removeUnusedvars.ts

import postcss from "postcss";

interface UseRecord {
	declarations: Set<postcss.Declaration>;
	dependencies: Set<string>;
	uses: number;
}

const varRegex = /var\(\s*(?<name>--[^ ,);]+)/g;

/**
 * Adapted from https://github.com/tomasklaen/postcss-prune-var/blob/57ad5041806b73479d0c558e0b1a918803cb7cbc/index.js
 */
export function removeUnusedCssVars(css: string) {
	const root = postcss.parse(css);

	const records = new Map<string, UseRecord>();

	const getRecord = (variable: string): UseRecord => {
		let record = records.get(variable);
		if (!record) {
			record = { declarations: new Set(), dependencies: new Set(), uses: 0 };
			records.set(variable, record);
		}
		return record;
	};

	const registerUse = (variable: string, ignoreList = new Set<string>()) => {
		const record = getRecord(variable);
		record.uses++;
		ignoreList.add(variable);
		for (const dependency of record.dependencies) {
			if (!ignoreList.has(dependency))
				registerUse(dependency, ignoreList);
		}
	};

	const registerDependency = (variable: string, dependency: string) => {
		const record = getRecord(variable);
		record.dependencies.add(dependency);
	};

	// Detect variable uses
	root.walkDecls((decl) => {
		const parent = decl.parent;
		if (!parent)
			return;

		if (parent.type === "rule" && (parent as postcss.Rule).selector === ":root")
			return;

		const isVar = decl.prop.startsWith("--");

		// Initiate record
		if (isVar)
			getRecord(decl.prop).declarations.add(decl);

		if (!decl.value.includes("var("))
			return;

		for (const match of decl.value.matchAll(varRegex)) {
			const variable = match.groups?.name.trim();
			if (!variable)
				continue;

			if (isVar)
				registerDependency(decl.prop, variable);
			else
				registerUse(variable);
		}
	});

	// Remove unused variables
	for (const { declarations, uses } of records.values()) {
		if (uses === 0) {
			for (const decl of declarations) {
				if (decl.parent?.nodes.length === 1)
					decl.parent?.remove();
				else decl.remove();
			}
		}
	}

	return root.toString();
}