feat(retry): introduce fibonacci delay strategy (#74)

Author: ns-vasilev

.swiftlint.yml

@@ -98,7 +98,7 @@ identifier_name:
   excluded:
     - id
     - URL
-
+    - n
 analyzer_rules:
   - unused_import
   - unused_declaration

README.md

@@ -104,6 +104,9 @@ case constant(retry: Int, duration: DispatchTimeInterval)
 /// A retry strategy with a linearly increasing delay.
 case linear(retry: UInt, duration: DispatchTimeInterval)
 
+/// A retry strategy with a Fibonacci-based delay progression.
+case fibonacci(retry: UInt, duration: DispatchTimeInterval)
+
 /// A retry strategy with exponential increase in duration between retries and added jitter.
 case exponential(
     retry: Int, 

Sources/Typhoon/Classes/RetrySequence/Strategies/FibonacciDelayStrategy.swift

@@ -0,0 +1,56 @@
+//
+// Typhoon
+// Copyright © 2026 Space Code. All rights reserved.
+//
+
+import Foundation
+
+/// A retry delay strategy that increases the delay
+/// following the Fibonacci sequence.
+///
+/// The delay is calculated as:
+/// `baseDuration * fibonacci(retryIndex + 1)`
+struct FibonacciDelayStrategy: IRetryDelayStrategy {
+    // MARK: - Properties
+
+    /// The base delay interval.
+    ///
+    /// Each retry multiplies this value by
+    /// the corresponding Fibonacci number.
+    let duration: DispatchTimeInterval
+
+    // MARK: - IRetryDelayStrategy
+
+    /// Calculates a delay based on the Fibonacci sequence.
+    ///
+    /// - Parameter retries: The current retry attempt index (starting from `0`).
+    /// - Returns: The delay in nanoseconds.
+    func delay(forRetry retries: UInt) -> UInt64? {
+        guard let seconds = duration.double else { return .zero }
+
+        let fib = fibonacci(retries + 1)
+        let delay = seconds * Double(fib)
+
+        return (delay * .nanosec).safeUInt64
+    }
+
+    // MARK: - Private
+
+    /// Returns the Fibonacci number for a given index.
+    ///
+    /// Uses an iterative approach to avoid recursion overhead.
+    private func fibonacci(_ n: UInt) -> UInt {
+        guard n > 1 else { return 1 }
+
+        var previous: UInt = 1
+        var current: UInt = 1
+
+        for _ in 2 ..< n {
+            let next = previous + current
+            previous = current
+            current = next
+        }
+
+        return current
+    }
+}

Sources/Typhoon/Classes/Strategy/RetryPolicyStrategy.swift

@@ -27,6 +27,17 @@ public enum RetryPolicyStrategy: Sendable {
     ///               the linear backoff interval.
     case linear(retry: UInt, duration: DispatchTimeInterval)
 
+    /// A retry strategy with a Fibonacci-based delay progression.
+    ///
+    /// The delay grows according to the Fibonacci sequence:
+    /// `duration * fibonacci(retryIndex + 1)`.
+    ///
+    /// - Parameters:
+    ///   - retry: The maximum number of retry attempts.
+    ///   - duration: The base delay used to calculate
+    ///               the Fibonacci backoff interval.
+    case fibonacci(retry: UInt, duration: DispatchTimeInterval)
+
     /// A retry strategy with exponential increase in duration between retries and added jitter.
     ///
     /// - Parameters:
@@ -52,6 +63,8 @@ public enum RetryPolicyStrategy: Sendable {
             retry
         case let .linear(retry, _):
             retry
+        case let .fibonacci(retry, _):
+            retry
         }
     }
 
@@ -64,6 +77,8 @@ public enum RetryPolicyStrategy: Sendable {
             duration
         case let .linear(_, duration):
             duration
+        case let .fibonacci(_, duration):
+            duration
         }
     }
 }
@@ -82,6 +97,8 @@ extension RetryPolicyStrategy {
             ConstantDelayStrategy(duration: duration)
         case let .linear(_, duration):
             LinearDelayStrategy(duration: duration)
+        case let .fibonacci(_, duration):
+            FibonacciDelayStrategy(duration: duration)
         }
     }
 }

Tests/TyphoonTests/UnitTests/RetryPolicyStrategyTests.swift

@@ -42,6 +42,14 @@ final class RetryPolicyStrategyTests: XCTestCase {
         // then
         XCTAssertEqual(duration, .second)
     }
+
+    func test_thatRetryPolicyStrategyReturnsDuration_whenTypeIsFibonacci() {
+        // when
+        let duration = RetryPolicyStrategy.fibonacci(retry: .retry, duration: .second).duration
+
+        // then
+        XCTAssertEqual(duration, .second)
+    }
 }
 
 // MARK: Constants

Tests/TyphoonTests/UnitTests/RetrySequenceTests.swift

@@ -33,6 +33,17 @@ final class RetrySequenceTests: XCTestCase {
         XCTAssertEqual(result, [1, 2, 3, 4, 5, 6, 7, 8])
     }
 
+    func test_thatRetrySequenceCreatesASequence_whenStrategyIsFibonacci() {
+        // given
+        let sequence = RetrySequence(strategy: .fibonacci(retry: .retry, duration: .nanosecond))
+
+        // when
+        let result: [UInt64] = sequence.map { $0 }
+
+        // then
+        XCTAssertEqual(result, [1, 1, 2, 3, 5, 8, 13, 21])
+    }
+
     func test_thatRetrySequenceCreatesASequence_whenStrategyIsExponential() {
         // given
         let sequence = RetrySequence(strategy: .exponential(retry: .retry, jitterFactor: .zero, duration: .nanosecond))