For the purpose of a game, I need to generate a save code that the user can note somewhere and use to reload his game state later (no persistent data possible).
The save code needs to be short like 6DZF1D3, (a base 36 or base 62 string).
Scores of game levels can be simplified as a string like 1232312321321321321, a sequence where each char is a level score in "stars" (1, 2 or 3 stars).
There will be around 30 game levels.
I would like to generate the shortest code possible for the user, so my first idea was to generate all possibilities inside an Array. Then generate the base 62 code of the key where the user is. But with 3^30 possibilities, this is generating an array with 2e+14 key/values, which is not good for memory and CPU.
Second thought, was to use a base 4 to 62 converter, but most of codes I found are using int or long which are limited in size and lower than 30 chars.
Do you have any idea of how to generate the shortest save code composed of alphanumeric chars ?
If a user should be able to write it down i would prefer a Base58 encoding. So, for 1-3 possible stars per level we need 2 bits to encode each level.
00 => 0 star (would mean last unplayed level reached)
01 => 1 star
10 => 2 stars
11 => 3 stars
We need 60 bits for 30 levels, all levels with 3 stars would be decimal 1152921504606846975. This, base58 encoded, would be 3gDmDv6tjHG, not too long, is it?!
Update:
@DrNootNoot I'm glad you found a way to solve your problem! But I had fun to hack a small piece of code for my mentioned base58 version. I adapted the two functions by Pavel Vladov you used.
Maybe someday someone else has a similar issue:
using System;
using System.Collections.Generic;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
string[] scoreArray = new string[30] { "1", "2", "3", "3", "1", "2", "2", "2", "3", "1", "1", "1", "2", "3", "2", "1", "2", "3", "1", "1", "1", "2", "2", "2", "1", "1", "2", "1", "2","3" };
ulong numScore = ScoreToDecimal(scoreArray);
string saveScore = UDecimalToBase58String(numScore);
Console.WriteLine("Score array: " + String.Join("-",scoreArray));
Console.WriteLine("Numeric score: " + Convert.ToString(numScore));
Console.WriteLine("Base58 score: " + saveScore);
ulong numScoreRestored = Base58StringToUDecimal(saveScore);
string[] scoreArrayRestored = DecimalToScore(numScoreRestored);
Console.WriteLine("From Base58 converted numeric score: " + Convert.ToString(numScoreRestored));
Console.WriteLine("From Base58 converted score array: " + String.Join("-", scoreArray));
Console.Read();
}
/// <summary>
/// Converts the stars-per-level array to a decimal value for the saved game.
/// </summary>
/// <param name="score">score array to convert. Max. 32 entries/levels.</param>
/// <returns></returns>
public static ulong ScoreToDecimal(string[] score)
{
int arrLength = score.Length;
if (arrLength > 32)
throw new ArgumentException("The score array must not be larger than 32 entries");
ulong result = 0;
for (int i = arrLength - 1; i >= 0; i--)
{
ulong singleScore = Convert.ToUInt64(score[i]);
if (singleScore > 3)
throw new ArgumentException(String.Format("Invalid score value. Max. allowed value is 3, but {0} was given at index {1}", singleScore, i), "score");
result += (singleScore << ((arrLength - 1 - i) * 2));
}
return result;
}
/// <summary>
/// Converts the decimal value of the saved game back to a stars-per-level array.
/// </summary>
/// <param name="decimalScore">Maximal 64-bit unsigned saved game number to convert.</param>
/// <returns></returns>
public static string[] DecimalToScore(ulong decimalScore)
{
List<string> result = new List<string>();
while(decimalScore > 0)
{
result.Add(Convert.ToString(decimalScore % 4));
decimalScore /= 4;
}
result.Reverse();
return result.ToArray();
}
/// <summary>
/// Adapted Unsigned-Base58-Version of Pavel Vladovs DecimalToArbitrarySystem function.
/// See: https://www.pvladov.com/2012/05/decimal-to-arbitrary-numeral-system.html
/// </summary>
/// <param name="decimalNumber"></param>
/// <returns></returns>
public static string UDecimalToBase58String(ulong decimalNumber)
{
const int BitsInLong = 64;
const int FixedRadix = 58;
const string Digits = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
if (decimalNumber == 0)
return "0";
int index = BitsInLong - 1;
ulong currentNumber = decimalNumber;
char[] charArray = new char[BitsInLong];
while (currentNumber != 0)
{
int remainder = (int)(currentNumber % FixedRadix);
charArray[index--] = Digits[remainder];
currentNumber = currentNumber / FixedRadix;
}
string result = new String(charArray, index + 1, BitsInLong - index - 1);
return result;
}
/// <summary>
/// Adapted Unsigned-Base58-Version of Pavel Vladovs ArbitraryToDecimalSystem function.
/// See: https://www.pvladov.com/2012/07/arbitrary-to-decimal-numeral-system.html
/// </summary>
/// <param name="base58String"></param>
/// <returns></returns>
public static ulong Base58StringToUDecimal(string base58String)
{
const int FixedRadix = 58;
const string Digits = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
if (String.IsNullOrEmpty(base58String))
return 0;
ulong result = 0;
ulong multiplier = 1;
for (int i = base58String.Length - 1; i >= 0; i--)
{
char c = base58String[i];
int digit = Digits.IndexOf(c);
if (digit == -1)
throw new ArgumentException(
"Invalid character in the arbitrary numeral system number",
"number");
result += (uint)digit * multiplier;
multiplier *= FixedRadix;
}
return result;
}
}
}
Regards