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diff --git a/js/src/jsapi-tests/testAvlTree.cpp b/js/src/jsapi-tests/testAvlTree.cpp
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+++ b/js/src/jsapi-tests/testAvlTree.cpp
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <set>
+#include <stdio.h>
+
+#include "ds/AvlTree.h"
+
+#include "jsapi-tests/tests.h"
+
+using namespace js;
+
+////////////////////////////////////////////////////////////////////////
+//                                                                    //
+// AvlTree testing interface.                                         //
+//                                                                    //
+////////////////////////////////////////////////////////////////////////
+
+// The "standard" AVL interface, `class AvlTree` at the end of
+// js/src/ds/AvlTree.h, is too restrictive to allow proper testing of the AVL
+// tree internals.  In particular it disallows insertion of duplicate values
+// and removal of non-present values, and lacks various secondary functions
+// such as for counting the number of nodes.
+//
+// So, for testing, we wrap an alternative interface `AvlTreeTestIF` around
+// the core implementation.
+
+template <class T, class C>
+class AvlTreeTestIF : public AvlTreeImpl<T, C> {
+  // Shorthands for names in the implementation (parent) class.
+  using Impl = AvlTreeImpl<T, C>;
+  using ImplTag = typename AvlTreeImpl<T, C>::Tag;
+  using ImplNode = typename AvlTreeImpl<T, C>::Node;
+  using ImplResult = typename AvlTreeImpl<T, C>::Result;
+  using ImplNodeAndResult = typename AvlTreeImpl<T, C>::NodeAndResult;
+
+ public:
+  explicit AvlTreeTestIF(LifoAlloc* alloc = nullptr) : Impl(alloc) {}
+
+  // Insert `v` if it isn't already there, else leave the tree unchanged.
+  // Returns true iff an insertion happened.
+  bool testInsert(const T& v) {
+    ImplNode* new_root = Impl::insert_worker(v);
+    if (!new_root) {
+      // OOM
+      MOZ_CRASH();
+    }
+    if (uintptr_t(new_root) == uintptr_t(1)) {
+      // Already present
+      return false;
+    }
+    Impl::root_ = new_root;
+    return true;
+  }
+
+  // Remove `v` if it is present.  Returns true iff a removal happened.
+  bool testRemove(const T& v) {
+    ImplNodeAndResult pair = Impl::delete_worker(Impl::root_, v);
+    ImplNode* new_root = pair.first;
+    ImplResult res = pair.second;
+    if (res == ImplResult::Error) {
+      // `v` isn't in the tree.
+      return false;
+    } else {
+      Impl::root_ = new_root;
+      return true;
+    }
+  }
+
+  // Count number of elements
+  size_t testSize_worker(ImplNode* n) const {
+    if (n) {
+      return 1 + testSize_worker(n->left) + testSize_worker(n->right);
+    }
+    return 0;
+  }
+  size_t testSize() const { return testSize_worker(Impl::root_); }
+
+  size_t testDepth_worker(ImplNode* n) const {
+    if (n) {
+      size_t depthL = testDepth_worker(n->left);
+      size_t depthR = testDepth_worker(n->right);
+      return 1 + (depthL > depthR ? depthL : depthR);
+    }
+    return 0;
+  }
+  size_t testDepth() const { return testDepth_worker(Impl::root_); }
+
+  bool testContains(const T& v) const {
+    ImplNode* node = Impl::find_worker(v);
+    return node != nullptr;
+  }
+
+  ImplNode* testGetRoot() const { return Impl::root_; }
+  ImplNode* testGetFreeList() const { return Impl::freeList_; }
+
+  bool testFreeListLooksValid(size_t maxElems) {
+    size_t numElems = 0;
+    ImplNode* node = Impl::freeList_;
+    while (node) {
+      numElems++;
+      if (numElems > maxElems) {
+        return false;
+      }
+      if (node->tag != ImplTag::Free || node->right != nullptr) {
+        return false;
+      }
+      node = node->left;
+    }
+    return true;
+  }
+
+  // For debugging only
+ private:
+  void testShow_worker(int depth, const ImplNode* node) const {
+    if (node) {
+      testShow_worker(depth + 1, node->right);
+      for (int i = 0; i < depth; i++) {
+        printf("   ");
+      }
+      char* str = node->item.show();
+      printf("%s\n", str);
+      free(str);
+      testShow_worker(depth + 1, node->left);
+    }
+  }
+
+ public:
+  // For debugging only
+  void testShow() const { testShow_worker(0, Impl::root_); }
+
+  // AvlTree::Iter is also public; it's just pass-through from AvlTreeImpl.
+};
+
+////////////////////////////////////////////////////////////////////////
+//                                                                    //
+// AvlTree test cases.                                                //
+//                                                                    //
+////////////////////////////////////////////////////////////////////////
+
+class CmpInt {
+  int x_;
+
+ public:
+  explicit CmpInt(int x) : x_(x) {}
+  ~CmpInt() {}
+  static int compare(const CmpInt& me, const CmpInt& other) {
+    if (me.x_ < other.x_) return -1;
+    if (me.x_ > other.x_) return 1;
+    return 0;
+  }
+  int get() const { return x_; }
+  char* show() const {
+    const size_t length = 16;
+    char* str = (char*)calloc(length, 1);
+    snprintf(str, length, "%d", x_);
+    return str;
+  }
+};
+
+bool TreeIsPlausible(const AvlTreeTestIF<CmpInt, CmpInt>& tree,
+                     const std::set<int>& should_be_in_tree, int UNIV_MIN,
+                     int UNIV_MAX) {
+  // Same cardinality
+  size_t n_in_set = should_be_in_tree.size();
+  size_t n_in_tree = tree.testSize();
+  if (n_in_set != n_in_tree) {
+    return false;
+  }
+
+  // Tree is not wildly out of balance.  Depth should not exceed 1.44 *
+  // log2(size).
+  size_t tree_depth = tree.testDepth();
+  size_t log2_size = 0;
+  {
+    size_t n = n_in_tree;
+    while (n > 0) {
+      n = n >> 1;
+      log2_size += 1;
+    }
+  }
+  // Actually a tighter limit than stated above.  For these test cases, the
+  // tree is either perfectly balanced or within one level of being so (hence
+  // the +1).
+  if (tree_depth > log2_size + 1) {
+    return false;
+  }
+
+  // Check that everything that should be in the tree is in it, and vice
+  // versa.
+  for (int i = UNIV_MIN; i < UNIV_MAX; i++) {
+    bool should_be_in = should_be_in_tree.find(i) != should_be_in_tree.end();
+
+    // Look it up with a null comparator (so `contains` compares
+    // directly)
+    bool is_in = tree.testContains(CmpInt(i));
+    if (is_in != should_be_in) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+template <typename T>
+bool SetContains(std::set<T> s, const T& v) {
+  return s.find(v) != s.end();
+}
+
+BEGIN_TEST(testAvlTree_main) {
+  static const int UNIV_MIN = 5000;
+  static const int UNIV_MAX = 5999;
+  static const int UNIV_SIZE = UNIV_MAX - UNIV_MIN + 1;
+
+  LifoAlloc alloc(4096);
+  AvlTreeTestIF<CmpInt, CmpInt> tree(&alloc);
+  std::set<int> should_be_in_tree;
+
+  // Add numbers to the tree, checking as we go.
+  for (int i = UNIV_MIN; i < UNIV_MAX; i++) {
+    // Idiotic but simple
+    if (i % 3 != 0) {
+      continue;
+    }
+    bool was_added = tree.testInsert(CmpInt(i));
+    should_be_in_tree.insert(i);
+    CHECK(was_added);
+    CHECK(TreeIsPlausible(tree, should_be_in_tree, UNIV_MIN, UNIV_MAX));
+  }
+
+  // Then remove the middle half of the tree, also checking.
+  for (int i = UNIV_MIN + UNIV_SIZE / 4; i < UNIV_MIN + 3 * (UNIV_SIZE / 4);
+       i++) {
+    // Note that here, we're asking to delete a bunch of numbers that aren't
+    // in the tree.  It should remain valid throughout.
+    bool was_removed = tree.testRemove(CmpInt(i));
+    bool should_have_been_removed = SetContains(should_be_in_tree, i);
+    CHECK(was_removed == should_have_been_removed);
+    should_be_in_tree.erase(i);
+    CHECK(TreeIsPlausible(tree, should_be_in_tree, UNIV_MIN, UNIV_MAX));
+  }
+
+  // Now add some numbers which are already in the tree.
+  for (int i = UNIV_MIN; i < UNIV_MIN + UNIV_SIZE / 4; i++) {
+    if (i % 3 != 0) {
+      continue;
+    }
+    bool was_added = tree.testInsert(CmpInt(i));
+    bool should_have_been_added = !SetContains(should_be_in_tree, i);
+    CHECK(was_added == should_have_been_added);
+    should_be_in_tree.insert(i);
+    CHECK(TreeIsPlausible(tree, should_be_in_tree, UNIV_MIN, UNIV_MAX));
+  }
+
+  // Then remove all numbers from the tree, in reverse order.
+  for (int ir = UNIV_MIN; ir < UNIV_MAX; ir++) {
+    int i = UNIV_MIN + (UNIV_MAX - ir);
+    bool was_removed = tree.testRemove(CmpInt(i));
+    bool should_have_been_removed = SetContains(should_be_in_tree, i);
+    CHECK(was_removed == should_have_been_removed);
+    should_be_in_tree.erase(i);
+    CHECK(TreeIsPlausible(tree, should_be_in_tree, UNIV_MIN, UNIV_MAX));
+  }
+
+  // Now the tree should be empty.
+  CHECK(should_be_in_tree.empty());
+  CHECK(tree.testSize() == 0);
+
+  // Now delete some more stuff.  Tree should still be empty :-)
+  for (int i = UNIV_MIN + 10; i < UNIV_MIN + 100; i++) {
+    CHECK(should_be_in_tree.empty());
+    CHECK(tree.testSize() == 0);
+    bool was_removed = tree.testRemove(CmpInt(i));
+    CHECK(!was_removed);
+    CHECK(TreeIsPlausible(tree, should_be_in_tree, UNIV_MIN, UNIV_MAX));
+  }
+
+  // The tree root should be NULL.
+  CHECK(tree.testGetRoot() == nullptr);
+  CHECK(tree.testGetFreeList() != nullptr);
+
+  // Check the freelist to the extent we can: it's non-circular, and the
+  // elements look plausible.  If it's not shorter than the specified length
+  // then it must have a cycle, since the tests above won't have resulted in
+  // more than 400 free nodes at the end.
+  CHECK(tree.testFreeListLooksValid(400 /*arbitrary*/));
+
+  // Test iteration, first in a tree with 9 nodes.  This tests the general
+  // case.
+  {
+    CHECK(tree.testSize() == 0);
+    for (int i = 10; i < 100; i += 10) {
+      bool was_inserted = tree.testInsert(CmpInt(i));
+      CHECK(was_inserted);
+    }
+
+    // Test iteration across the whole tree.
+    AvlTreeTestIF<CmpInt, CmpInt>::Iter iter(&tree);
+    // `expect` produces (independently) the next expected number.  `remaining`
+    // counts the number items of items remaining.
+    int expect = 10;
+    int remaining = 9;
+    while (iter.hasMore()) {
+      CmpInt ci = iter.next();
+      CHECK(ci.get() == expect);
+      expect += 10;
+      remaining--;
+    }
+    CHECK(remaining == 0);
+
+    // Test iteration from a specified start point.
+    for (int i = 10; i < 100; i += 10) {
+      for (int j = i - 1; j <= i + 1; j++) {
+        // Set up `expect` and `remaining`.
+        remaining = (100 - i) / 10;
+        switch (j % 10) {
+          case 0:
+            expect = j;
+            break;
+          case 1:
+            expect = j + 9;
+            remaining--;
+            break;
+          case 9:
+            expect = j + 1;
+            break;
+          default:
+            MOZ_CRASH();
+        }
+        AvlTreeTestIF<CmpInt, CmpInt>::Iter iter(&tree, CmpInt(j));
+        while (iter.hasMore()) {
+          CmpInt ci = iter.next();
+          CHECK(ci.get() == expect);
+          expect += 10;
+          remaining--;
+        }
+        CHECK(remaining == 0);
+      }
+    }
+  }
+
+  // Now with a completely empty tree.
+  {
+    AvlTreeTestIF<CmpInt, CmpInt> emptyTree(&alloc);
+    CHECK(emptyTree.testSize() == 0);
+    // Full tree iteration gets us nothing.
+    AvlTreeTestIF<CmpInt, CmpInt>::Iter iter1(&emptyTree);
+    CHECK(!iter1.hasMore());
+    // Starting iteration with any number gets us nothing.
+    AvlTreeTestIF<CmpInt, CmpInt>::Iter iter2(&emptyTree, CmpInt(42));
+    CHECK(!iter2.hasMore());
+  }
+
+  // Finally with a one-element tree.
+  {
+    AvlTreeTestIF<CmpInt, CmpInt> unitTree(&alloc);
+    bool was_inserted = unitTree.testInsert(CmpInt(1337));
+    CHECK(was_inserted);
+    CHECK(unitTree.testSize() == 1);
+    // Try full tree iteration.
+    AvlTreeTestIF<CmpInt, CmpInt>::Iter iter3(&unitTree);
+    CHECK(iter3.hasMore());
+    CmpInt ci = iter3.next();
+    CHECK(ci.get() == 1337);
+    CHECK(!iter3.hasMore());
+    for (int i = 1336; i <= 1338; i++) {
+      int remaining = i < 1338 ? 1 : 0;
+      int expect = i < 1338 ? 1337 : 99999 /*we'll never use this*/;
+      AvlTreeTestIF<CmpInt, CmpInt>::Iter iter4(&unitTree, CmpInt(i));
+      while (iter4.hasMore()) {
+        CmpInt ci = iter4.next();
+        CHECK(ci.get() == expect);
+        remaining--;
+        // expect doesn't change, we only expect it (or nothing)
+      }
+      CHECK(remaining == 0);
+    }
+  }
+
+  return true;
+}
+END_TEST(testAvlTree_main)