Rune literals are just 32-bit integer values (however they’re untyped constants, so their type can change). They represent unicode codepoints. For example, the rune literal 'a' is actually the number 97.
Therefore your program is pretty much equivalent to:
package main
import "fmt"
func SwapRune(r rune) rune {
switch {
case 97 <= r && r <= 122:
return r - 32
case 65 <= r && r <= 90:
return r + 32
default:
return r
}
}
func main() {
fmt.Println(SwapRune('a'))
}
It should be obvious, if you were to look at the Unicode mapping, which is identical to ASCII in that range. Furthermore, 32 is in fact the offset between the uppercase and lowercase codepoint of the character. So by adding 32 to 'A', you get 'a' and vice versa.