@@ -0,0 +1,94 @@
+import ava from 'ava';
+import parser from '../index.js';
+
+// Regression tests for the quadratic parsing of flat selectors: `.` and `#` are
+// not word delimiters, so `.a.a.a...` reaches splitWord as a single word token
+// carrying n class indexes. Three passes used to scan those index arrays
+// linearly for every index, making the parse O(n^2):
+//
+//   1. uniqs(), deduplicating with filter + indexOf
+//   2. the indices.forEach loop, calling hasClass.indexOf / hasId.indexOf
+//   3. the Sass-interpolation filter, calling interpolations.indexOf
+//
+// Nesting depth is 0 for such a selector, so maxNestingDepth offers no
+// protection. All three now use Set membership tests.
+
+const N = 60000;
+
+// Fastest of a few runs, to shrug off GC pauses and scheduling noise.
+function fastest (input) {
+    let best = Infinity;
+    for (let i = 0; i < 3; i++) {
+        const start = process.hrtime.bigint();
+        parser().astSync(input);
+        best = Math.min(best, Number(process.hrtime.bigint() - start) / 1e6);
+    }
+    return best;
+}
+
+// Compare the hostile payload (one word token holding N indexes) against a
+// benign control of the same repeated atom, space separated, which forces the
+// same amount of real parsing work spread over N word tokens. Deriving the
+// budget from a measurement taken on the same machine keeps this stable across
+// CI runners and under coverage instrumentation, where an absolute time budget
+// or a raw input-scaling ratio would not be.
+//
+// The control does strictly more work than the payload once parsing is linear,
+// so a healthy ratio sits below 1. It reached 4-8 while the passes were
+// quadratic. The threshold sits well clear of both.
+function costRatio (atom) {
+    const hostile = fastest(atom.repeat(N));
+    const benign = fastest(`${atom} `.repeat(N));
+    return hostile / Math.max(benign, 1);
+}
+
+for (const [label, atom] of [
+    ['flat class selectors', '.a'],
+    ['flat id selectors', '#a'],
+    ['repeated Sass interpolations', '#{a}'],
+]) {
+    ava(`${label} parse in time proportional to their size`, t => {
+        const ratio = costRatio(atom);
+        t.true(
+            ratio < 2,
+            `parsing "${atom}" repeated ${N} times as one token cost ${ratio.toFixed(1)}x the same atoms parsed separately, which suggests a super-linear pass`
+        );
+    });
+}
+
+ava('a large flat selector still produces one node per class', t => {
+    const tree = parser().astSync('.a'.repeat(20000));
+    const nodes = tree.first.nodes;
+    t.is(nodes.length, 20000);
+    t.true(nodes.every(node => node.type === 'class' && node.value === 'a'));
+});
+
+// The leading 0 pushed into the index list collides with hasClass[0] or
+// hasId[0] whenever the word starts with `.` or `#`, so the deduplication in
+// uniqs() is load-bearing: dropping it emits a spurious first node covering the
+// whole word. These pin that down.
+ava('deduplication keeps a leading class from being emitted twice', t => {
+    t.deepEqual(
+        parser().astSync('.a.b').first.nodes.map(node => [node.type, node.value]),
+        [
+            ['class', 'a'],
+            ['class', 'b'],
+        ]
+    );
+});
+
+ava('deduplication keeps a leading id from being emitted twice', t => {
+    t.deepEqual(
+        parser().astSync('#x.y').first.nodes.map(node => [node.type, node.value]),
+        [
+            ['id', 'x'],
+            ['class', 'y'],
+        ]
+    );
+});
+
+ava('a single leading class is emitted once', t => {
+    const nodes = parser().astSync('.a').first.nodes;
+    t.is(nodes.length, 1);
+    t.is(nodes[0].type, 'class');
+});
@@ -107,9 +107,9 @@ function indexesOf (array, item) {
 }
 
 function uniqs () {
-    const list = Array.prototype.concat.apply([], arguments);
-
-    return list.filter((item, i) => i === list.indexOf(item));
+    // A Set keeps the first-occurrence order the previous filter/indexOf pass
+    // produced, without its quadratic cost on long index lists.
+    return Array.from(new Set(Array.prototype.concat.apply([], arguments)));
 }
 
 export default class Parser {
@@ -881,8 +881,15 @@ export default class Parser {
         // Eliminate Sass interpolations from the list of id indexes
         const interpolations = indexesOf(word, '#{');
         if (interpolations.length) {
-            hasId = hasId.filter(hashIndex => !~interpolations.indexOf(hashIndex));
+            const interpolationIndexes = new Set(interpolations);
+            hasId = hasId.filter(hashIndex => !interpolationIndexes.has(hashIndex));
         }
+        // Membership tests are built once. Scanning the arrays per index would
+        // make a flat selector such as `.a.a.a...` quadratic in its number of
+        // class/id indexes, which is enough to pin a CPU core on
+        // attacker-supplied input.
+        const classIndexes = new Set(hasClass);
+        const idIndexes = new Set(hasId);
         let indices = sortAsc(uniqs([0, ...hasClass, ...hasId]));
         indices.forEach((ind, i) => {
             const index = indices[i + 1] || word.length;
@@ -899,14 +906,14 @@ export default class Parser {
                 current[3],
                 current[2] + (index - 1)
             );
-            if (~hasClass.indexOf(ind)) {
+            if (classIndexes.has(ind)) {
                 let classNameOpts = {
                     value: value.slice(1),
                     source,
                     sourceIndex,
                 };
                 node = new ClassName(unescapeProp(classNameOpts, "value"));
-            } else if (~hasId.indexOf(ind)) {
+            } else if (idIndexes.has(ind)) {
                 let idOpts = {
                     value: value.slice(1),
                     source,