Add implementation of AsyncSequence.enumerated() which is awaiting merge into apple/swift-async-algorithms version 1.1.0. (apple/swift-async-algorithms#261).

Author: Matthew Judy

Sources/Shared/AsyncSequence+Enumerated.swift

@@ -0,0 +1,78 @@
+//
+//  File.swift
+//  
+//
+//  Created by Matthew Judy on 12/21/23.
+//
+//  TODO: Use the implementation found in swift-async-algorithms when this PR is merged:
+//  https://github.com/apple/swift-async-algorithms/pull/261
+//
+
+
+
+/// An enumeration of the elements of an AsyncSequence.
+///
+/// `AsyncEnumeratedSequence` generates a sequence of pairs (*n*, *x*), where *n*s are
+/// consecutive `Int` values starting at zero, and *x*s are the elements from an
+/// base AsyncSequence.
+///
+/// To create an instance of `EnumeratedSequence`, call `enumerated()` on an
+/// AsyncSequence.
+public struct AsyncEnumeratedSequence<Base: AsyncSequence>
+{
+    @usableFromInline
+    let base: Base
+
+    @usableFromInline
+    init(_ base: Base)
+    {
+        self.base = base
+    }
+}
+
+
+extension AsyncEnumeratedSequence: AsyncSequence
+{
+    public typealias Element = (Int, Base.Element)
+
+    public struct AsyncIterator: AsyncIteratorProtocol
+    {
+        @usableFromInline
+        var baseIterator: Base.AsyncIterator
+        @usableFromInline
+        var index: Int
+
+        @usableFromInline
+        init(baseIterator: Base.AsyncIterator)
+        {
+            self.baseIterator = baseIterator
+            self.index = 0
+        }
+
+        @inlinable
+        public mutating func next() async rethrows -> AsyncEnumeratedSequence.Element?
+        {
+            let value = try await self.baseIterator.next().map { (self.index, $0) }
+            self.index += 1
+            return value
+        }
+    }
+
+    @inlinable
+    public __consuming func makeAsyncIterator() -> AsyncIterator
+    {
+        return .init(baseIterator: self.base.makeAsyncIterator())
+    }
+}
+
+
+extension AsyncEnumeratedSequence: Sendable where Base: Sendable {}
+
+
+extension AsyncSequence
+{
+    /// Return an enumaterated AsyncSequence
+    public func enumerated() -> AsyncEnumeratedSequence<Self> { return AsyncEnumeratedSequence(self) }
+}
+
+