Cannot decode Caesar-Encrypted messages - go

Question

I have to write a function for school that decodes Caesar-Encrypted messages. Our teacher gave us the code that encodes the messages, and asked us to write one based on the givenb function that decodes any encrypted message. (The key/shift wasn't given but was explicitly not 3 as in the the encode example, we should figure out ourselves. With online tools i know the key is 17 but for the purpose ofthe question i chose the given example with the known shift of 3)
The given Function with the Shift 3:
func main () {
var textN string = "Wenn man jetzt diesen Text Cäsar-Codieren möchte, dann macht man das so!" // Variable was given by tacher
var textC string = "Zhqq pdq mhwdw glhvhq Whbw Fävdu-Frglhuhq pöfkwh, gdqq pdfkw pdq gdv vr!" // Variable was given by tacher
fmt.Println(textN)
encode(textN)
decode(textC)
}
// The given Function with the shift 3:
func encode (text string){
var ergebnis string
for _,w:=range(text){
if w>='A' && w<='Z' {
if w+3>'Z'{
ergebnis += string((w+3)-'Z'+'A')
} else {
ergebnis += string(w+3)
}
} else if w>='a' && w<='z'{
if w+3>'z' {
ergebnis += string((w+3)-'z'+'a')
} else {
ergebnis += string(w+3)
}
} else {
ergebnis += string(w)
}
}
fmt.Println(ergebnis)
}
// My decode funtion with the "backwards shift" 3:
func decode (text string) {
var ergebnis string
for _,w:=range(text){
if w >='A' && w <='Z' {
if w-3 < 'A' {
// fmt.Println(string(w-3)) // Search for Mistakes made by me
ergebnis += string((w-3)+'Z'-'A')
} else {
// fmt.Println(string(w-3)) // Search for Mistakes made by me
ergebnis += string(w-3)
}
} else if w>='a' && w<='z'{
if w-3 < 'a' {
// fmt.Println(string(w-3)) // Search for Mistakes made by me
ergebnis += string((w-3)+'z'-'a')
} else {
// fmt.Println(string(w-3)) // Search for Mistakes made by me
ergebnis += string(w-3)
}
} else{
ergebnis += string(w)
}
}
fmt.Println(ergebnis)
}
Now the problem is: textN isn't equal to decode(textC), my code seems to fail at lowercase letters that shifted back 3 letters don't become the decoded letters they should be. I saw this at "z" and "x" and i dont have a clue why. I tried highering/lowering the shift but that didn't work, changed plusses and minusses and bigger lower signs. I don't know what to try and am thankful in advance.

Answer 1

there is one solution you can use, i can provide other slower one, but more interesting if you want.
func Chipher(input string, shift int) string {
bts := []rune(input)
var cursor *rune
// i em using it like this to remove repetition but its little advanced
// it may make you suspicious to teacher
shiftFn := func(start, end rune) bool {
r := *cursor
// not in range we cannot shift it
if r < start || r > end {
return false
}
res := start + (r-start+rune(shift))%(end-start)
if res < start {
res += end - start + 1
}
*cursor = res
return true
}
for i := range bts {
cursor = &bts[i]
// this is little trick, if one of expresions returns true, expressions
// after will not get executed as result would be true anyway
_ = shiftFn('a', 'z') || shiftFn('A', 'Z') || shiftFn('0', '9')
}
return string(bts)
}
now this is all beautiful but we have to do tests as well
func TestChipher(t *testing.T) {
testCases := []struct {
desc string
input, output string
shift int
}{
{
desc: "simple shift",
input: "abcd",
shift: 1,
output: "bcde",
},
{
desc: "negative shift",
input: "abcd",
shift: -1,
output: "zabc",
},
{
desc: "numbers",
input: "0123",
shift: 1,
output: "1234",
},
{
desc: "capital letters",
input: "ABCD",
shift: 1,
output: "BCDE",
},
{
desc: "big shift",
input: "ABCD",
shift: 1000,
output: "ABCD",
},
}
for _, tC := range testCases {
t.Run(tC.desc, func(t *testing.T) {
res := Chipher(tC.input, tC.shift)
if res != tC.output {
t.Errorf("\n%s :result\n%s :expected", res, tC.output)
}
})
}
}
hope you will learn new things from this