1 files changed, 73 insertions, 0 deletions
diff --git a/packages/astro/src/assets/fonts/implementations/font-metrics-resolver.ts b/packages/astro/src/assets/fonts/implementations/font-metrics-resolver.ts
new file mode 100644
index 000000000..4541479d0
--- /dev/null
+++ b/packages/astro/src/assets/fonts/implementations/font-metrics-resolver.ts
@@ -0,0 +1,73 @@
+import { fromBuffer, type Font } from '@capsizecss/unpack';
+import type { CssRenderer, FontFetcher, FontMetricsResolver } from '../definitions.js';
+import type { FontFaceMetrics } from '../types.js';
+import { renderFontSrc } from '../utils.js';
+
+// Source: https://github.com/unjs/fontaine/blob/main/src/metrics.ts
+function filterRequiredMetrics({
+	ascent,
+	descent,
+	lineGap,
+	unitsPerEm,
+	xWidthAvg,
+}: Pick<Font, 'ascent' | 'descent' | 'lineGap' | 'unitsPerEm' | 'xWidthAvg'>) {
+	return {
+		ascent,
+		descent,
+		lineGap,
+		unitsPerEm,
+		xWidthAvg,
+	};
+}
+
+// Source: https://github.com/unjs/fontaine/blob/f00f84032c5d5da72c8798eae4cd68d3ddfbf340/src/css.ts#L7
+function toPercentage(value: number, fractionDigits = 4) {
+	const percentage = value * 100;
+	return `${+percentage.toFixed(fractionDigits)}%`;
+}
+
+export function createCapsizeFontMetricsResolver({
+	fontFetcher,
+	cssRenderer,
+}: {
+	fontFetcher: FontFetcher;
+	cssRenderer: CssRenderer;
+}): FontMetricsResolver {
+	const cache: Record<string, FontFaceMetrics | null> = {};
+
+	return {
+		async getMetrics(name, { hash, url }) {
+			cache[name] ??= filterRequiredMetrics(await fromBuffer(await fontFetcher.fetch(hash, url)));
+			return cache[name];
+		},
+		// Source: https://github.com/unjs/fontaine/blob/f00f84032c5d5da72c8798eae4cd68d3ddfbf340/src/css.ts#L170
+		generateFontFace({
+			metrics,
+			fallbackMetrics,
+			name: fallbackName,
+			font: fallbackFontName,
+			properties,
+		}) {
+			// Calculate size adjust
+			const preferredFontXAvgRatio = metrics.xWidthAvg / metrics.unitsPerEm;
+			const fallbackFontXAvgRatio = fallbackMetrics.xWidthAvg / fallbackMetrics.unitsPerEm;
+			const sizeAdjust = preferredFontXAvgRatio / fallbackFontXAvgRatio;
+
+			const adjustedEmSquare = metrics.unitsPerEm * sizeAdjust;
+
+			// Calculate metric overrides for preferred font
+			const ascentOverride = metrics.ascent / adjustedEmSquare;
+			const descentOverride = Math.abs(metrics.descent) / adjustedEmSquare;
+			const lineGapOverride = metrics.lineGap / adjustedEmSquare;
+
+			return cssRenderer.generateFontFace(fallbackName, {
+				src: renderFontSrc([{ name: fallbackFontName }]),
+				'size-adjust': toPercentage(sizeAdjust),
+				'ascent-override': toPercentage(ascentOverride),
+				'descent-override': toPercentage(descentOverride),
+				'line-gap-override': toPercentage(lineGapOverride),
+				...properties,
+			});
+		},
+	};
+}