Add Max2Sequence to be used in ConnectionPool (#419)

Sources/ConnectionPoolModule/Max2Sequence.swift

@@ -0,0 +1,95 @@
+// A `Sequence` that can contain at most two elements. However it does not heap allocate.
+@usableFromInline
+struct Max2Sequence<Element>: Sequence {
+    @usableFromInline
+    private(set) var first: Element?
+    @usableFromInline
+    private(set) var second: Element?
+
+    @inlinable
+    var count: Int {
+        if self.first == nil { return 0 }
+        if self.second == nil { return 1 }
+        return 2
+    }
+
+    @inlinable
+    var isEmpty: Bool {
+        self.first == nil
+    }
+
+    @inlinable
+    init(_ first: Element?, _ second: Element? = nil) {
+        if let first = first {
+            self.first = first
+            self.second = second
+        } else {
+            self.first = second
+            self.second = nil
+        }
+    }
+
+    @inlinable
+    init() {
+        self.first = nil
+        self.second = nil
+    }
+
+    @inlinable
+    func makeIterator() -> Iterator {
+        Iterator(first: self.first, second: self.second)
+    }
+
+    @usableFromInline
+    struct Iterator: IteratorProtocol {
+        @usableFromInline
+        let first: Element?
+        @usableFromInline
+        let second: Element?
+
+        @usableFromInline
+        private(set) var index: UInt8 = 0
+
+        @inlinable
+        init(first: Element?, second: Element?) {
+            self.first = first
+            self.second = second
+            self.index = 0
+        }
+
+        @inlinable
+        mutating func next() -> Element? {
+            switch self.index {
+            case 0:
+                self.index += 1
+                return self.first
+            case 1:
+                self.index += 1
+                return self.second
+            default:
+                return nil
+            }
+        }
+    }
+
+    @inlinable
+    mutating func append(_ element: Element) {
+        precondition(self.second == nil)
+        if self.first == nil {
+            self.first = element
+        } else if self.second == nil {
+            self.second = element
+        } else {
+            fatalError("Max2Sequence can only hold two Elements.")
+        }
+    }
+
+    @inlinable
+    func map<NewElement>(_ transform: (Element) throws -> (NewElement)) rethrows -> Max2Sequence<NewElement> {
+        try Max2Sequence<NewElement>(self.first.flatMap(transform), self.second.flatMap(transform))
+    }
+}
+
+extension Max2Sequence: Equatable where Element: Equatable {}
+extension Max2Sequence: Hashable where Element: Hashable {}
+extension Max2Sequence: Sendable where Element: Sendable {}

Tests/ConnectionPoolModuleTests/Max2SequenceTests.swift

@@ -0,0 +1,60 @@
+@testable import _ConnectionPoolModule
+import XCTest
+
+final class Max2SequenceTests: XCTestCase {
+    func testCountAndIsEmpty() async {
+        var sequence = Max2Sequence<Int>()
+        XCTAssertEqual(sequence.count, 0)
+        XCTAssertEqual(sequence.isEmpty, true)
+        sequence.append(1)
+        XCTAssertEqual(sequence.count, 1)
+        XCTAssertEqual(sequence.isEmpty, false)
+        sequence.append(2)
+        XCTAssertEqual(sequence.count, 2)
+        XCTAssertEqual(sequence.isEmpty, false)
+    }
+
+    func testOptionalInitializer() {
+        let emptySequence = Max2Sequence<Int>(nil, nil)
+        XCTAssertEqual(emptySequence.count, 0)
+        XCTAssertEqual(emptySequence.isEmpty, true)
+        var emptySequenceIterator = emptySequence.makeIterator()
+        XCTAssertNil(emptySequenceIterator.next())
+        XCTAssertNil(emptySequenceIterator.next())
+        XCTAssertNil(emptySequenceIterator.next())
+
+        let oneElemSequence1 = Max2Sequence<Int>(1, nil)
+        XCTAssertEqual(oneElemSequence1.count, 1)
+        XCTAssertEqual(oneElemSequence1.isEmpty, false)
+        var oneElemSequence1Iterator = oneElemSequence1.makeIterator()
+        XCTAssertEqual(oneElemSequence1Iterator.next(), 1)
+        XCTAssertNil(oneElemSequence1Iterator.next())
+        XCTAssertNil(oneElemSequence1Iterator.next())
+
+        let oneElemSequence2 = Max2Sequence<Int>(nil, 2)
+        XCTAssertEqual(oneElemSequence2.count, 1)
+        XCTAssertEqual(oneElemSequence2.isEmpty, false)
+        var oneElemSequence2Iterator = oneElemSequence2.makeIterator()
+        XCTAssertEqual(oneElemSequence2Iterator.next(), 2)
+        XCTAssertNil(oneElemSequence2Iterator.next())
+        XCTAssertNil(oneElemSequence2Iterator.next())
+
+        let twoElemSequence = Max2Sequence<Int>(1, 2)
+        XCTAssertEqual(twoElemSequence.count, 2)
+        XCTAssertEqual(twoElemSequence.isEmpty, false)
+        var twoElemSequenceIterator = twoElemSequence.makeIterator()
+        XCTAssertEqual(twoElemSequenceIterator.next(), 1)
+        XCTAssertEqual(twoElemSequenceIterator.next(), 2)
+        XCTAssertNil(twoElemSequenceIterator.next())
+    }
+
+    func testMap() {
+        let twoElemSequence = Max2Sequence<Int>(1, 2).map({ "\($0)" })
+        XCTAssertEqual(twoElemSequence.count, 2)
+        XCTAssertEqual(twoElemSequence.isEmpty, false)
+        var twoElemSequenceIterator = twoElemSequence.makeIterator()
+        XCTAssertEqual(twoElemSequenceIterator.next(), "1")
+        XCTAssertEqual(twoElemSequenceIterator.next(), "2")
+        XCTAssertNil(twoElemSequenceIterator.next())
+    }
+}