Runes and Bytes

A Go string is a read-only slice of bytes, and its bytes are UTF-8. That one sentence explains most of what surprises people about strings in Go.

len(s) counts bytes. Indexing, s[0], gives you a single byte. Neither one knows anything about characters, so for "héllo" you get 6 and the first byte of h, and for "日本語" you get 9.

A rune is a Unicode code point: the thing you probably mean when you say "character". It is an alias for int32. Two operations decode UTF-8 for you:

for i, r := range s { ... }  // i is a BYTE offset, r is a rune
runes := []rune(s)           // decodes the whole string at once

Ranging over a string steps one rune at a time, but the index it hands you is where that rune starts in bytes, not which rune it is. That gap is where the bugs live.

Task

Finish the four functions in main.go.

1. ByteLen(s string) int - how many bytes.
2. RuneLen(s string) int - how many runes.
3. RuneAt(s string, i int) (rune, bool) - the rune at position i counted in runes. RuneAt("héllo", 1) is 'é', not the second byte of é. Out of range, including a negative i, returns false.
4. ReverseRunes(s string) string - reverse the string without cutting a multi-byte rune in half. "héllo" reverses to "olléh", and reversing twice must give the original back.

Hints

• for range s with no variables at all is legal and is all RuneLen needs. utf8.RuneCountInString(s) from unicode/utf8 is the standard-library way to say the same thing.

• In RuneAt, ignore the index range gives you and keep your own counter, or convert with []rune(s) and index that. The byte offset is not the rune position: in "héllo" the l after é starts at byte 3.

• Reversing bytes is the classic wrong answer. s[len(s)-1-i] walks backwards through bytes, and the two bytes of é come out swapped, which is no longer valid UTF-8. Convert to []rune first.

• Swapping two elements needs no temporary variable:

  runes[i], runes[j] = runes[j], runes[i]
  

  and a for can advance two things at once with for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1.

• string(runes) converts a []rune back to a string. string(r) on a single rune works too, but note that string(65) is "A" and not "65": turning a number into its digits is strconv, which you will meet later.