Question

I need to convert an integer into a base64-character representation. I’m using OxA3’s answer on this thread: Quickest way to convert a base 10 number to any base in .NET?

How do I inverse this to get my original integer back, given a string?

Answer 1

Joel Mueller’s answer should guide you the base-64 case.

In response to the preliminary code you’ve provided in your own answer, you can definitely improve its efficiency by changing the code to accomplish what your for loop is doing (effectively an O(N) IndexOf) to use a hash lookup (which should make it O(1)).

I am basing this on the assumption that baseChars is a field that you initialize in your class’s constructor. If this is correct, make the following adjustment:

private Dictionary<char, int> baseChars;

// I don't know what your class is called.
public MultipleBaseNumberFormatter(IEnumerable<char> baseCharacters)
{
    // check for baseCharacters != null and Count > 0

    baseChars = baseCharacters
        .Select((c, i) => new { Value = c, Index = i })
        .ToDictionary(x => x.Value, x => x.Index);
}

Then in your StringToInt method:

char next = encodedString[currentChar];

// No enumerating -- we've gone from O(N) to O(1)!
if (!characterIndices.TryGetValue(next, out nextCharIndex))
{
    throw new ArgumentException("Input includes illegal characters.");
}