Add element-swapping support

Author: CTMacUser

Sources/EmbeddedIntegerCollection/EmbeddedIntegerCollection.swift

@@ -170,6 +170,11 @@ extension EmbeddedIntegerCollection: RandomAccessCollection, MutableCollection {
     }
   }
 
+  public mutating func swapAt(_ i: Int, _ j: Int) {
+    let flipMask = Wrapped(self[i] ^ self[j])
+    word ^= flipMask << abs(i) | flipMask << abs(j)
+  }
+
   @inlinable
   public var indices: Indices {
     .init(every: Element.bitWidth, over: startIndex..<endIndex)

Tests/EmbeddedIntegerCollectionTests/EmbeddedIntegerCollectionTests.swift

@@ -197,3 +197,29 @@ func moreIndexChecks(_ startingBitRange: EmbeddedIteratorDirection) async throws
   }
   #expect(manualIndices.elementsEqual(collectionIndices))
 }
+
+@Test("Element swapping", arguments: EmbeddedIteratorDirection.allCases)
+func elementSwap(_ startingBitRange: EmbeddedIteratorDirection) async throws {
+  var collection = EmbeddedIntegerCollection<UInt32, UInt8>(
+    iteratingFrom: startingBitRange
+  )
+  let firstIndex = collection.startIndex
+  let secondIndex = collection.index(after: firstIndex)
+  let thirdIndex = collection.index(after: secondIndex)
+  let fourthIndex = collection.index(after: thirdIndex)
+  assert(collection.index(after: fourthIndex) == collection.endIndex)
+  collection[firstIndex] = 0x40
+  collection[secondIndex] = 0x41
+  collection[thirdIndex] = 0x42
+  collection[fourthIndex] = 0x43
+
+  #expect(collection.elementsEqual([0x40, 0x41, 0x42, 0x43]))
+  collection.swapAt(secondIndex, secondIndex)
+  #expect(collection.elementsEqual([0x40, 0x41, 0x42, 0x43]))
+  collection.swapAt(firstIndex, thirdIndex)
+  #expect(collection.elementsEqual([0x42, 0x41, 0x40, 0x43]))
+  collection.swapAt(firstIndex, thirdIndex)
+  #expect(collection.elementsEqual([0x40, 0x41, 0x42, 0x43]))
+  collection.swapAt(fourthIndex, secondIndex)
+  #expect(collection.elementsEqual([0x40, 0x43, 0x42, 0x41]))
+}