Given an arbitrary set of letters
String range = "0123456789abcdefghijklmnopABCD#";
I am looking for 2 methods to encode/decode from long <-> String
String s = encode( range, l );
and
long l = decode( range, s );
So decode(range, encode(range, 123456789L)) == 123456789L
And if range is "0123456789" thats the usual way of encoding.
This is simply a matter of performing base conversion. Simply convert the long to the appropriate numeric base, corresponding to the number of characters in your string, and use the range string as your set of "digits".
For example, suppose you have the string "0123456789ABCDEF", then this means you must convert to base 16, hexadecimal. If the string is "01234567", then you convert to base 8, octal.
result = "";
while (number > 0)
{
result = range[(number % range.length)] + result;
number = number / 16; //integer division, decimals discarded
}
For going back, take the first character, find its position in the string, and add it to the result. Then, for each subsequent character, multiply the current result by the base before adding the position of the next character.
result = 0;
for (int i = 0; i < input.length; i++)
{
result = result * range.length;
result = range.indexOf(input[i])
}
The following code does what you need:
static long decode(String s, String symbols) {
final int B = symbols.length();
long num = 0;
for (char ch : s.toCharArray()) {
num *= B;
num += symbols.indexOf(ch);
}
return num;
}
static String encode(long num, String symbols) {
final int B = symbols.length();
StringBuilder sb = new StringBuilder();
while (num != 0) {
sb.append(symbols.charAt((int) (num % B)));
num /= B;
}
return sb.reverse().toString();
}
public static void main(String[] args) {
String range = "0123456789abcdefghijklmnopABCD#";
System.out.println(decode(encode(123456789L, range), range));
// prints "123456789"
System.out.println(encode(255L, "0123456789ABCDEF"));
// prints "FF"
System.out.println(decode("100", "01234567"));
// prints "64"
}
Note that this is essentially base conversion with a custom set of symbols.