reinstate formatting of atrule prelude (1:1 copy

Author: bartveneman

index.ts

@@ -389,11 +389,33 @@ export function format(css: string, { minify = false, tab_size = undefined }: Fo
 		return lines.join(NEWLINE)
 	}
 
+	/**
+	 * Pretty-printing atrule preludes takes an insane amount of rules,
+	 * so we're opting for a couple of 'good-enough' string replacements
+	 * here to force some nice formatting.
+	 * Should be OK perf-wise, since the amount of atrules in most
+	 * stylesheets are limited, so this won't be called too often.
+	 */
+	function print_atrule_prelude(prelude: string): string {
+		return prelude
+			.replace(/\s*([:,])/g, prelude.toLowerCase().includes('selector(') ? '$1' : '$1 ') // force whitespace after colon or comma, except inside `selector()`
+			.replace(/\)([a-zA-Z])/g, ') $1') // force whitespace between closing parenthesis and following text (usually and|or)
+			.replace(/\s*(=>|<=)\s*/g, ' $1 ') // force whitespace around => and <=
+			.replace(/([^<>=\s])([<>])([^<>=\s])/g, `$1${OPTIONAL_SPACE}$2${OPTIONAL_SPACE}$3`) // add spacing around < or > except when it's part of <=, >=, =>
+			.replace(/\s+/g, OPTIONAL_SPACE) // collapse multiple whitespaces into one
+			.replace(/calc\(\s*([^()+\-*/]+)\s*([*/+-])\s*([^()+\-*/]+)\s*\)/g, (_, left, operator, right) => {
+				// force required or optional whitespace around * and / in calc()
+				let space = operator === '+' || operator === '-' ? SPACE : OPTIONAL_SPACE
+				return `calc(${left.trim()}${space}${operator}${space}${right.trim()})`
+			})
+			.replace(/selector|url|supports|layer\(/gi, (match) => match.toLowerCase()) // lowercase function names
+	}
+
 	function print_atrule(node: CSSNode): string {
 		let lines = []
 		let name = [`@`, node.name.toLowerCase()]
 		if (node.prelude !== null) {
-			name.push(SPACE, node.prelude)
+			name.push(SPACE, print_atrule_prelude(node.prelude))
 		}
 		if (node.block === null) {
 			name.push(SEMICOLON)