<br /> If(h > 2147483647) h %= t;//If java int overflows, take the modulo<br> }<br> /*var t = -2147483648 * 2;<br> ​​​​while (h > 2147483647) {<br>              h = t<br>         }*/<br>          return h;<br> }<br> alert(hashCode('C# How to implement N threads executing concurrently at the same time and the rest in the queue')); //1107373715<br> </script>