Optimize spacing variants' building

font-src/derive-spacing.mjs

@@ -0,0 +1,80 @@
+import fs from "fs";
+
+import { FontIo, Ot } from "ot-builder";
+
+import { assignFontNames, createNamingDictFromArgv } from "./gen/meta/naming.mjs";
+
+export default main;
+
+async function main(argv) {
+	const font = await readTTF(argv);
+
+	const naming = createNamingDictFromArgv(argv);
+	assignFontNames(font, naming, false);
+
+	switch (argv.shape.spacing) {
+		case "term":
+			deriveTerm(font);
+			break;
+		case "fixed":
+			deriveTerm(font);
+			deriveFixed(font);
+			break;
+	}
+
+	await saveTTF(argv, font);
+}
+
+// To derive -Term variants, simply apply NWID
+function deriveTerm(font) {
+	const gsub = font.gsub;
+	let nwidMap = new Map();
+	for (const feature of gsub.features) {
+		if (feature.tag !== "NWID") continue;
+		for (const lookup of feature.lookups) {
+			if (!(lookup instanceof Ot.Gsub.Single)) continue;
+			for (const [from, to] of lookup.mapping) {
+				nwidMap.set(from, to);
+			}
+		}
+	}
+
+	for (const [ch, g] of [...font.cmap.unicode.entries()]) {
+		const narrow = nwidMap.get(g);
+		if (narrow) font.cmap.unicode.set(ch, narrow);
+	}
+	for (const [ch, vs, g] of [...font.cmap.vs.entries()]) {
+		const narrow = nwidMap.get(g);
+		if (narrow) font.cmap.vs.set(ch, vs, narrow);
+	}
+}
+
+// In FontConfig, a font is considered "monospace" if and only if all encoded non-combining
+// characters  (AW > 0) have the same width. We use this method to validate whether our
+// "Fixed" subfamilies are properly built.
+function deriveFixed(font) {
+	const unitWidth = font.os2.xAvgCharWidth;
+	for (const [ch, g] of [...font.cmap.unicode.entries()]) {
+		const advanceWidth = g.horizontal.end - g.horizontal.start;
+		if (!(advanceWidth === 0 || advanceWidth === unitWidth)) font.cmap.unicode.delete(ch);
+	}
+	for (const [ch, vs, g] of [...font.cmap.vs.entries()]) {
+		const advanceWidth = g.horizontal.end - g.horizontal.start;
+		if (!(advanceWidth === 0 || advanceWidth === unitWidth)) font.cmap.vs.delete(ch, vs);
+	}
+}
+
+async function readTTF(argv) {
+	const buf = await fs.promises.readFile(argv.i);
+	const sfnt = FontIo.readSfntOtf(buf);
+	const font = FontIo.readFont(sfnt, Ot.ListGlyphStoreFactory);
+	return font;
+}
+async function saveTTF(argv, font) {
+	const sfnt = FontIo.writeFont(font, {
+		glyphStore: { statOs2XAvgCharWidth: false },
+		generateDummyDigitalSignature: true
+	});
+	const buf = FontIo.writeSfntOtf(sfnt);
+	await fs.promises.writeFile(argv.o, buf);
+}