## Notes

Cyclic permutation is a method for generating hash functions from data.

The basic idea is to generate a binary representation of the data (typically stored in a 32-bit integer, so wrapped into 32 bits) with a series of 32 0s and 1s, then to cyclically shift the digits of this binary number. If the data are the same, they should result in the same binary number, which will be shifted into the same resulting number when the hash code is computed. Collisions will happen if two objects end up having the same binary representation.

For example, to cyclically shift the digits of 10001111 2 digits to the right, the number would become 111000011.

A simple implementation of the cyclic permutation hash code calculation in Java is given as a static method below:

```	public static int hashCode(String s) {
int shift = 5;
int h = 0;
for(int i=0; i<s.length(); i++) {
h = (h<<(shift)) | (h>>>(32-shift));
h += (int)(s.charAt(i));
}
return h;
}
```

The value of the shift can be anywhere from 0 (in which case, an object's hash code is simply its binary representation) to 31 (if the shift is 32, you're back to 0). The actual frequency of collisions depends heavily on this shift value. For example, here is the output of a program that uses a collection of about 7,000 words form James Joyce's Ulysses and computes a hash code for each word, counting the number of collisions:

```\$ javac HashFunctions.java && java HashFunctions
Shift: 0 	Hash codes: 7313		Collisions: 6075
Shift: 1 	Hash codes: 7313		Collisions: 1678
Shift: 2 	Hash codes: 7313		Collisions: 338
Shift: 3 	Hash codes: 7313		Collisions: 81
Shift: 4 	Hash codes: 7313		Collisions: 13
Shift: 5 	Hash codes: 7313		Collisions: 2
Shift: 6 	Hash codes: 7313		Collisions: 1
Shift: 7 	Hash codes: 7313		Collisions: 2
Shift: 8 	Hash codes: 7313		Collisions: 8
Shift: 9 	Hash codes: 7313		Collisions: 2
Shift: 10 	Hash codes: 7313		Collisions: 27
Shift: 11 	Hash codes: 7313		Collisions: 60
Shift: 12 	Hash codes: 7313		Collisions: 5
Shift: 13 	Hash codes: 7313		Collisions: 2
Shift: 14 	Hash codes: 7313		Collisions: 17
Shift: 15 	Hash codes: 7313		Collisions: 126
Shift: 16 	Hash codes: 7313		Collisions: 752
Shift: 17 	Hash codes: 7313		Collisions: 60
Shift: 18 	Hash codes: 7313		Collisions: 12
Shift: 19 	Hash codes: 7313		Collisions: 0
Shift: 20 	Hash codes: 7313		Collisions: 6
Shift: 21 	Hash codes: 7313		Collisions: 51
Shift: 22 	Hash codes: 7313		Collisions: 25
Shift: 23 	Hash codes: 7313		Collisions: 3
Shift: 24 	Hash codes: 7313		Collisions: 8
Shift: 25 	Hash codes: 7313		Collisions: 3
Shift: 26 	Hash codes: 7313		Collisions: 2
Shift: 27 	Hash codes: 7313		Collisions: 3
Shift: 28 	Hash codes: 7313		Collisions: 23
Shift: 29 	Hash codes: 7313		Collisions: 114
Shift: 30 	Hash codes: 7313		Collisions: 671
Shift: 31 	Hash codes: 7313		Collisions: 2826
```

As you can see, some keys result in just a few collisions, while others result in thousands.