package main

import (
    "fmt"
    "unicode"
    "unicode/utf8"
)

// no encoding
func reverseBytes(s string) string {
    r := make([]byte, len(s))
    for i := 0; i < len(s); i++ {
        r[i] = s[len(s)-1-i]
    }
    return string(r)
}

// reverseCodePoints interprets its argument as UTF-8 and ignores bytes
// that do not form valid UTF-8.  return value is UTF-8.
func reverseCodePoints(s string) string {
    r := make([]rune, len(s))
    start := len(s)
    for _, c := range s {
        // quietly skip invalid UTF-8
        if c != utf8.RuneError {
            start--
            r[start] = c
        }
    }
    return string(r[start:])
}

// reversePreservingCombiningCharacters interprets its argument as UTF-8
// and ignores bytes that do not form valid UTF-8.  return value is UTF-8.
func reversePreservingCombiningCharacters(s string) string {
    if s == "" {
        return ""
    }
    p := []rune(s)
    r := make([]rune, len(p))
    start := len(r)
    for i := 0; i < len(p); {
        // quietly skip invalid UTF-8
        if p[i] == utf8.RuneError {
            i++
            continue
        }
        j := i + 1
        for j < len(p) && (unicode.Is(unicode.Mn, p[j]) ||
            unicode.Is(unicode.Me, p[j]) || unicode.Is(unicode.Mc, p[j])) {
            j++
        }
        for k := j - 1; k >= i; k-- {
            start--
            r[start] = p[k]
        }
        i = j
    }
    return (string(r[start:]))
}

func main() {
    test("asdf")
    test("as⃝df̅")
}

func test(s string) {
    fmt.Println("\noriginal:      ", []byte(s), s)
    r := reverseBytes(s)
    fmt.Println("reversed bytes:", []byte(r), r)
    fmt.Println("original code points:", []rune(s), s)
    r = reverseCodePoints(s)
    fmt.Println("reversed code points:", []rune(r), r)
    r = reversePreservingCombiningCharacters(s)
    fmt.Println("combining characters:", []rune(r), r)
}