[Golang] 1000-digit Fibonacci number - Problem 25 - Project Euler

package main

import (
      "fmt"
)

const MaxDigits = 1200
const BASE = 10

func MakePositiveInt(s string) (n [MaxDigits]int) {
      // make n zero
      for i := 0; i < MaxDigits; i++ {
              n[i] = 0
      }

      for index, digit := range s {
              i := len(s) - index - 1
              switch digit {
              case '0':
                      n[i] = 0
              case '1':
                      n[i] = 1
              case '2':
                      n[i] = 2
              case '3':
                      n[i] = 3
              case '4':
                      n[i] = 4
              case '5':
                      n[i] = 5
              case '6':
                      n[i] = 6
              case '7':
                      n[i] = 7
              case '8':
                      n[i] = 8
              case '9':
                      n[i] = 9
              default:
                      panic("invalid digit in number string")
              }
      }

      return
}

func AddPositiveInt(a, b [MaxDigits]int) (c [MaxDigits]int) {
      var carry, sum = 0, 0

      // make c zero
      for i := 0; i < MaxDigits; i++ {
              c[i] = 0
      }

      for i := 0; i < MaxDigits; i++ {
              sum = a[i] + b[i] + carry

              if sum >= BASE {
                      carry = 1
                      sum -= BASE
              } else {
                      carry = 0
              }

              c[i] = sum
      }

      if carry != 0 {
              panic("overflow in addition")
      }

      return
}

func PositiveIntToString(a [MaxDigits]int) (result string) {
      isLeadingZero := true
      for i := MaxDigits - 1; i >= 0; i-- {
              if isLeadingZero && a[i] == 0 {
                      continue
              } else {
                      isLeadingZero = false
                      switch a[i] {
                      case 0:
                              result += "0"
                      case 1:
                              result += "1"
                      case 2:
                              result += "2"
                      case 3:
                              result += "3"
                      case 4:
                              result += "4"
                      case 5:
                              result += "5"
                      case 6:
                              result += "6"
                      case 7:
                              result += "7"
                      case 8:
                              result += "8"
                      case 9:
                              result += "9"
                      default:
                              panic("invalid digit in int array")
                      }
              }
      }
      return
}

func main() {
      Fn1 := MakePositiveInt(`1`)
      Fn2 := MakePositiveInt(`1`)
      Fn := AddPositiveInt(Fn1, Fn2)
      index := 3

      for len(PositiveIntToString(Fn)) < 1000 {
              Fn2 = Fn1
              Fn1 = Fn
              Fn = AddPositiveInt(Fn1, Fn2)
              index++
      }
      fmt.Println(PositiveIntToString(Fn))
      fmt.Println("index: ", index)
}