Ensure CommonPrefix.Iterator keeps returning nil after it has done so once

Author: Tim Vermeulen

Sources/Algorithms/CommonPrefix.swift

@@ -44,6 +44,9 @@ extension CommonPrefix: Sequence {
     @usableFromInline
     internal let areEquivalent: (Base.Element, Other.Element) -> Bool
 
+    @usableFromInline
+    internal var isDone = false
+    
     @inlinable
     internal init(
       base: Base.Iterator,
@@ -56,11 +59,13 @@ extension CommonPrefix: Sequence {
     }
 
     public mutating func next() -> Base.Element? {
-      if let next = base.next(),
+      if !isDone,
+         let next = base.next(),
          let otherNext = other.next(),
          areEquivalent(next, otherNext) {
         return next
       } else {
+        isDone = true
         return nil
       }
     }

Tests/SwiftAlgorithmsTests/CommonPrefixTests.swift

@@ -43,6 +43,15 @@ final class CommonPrefixTests: XCTestCase {
     XCTAssertLazyCollection([1, 2, 3].lazy.commonPrefix(with: [4, 5, 6]))
   }
 
+  func testCommonPrefixIteratorKeepsReturningNil() {
+    var iter = AnySequence("12A34").commonPrefix(with: "12B34").makeIterator()
+    XCTAssertEqual(iter.next(), "1")
+    XCTAssertEqual(iter.next(), "2")
+    XCTAssertEqual(iter.next(), nil)
+    XCTAssertEqual(iter.next(), nil)
+    XCTAssertEqual(iter.next(), nil)
+  }
+  
   func testCommonSuffix() {
     func testCommonSuffix(of a: String, and b: String, equals c: String) {
       XCTAssertEqualSequences(a.commonSuffix(with: b, by: ==), c)