feat(appliedFontsStore): create separate getEffectiveConcurrency function with proper tests

src/entities/Font/model/store/appliedFontsStore/appliedFontsStore.svelte.ts

@@ -1,4 +1,5 @@
 import { SvelteMap } from 'svelte/reactivity';
+import { getEffectiveConcurrency } from './utils/getEffectiveConcurrency/getEffectiveConcurrency';
 
 /** Loading state of a font. Failed loads may be retried up to MAX_RETRIES. */
 export type FontStatus = 'loading' | 'loaded' | 'error';
@@ -200,7 +201,7 @@ export class AppliedFontsManager {
         }
 
         // Phase 1: Concurrent fetching (I/O bound, non-blocking)
-        const concurrency = this.#getEffectiveConcurrency();
+        const concurrency = getEffectiveConcurrency();
         const buffers = new Map<string, ArrayBuffer>();
 
         for (let i = 0; i < entries.length; i += concurrency) {

src/entities/Font/model/store/appliedFontsStore/utils/getEffectiveConcurrency/getEffectiveConcurrency.test.ts

@@ -0,0 +1,41 @@
+import {
+    beforeEach,
+    describe,
+    expect,
+    it,
+} from 'vitest';
+import {
+    Concurrency,
+    getEffectiveConcurrency,
+} from './getEffectiveConcurrency';
+
+describe('getEffectiveConcurrency', () => {
+    beforeEach(() => {
+        const nav = navigator as any;
+        nav.connection = null;
+    });
+
+    it('should return MAX when connection is not available', () => {
+        const nav = navigator as any;
+        nav.connection = null;
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MAX);
+    });
+
+    it('should return MIN for slow-2g or 2g connection', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: 'slow-2g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MIN);
+    });
+
+    it('should return AVERAGE for 3g connection', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: '3g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.AVERAGE);
+    });
+
+    it('should return MAX for other connection types', () => {
+        const nav = navigator as any;
+        nav.connection = { effectiveType: '4g' };
+        expect(getEffectiveConcurrency()).toBe(Concurrency.MAX);
+    });
+});

src/entities/Font/model/store/appliedFontsStore/utils/getEffectiveConcurrency/getEffectiveConcurrency.ts

@@ -0,0 +1,26 @@
+export enum Concurrency {
+    MIN = 1,
+    AVERAGE = 2,
+    MAX = 4,
+}
+
+/**
+ * Calculates the amount of fonts for concurrent download based on the user internet connection
+ */
+export function getEffectiveConcurrency(): number {
+    const nav = navigator as any;
+    const connection = nav.connection;
+    if (!connection) {
+        return Concurrency.MAX;
+    }
+
+    switch (connection.effectiveType) {
+        case 'slow-2g':
+        case '2g':
+            return Concurrency.MIN;
+        case '3g':
+            return Concurrency.AVERAGE;
+        default:
+            return Concurrency.MAX;
+    }
+}