[Golang] Factorial digit sum - Problem 20 - Project Euler

package main

import (
      "fmt"
      "strconv"
)

const MaxDigits = 200
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 MultiplyOneDigit(a [MaxDigits]int, n int) (b [MaxDigits]int) {
      var carry = 0

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

      for i := 0; i < MaxDigits; i++ {
              b[i] = n * a[i]

              b[i] += carry

              if b[i] >= BASE {
                      carry = b[i] / BASE
                      b[i] %= BASE
              } else {
                      carry = 0
              }
      }

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

      return
}

func ShiftLeft(a [MaxDigits]int, n int) [MaxDigits]int {
      var i int

      for i = MaxDigits - 1; i >= n; i-- {
              a[i] = a[i-n]
      }
      for i >= 0 {
              a[i] = 0
              i -= 1
      }

      return a
}

func MultiplyPositiveInt(a, b [MaxDigits]int) (c [MaxDigits]int) {
      // make c zero
      for i := 0; i < MaxDigits; i++ {
              c[i] = 0
      }

      for i := 0; i < MaxDigits; i++ {
              tmp := MultiplyOneDigit(b, a[i])
              tmp = ShiftLeft(tmp, i)
              c = AddPositiveInt(c, tmp)
      }

      return
}

func PrintPositiveInt(a [MaxDigits]int) {
      isLeadingZero := true
      for i := MaxDigits - 1; i >= 0; i-- {
              if isLeadingZero && a[i] == 0 {
                      continue
              } else {
                      isLeadingZero = false
                      fmt.Print(a[i])
              }
      }
      fmt.Println("\n")
}

func main() {
      result := MakePositiveInt("1")
      for i := 2; i <= 100; i++ {
              s := strconv.Itoa(i)
              tmp := MakePositiveInt(s)
              result = MultiplyPositiveInt(result, tmp)
      }
      PrintPositiveInt(result)

      sum := 0
      for i := 0; i < MaxDigits; i++ {
              sum += result[i]
      }
      fmt.Println(sum)
}