`SortedSet` returns different results for equal indexes.

[Moriquendi]

I encountered the issue when trying to get an absolute position of the element and got incorrect results.
I started a conversation on forum because I wasn't sure if it's a bug.

let index = set.index(of: X)
print(index == set.startIndex) // true
print(set[index] == set[set.startIndex]) // false

Information

Package version: Branch feature/SortedCollections
Platform version: macOS 12.2 Beta (21D5025f)
Swift version:

swift-driver version: 1.26.21 Apple Swift version 5.5.2 (swiftlang-1300.0.47.5 clang-1300.0.29.30)
Target: arm64-apple-macosx12.0

Checklist

• If possible, I've reproduced the issue using the main branch of this package.
• I've searched for existing GitHub issues.

Steps to Reproduce

Sample code:

var set = SortedSet<Int>()
        
for i in 0..<50 {
   set.insert(i)
}
        
set.forEach {
   let i = set.index(of: $0)!
   let v = set[i]
   let d = set.distance(from: set.startIndex, to: i)
   print("\(v) --> \(d)")

   if i == set.startIndex {
      print("Current `index` equals `startIndex`.")
      print("set[index] = \(set[i]). set[startIndex] = \(set[set.startIndex])")
   }
}

Output:

0 --> 0
Current index equals startIndex.
set[index] = 0. set[startIndex] = 0
1 --> 1
2 --> 0
Current index equals startIndex.
set[index] = 2. set[startIndex] = 0
3 --> 3
4 --> 4
5 --> 1
6 --> 6
7 --> 7
8 --> 0
...

Expected behavior

When indexes are equal, getting a value for those indexes should return the same value.

Actual behavior

Even though indexes are equal, getting a value for them returns different results.

[lorentey]

Yep, this is definitely a bug!

(Beware, SortedSet isn't ready for production use yet. These reports are very much appreciated, though!)

[Moriquendi]

(Beware, SortedSet isn't ready for production use yet. These reports are very much appreciated, though!)

Yup, I saw the Readme notice but I needed a SortedSet in my app and thought I'll give Swift Collections a try :)
In the future, if time allows, I'll try to contribute fixes on my own. But in the meantime, I hope these bug reports will be helpful :)

[SaudKadiri]

After spending some time I figured out:

1. The bug is across the SortedCollections module i.e in SortedSet, SortedDictionary and _BTree(root cause of the bug lies in here).

• Observed in SortedSet

func test_indexOf() {
  var set = SortedSet<Int>()
      
  for i in 0..<50 {
    set.insert(i)
  }
  for (item, index) in zip(set, set.indices) {
      let i = set.index(of: item)!
      if i != index {
        print("Some issue with index(of:)")
        break
      }
  }
}

Output:

Some issue with index(of:)

• Observed in SortedDictionary

func test_indexForKey() {
  var sortedDict: SortedDictionary<Int, String> = [:]
  for i in 0..<50 {
    sortedDict[i] = "\(i)"
  }
  for (index, item) in zip(sortedDict.indices, sortedDict) {
    let i = sortedDict.index(forKey: item.key)!
    if i != index {
      print("Some issue with index(forKey:)")
      break
    } 
  }
}

Output:

Some issue with index(forKey:)

2. SortedSet.index(of:) and SortedDictionary.index(forKey:) are basically a call to _BTrees's findAnyIndex(forKey key: Key) -> Index? function. So let's test if _BTree.findAnyIndex(forKey:) is correct. I used this test case.

func test_findAnyIndex() {
  var tree = _BTree<Int, String>()
  for (key, value) in (0..<50).map({ ($0, "\($0)") }) {
    tree.updateAnyValue(value, forKey: key)
  }
  for (index, item) in zip(tree.indices, tree) { 
    let i = tree.findAnyIndex(forKey: item.key)
    if i != index {
      print("Problems with findAnyIndex")
      break
    }
  }
}

Output:

Problems with findAnyIndex

3. I then wrote my own findAnyIndex as such (O(log n) implementation):

extension _BTree {
  @inlinable
  internal func findAnyIndex(forKey key: Key) -> Index? {
    var (lo, hi) = (startIndex, endIndex)
    while lo < hi {
        let mid = index(lo, offsetBy: distance(from: lo, to: hi)/2)
        if self[mid].key < key {
            lo = index(after: mid)
        } else {
            hi = mid
        }
    }
    return lo < endIndex && self[lo].key == key ? lo : nil
  }
}

and all the bugs vanished.
The test case in the issue gives the following output now.

0 --> 0
Current `index` equals `startIndex`.
set[index] = 0. set[startIndex] = 0
1 --> 1
2 --> 2
3 --> 3
4 --> 4
5 --> 5
6 --> 6
7 --> 7
8 --> 8
9 --> 9
10 --> 10
11 --> 11
12 --> 12
13 --> 13
14 --> 14
15 --> 15
16 --> 16
17 --> 17
18 --> 18
19 --> 19
20 --> 20
21 --> 21
22 --> 22
23 --> 23
24 --> 24
25 --> 25
26 --> 26
27 --> 27
28 --> 28
29 --> 29
30 --> 30
31 --> 31
32 --> 32
33 --> 33
34 --> 34
35 --> 35
36 --> 36
37 --> 37
38 --> 38
39 --> 39
40 --> 40
41 --> 41
42 --> 42
43 --> 43
44 --> 44
45 --> 45
46 --> 46
47 --> 47
48 --> 48
49 --> 49

And also the test cases I made are all working just fine now.

Hope this helps!🤝