Add quick reuse of default weight/width/slope grades in customized build (#1215).

README.md

@@ -2955,19 +2955,19 @@ Subsection `variants` is used to configure character variants in the font. Prope
 
 #### Configuring Weights, Widths and Slopes
 
-Subsection `weights` is used to change the weight grades that the custom family needs. It is a dictionary of sub-objects with properties:
+Subsection `weights` is used to change the weight grades that the custom family needs. It is a dictionary of either strings formatted in `default.<weight>` format, meaning reusing a default weight grade, or sub-objects with properties:
 
 * `shape`: Number, configures the weight grade of the glyphs' shapes.
 * `menu`: Integer, configures the weight grade used when naming fonts.
 * `css`: Integer, configures the weight grade used in web font CSS.
 
-Subsection `widths` is used to change the width grades that the custom family needs. It is a dictionary of sub-objects with properties:
+Subsection `widths` is used to change the width grades that the custom family needs. It is a dictionary of either strings formatted in `default.<width>` format, meaning reusing a default width grade, or sub-objects with properties:
 
 * `shape`: Number, configures the width of the glyphs' shapes, measured in 1/1000 em.
 * `menu`: Integer, configures the width grade used when naming fonts. The valid values are `1` to `9`, inclusive.
 * `css`: String, configures the [font-stretch](https://developer.mozilla.org/en-US/docs/Web/CSS/font-stretch) value used in web font CSS.
 
-Subsection `slopes` is used to change the slope angles and grades that the custom family needs. It is a dictionary of sub-objects with properties:
+Subsection `slopes` is used to change the slope angles and grades that the custom family needs. It is a dictionary of either strings formatted in `default.<slope>` format, meaning reusing a default slope grade, or sub-objects with properties:
 
 * `angle`: Number, configures the slope angle in degrees. The valid vales are `0` to `15`, inclusive.
 * `shape`: String from `upright`, `italic` or `oblique`. Configures the slope used for variant selection. 

changes/10.2.0.md

@@ -2,3 +2,4 @@
  * Refine the alignment of Number Sign (`#`) to align with numbers (#1210).
  * Add tailless variants for Greek small Mu (`μ`) (#1211).
  * Remove unnecessary serifs of Greek Capital Ksi (`Ξ`) (#1212).
+ * Add quick reuse of default weight/width/slope grades in customized build (#1215).

verdafile.js

@@ -925,9 +925,29 @@ function validateAndShimBuildPlans(prefix, bp, dWeights, dSlopes, dWidths) {
 		);
 	}
 
-	bp.weights = bp.weights || dWeights;
+	bp.weights = shimBpAspect("weights", bp.weights, dWeights);
-	bp.slopes = bp.slopes || bp.slants || dSlopes;
+	bp.slopes = shimBpAspect("slopes", bp.slopes || bp.slants, dSlopes);
-	bp.widths = bp.widths || dWidths;
+	bp.widths = shimBpAspect("widths", bp.widths, dWidths);
+}
+function shimBpAspect(aspectName, aspect, defaultAspect) {
+	if (!aspect) return defaultAspect;
+	const result = {};
+	for (const [k, v] of Object.entries(aspect)) {
+		shimBpAspectKey(aspectName, result, k, v, defaultAspect);
+	}
+	return result;
+}
+function shimBpAspectKey(aspectName, sink, k, v, defaultAspect) {
+	if (typeof v === "string") {
+		if (!/^default\./.test(v))
+			throw new Error(`Invalid configuration '${v}' for ${aspectName}.${k}'`);
+		const remappingKey = v.slice("default.".length);
+		if (!defaultAspect[remappingKey])
+			throw new Error(`Invalid configuration '${v}' for ${aspectName}.${k}'`);
+		sink[k] = defaultAspect[remappingKey];
+	} else {
+		sink[k] = v;
+	}
 }
 
 // Recommended weight validation