Why are Data.endIndex and Data.count different?

You should open an Apple Feedback for the documentation of Data.endIndex. It's incorrect.

The startIndex of Data is not promised to be zero, and this is an example of when it isn't. Using the Int subscript on Data is unfortunately very dangerous unless you know precisely how the Data was constructed (and specifically that it has a zero index).

Data uniquely mixes two facts that make it tricky to use correctly:

• It is its own Slice
• Its Index is Int

For some discussion of this, and suggested patterns, see Data.popFirst(), removeFirst() adjust indices. Also see Data ranged subscribe strange behavior for another version of this question.

When you use an expression like array[2..<array.endIndex] you are creating a slice. A slice is a sort of window onto an array (or something similar to an array). Its startIndex is not necessarily 0 and its endIndex is not necessarily one after the last index of the original.

Example:

let arr = Array(1...10)
print(arr.startIndex) // 0
print(arr.endIndex) // 10
let slice = arr[2...4]
print(slice.startIndex) // 2
print(slice.endIndex) // 5
print(slice.count) // 3

You see how this works? The slice has its own logic. Its size (count) is the size of the slice, but its index numbers come from the original array, because the slice is nothing but a pointer into a section of the original array. It has no independent existence; it is just a way of seeing, as it were.

An important consequence is that slice[0] will crash: the first available index of slice is 2, as we have already been told. This is why it is crucial to know whether you're dealing with an original array or a slice.

However, at least you have reason to know that this issue might exist, because slice has a special type — Array<Int>.SubSequence, meaning an ArraySlice. But the fact that you are encountering this by way of Data makes it more tricky, because trimmed is typed as a Data, not as a DataSlice! It is in fact a Data.SubSequence, but you have no simple way of finding that out! That's because Data.SubSequence is typealiased to Data itself. This is to be regarded as a flaw in the Data implementation.

Nevertheless, it is exactly the same phenomenon. These answers should look strangely familiar:

let str = "This is a swift bug"
let data = Data(str.utf8)
let trimmed = data[2...4]
print(trimmed.startIndex) // 2
print(trimmed.endIndex) // 5
print(trimmed.count) // 3

The best way to solve this is Don't Do That. To take a subrange of a Data as a true Data, use subdata:

let trimmed2 = data.subdata(in: 2..<5)
print(trimmed2.startIndex) // 0, and so on; it's an independent copy