Read hdmx device records at their padded size

CHANGELOG.md

@@ -6,6 +6,7 @@
 - [BREAKING] Node builds now targets Node 22+, Browser build targets Firefox 115+, Safari 16+, Chrome 120+
 - Replace cache decorators with specialized lazy caching
 - Fix crash when using TrueType font with empty glyphs (#353, #378, #303, #312). Based on #379
+- Read hdmx device records at their padded size
 - Map space and missing code points correctly with a legacy Mac cmap (#394)
 - Keep a glyph's variation metrics stable when several composites use it
 - Return the requested code points for glyphs shared by several code points (#342) (#395)

src/tables/hdmx.js

@@ -4,6 +4,10 @@ let DeviceRecord = new r.Struct({
   pixelSize: r.uint8,
   maximumWidth: r.uint8,
   widths: new r.Array(r.uint8, (t) => t.parent.parent.maxp.numGlyphs),
+  // Each record is padded to sizeDeviceRecord bytes, a multiple of four
+  padding: new r.Reserved(r.uint8, (t) =>
+    Math.max(0, t.parent.sizeDeviceRecord - t._currentOffset),
+  ),
 });
 
 // The Horizontal Device Metrics table stores integer advance widths scaled to particular pixel sizes

test/device_metrics.js

@@ -0,0 +1,52 @@
+import * as fontkit from 'fontkit';
+import assert from 'assert';
+
+// Builds a minimal SFNT font that only holds the given tables
+function buildFont(tables) {
+  let tags = Object.keys(tables);
+  let offset = 12 + tags.length * 16;
+  let length = tags.reduce((sum, tag) => sum + tables[tag].length, offset);
+  let buffer = new Uint8Array(length);
+  let view = new DataView(buffer.buffer);
+
+  view.setUint32(0, 0x00010000);
+  view.setUint16(4, tags.length);
+
+  tags.forEach((tag, i) => {
+    let entry = 12 + i * 16;
+    for (let j = 0; j < 4; j++) {
+      buffer[entry + j] = tag.charCodeAt(j);
+    }
+
+    view.setUint32(entry + 8, offset);
+    view.setUint32(entry + 12, tables[tag].length);
+    buffer.set(tables[tag], offset);
+    offset += tables[tag].length;
+  });
+
+  return buffer;
+}
+
+describe('device metrics', function () {
+  it('should read hdmx records padded to sizeDeviceRecord', function () {
+    // With 5 glyphs, a record holds 7 bytes of data and 1 byte of padding
+    let maxp = new Uint8Array(32);
+    new DataView(maxp.buffer).setUint16(4, 5);
+
+    // prettier-ignore
+    let hdmx = new Uint8Array([
+      0, 0, // version
+      0, 2, // numRecords
+      0, 0, 0, 8, // sizeDeviceRecord
+      11, 7, 3, 4, 5, 6, 7, 0, // pixelSize, maximumWidth, widths, padding
+      12, 9, 5, 6, 7, 8, 9, 0,
+    ]);
+
+    let font = fontkit.create(buildFont({ maxp, hdmx }));
+
+    assert.deepEqual(font.hdmx.records, [
+      { pixelSize: 11, maximumWidth: 7, widths: [3, 4, 5, 6, 7] },
+      { pixelSize: 12, maximumWidth: 9, widths: [5, 6, 7, 8, 9] },
+    ]);
+  });
+});