How to Reverse or Rotate in Golang

The challenge

The input is a string str of digits. Cut the string into chunks (a chunk here is a substring of the initial string) of size sz (ignore the last chunk if its size is less than sz).

If a chunk represents an integer such as the sum of the cubes of its digits is divisible by 2, reverse that chunk; otherwise rotate it to the left by one position. Put together these modified chunks and return the result as a string.

If

• sz is <= 0 or if str is empty return ""
• sz is greater (>) than the length of str it is impossible to take a chunk of size sz hence return “”.

Examples:
revrot("123456987654", 6) --> "234561876549"
revrot("123456987653", 6) --> "234561356789"
revrot("66443875", 4) --> "44668753"
revrot("66443875", 8) --> "64438756"
revrot("664438769", 8) --> "67834466"
revrot("123456779", 8) --> "23456771"
revrot("", 8) --> ""
revrot("123456779", 0) --> "" 
revrot("563000655734469485", 4) --> "0365065073456944"

Example of a string rotated to the left by one position:
s = "123456" gives "234561".

The solution in Golang

package solution

func cubeSum(str []rune) int {
  sum := 0
  for _, dr := range str {
    d := int(dr) - 48
    sum += d * d * d
  }
  return sum
}
func reverse(s []rune) {
  for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
    s[i], s[j] = s[j], s[i]
  }
}
func shift(s []rune) {
  first := s[0]
  for i := range s[1:] {
    s[i] = s[i+1]
  }
  s[len(s)-1] = first
}
func Revrot(s string, n int) string {
  if n <= 0 || n > len(s) {
    return ""
  }
  sr := []rune(s)
  i := 0
  for ; i+n <= len(sr); i += n {
    chunk := sr[i:i+n]
    if cubeSum(chunk)%2 == 0 {
      reverse(chunk)
    } else {
      shift(chunk)
    }
  }
  return string(sr[:i])
}

Test cases to validate our solution

package our_test
import (
  . "github.com/onsi/ginkgo"
  . "github.com/onsi/gomega"
)

func dotest(s string, n int, exp string) {
    var ans = Revrot(s, n)
    Expect(ans).To(Equal(exp))
}

var _ = Describe("Tests Revrot", func() {

    It("should handle basic cases", func() {
        dotest("1234", 0, "")
        dotest("", 0, "")
        dotest("1234", 5, "")
        var s = "733049910872815764"
        dotest(s, 5, "330479108928157")
        
    })
})