Compress large Integers into smallest possible string

Question

I have a bunch of 10 digit integers that I\'m passing in a URL. Something like: \"4294965286\", \"2292964213\". They will always be positive and always be 10 digits.

Answer 1

I liked @user166390 answer but I preferred a most-to-least format and thought the code could be improved since the use of dictionary is unnecessary in encode and don't need to be generated on every decode. Also I added an exception and changed to ulong since negative values are not supported.

If somebody has further performance improvements feel free to write. Maybe if there is a better alternative to StringBuilder

Here is the code modified by me.

        public static string EncodeNumber(ulong input)
        {
            return EncodeNumber(input, Mapping85Bit);
        }

        // This does not "pad" values
        private static string EncodeNumber(ulong inp, char[] map)
        {
            // use ulong count instead of int since does not matter on x64 operating system.
            ulong cnt = (ulong)map.Length;
            // value -> character
            if (inp == 0)
            {
                return map[0].ToString();
            }
            var sb = new StringBuilder();
            while (inp > 0)
            {
                // encoded most-to-least significant
                ulong val = inp % cnt;
                inp = inp / cnt;
                sb.Insert(0, map[(int)val]);
            }
            return sb.ToString();
        }

        public static ulong DecodeNumber(string encoded)
        {
            return DecodeNumber(encoded, Mapping85Bit, Mapping85BitDict);
        }

        private static ulong DecodeNumber(string encoded, char[] map, Dictionary charMapDict)
        {
            // use ulong count instead of int since does not matter on x64 operating system.
            ulong b = (ulong)map.Length;
            ulong res = 0;
            for (var i = 0; i < encoded.Length; i++)
            {
                char ch = encoded[i];
                if(!charMapDict.TryGetValue(ch, out ulong val))
                {
                    throw new ArgumentException($"Invalid encoded number: '{encoded}'. '{ch}' is not a valid character for this encoding.");
                }
                res = (res * b) + val;
            }
            return res;
        }



        // Windows file system reserved characters:     < > : " / \ | = * 

        /// 

        /// Compatible with file system. Originates from ASCII table except starting like Base64Url and except windows path reserved chars. Skipped '/' and '\' to prevent path problems. Skipped ' for sql problems.
        /// https://www.ascii-code.com/
        /// Does not need to be encoded for json since it doesn't use \ and ". No encoding also needed for xml since < > are also not used. That is why it is also different to https://en.wikipedia.org/wiki/Ascii85
        /// 
        public static readonly char[] Mapping85Bit = new char[] {
            'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
            'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
            'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd',
            'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
            'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
            'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7',
            '8', '9', '-', '_', ' ', '!', '#', '$', '%', '&',
            '(', ')', '+', ',', '.', ';', '?', '@', '[', ']',
            '^', '`', '{', '}', '~'
        };
        private static readonly Dictionary Mapping85BitDict = Mapping85Bit.Select((v, i) => new { Value = v, Index = (ulong)i }).ToDictionary(i => i.Value, i => i.Index);

    [Test]
    public void EncodeTest()
    {
        // 85Bit Encoding:
        Assert.AreEqual(EncodeNumber(85), "BA");
        Assert.AreEqual(EncodeNumber(86), "BB");
        Assert.AreEqual(EncodeNumber(3), "D");
        Assert.AreEqual(EncodeNumber(84), "~");

        Assert.AreEqual(EncodeNumber(0), "A");

        Assert.AreEqual(DecodeNumber("BA"), 85);

        Assert.AreEqual(DecodeNumber("BA"), 85);
        Assert.AreEqual(DecodeNumber("`"), 81);
    }