How to convert AnyBase to Base10?

Question

I found some code to encode a Base10-String with to a custom BaseString:

func stringToCustomBase(encode: Int, alphabet: String) -> String {
    var base = alphabet.count, int = encode, result = ""
    repeat {
        let index = alphabet.index(alphabet.startIndex, offsetBy: (int % base))
        result = [alphabet[index]] + result
        int /= base
    } while (int > 0)
    return result
}

... calling it with this lines:

let encoded = stringToCustomBase(encode: 9291, alphabet: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")
print(encoded)

---

The encoding above works pretty good. But what about decoding the encoded string?

So because I got no idea how to decode a (in this case Base62 [alphabet.count=62]) to a human readable string (in this case [Base10]) any help would be super appreciated.

---

PS: (A full code solution is not required, I can also come up with some kind of pseudo-code or maybe just a few-lines of code)

---

This is what I've tried so far:

func reVal(num: Int) -> Character {
    if (num >= 0 && num <= 9) {
        return Character("\(num)")
    }
    return Character("\(num - 10)A");
}


func convertBack() {
    var index = 0;
    let encoded = "w2RDn3"
    var decoded = [Character]()
    var inputNum = encoded.count
    repeat {
        index+=1
        decoded[index] = reVal(num: inputNum % 62)
        //encoded[index] = reVal(inputNum % 62);
        inputNum /= 62;

    }  while (inputNum > 0)
    print(decoded);
}

Answer 1

Based on the original algorithm you need to iterate through each character of the encoded string, find the location of that character within the alphabet, and calculate the new result.

Here are both methods and some test code:

func stringToCustomBase(encode: Int, alphabet: String) -> String {
    var base = alphabet.count, string = encode, result = ""
    repeat {
        let index = alphabet.index(alphabet.startIndex, offsetBy: (string % base))
        result = [alphabet[index]] + result
        string /= base
    } while (string > 0)
    return result
}

func customBaseToInt(encoded: String, alphabet: String) -> Int? {
    let base = alphabet.count
    var result = 0
    for ch in encoded {
        if let index = alphabet.index(of: ch) {
            let mult = result.multipliedReportingOverflow(by: base)
            if (mult.overflow) {
                 return nil
            } else {
                let add = mult.partialValue.addingReportingOverflow(alphabet.distance(from: alphabet.startIndex, to: index))
                if (add.overflow) {
                    return nil
                } else {
                    result = add.partialValue
                }
            }
        } else {
            return nil
        }
    }

    return result
}


let startNum = 234567
let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
let codedNum = stringToCustomBase(encode: startNum, alphabet: alphabet)
let origNun = customBaseToInt(encoded: codedNum, alphabet: alphabet)

I made the customBaseToInt method return an optional result in case there are characters in the encoded value that are not in the provided alphabet.