String / Int Conversions

string_int_conversions.swift

//: Playground - noun: a place where people can play

import UIKit

extension Character {
    var asciiValue: Int {
        get {
            let s = String(self).unicodeScalars
            return Int(s[s.startIndex].value)
        }
    }
}

extension String {
    var asInt: Int {
        let base = Array("0").first!.asciiValue
        var result = 0
        let negative = self.hasPrefix("-")
        let startIndex = negative ? 1 : 0
        let stringAsArray = Array(self)
        for index in startIndex..<stringAsArray.count {
            let c = stringAsArray[index]
            let delta = c.asciiValue - base
            result *= 10 // Horner's Rule http://mathworld.wolfram.com/HornersRule.html
            result += delta
        }
        if negative {
            result *= -1
        }
        return result
    }
}

extension Int {
    var asString: String {
        guard self != 0 else {
            return "0"
        }

        var chars: [Character] = []
        let base = Array("0").first!.asciiValue

        var value = self > 0 ? self : -self // We need to convert negatives to positive

        while value != 0 {
            let ascii = (value % 10) + base
            if let unicodeScalar = UnicodeScalar(ascii) {
                let c = Character(unicodeScalar)
                chars.append(c)
            }
            value /= 10
        }

        if self < 0 {
            let neg = Character("-")
            chars.append(neg)
        }

        // reverse buffer
        chars.reverse()

        let result = String(chars)
        return result
    }
}

let toTest: [String: Int] = [
    "42": 42,
    "-42": -42,
    "1": 1,
    "-1": -1,
    "0": 0
]

for (k, v) in toTest {
    let asInt = k.asInt
    assert(asInt == v)

    let asStr = v.asString
    assert(asStr == k)
}