Why does Swift String.Index keeps its index value 4 times bigger than real?

user_3600264 0
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I was trying to implement Boyer-Moore algorithm in Swift Playground and I used Swift String.Index a lot and something that started to bother me is why indexes are kept 4 times bigger that what it seems they should be.

For example:

let why = "is s on 4th position not 1st".index(of: "s")

This code in Swift Playground will generate _compoundOffset 4 not 1. I'm sure there is a reason for doing this, but I couldn't find explanation anywhere.

It's not a duplicate of any question that explains how to get index of char in Swift, I know that, I used index(of:) function just to illustrate the question. I wanted to know why value of 2nd char is 4 not 1 when using String.Index.

So I guess the way it keeps indexes is private and I don't need to know the inside implementation, it's probably connected with UTF16 and UTF32 coding.

swift-playground swift string
1 Answers

First of all, don’t ever assume _compoundOffset to be anything else than an implementation detail. _compoundOffset is an internal property of String.Index that uses bit masking to store two values in this one number:

  • The encodedOffset, which is the index's byte offset in terms of UTF-16 code units. This one is public and can be relied on. In your case encodedOffset is 1 because that's the offset for that character, as measured in UTF-16 code units. Note that the encoding of the string in memory doesn't matter! encodedOffset is always UTF-16.

  • The transcodedOffset, which stores the index's offset inside the current UTF-16 code unit. This is also an internal property that you can't access. The value is usually 0 for most indices, unless you have an index into the string's UTF-8 view that refers to a code unit which doesn't fall on a UTF-16 boundary. In that case, the transcodedOffset will store the offset in bytes from the encodedOffset.

Now why is _compoundOffset == 4? Because it stores the transcodedOffset in the two least significant bits and the encodedOffset in the 62 most significant bits. So the bit pattern for encodedOffset == 1, transcodedOffset == 0 is 0b100, which is 4.

You can verify all this in the source code for String.Index.

user_1168620
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