calculating a backoff value for auto-reconnect timing (#11)

Sources/AutomergeRepo/Backoff.swift

@@ -0,0 +1,27 @@
+import Foundation
+
+public enum Backoff {
+    // fibonacci numbers for 0...15
+    static let fibonacci = [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610]
+
+    public static func delay(_ step: UInt, withJitter: Bool) -> Int {
+        let boundedStep = Int(min(15, step))
+
+        if withJitter {
+            // pick a range of +/- values that's one fibonacci step lower
+            let jitterStep = max(min(15,boundedStep - 1),0)
+            if jitterStep < 1 {
+                // picking a random number between -0 and 0 is just silly, and kinda wrong
+                // so just return the fibonacci number
+                return Self.fibonacci[boundedStep]
+            }
+            let jitterRange = -1*Self.fibonacci[jitterStep]...Self.fibonacci[jitterStep]
+            let selectedValue = Int.random(in: jitterRange)
+            // no delay should be less than 0
+            let adjustedValue = max(0, Self.fibonacci[boundedStep] + selectedValue)
+            // max value is 987, min value is 0
+            return adjustedValue
+        }
+        return Self.fibonacci[boundedStep]
+    }
+}

Tests/AutomergeRepoTests/BackoffTests.swift

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+import AutomergeRepo
+import XCTest
+
+final class BackoffTests: XCTestCase {
+    func testSimpleBackoff() throws {
+        XCTAssertEqual(0, Backoff.delay(0, withJitter: false))
+        XCTAssertEqual(1, Backoff.delay(1, withJitter: false))
+        XCTAssertEqual(1, Backoff.delay(2, withJitter: false))
+        XCTAssertEqual(2, Backoff.delay(3, withJitter: false))
+        XCTAssertEqual(3, Backoff.delay(4, withJitter: false))
+        XCTAssertEqual(610, Backoff.delay(15, withJitter: false))
+        XCTAssertEqual(610, Backoff.delay(16, withJitter: false))
+        XCTAssertEqual(610, Backoff.delay(100, withJitter: false))
+    }
+
+    func testWithJitterBackoff() throws {
+        XCTAssertEqual(0, Backoff.delay(0, withJitter: true))
+        XCTAssertEqual(1, Backoff.delay(1, withJitter: true))
+        for i: UInt in 2...50 {
+            XCTAssertTrue(Backoff.delay(i, withJitter: true) <= 987)
+            print(Backoff.delay(i, withJitter: true))
+        }
+    }
+
+}