how can i generate a unique int from a unique string?

Question

I have an object with a String that holds a unique id . (such as \"ocx7gf\" or \"67hfs8\") I need to supply it an implementation of int hascode() which will be unique obvio

Answer 1

You can't get a unique integer from a String of unlimited length. There are 4 billionish (2^32) unique integers, but an almost infinite number of unique strings.

String.hashCode() will not give you unique integers, but it will do its best to give you differing results based on the input string.

EDIT

Your edited question says that String.hashCode() is not recommended. This is not true, it is recommended, unless you have some special reason not to use it. If you do have a special reason, please provide details.

Answer 2

Looks like you've got a base-36 number there (a-z + 0-9). Why not convert it to an int using Integer.parseInt(s, 36)? Obviously, if there are too many unique IDs, it won't fit into an int, but in that case you're out of luck with unique integers and will need to get by using String.hashCode(), which does its best to be close to unique.

Answer 3

Unless your strings are limited in some way or your integers hold more bits than the strings you're trying to convert, you cannot guarantee the uniqueness.

Let's say you have a 32 bit integer and a 64-character character set for your strings. That means six bits per character. That will allow you to store five characters into an integer. More than that and it won't fit.

Answer 4

One way to do it is assign each letter a value, and each place of the string it's own multiple ie a = 1, b = 2, and so on, then everything in the first digit (read left to right) would be multiplied by a prime number, the next the next prime number and so on, such that the final digit was multiplied by a prime larger than the number of possible subsets in that digit (26+1 for a space or 52+1 with capitols and so on for other supported characters). If the number is mapped back to the first digits (leftmost character) any number you generate from a unique string mapping back to 1 or 6 whatever the first letter will be, gives a unique value.

Dog might be 30,3(15),101(7) or 782, while God 33,3(15),101(4) or 482. More importantly than unique strings being generated they can be useful in generation if the original digit is kept, like 30(782) would be unique to some 12(782) for the purposes of differentiating like strings if you ever managed to go over the unique possibilities. Dog would always be Dog, but it would never be Cat or Mouse.

Answer 5

No, you don't need to have an implementation that returns a unique value, "obviously", as obviously the majority of implementations would be broken.

What you want to do, is to have a good spread across bits, especially for common values (if any values are more common than others). Barring special knowledge of your format, then just using the hashcode of the string itself would be best.

With special knowledge of the limits of your id format, it may be possible to customise and result in better performance, though false assumptions are more likely to make things worse than better.

Edit: On good spread of bits.

As stated here and in other answers, being completely unique is impossible and hash collisions are possible. Hash-using methods know this and can deal with it, but it does impact upon performance, so we want collisions to be rare.

Further, hashes are generally re-hashed so our 32-bit number may end up being reduced to e.g. one in the range 0 to 22, and we want as good a distribution within that as possible to.

We also want to balance this with not taking so long to compute our hash, that it becomes a bottleneck in itself. An imperfect balancing act.

A classic example of a bad hash method is one for a co-ordinate pair of X, Y ints that does:

return X ^ Y;

While this does a perfectly good job of returning 2^32 possible values out of the 4^32 possible inputs, in real world use it's quite common to have sets of coordinates where X and Y are equal ({0, 0}, {1, 1}, {2, 2} and so on) which all hash to zero, or matching pairs ({2,3} and {3, 2}) which will hash to the same number. We are likely better served by:

return ((X << 16) | (x >> 16)) ^ Y;

Now, there are just as many possible values for which this is dreadful than for the former, but it tends to serve better in real-world cases.

Of course, there is a different job if you are writing a general-purpose class (no idea what possible inputs there are) or have a better idea of the purpose at hand. For example, if I was using Date objects but knew that they would all be dates only (time part always midnight) and only within a few years of each other, then I might prefer a custom hash code that used only the day, month and lower-digits of the years, over the standard one. The writer of Date though can't work on such knowledge and has to try to cater for everyone.

Hence, If I for instance knew that a given string is always going to consist of 6 case-insensitive characters in the range [a-z] or [0-9] (which yours seem to, but it isn't clear from your question that it does) then I might use an algorithm that assigned a value from 0 to 35 (the 36 possible values for each character) to each character, and then walk through the string, each time multiplying the current value by 36 and adding the value of the next char.

Assuming a good spread in the ids, this would be the way to go, especially if I made the order such that the lower-significant digits in my hash matched the most frequently changing char in the id (if such a call could be made), hence surviving re-hashing to a smaller range well.

However, lacking such knowledge of the format for sure, I can't make that call with certainty, and I could well be making things worse (slower algorithm for little or even negative gain in hash quality).

One advantage you have is that since it's an ID in itself, then presumably no other non-equal object has the same ID, and hence no other properties need be examined. This doesn't always hold.

Answer 6

represent each string character by a five-digit binary digit, eg. a by 00001 b by 00010 etc. thus 32 combinations are possible, for example, cat might be written as 00100 00001 01100, then convert this binary into decimal, eg. this would be 4140, thus cat would be 4140, similarly, you can get cat back from 4140 by converting it to binary first and Map the five digit binary to string