⚡️ Introduce cache for loaded fonts

To reduce the number of calls to the font loader and reuse loaded fonts,
this commit introduces a cache for loaded fonts. The caching includes
fontkit fonts which are now also reused.

src/fonts.test.ts

@@ -65,7 +65,7 @@ describe('fonts', () => {
       fontLoader = {
         loadFont: jest.fn(async (selector: FontSelector) => {
           if (selector.fontFamily === 'Test') return testFont;
-          throw new Error('No font defined');
+          throw new Error('No such font defined');
         }) as any,
       };
       jest.spyOn(fontkit, 'create').mockReturnValue({ fake: true } as any);
@@ -79,7 +79,7 @@ describe('fonts', () => {
       const store = createFontStore(fontLoader);
 
       await expect(store.selectFont({ fontFamily: 'foo' })).rejects.toThrowError(
-        "Could not load font for 'foo', style=normal, weight=normal: No font defined"
+        "Could not load font for 'foo', style=normal, weight=normal: No such font defined"
       );
     });
 
@@ -96,6 +96,40 @@ describe('fonts', () => {
         fkFont: { fake: true },
       });
     });
+
+    it('calls font loader only once per selector', async () => {
+      const store = createFontStore(fontLoader);
+
+      await store.selectFont({ fontFamily: 'Test' });
+      await store.selectFont({ fontFamily: 'Test', fontStyle: 'italic' });
+      await store.selectFont({ fontFamily: 'Test' });
+      await store.selectFont({ fontFamily: 'Test', fontStyle: 'italic' });
+
+      expect(fontLoader.loadFont).toHaveBeenCalledTimes(2);
+    });
+
+    it('returns same font object for concurrent calls', async () => {
+      const store = createFontStore(fontLoader);
+
+      const [font1, font2] = await Promise.all([
+        store.selectFont({ fontFamily: 'Test' }),
+        store.selectFont({ fontFamily: 'Test' }),
+      ]);
+
+      expect(font1).toBe(font2);
+    });
+
+    it('caches errors from font loader', async () => {
+      const store = createFontStore(fontLoader);
+
+      await expect(store.selectFont({ fontFamily: 'foo' })).rejects.toThrowError(
+        "Could not load font for 'foo', style=normal, weight=normal: No such font defined"
+      );
+      await expect(store.selectFont({ fontFamily: 'foo' })).rejects.toThrowError(
+        "Could not load font for 'foo', style=normal, weight=normal: No such font defined"
+      );
+      expect(fontLoader.loadFont).toHaveBeenCalledTimes(1);
+    });
   });
 
   describe('weightToNumber', () => {

src/fonts.ts

@@ -3,7 +3,7 @@ import { CustomFontSubsetEmbedder, PDFDocument, PDFFont, PDFRef } from 'pdf-lib'
 
 import { parseBinaryData } from './binary-data.js';
 import { FontStyle, FontWeight } from './content.js';
-import { FontLoader } from './font-loader.js';
+import { FontLoader, LoadedFont } from './font-loader.js';
 import { printValue } from './print-value.js';
 import {
   optional,
@@ -74,12 +74,23 @@ export function registerFont(font: Font, pdfDoc: PDFDocument) {
 }
 
 export function createFontStore(fontLoader: FontLoader): FontStore {
+  const fontCache: Record<string, Promise<Font>> = {};
+
   return {
     selectFont,
   };
 
-  async function selectFont(selector: FontSelector): Promise<Font> {
-    let loadedFont;
+  function selectFont(selector: FontSelector): Promise<Font> {
+    const cacheKey = [
+      selector.fontFamily ?? 'any',
+      selector.fontStyle ?? 'normal',
+      selector.fontWeight ?? 'normal',
+    ].join(':');
+    return (fontCache[cacheKey] ??= loadFont(selector));
+  }
+
+  async function loadFont(selector: FontSelector): Promise<Font> {
+    let loadedFont: LoadedFont;
     try {
       loadedFont = await fontLoader.loadFont(selector);
     } catch (error) {