Fix RandomAccessCollection + lowerBound API (#579)

Author: Kyle-Ye

Sources/OpenSwiftUICore/Util/StandardLibraryAdditions.swift

@@ -593,28 +593,35 @@ extension BidirectionalCollection where Self: MutableCollection, Element: Compar
     }
 }
 
-// MARK: - RandomAccessCollection Extensions [Copilot]
+// MARK: - RandomAccessCollection Extensions [6.5.4]
 
 extension RandomAccessCollection {
     package func lowerBound(_ predicate: (Element) -> Bool) -> Index {
-        var left = startIndex
-        var right = endIndex
-        
-        while left < right {
-            let mid = index(left, offsetBy: distance(from: left, to: right) / 2)
+        var result = startIndex
+        var remaining = count
+        guard remaining >= 1 else {
+            return result
+        }
+        repeat {
+            let half = remaining / 2
+            var mid = result
+            formIndex(&mid, offsetBy: half)
+
             if predicate(self[mid]) {
-                right = mid
+                result = mid
+                formIndex(after: &result)
+                remaining &+= -(half + 1)
             } else {
-                left = index(after: mid)
+                remaining = half
             }
-        }
-        return left
+        } while remaining > 0
+        return result
     }
 }
 
 extension RandomAccessCollection where Element: Comparable {
     package func lowerBound(of value: Element) -> Index {
-        lowerBound { $0 >= value }
+        lowerBound { $0 < value }
     }
 }
 

Tests/OpenSwiftUICoreTests/Util/StandardLibraryAdditionsTests.swift

@@ -310,6 +310,24 @@ struct Cache3Tests {
     }
 }
 
+// MARK: - RandomAccessCollection + lowerBound
+
+struct RandomAccessCollectionLowerBoundTests {
+    @Test
+    func customPredicate() {
+        let array = [1, 3, 5, 7, 9, 11, 13, 15]
+        let index = array.lowerBound { $0 <= 9 }
+        #expect(index == 5)
+    }
+
+    @Test
+    func defaultPredicate() {
+        let array = [1, 3, 5, 7, 9, 11, 13, 15]
+        let index = array.lowerBound(of: 7)
+        #expect(index == 3)
+    }
+}
+
 // MARK: - CollectionOfTwoTests
 
 struct CollectionOfTwoTests {