Simple base encoding and decoding in Swift.

BaseFunctions.swift

import Foundation

enum StringSearchError: Swift.Error {
    case didNotFind(Character)
}

extension String {
    
    fileprivate static let DefaultDigits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
    fileprivate static let DefaultEmoji = "🤖❤️☠️🤓🦁🍆🧀😎🙄🤢🧠🧶🙈🙉🙊🔥🥕⚽️💊🎶🧶"
    
    func character<T>(at index: T) -> String where T: FixedWidthInteger {
        return String(self[self.index(self.startIndex, offsetBy: Int(index))])
    }
    
    func position(of char: Character) throws -> Int {
        guard let idx = self.firstIndex(of: char) else { throw StringSearchError.didNotFind(char) }
        return Int( self.distance(from: self.startIndex, to: idx) )
    }
}

enum BaseError: Swift.Error {
    case outOfRange
}

extension FixedWidthInteger where Self.Magnitude : UnsignedInteger {
    
    func base36() -> String { return try! self.base(36) }
    func base62() -> String { return try! self.base(62) }
    func baseEmoji() -> String { return try! self.base(String.DefaultEmoji.count, using: String.DefaultEmoji) }
    
    func base<T>(_ redux: T, using digits: String = .DefaultDigits) throws -> String where T: FixedWidthInteger {
        
        guard 1...digits.count ~= Int(redux) else { throw BaseError.outOfRange }
        
        var result = ""
        var value = Int(self)
        let rdx = Int(redux)
        
        repeat {
            result = digits.character(at: value % rdx) + result
            value /= rdx
        } while (value > 0)
        
        return result
    }
}

extension String {
    
    func decode36() -> Int { return try! self.decode(36) }
    func decode62() -> Int { return try! self.decode(62) }
    
    func decode(_ redux: Int, using digits: String = .DefaultDigits) throws -> Int {
        
        let decoded = try self.reduce(0) {
            let value = Double( try digits.position(of: $1) )
            return Int( Double($0) * Double(redux) + value )
        }
        
        return decoded
    }
}