updated fixed window algo and test cases

Author: Yufei Wu

src/rateLimiters/fixedWindow.ts

@@ -83,18 +83,18 @@ class FixedWindow implements RateLimiter {
         }
         const window: RedisWindow = await JSON.parse(windowJSON);
 
+        const updatedUserWindow = this.updateTimeWindow(window, timestamp);
+        updatedUserWindow.currentTokens += tokens;
         // update the currentToken until reaches its capacity
-        window.currentTokens += tokens;
-        if (window.currentTokens > this.capacity) {
-            window.currentTokens = this.capacity;
+        if (updatedUserWindow.currentTokens > this.capacity) {
+            updatedUserWindow.currentTokens -= tokens;
             return {
                 success: false,
-                tokens: this.capacity - window.currentTokens,
+                tokens: this.capacity - updatedUserWindow.currentTokens,
             };
         }
 
         // update a new time window, check the current capacity situation
-        const updatedUserWindow = this.updateTimeWindow(window, timestamp);
         if (tokens > this.capacity) {
             await this.client.setex(uuid, keyExpiry, JSON.stringify(updatedUserWindow));
             return { success: false, tokens: this.capacity };
@@ -119,8 +119,13 @@ class FixedWindow implements RateLimiter {
             fixedWindowStart: window.fixedWindowStart,
         };
         if (timestamp >= window.fixedWindowStart + this.windowSize) {
-            updatedUserWindow.fixedWindowStart = window.fixedWindowStart + this.windowSize;
-            updatedUserWindow.currentTokens = 0;
+            if (timestamp >= window.fixedWindowStart + this.windowSize * 2) {
+                updatedUserWindow.fixedWindowStart = timestamp;
+                updatedUserWindow.currentTokens = 0;
+            } else {
+                updatedUserWindow.fixedWindowStart = window.fixedWindowStart + this.windowSize;
+                updatedUserWindow.currentTokens = 0;
+            }
         }
         return updatedUserWindow;
     };

test/rateLimiters/fixedWindow.test.ts

@@ -72,7 +72,7 @@ describe('Test FixedWindow Rate Limiter', () => {
                 ).toBe(false);
                 expect(
                     (await limiter.processRequest(user2, timestamp, partialWithdraw)).tokens
-                ).toBe(0);
+                ).toBe(4);
             });
         });
         describe('after a BLOCKED request...', () => {
@@ -93,7 +93,7 @@ describe('Test FixedWindow Rate Limiter', () => {
                 const result = await limiter.processRequest(user2, timestamp + WINDOW_SIZE - 1, 2);
 
                 expect(result.success).toBe(false);
-                expect(result.tokens).toBe(0);
+                expect(result.tokens).toBe(1);
 
                 // expect current tokens in the window to still be 9
                 expect((await getWindowFromClient(client, user2)).currentTokens).toBe(9);
@@ -109,19 +109,44 @@ describe('Test FixedWindow Rate Limiter', () => {
                 const noAccess = await limiter.processRequest(
                     user3,
                     timestamp + WINDOW_SIZE - 1,
-                    1
+                    2
                 );
 
-                // expect cannot pass any request
+                // expect not passing any request
                 expect(noAccess.tokens).toBe(0);
                 expect(noAccess.success).toBe(false);
 
-                const newRequest = 3;
-                await setTokenCountInClient(client, user3, newRequest, timestamp + WINDOW_SIZE + 1);
-                const newAccess = await limiter.processRequest(user3, timestamp, 1);
+                const newRequest = 1;
+                expect(
+                    (await limiter.processRequest(user3, timestamp + WINDOW_SIZE, newRequest))
+                        .success
+                ).toBe(true);
+                const count = await getWindowFromClient(client, user3);
+                expect(count.currentTokens).toBe(1);
+            });
+            test('Request will be passed after two window sizes', async () => {
+                const fullRequest = 10;
+                await setTokenCountInClient(client, user3, fullRequest, timestamp);
+                const noAccess = await limiter.processRequest(
+                    user3,
+                    timestamp + WINDOW_SIZE - 1,
+                    2
+                );
+
+                // expect not passing any request
+                expect(noAccess.tokens).toBe(0);
+                expect(noAccess.success).toBe(false);
+
+                const newRequest = 6;
+                // check if current time is over one window size
+                const newAccess = await limiter.processRequest(
+                    user3,
+                    timestamp + WINDOW_SIZE * 2,
+                    newRequest
+                );
 
+                expect(newAccess.tokens).toBe(4);
                 expect(newAccess.success).toBe(true);
-                expect(newAccess.tokens).toBe(6);
             });
         });
     });