1 files changed, 79 insertions, 0 deletions
diff --git a/src/compiler/optimize/postcss-scoped-styles/index.ts b/src/compiler/optimize/postcss-scoped-styles/index.ts
new file mode 100644
index 000000000..7949f63b5
--- /dev/null
+++ b/src/compiler/optimize/postcss-scoped-styles/index.ts
@@ -0,0 +1,79 @@
+import { Plugin } from 'postcss';
+
+interface AstroScopedOptions {
+  className: string;
+}
+
+interface Selector {
+  start: number;
+  end: number;
+  value: string;
+}
+
+const CSS_SEPARATORS = new Set([' ', ',', '+', '>', '~']);
+
+/**
+ * Scope Selectors
+ * Given a selector string (`.btn>span,.nav>span`), add an additional CSS class to every selector (`.btn.myClass>span.myClass,.nav.myClass>span.myClass`)
+ * @param {string} selector The minified selector string to parse. Cannot contain arbitrary whitespace (other than child selector syntax).
+ * @param {string} className The CSS class to apply.
+ */
+export function scopeSelectors(selector: string, className: string) {
+  const c = className.replace(/^\.?/, '.'); // make sure class always has leading '.'
+  const selectors: Selector[] = [];
+  let ss = selector; // final output
+
+  // Pass 1: parse selector string; extract top-level selectors
+  let start = 0;
+  let lastValue = '';
+  for (let n = 0; n < ss.length; n++) {
+    const isEnd = n === selector.length - 1;
+    if (isEnd || CSS_SEPARATORS.has(selector[n])) {
+      lastValue = selector.substring(start, isEnd ? undefined : n);
+      if (!lastValue) continue;
+      selectors.push({ start, end: isEnd ? n + 1 : n, value: lastValue });
+      start = n + 1;
+    }
+  }
+
+  // Pass 2: starting from end, transform selectors w/ scoped class
+  for (let i = selectors.length - 1; i >= 0; i--) {
+    const { start, end, value } = selectors[i];
+    const head = ss.substring(0, start);
+    const tail = ss.substring(end);
+
+    // replace '*' with className
+    if (value === '*') {
+      ss = head + c + tail;
+      continue;
+    }
+
+    // leave :global() alone!
+    if (value.startsWith(':global(')) {
+      ss = head + ss.substring(start, end).replace(':global(', '').replace(')', '') + tail;
+      continue;
+    }
+
+    // scope everything else
+    let newSelector = ss.substring(start, end);
+    const pseudoIndex = newSelector.indexOf(':');
+    if (pseudoIndex > 0) {
+      // if there‘s a pseudoclass (:focus)
+      ss = head + newSelector.substring(start, pseudoIndex) + c + newSelector.substr(pseudoIndex) + tail;
+    } else {
+      ss = head + newSelector + c + tail;
+    }
+  }
+
+  return ss;
+}
+
+/** PostCSS Scope plugin */
+export default function astroScopedStyles(options: AstroScopedOptions): Plugin {
+  return {
+    postcssPlugin: '@astro/postcss-scoped-styles',
+    Rule(rule) {
+      rule.selector = scopeSelectors(rule.selector, options.className);
+    },
+  };
+}