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build/node_modules/postcss-discard-overridden/index.js

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+import postcss from 'postcss';
+
+const OVERRIDABLE_RULES = ['keyframes', 'counter-style'];
+const SCOPE_RULES = ['media', 'supports'];
+
+function isOverridable(name) {
+    return OVERRIDABLE_RULES.indexOf(postcss.vendor.unprefixed(name)) !== -1;
+}
+
+function isScope(name) {
+    return SCOPE_RULES.indexOf(postcss.vendor.unprefixed(name)) !== -1;
+}
+
+function getScope(node) {
+    let current = node.parent;
+    let chain = [node.name, node.params];
+    do {
+        if (current.type === 'atrule' && isScope(current.name)) {
+            chain.unshift(current.name + ' ' + current.params);
+        }
+        current = current.parent;
+    } while (current);
+    return chain.join('|');
+}
+
+export default postcss.plugin('postcss-discard-overridden', () => {
+    return css => {
+        let cache = {};
+        let rules = [];
+        css.walkAtRules(rule => {
+            if (rule.type === 'atrule' && isOverridable(rule.name)) {
+                let scope = getScope(rule);
+                cache[scope] = rule;
+                rules.push({
+                    node: rule,
+                    scope
+                });
+            }
+        });
+        rules.forEach(rule => {
+            if (cache[rule.scope] !== rule.node) {
+                rule.node.remove();
+            }
+        });
+    };
+});