How do sites like goo.gl or jsfiddle generate their URL codes?

Question

I would like to generate a code like goo.gl and jsfiddle websites (http://jsfiddle.net/XzKvP/).

I tried different things that give me too large of a guid,

Answer 1

You can think of the five-letter code as a number in base-62 notation: your "digits" are 26 lowercase and 26 uppercase letters, and digits from 0 to 9. (26+26+10) digits in total. Given a number from 0 to 62^5 (which equals 916132832) (say, your primary key) you can do the conversion to a five-digit base-62 as follows:

private static char Base62Digit(int d) {
    if (d < 26) {
        return (char)('a'+d);
    } else if (d < 52) {
        return (char)('A'+d-26);
    } else if (d < 62) {
        return (char)('0'+d-52);
    } else {
        throw new ArgumentException("d");
    }
}

static string ToBase62(int n) {
    var res = "";
    while (n != 0) {
        res = Base62Digit(n%62) + res;
        n /= 62;
    }
    return res;
}

private static int Base62Decode(char c) {
    if (c >= '0' && c <= '9') {
        return 52 + c - '0';
    } else if (c >= 'A' && c <= 'Z') {
        return 26 + c - 'A';
    } else if (c >= 'a' && c <= 'z') {
        return c - 'a';
    } else {
        throw new ArgumentException("c");
    }
}

static int FromBase62(string s) {
    return s.Aggregate(0, (current, c) => current*62 + Base62Decode(c));
}

Here is how to generate cryptographically strong random numbers (you need to add a reference to System.Security):

private static readonly RNGCryptoServiceProvider crypto =
    new RNGCryptoServiceProvider();

private static int NextRandom() {
    var buf = new byte[4];
    crypto.GetBytes(buf);
    return buf.Aggregate(0, (p, v) => (p << 8) + v) & 0x3FFFFFFF;
}