Hasan Hüseyin Yücel - 1 year ago 38

Java Question

*Develop a hash function to generate an index value between 0-4999 inclusive for a given traffic license number. Your hash function should generate as few as possible collisions. Hash function should use the properties of license numbers. Hash method should take the license number as a single String and return an index value. We assume that the license numbers to be in the following format: City code is a number between 10 and 99 inclusive. Three letters are any letter combination from English alphabet with 26 chars. Two digits number is a number between 10 and 99 inclusive.*

I wrote something about this question but, collisions are a lot (1800 for 5k)

`static long printValue(String s) {`

long result = 0;

for (int i = 0; i < s.length(); i++) {

result += Math.pow(27, MAX_LENGTH - i - 1) * (1 + s.charAt(i) - 'A');

}

result = result % 5009;

return (int) result;

}

public int hashF(String str) {

String a = str.substring(0, 2);

String b = str.substring(5, 7);

String middle = str.substring(2, 5);

int q = (int) printValue(middle);

String last = a + q + b;

int index = Integer.parseInt(last);

index = index % 5009;

return index;

}

Link for orjinal file of licence numbers.

These are some examples from file of traffic licence number. Collisions must be 300 (maximum).

65HNM25

93DTV23

94WPX23

31RKK46

15YXX90

31MDV74

45BOG99

65JRM50

77VXR55

39TKY41

80MJU73

63QYE57

38FCO80

45ORI16

17CHN73

70SXR63

87CVM74

27EEE85

32PFJ91

50PBA66

70TVK72

15YLS20

80MPM74

21ZRN20

36VVE84

58IDW24

77VDC89

19BVK93

28SUF63

Answer

Your problem is not your code, but mathematics. Even a (perfect for you, but not very useful) hash code that produces consecutive hashes that are then mod 5000, ie

```
10AAA10 -> 0
10AAA11 -> 1
... etc
99ZZZ99 -> 600 (90 * 26 * 26 * 26 * 90) % 5000
```

will statistically produce over 1800 collisions and is no better than the simplest implementation, which is to use String's hashCode:

```
int hash = Math.abs(number.hashCode() % 5000);
```

It's a silly exercise, as it has no real world use.