Add Retry implementation & tests

Sources/Retry/Retry.swift

@@ -1,2 +1,38 @@
-// The Swift Programming Language
-// https://docs.swift.org/swift-book
+import Foundation
+
+
+enum Retry {
+    public enum Error: Swift.Error, Equatable {
+        case retryLimitExceeded(lastError: String?)
+    }
+
+    public static func backedOffDelay(baseDelay: Double, attempt: Int) -> UInt32 {
+        (pow(2, max(0, attempt - 1)) * Decimal(baseDelay) as NSDecimalNumber).uint32Value
+    }
+
+    public static func attempt<T>(_ label: String,
+                                  delay: Double = 5,
+                                  retries: Int = 5,
+                                  _ block: () throws -> T) throws -> T {
+        var retriesLeft = retries
+        var currentTry = 1
+        var lastError: String?
+        while true {
+            if currentTry > 1 {
+                print("\(label) (attempt \(currentTry))")
+            }
+            do {
+                return try block()
+            } catch {
+                lastError = "\(error)"
+                guard retriesLeft > 0 else { break }
+                let delay = backedOffDelay(baseDelay: delay, attempt: currentTry)
+                print("Retrying in \(delay) seconds ...")
+                sleep(delay)
+                currentTry += 1
+                retriesLeft -= 1
+            }
+        }
+        throw Error.retryLimitExceeded(lastError: lastError)
+    }
+}

Tests/RetryTests/RetryTests.swift

@@ -2,11 +2,62 @@ import XCTest
 @testable import Retry
 
 final class RetryTests: XCTestCase {
-    func testExample() throws {
-        // XCTest Documentation
-        // https://developer.apple.com/documentation/xctest
 
-        // Defining Test Cases and Test Methods
-        // https://developer.apple.com/documentation/xctest/defining_test_cases_and_test_methods
+    func test_backedOffDelay() throws {
+        XCTAssertEqual(Retry.backedOffDelay(baseDelay: 1, attempt: 0), 1)
+        XCTAssertEqual(Retry.backedOffDelay(baseDelay: 1, attempt: 1), 1)
+        XCTAssertEqual(Retry.backedOffDelay(baseDelay: 1, attempt: 2), 2)
+        XCTAssertEqual(Retry.backedOffDelay(baseDelay: 1, attempt: 3), 4)
+        XCTAssertEqual(Retry.backedOffDelay(baseDelay: 1, attempt: 4), 8)
     }
+
+    func test_attempt_immediate_success() throws {
+        var called = 0
+
+        // MUT
+        try Retry.attempt("", delay: 0, retries: 3) {
+            called += 1
+        }
+
+        // validation
+        XCTAssertEqual(called, 1)
+    }
+
+    func test_attempt_success_after_retry() throws {
+        var called = 0
+        struct Error: Swift.Error { }
+
+        // MUT
+        try Retry.attempt("", delay: 0, retries: 3) {
+            called += 1
+            if called < 3 {
+                throw Error()
+            }
+        }
+
+        // validation
+        XCTAssertEqual(called, 3)
+    }
+
+    func test_attempt_retryLimitExceeded() throws {
+        var called = 0
+        struct Error: Swift.Error, CustomStringConvertible {
+            var description: String { "test error" }
+        }
+
+        // MUT
+        XCTAssertThrowsError(
+            try Retry.attempt("", delay: 0, retries: 3) {
+                called += 1
+                throw Error()
+            }
+
+        ) { error in
+            XCTAssertEqual(error as? Retry.Error, .retryLimitExceeded(lastError: "test error"))
+        }
+
+        // validation
+        XCTAssertEqual(called, 4)
+    }
+
 }