UTF8 String length and indices in Go vs Swift - swift

Question

I have apps in Go and Swift which process strings, such as finding substrings and their indices. At first it worked nicely even with multi-byte characters (e.g. emojis), using to Go's utf8.RuneCountInString() and Swift's native String.
But there are some UTF8 characters that break the string length and indices for substrings, e.g. a string "Lorem 😂😃✌️🤔 ipsum":
Go's utf8.RuneCountInString("Lorem 😂😃✌️🤔 ipsum") returns 17 and the start index of ipsum is 12.
Swift's "Lorem 😂😃✌️🤔 ipsum".count returns 16 and the start index of ipsum is 11.
Using Swift String's utf8, utf16 or casting to NSString gives also different lengths and indices. There are also other emojis composed from multiple other emoji's like 👨‍👩‍👧‍👦 which gives even funnier numbers.
This is with Go 1.8 and Swift 4.1.
Is there any way to get the same string lengths and substrings' indices with same values with Go and Swift?
EDIT
I created a Swift String extension based on #MartinR's great answer:
extension String {
func runesRangeToNSRange(from: Int, to: Int) -> NSRange {
let length = to - from
let start = unicodeScalars.index(unicodeScalars.startIndex, offsetBy: from)
let end = unicodeScalars.index(start, offsetBy: length)
let range = start..<end
return NSRange(range, in: self)
}
}

Answer 1

In Swift a Character is an “extended grapheme cluster,” and each of "😂", "😃", "✌️", "🤔", "👨‍👩‍👧‍👦" counts as a single character.
I have no experience with Go, but as I understand it from Strings, bytes, runes and characters in Go,
a “rune” is a Unicode code point, which essentially corresponds to a UnicodeScalar in Swift.
In your example, the difference comes from "✌️" which
counts as a single Swift character, but is built from two Unicode scalars:
print("✌️".count) // 1
print("✌️".unicodeScalars.count) // 2
Here is an example how you can compute the length and offsets in
terms of Unicode scalars:
let s = "Lorem 😂😃✌️🤔 ipsum"
print(s.unicodeScalars.count) // 17
if let idx = s.range(of: "ipsum") {
print(s.unicodeScalars.distance(from: s.startIndex, to: idx.lowerBound)) // 12
}
As you can see, this gives the same numbers as in your example from Go.

Answer 2

A rune in Go identifies a specific UTF-8 code point; that does not necessarily mean it maps 1:1 to visually distinct characters. Some characters may be made up of multiple runes/code points, therefor counting runes may not give you what you'd expect from a visual inspection of the string. I don't know what "some text".count actually counts in Swift so I can't offer any comparison there.