Generate 5 length unique code from natural number

Question

I need to generate a unique code with 5 lengths with the given number. In other words, I need to encode natural number to 5 length unique code

I wanna give fixed-length rememberable code to the customer, and keep the sequential number in the database and encode or decode when needed.

The given number can be in the range of 1 to 9999999. But the result always must be 5 lengths.

for example

1 => a56er or 2 => c7gh4

Uniqueness is important I googled a lot and I can't find a solution.

Answer 1

The given number can be in the range of 1 to 9999999

Right. So you need to encode 24 bits of information, and you have 5 characters in which to do that - so you need 5 bits per character. That's pleasantly in the range of "only digits and lower case ASCII characters" and you can even remove points of confusion like "o/0" and "i/1".

Note that this isn't in any way "secure" - it's entirely predictable and reversible. If you don't want customers being able to reverse engineer their sequence number from the encoded form, it won't work. But it's a simple way of encoding a number as a fixed-length string.

Sample code showing encoding and decoding:

using System;
using System.Globalization;

public class Test
{
    static void Main()
    {
        EncodeDecode(10);
        EncodeDecode(100);
        EncodeDecode(1000);
        EncodeDecode(10000);
        EncodeDecode(100000);
        EncodeDecode(1000000);
        EncodeDecode(9999999);
        
        void EncodeDecode(int number)
        {
            string encoded = EncodeBase32(number);
            int decoded = DecodeBase32(encoded);
            Console.WriteLine($"{number} => {encoded} => {decoded}");
        }
   }
    
    private const string Base32Alphabet = 
        "23456789abcdefghjklmnpqrstuvwxyz";
    private static string EncodeBase32(int number)
    {
        // TODO: Range validation
        char[] chars = new char[5];
        for (int i = 0; i < 5; i++)
        {
            chars[i] = Base32Alphabet[number & 0x1f];
            number = number >> 5;
        }
        return new string(chars);
    }
    
    private static int DecodeBase32(string text)
    {
        if (text.Length != 5)
        {
            throw new ArgumentException("Invalid input: wrong length");
        }
        int number = 0;
        for (int i = 4; i >= 0; i--)
        {
            number = number << 5;
            int index = Base32Alphabet.IndexOf(text[i]);
            if (index == -1)
            {
                throw new ArgumentException("Invalid input: invalid character");
            }
            number |= index;
        }
        return number;
    }
}

Output:

10 => c2222 => 10
100 => 65222 => 100
1000 => az222 => 1000
10000 => jsb22 => 10000
100000 => 2p352 => 100000
1000000 => 2ljy2 => 1000000
9999999 => zm7kb => 9999999