Theoretical background
You need a Bijective Function f. This is necessary so that you can find a inverse function g('abc') = 123 for your f(123) = 'abc' function. This means:
- There must be no x1, x2 (with x1 ≠ x2) that will make f(x1) = f(x2),
- and for every y you must be able to find an x so that f(x) = y.
How to convert the ID to a shortened URL
- Think of an alphabet we want to use. In your case that's
[a-zA-Z0-9]. It contains 62 letters. - Take an auto-generated, unique numerical key (the auto-incremented
idof a MySQL table for example).For this example I will use 12510 (125 with a base of 10). - Now you have to convert 12510 to X62 (base 62).12510 = 2×621 + 1×620 =
[2,1]This requires use of integer division and modulo. A pseudo-code example:digits = [] while num > 0 remainder = modulo(num, 62) digits.push(remainder) num = divide(num, 62) digits = digits.reverseNow map the indices 2 and 1 to your alphabet. This is how your mapping (with an array for example) could look like:0 → a 1 → b ... 25 → z ... 52 → 0 61 → 9With 2 → c and 1 → b you will receive cb62 as the shortened URL.http://shor.ty/cb
How to resolve a shortened URL to the initial ID
The reverse is even easier. You just do a reverse lookup in your alphabet.
- e9a62 will be resolved to "4th, 61st, and 0th letter in alphabet".e9a62 =
[4,61,0]= 4×622 + 61×621 + 0×620 = 1915810 - Now find your database-record with
WHERE id = 19158and do the redirect.
C# version:
public class UrlShortener
{
private static String ALPHABET = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
private static int BASE = 62;
public static String encode(int num)
{
StringBuilder sb = new StringBuilder();
while ( num > 0 )
{
sb.Append( ALPHABET[( num % BASE )] );
num /= BASE;
}
StringBuilder builder = new StringBuilder();
for (int i = sb.Length - 1; i >= 0; i--)
{
builder.Append(sb[i]);
}
return builder.ToString();
}
public static int decode(String str)
{
int num = 0;
for ( int i = 0, len = str.Length; i < len; i++ )
{
num = num * BASE + ALPHABET.IndexOf( str[(i)] );
}
return num;
}
}