libc/src/__support/freetrie.cpp

//===-- Implementation for freetrie ---------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "freetrie.h"
#include "src/__support/libc_assert.h"

namespace LIBC_NAMESPACE_DECL {

void FreeTrie::remove(Node *node) {
  LIBC_ASSERT(!empty() && "cannot remove from empty trie");
  node->integrity_check();
  FreeList list = node;
  list.pop();
  Node *new_node = static_cast<Node *>(list.begin());
  if (!new_node) {
    // The freelist is empty. Replace the subtrie root with an arbitrary leaf.
    // This is legal because there is no relationship between the size of the
    // root and its children.
    Node *leaf = node;
    while (leaf->lower || leaf->upper) {
      leaf->integrity_check();
      leaf = leaf->lower ? leaf->lower : leaf->upper;
    }
    leaf->integrity_check();
    if (leaf == node) {
      // If the root is a leaf, then removing it empties the subtrie.
      replace_node(node, nullptr);
      return;
    }

    replace_node(leaf, nullptr);
    new_node = leaf;
  }

  if (!is_head(node))
    return;

  // Copy the trie links to the new head.
  new_node->lower = node->lower;
  new_node->upper = node->upper;
  new_node->parent = node->parent;
  replace_node(node, new_node);
}

void FreeTrie::replace_node(Node *node, Node *new_node) {
  LIBC_ASSERT(is_head(node) && "only head nodes contain trie links");

  if (node->parent) {
    Node *&parent_child =
        node->parent->lower == node ? node->parent->lower : node->parent->upper;
    LIBC_ASSERT(parent_child == node &&
                "no reference to child node found in parent");
    parent_child = new_node;
  } else {
    LIBC_ASSERT(root_ == node && "non-root node had no parent");
    root_ = new_node;
  }
  if (node->lower)
    node->lower->parent = new_node;
  if (node->upper)
    node->upper->parent = new_node;
}

void FreeTrie::integrity_check() const {
  auto integrity_check_trie_node = [&](auto &self, const Node *node) -> void {
    if (!node)
      return;
    node->integrity_check();
    FreeList list = const_cast<Node *>(node);
    list.integrity_check();
    self(self, node->lower);
    self(self, node->upper);
  };
  integrity_check_trie_node(integrity_check_trie_node, root());
}

} // namespace LIBC_NAMESPACE_DECL