Encoding a number, C# implementation of z-base-32 or something else?

Question

I need to encode/decode an integer which is up to 9 digits long but most often 7 digits long. I'd like to make it easier to communicate/memorise - it will be communicated by phone, copied & pasted, keyed in from a card/memory/label, hand-written on labels and generally abused!

I'm looking to reduce the number of digits (whilst adding a checksum) using a base 32 scheme. I'm most in favour of z-base-32 (over the RFC4648 implementation) because of some of the design goals (e.g. handwriting) and choice of alphabet (lower-case, permuted to favour characters that are easier to read, write, speak, and remember). However, I can't find a C# implementation and I'm concerned about porting from the existing Python implementation.

Does anyone have a C# implementation? Alternatively, does anyone have a set of test cases (other than the examples in the spec) that I can use to validate a port?

I'm open to suggestions about alternative encoding schemes.

Answer 1

I started with the code provided by Dead account and made some changes based on my tests. I hope this is useful.

/// <summary>
/// Author: Ben Maddox
/// </summary>
public class ZBase32Encoder
{
    /*
     * Accepted characters based on code from: 
     * http://www.codeproject.com/KB/recipes/Base32Encoding.aspx?display=Print
     */
    public const string AcceptedCharacters = "ybndrfg8ejkmcpqxot1uwisza345h769";

    public static string Encode(int input)
    {
        string result = "";

        if (input == 0)
        {
            result += AcceptedCharacters[0];
        }
        else
        {
            while (input > 0)
            {
                //Must make sure result is in the correct order
                result = AcceptedCharacters[input%AcceptedCharacters.Length] + result;
                input /= AcceptedCharacters.Length;
            }
        }

        return result;
    }

    public static int Decode(string input)
    {
        var inputString = input.ToLower();

        int result = 0;
        for (int i = 0; i < inputString.Length; i++)
        {
            result *= AcceptedCharacters.Length;
            var character = inputString[i];
            result += AcceptedCharacters.IndexOf(character);
        }
        return result;
    }

    public static int Decode(char data)
    {
        return Decode(data.ToString());
    }
}

And here are the tests I used. MS Test with the Should assertions library.

[TestClass]
public class ZBase32EncoderTests
{

    [TestMethod]
    public void Encoding_0_ReturnsFirstCharacter()
    {
        var result = ZBase32Encoder.Encode(0);
        result.ShouldEqual(ZBase32Encoder.AcceptedCharacters[0].ToString());
    }

    [TestMethod]
    public void Encoding_1_ReturnsSecondCharacter()
    {
        var result = ZBase32Encoder.Encode(1);
        result.ShouldEqual(ZBase32Encoder.AcceptedCharacters[1].ToString());
    }

    [TestMethod]
    public void Encoding_32_ReturnsSecondAndFirstValues()
    {
        var result = ZBase32Encoder.Encode(32);
        result.ShouldEqual(ZBase32Encoder.AcceptedCharacters[1].ToString() + ZBase32Encoder.AcceptedCharacters[0].ToString());
    }

    [TestMethod]
    public void Encoding_64_ReturnsThirdAndFirstValues()
    {
        var result = ZBase32Encoder.Encode(64);
        result.ShouldEqual(ZBase32Encoder.AcceptedCharacters[2].ToString() + ZBase32Encoder.AcceptedCharacters[0].ToString());
    }

    [TestMethod]
    public void Encoding_65_ReturnsThirdAndSecondValues()
    {
        var result = ZBase32Encoder.Encode(65);
        result.ShouldEqual(ZBase32Encoder.AcceptedCharacters[2].ToString() + ZBase32Encoder.AcceptedCharacters[1].ToString());
    }



    [TestMethod]
    public void Decoding_FirstCharacter_Returns_0()
    {
        var inputCharacter = ZBase32Encoder.AcceptedCharacters[0];
        var result = ZBase32Encoder.Decode(inputCharacter);
        result.ShouldEqual(0);
    }

    [TestMethod]
    public void Decoding_SecondCharacter_Returns_1()
    {
        var inputCharacter = ZBase32Encoder.AcceptedCharacters[1];
        var result = ZBase32Encoder.Decode(inputCharacter);
        result.ShouldEqual(1);
    }

    [TestMethod]
    public void Decoding_SecondAndFirstValues_Shows_32()
    {
        var inputCharacters = ZBase32Encoder.AcceptedCharacters[1].ToString() + ZBase32Encoder.AcceptedCharacters[0];
        var result = ZBase32Encoder.Decode(inputCharacters);
        result.ShouldEqual(32);
    }

    [TestMethod]
    public void Decoding_ThirdAndFirstCharacters_Shows_64()
    {
        var inputCharacters = ZBase32Encoder.AcceptedCharacters[2].ToString() + ZBase32Encoder.AcceptedCharacters[0];
        var result = ZBase32Encoder.Decode(inputCharacters);
        result.ShouldEqual(64);
    }
}

Answer 2

You might want to use your own encode / decode routine?

Encode:

 string acceptedChar = "ABCDEFGHJKLMNPQRSTUWXZ0123456789";
 int yourNumber = 12345678;

 string response = "";
 while (yourNumber > 0)
 {
      response += acceptedChar[yourNumber % acceptedChar.Length];
      yourNumber /= acceptedChar.Length;
 }

Decode:

 string acceptedChar = "ABCDEFGHJKLMNPQRSTUWXZ0123456789";
 string inputStr = "ABCD";

 int yourNumber = 0;
 for (int i = inputStr.Length; i > 0; i--)
 {
     yourNumber *= acceptedChar.Length;
     yourNumber += acceptedChar.IndexOf(inputStr[i]);
 }    

(Untested code)

Answer 3

If you look at your cell phone keyboard, the number 1985239 can be represented using these characters (a,b,c), (w,x,y,z), (t,u,v), ... Try to find an algorithm that would generate more-or-less English-like words given an ordered set of unordered sets of characters -- these will be easier to memorize.