Share standard font metrics across documents

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Unreleased
 
+- Reuse the glyph width and kerning tables of a standard font across documents instead of building them again for every document that uses it, which made a short document using two standard fonts more than twice as slow to create
 - Fix `doc.table()` parsing a font passed as binary data (e.g. registered with `registerFont(name, uint8Array)`) again for every cell, when it restores the font after measuring and drawing the cell, which made tables with such fonts slower. A font loaded from the same `Uint8Array` is now reused without parsing it again
 - Add `lineBreak: 'whitespace'` to `doc.text()`, which breaks a line only at whitespace, a soft hyphen or a zero-width space. The Unicode line breaking rules the wrapper follows also allow a break after a hyphen or a slash, so `design-system` or `https://github.com/foliojs/pdfkit` could be cut at a line end, and a text extractor that joins the lines, such as a résumé parser, reads two words
 - Add an `objectStreams` option, which packs the objects that are not streams into compressed object streams and writes a cross-reference stream instead of the xref table, making documents with many such objects, e.g. tagged ones, much smaller. It requires a `pdfVersion` of `1.5` or higher, and cannot be used with encryption or PDF/A-1. Fixes #1810

lib/font/standard.js

@@ -1,13 +1,21 @@
 import AFMFont from './afm';
 import PDFFont from '../font';
 
+// AFMFont only reads its data, so every document can share the one built for
+// a given font data instead of rebuilding its glyph width and kerning tables.
+const afmFonts = new WeakMap();
+
 class StandardFont extends PDFFont {
   constructor(document, fontData, id) {
     super();
     this.document = document;
     this.name = fontData.name;
     this.id = id;
-    this.font = new AFMFont(fontData);
+    this.font = afmFonts.get(fontData);
+    if (!this.font) {
+      this.font = new AFMFont(fontData);
+      afmFonts.set(fontData, this.font);
+    }
     ({
       ascender: this.ascender,
       descender: this.descender,

tests/unit/font.spec.js

@@ -454,3 +454,13 @@ describe('binary font sources', () => {
     expect(doc._font.id).toBe(first.id);
   });
 });
+
+describe('standard font metrics', () => {
+  test('are shared across documents', () => {
+    const first = new PDFDocument().font('Helvetica-Bold')._font;
+    const second = new PDFDocument().font('Helvetica-Bold')._font;
+
+    expect(second).not.toBe(first);
+    expect(second.font).toBe(first.font);
+  });
+});