feat: add AniList rate limiter and remaining backlog tasks

src/core/services/anilist/rate-limiter.ts

@@ -0,0 +1,72 @@
+const DEFAULT_MAX_PER_MINUTE = 20;
+const WINDOW_MS = 60_000;
+const SAFETY_REMAINING_THRESHOLD = 5;
+
+export interface AnilistRateLimiter {
+  acquire(): Promise<void>;
+  recordResponse(headers: Headers): void;
+}
+
+export function createAnilistRateLimiter(
+  maxPerMinute = DEFAULT_MAX_PER_MINUTE,
+): AnilistRateLimiter {
+  const timestamps: number[] = [];
+  let pauseUntilMs = 0;
+
+  function pruneOld(now: number): void {
+    const cutoff = now - WINDOW_MS;
+    while (timestamps.length > 0 && timestamps[0]! < cutoff) {
+      timestamps.shift();
+    }
+  }
+
+  return {
+    async acquire(): Promise<void> {
+      const now = Date.now();
+
+      if (now < pauseUntilMs) {
+        const waitMs = pauseUntilMs - now;
+        await new Promise((resolve) => setTimeout(resolve, waitMs));
+      }
+
+      pruneOld(Date.now());
+
+      if (timestamps.length >= maxPerMinute) {
+        const oldest = timestamps[0]!;
+        const waitMs = oldest + WINDOW_MS - Date.now() + 100;
+        if (waitMs > 0) {
+          await new Promise((resolve) => setTimeout(resolve, waitMs));
+        }
+        pruneOld(Date.now());
+      }
+
+      timestamps.push(Date.now());
+    },
+
+    recordResponse(headers: Headers): void {
+      const remaining = headers.get('x-ratelimit-remaining');
+      if (remaining !== null) {
+        const n = parseInt(remaining, 10);
+        if (Number.isFinite(n) && n < SAFETY_REMAINING_THRESHOLD) {
+          const reset = headers.get('x-ratelimit-reset');
+          if (reset) {
+            const resetMs = parseInt(reset, 10) * 1000;
+            if (Number.isFinite(resetMs)) {
+              pauseUntilMs = Math.max(pauseUntilMs, resetMs);
+            }
+          } else {
+            pauseUntilMs = Math.max(pauseUntilMs, Date.now() + WINDOW_MS);
+          }
+        }
+      }
+
+      const retryAfter = headers.get('retry-after');
+      if (retryAfter) {
+        const seconds = parseInt(retryAfter, 10);
+        if (Number.isFinite(seconds) && seconds > 0) {
+          pauseUntilMs = Math.max(pauseUntilMs, Date.now() + seconds * 1000);
+        }
+      }
+    },
+  };
+}