BigInt completed

BigInt.swift

import UIKit
import Foundation

struct BigInt {
    
    var value: String
    
    func multiply(right: BigInt) -> BigInt {
        var leftCharacterArray = value.characters.reversed().map { Int(String($0))! }
        var rightCharacterArray = right.value.characters.reversed().map { Int(String($0))! }
        var result = [Int](repeating: 0, count: leftCharacterArray.count+rightCharacterArray.count)
        
        for leftIndex in 0..<leftCharacterArray.count {
            for rightIndex in 0..<rightCharacterArray.count {
                
                let resultIndex = leftIndex + rightIndex
                
                result[resultIndex] = leftCharacterArray[leftIndex] * rightCharacterArray[rightIndex] + (resultIndex >= result.count ? 0 : result[resultIndex])
                if result[resultIndex] > 9 {
                    result[resultIndex + 1] = (result[resultIndex] / 10) + (resultIndex+1 >= result.count ? 0 : result[resultIndex + 1])
                    result[resultIndex] -= (result[resultIndex] / 10) * 10
                }
                
            }
            
        }
        
        result = Array(result.reversed())

        while result.count > 0 && result.first == 0 {
            result.removeFirst(1)
        }
        
        return  BigInt(value: result.map { String($0) }.joined(separator: ""))
    }
    
}

func * (left: BigInt, right: BigInt) -> BigInt { return left.multiply(right: right) }

Maybe add this before returning to remove trailing zeroes,

    while result.count > 0 && result.first == 0 {
      result.removeFirst(1)
    }

And here's a naive addition function,

func add(right: BigInt) -> BigInt {
    var a = value.reversed().map { Int(String($0))! }
    var b = right.value.reversed().map { Int(String($0))! }
    var result: [Int] = []
    let numDigits = max(a.count, b.count)
    for i in 0..<numDigits {
      let ai = i < a.count ? a[i] : 0
      let bi = i < b.count ? b[i] : 0
      let ri = i < result.count ? result[i] : 0
      let sum = ai + bi + ri
      if i < result.count {
        result[i] = sum % 10
      } else {
        result.append(sum % 10)            
      }
      if sum / 10 > 0 {
        if i + 1 < result.count {
          result[i+1] = sum / 10
        } else {
          result.append(sum / 10)
        }
      }
    }
    result = Array(result.reversed())
    return  BigInt(value: result.map { String($0) }.joined(separator: ""))
}


func + (left: BigInt, right: BigInt) -> BigInt { return left.add(right: right) }

Ah not bad! Made the change. Thank you.