Add calculateRetryDelayFactor helper function

This is used to calculate the exponentially increasing delay. The
function is used to calculate the reconnection delay - but does not keep
track of the current reconnection attempt. Keeping track of the current
reconnection attempt is reserved for the next few commits.

__tests__/index/retryConnectionDelay.test.ts

@@ -1,6 +1,8 @@
 // File Dependencies
 import Sarus from "../../src/index";
 import { WS } from "jest-websocket-mock";
+import { calculateRetryDelayFactor } from "../../src/index";
+import type { ExponentialBackoffParams } from "../../src/index";
 
 const url = "ws://localhost:1234";
 
@@ -61,3 +63,30 @@ describe("retry connection delay", () => {
     });
   });
 });
+
+describe("Exponential backoff delay", () => {
+  it("will never be more than 8000 ms with rate set to 2", () => {
+    // The initial delay shall be 1 s
+    const initialDelay = 1000;
+    const exponentialBackoffParams: ExponentialBackoffParams = {
+      backoffRate: 2,
+      // We put the ceiling at exactly 8000 ms
+      backoffLimit: 8000,
+    };
+    expect(
+      calculateRetryDelayFactor(exponentialBackoffParams, initialDelay, 0),
+    ).toBe(1000);
+    expect(
+      calculateRetryDelayFactor(exponentialBackoffParams, initialDelay, 1),
+    ).toBe(2000);
+    expect(
+      calculateRetryDelayFactor(exponentialBackoffParams, initialDelay, 2),
+    ).toBe(4000);
+    expect(
+      calculateRetryDelayFactor(exponentialBackoffParams, initialDelay, 3),
+    ).toBe(8000);
+    expect(
+      calculateRetryDelayFactor(exponentialBackoffParams, initialDelay, 4),
+    ).toBe(8000);
+  });
+});

src/index.ts

@@ -78,6 +78,29 @@ export interface ExponentialBackoffParams {
   backoffLimit: number;
 }
 
+/*
+ * Calculate the exponential backoff delay for a given number of connection
+ * attempts.
+ * @param {ExponentialBackoffParams} params - configuration parameters for
+ * exponential backoff.
+ * @param {number} initialDelay - the initial delay before any backoff is
+ * applied
+ * @param {number} connectionAttempts - the number of connection attempts
+ * that have previously failed, excluding the original connection attempt that
+ * succeeded
+ * @returns {void} - set does not return
+ */
+export function calculateRetryDelayFactor(
+  params: ExponentialBackoffParams,
+  initialDelay: number,
+  connectionAttempts: number,
+): number {
+  return Math.min(
+    initialDelay * Math.pow(params.backoffRate, connectionAttempts),
+    params.backoffLimit,
+  );
+}
+
 export interface SarusClassParams {
   url: string;
   binaryType?: BinaryType;
@@ -363,12 +386,20 @@ export default class Sarus {
   }
 
   /**
-   * Reconnects the WebSocket client based on the retryConnectionDelay setting.
+   * Reconnects the WebSocket client based on the retryConnectionDelay and
+   * ExponentialBackoffParam setting.
    */
   reconnect() {
     const self = this;
-    const { retryConnectionDelay } = self;
-    setTimeout(self.connect, retryConnectionDelay);
+    const { retryConnectionDelay, exponentialBackoff } = self;
+
+    // If no exponential backoff is enabled, retryConnectionDelay will
+    // be scaled by a factor of 1 and it will stay the original value.
+    const delay = exponentialBackoff
+      ? calculateRetryDelayFactor(exponentialBackoff, retryConnectionDelay, 0)
+      : retryConnectionDelay;
+
+    setTimeout(self.connect, delay);
   }
 
   /**