include/swift/Basic/PrefixMap.h - third_party/swift - Git at Google

 //===--- PrefixMap.h - A ternary tree ---------------------------*- C++ -*-===//
 //
 // This source file is part of the Swift.org open source project
 //
 // Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines a data structure which stores a map whose keys are
 //  sequences of comparable values.  Specifically, it implements the trie
 //  variant known as a ternary search tree.  In performance, it is similar
 //  to a binary tree; however, it has two properties specific to the use of
 //  homogeneous sequences as keys:
 //
 //    - Individual entries do not necessarily store the entire key; instead,
 //      the key data may be spread over a sequence of nodes.  This causes the
 //      tree to be much more space-compact when keys share common prefixes.
 //      This does require an extra pointer of storage in each node.
 //
 //      Unlike some traditional presentations of ternary trees, this
 //      implementation allows more than one key element per node.
 //
 //    - It is efficient to find entries that share a common prefix with
 //      a given key.
 //
 //  FIXME: The current implementation doesn't rebalance siblings.
 //
 //===----------------------------------------------------------------------===//

 #ifndef SWIFT_BASIC_PREFIXMAP_H
 #define SWIFT_BASIC_PREFIXMAP_H

 #include "swift/Basic/Debug.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/type_traits.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <iterator>

 namespace swift {

 void printOpaquePrefixMap(raw_ostream &out, void *root,
                           void (*printNode)(raw_ostream &, void*));
 template <class KeyElementType> class PrefixMapKeyPrinter;

 /// A map whose keys are sequences of comparable values, optimized for
 /// finding a mapped value for the longest matching initial subsequence.
 template <class KeyElementType, class ValueType,
           size_t InlineKeyCapacity = std::max(
               (sizeof(void *) - 1) / sizeof(KeyElementType), size_t(1))>
 class PrefixMap {
 public:
   using KeyType = ArrayRef<KeyElementType>;

   static_assert(IsTriviallyCopyable<KeyElementType>::value,
                 "key element type must be trivially copyable");
   static_assert(IsTriviallyConstructible<KeyElementType>::value,
                 "key element type must be trivially default-initializable");

 private:
   template <typename T>
   union UninitializedStorage {
     alignas(T) char Storage[sizeof(T)];

     template <typename... A>
     void emplace(A && ...value) {
       ::new((void*) &Storage) T(std::forward<A>(value)...);
     }

     T &get() { return *reinterpret_cast<T*>(Storage); }
     const T &get() const { return *reinterpret_cast<T*>(Storage); }
   };

   // We expect to see a lot of entries for keys like:
   //   ab
   //   abcdef
   //   abcdefg
   //   abcdefh
   // corresponding to closely-related paths and different prefixes
   // of the same path.  This basically demands something like a trie.
   //
   // The rules of our data structure are:
   //   - A node's complete key is the concatenation of its non-local prefix
   //     and its local key.  The non-local prefix is the concatenation of
   //     of the local keys of the Further links followed from the root.
   //   - A node's local key contains no common prefix with the local keys of
   //     its left or right siblings.

   struct Node;
   struct NodeBase {
     // The initial layout of this struct is assumed in the out-of-line
     // printing code; you'll need to modify it.

     // Left and right siblings: nodes which share the same non-local
     // prefix as this one, but which share no common local prefix with it.
     Node *Left = nullptr;
     Node *Right = nullptr;

     // Further children: nodes whose non-local prefix is the concatenation
     // of the non-local prefix of this node and its local key.
     Node *Further = nullptr;

     KeyElementType Key[InlineKeyCapacity];
     unsigned char KeyLength : 7;
     unsigned char HasValue : 1;
     static_assert(InlineKeyCapacity < (1 << 7),
                   "can't store inline key length in bit-field");

     NodeBase() : KeyLength(0), HasValue(false) {}

     KeyType getLocalKey() const { return { Key, KeyLength }; }
   };
   struct Node : NodeBase {
   private:
     UninitializedStorage<ValueType> Value;

   public:
     Node() { /* leave Value uninitialized */ }

     // We split NodeBase out so that we can just delegate to something that
     // copies all the other fields.
     Node(const Node &other) : NodeBase(other) {
       if (this->HasValue) {
         Value.emplace(other.Value.get());
       }
     }

     ValueType &get() {
       assert(this->HasValue);
       return Value.get();
     }
     const ValueType &get() const {
       assert(this->HasValue);
       return Value.get();
     }

     template <typename... A>
     void emplace(A && ...value) {
       assert(!this->HasValue);
       Value.emplace(std::forward<A>(value)...);
       this->HasValue = true;
     }
   };

   /// Splits a node in two.  The second part must always be non-empty.
   ///
   ///   ref -> cur 'abcdef' -> ...
   /// =>
   ///   ref -> split 'abc' -> cur 'def' -> ...
   ///
   /// Returns the node that stores the common prefix as its key.
   ///
   /// TODO: consider just reorganizing cur and its Further node if
   /// cur doesn't have a value or left/right children, e.g.
   ///   ref -> cur 'abcdefg' -> further 'hij'
   /// =>
   ///   ref -> cur 'abc' -> further 'defghij'
   static Node *splitNode(Node **ref, size_t splitIndex) {
     auto cur = *ref;
     assert(splitIndex < cur->KeyLength &&
            "split index would leave second node with empty key");

     auto split = new Node();
     split->Further = cur;

     // Move the sibling links of cur onto split unless we're giving split
     // an empty local key, which is the only case where the siblings will
     // have split's key as a prefix.
     if (splitIndex != 0) {
       split->Left = cur->Left;
       split->Right = cur->Right;
       cur->Left = nullptr;
       cur->Right = nullptr;
     }

     // Initialize the key of the split node.
     split->KeyLength = splitIndex;
     memcpy(split->Key, cur->Key, splitIndex * sizeof(KeyElementType));

     // Slide cur's key if the split point wasn't the start.
     if (splitIndex != 0) {
       cur->KeyLength -= splitIndex;
       memmove(cur->Key, cur->Key + splitIndex,
               cur->KeyLength * sizeof(KeyElementType));
     }

     *ref = split;
     return split;
   }

   /// Try to find a node corresponding to a prefix of the given key.
   ///
   /// If 'remainingLookupKey' is non-null, the returned node will correspond
   /// to the longest prefix that had a value set, and 'remainingLookupKey'
   /// will be set to the part of the key that was not matched.  The tree
   /// will not be modified.
   ///
   /// If 'remainingLookupKey' is null, the returned node will correspond
   /// to the complete lookup key.  This node will be created if necessary.
   Node *getOrCreatePrefixNode(KeyType lookupKey,
                               KeyType *remainingLookupKey) {
     // Invariant: 'best' contains the last node we followed the Further
     // link of that had a value.  This is used only when not creating
     // an exact match.
     Node *best = nullptr;

     Node **next = &Root;
     while (Node * const cur = *next) {
       KeyType curKey = cur->getLocalKey();

       // Compare the lookup key with the stored key in the node.
       size_t len = std::min(curKey.size(), lookupKey.size());
       size_t i = 0;
       for (; i != len; ++i) {
          if (lookupKey[i] != curKey[i]) break;
       }

       // If we didn't reach the end of the common length, then we have two
       // basic cases:
       //    looking up 'def' in 'abc' or 'ghi'
       //    looking up 'abd' in 'abc' or 'abce'
       if (i != len) {
         assert(len != 0);
         bool goLeft = (lookupKey[i] < curKey[i]);

         // If there's no common prefix, just go to the appropriate side.
         if (i == 0) {
           next = (goLeft ? &cur->Left : &cur->Right);
           continue;
         }

         // Otherwise, there's a common prefix, but it's not cur's entire
         // key, so there's no node at that common prefix.  That means that
         // there can't be an exact match.  So if we don't need to return
         // one, we can just stop here.
         if (remainingLookupKey)
           return best;

         // Otherwise, we need to split cur at the appropriate place.
         Node *split = splitNode(next, i);

         // Continue from a sibling of its Further link.
         lookupKey = lookupKey.slice(i);
         next = (goLeft ? &split->Further->Left : &split->Further->Right);
         assert(*next == nullptr);
         break;
       }

       // If we did reach the end of the common length, then we have three
       // cases:
       //   looking up 'abc' in 'abc'
       //   looking up 'abc' in 'abcdef'
       //   looking up 'abc' in 'ab'

       // We might have exhausted the node's local key.
       // (This could be empty.)
       if (len == curKey.size()) {
         lookupKey = lookupKey.slice(len);

         // Remember this as the best mapped match if it has a value.
         if (remainingLookupKey && cur->HasValue) {
           best = cur;
           *remainingLookupKey = lookupKey;
         }

         // If we've exhausted the lookup key, too, we have an exact match.
         if (lookupKey.empty()) {
           return (remainingLookupKey ? best : cur);
         }

         // Otherwise, we have a suffix match; continue along the Further
         // path.
         next = &cur->Further;
         continue;
       }

       // Otherwise, we matched a prefix of the node's key.
       assert(lookupKey.size() < curKey.size());

       // If we don't need to create an exact node, don't claim anything
       // more from the key and just use our deepest match.
       if (remainingLookupKey) {
         return best;
       }

       // Okay, we're supposed to return a node that exactly matches the key.
       // Split 'cur'.
       Node *split = splitNode(next, len);
       return split;
     }

     // Okay, we reached the end of our search.  If we don't need an exact
     // match, return our best match so far.
     if (remainingLookupKey) {
       return best;
     }

     // Otherwise, create nodes until we're out of lookup key.
     // TODO: balance the subtree when creating nodes to the left or right.
     do {
       best = new Node();
       *next = best;
       next = &best->Further;

       auto len = std::min(InlineKeyCapacity, lookupKey.size());
       best->KeyLength = len;
       memcpy(best->Key, lookupKey.data(), len * sizeof(KeyElementType));
       lookupKey = lookupKey.slice(len);
     } while (!lookupKey.empty());

     return best;
   }

   /// Delete all the nodes in a subtree.
   static void deleteTree(Node *root) {
     if (!root) return;

     SmallVector<Node *, 8> stack;
     auto pushChildrenAndDelete = [&](Node *node) {
       if (node->Left) stack.push_back(node->Left);
       if (node->Right) stack.push_back(node->Right);
       if (node->Further) stack.push_back(node->Further);
       delete node;
     };
     pushChildrenAndDelete(root);
     while (!stack.empty()) {
       pushChildrenAndDelete(stack.pop_back_val());
     }
   }

   /// Copy all the nodes in a subtree.
   static Node *cloneTree(Node *root) {
     if (!root) return nullptr;

     SmallVector<Node **, 8> stack;
     auto copyAndPushChildren = [&](Node **ptr) {
       assert(*ptr);
       Node *copy = new Node(**ptr);
       *ptr = copy;
       if (copy->Left) stack.push_back(&copy->Left);
       if (copy->Right) stack.push_back(&copy->Right);
       if (copy->Further) stack.push_back(&copy->Further);
     };
     copyAndEnqueueChildren(&root);
     while (!stack.empty()) {
       copyAndEnqueueChildren(stack.pop_back_val());
     }
     return root;
   }

   Node *Root = nullptr;

 public:
   PrefixMap() {}

   PrefixMap(const PrefixMap &other) : Root(cloneTree(other.Root)) {}
   PrefixMap(PrefixMap &&other) : Root(other.Root) {
     other.Root = nullptr;
   }
   PrefixMap &operator=(const PrefixMap &other) {
     deleteTree(Root);
     Root = cloneTree(other.Root);
     return *this;
   }
   PrefixMap &operator=(PrefixMap &&other) {
     deleteTree(Root);
     Root = other.Root;
     other.Root = nullptr;
     return *this;
   }

   ~PrefixMap() {
     deleteTree(Root);
   }

   /// Are there any entries in this map?
   bool empty() const {
     // The only way to create nodes is to insert an entry, and we don't
     // yet support delete, so having any nodes means we're non-empty.
     return Root == nullptr;
   }

   /// Return the number of entries in this map.
   size_t size() const {
     if (!Root) return 0;

     size_t result = 0;
     SmallVector<Node *, 16> stack;
     auto handleNode = [&](Node *node) {
       if (node->HasValue) result++;
       if (node->Left) stack.push_back(node->Left);
       if (node->Right) stack.push_back(node->Right);
       if (node->Further) stack.push_back(node->Further);
     };
     handleNode(Root);
     while (!stack.empty())
       handleNode(stack.pop_back_val());
     return result;
   }

   /// An input iterator over the entries in the map.
   ///
   /// This iterator stores a stack of the access path to the entry and
   /// is therefore expensive to copy, and its operator* returns a proxy
   /// object with a reference to its internal storage.  These are both
   /// legal for C++ input iterators, but still, be careful.
   class const_iterator {
   protected:
     enum Position {
       /// We are visiting the node's left subtree.
       Left,
       /// If the node is on the top of the stack, we are visiting its value.
       /// Otherwise, we are visiting its further subtree.
       Further,
       // We remove the node from the stack when visiting its right subtree.
     };
     using NodeAndPosition = llvm::PointerIntPair<Node*, 1, Position>;
     SmallVector<NodeAndPosition, 8> Stack;
   public:
     const_iterator() {}
     explicit const_iterator(Node *node) { enter(node); }

     /// A proxy object referencing a valid entry in the map.
     class ConstEntryProxy {
       ArrayRef<NodeAndPosition> Path;
     public:
       explicit ConstEntryProxy(ArrayRef<NodeAndPosition> path) : Path(path) {
         assert(!Path.empty() && Path.back().getPointer()->HasValue);
       }

       /// Return the value of the entry.  The returned reference is valid
       /// as long as the entry remains in the map.
       const ValueType &getValue() const {
         return Path.back().getPointer()->get();
       }

       /// Read the value's key into the given buffer.
       KeyType getKey(SmallVectorImpl<KeyElementType> &buffer) const {
         buffer.clear();
         for (auto &entry : Path.drop_back()) {
           if (entry.getInt() != Further) continue;
           auto key = entry.getPointer()->getLocalKey();
           buffer.append(key.begin(), key.end());
         }
         auto key = Path.back().getPointer()->getLocalKey();
         buffer.append(key.begin(), key.end());
         return buffer;
       }
     };

     /// Return a proxy value for the entry.  The returned proxy is invalidated
     /// by any change to the underlying iterator.
     ConstEntryProxy operator*() const {
       assert(!Stack.empty());
       assert(Stack.back().getPointer()->HasValue);
       return ConstEntryProxy(Stack);
     }

     /// Advance the iterator.
     const_iterator &operator++() {
       assert(!Stack.empty());

       while (true) {
         if (Stack.back().getInt() == Left) {
           Stack.back().setInt(Further);
           if (Stack.back().getPointer()->HasValue) return *this;
         }
         if (enter(Stack.back().getPointer()->Further)) return *this;

         do {
           Node *top = Stack.pop_back_val().getPointer();
           if (enter(top->Right)) return *this;
           if (Stack.empty()) return *this;
         } while (Stack.back().getInt() == Further);
       }
     }
     const_iterator operator++(int) {
       const_iterator copy = *this;
       operator++();
       return copy;
     }

     bool operator==(const const_iterator &other) const {
       return Stack == other.Stack;
     }
     bool operator!=(const const_iterator &other) const {
       return !operator==(other);
     }

     using difference_type = ptrdiff_t;
     using iterator_category = std::input_iterator_tag;
     using value_type = ConstEntryProxy;
     using pointer = ConstEntryProxy;
     using reference = ConstEntryProxy;

   private:
     bool enter(Node *node) {
       if (!node) return false;
       Stack.push_back({node, Left});
       if (enter(node->Left)) return true;
       Stack.back().setInt(Further);
       if (node->HasValue) return true;
       if (enter(node->Further)) return true;
       Stack.pop_back();
       return enter(node->Right);
     }
   };

   /// An input iterator over the entries in this map.
   ///
   /// This iterator stores a stack of the access path to the entry and
   /// is therefore expensive to copy, and its operator* returns a proxy
   /// object with a reference to its internal storage.  These are both
   /// legal for C++ input iterators, but still, be careful.
   struct iterator : const_iterator {
     iterator() : const_iterator() {}
     explicit iterator(Node *node) : const_iterator(node) {}

     struct EntryProxy : const_iterator::ConstEntryProxy {
       explicit EntryProxy(
                        ArrayRef<typename const_iterator::NodeAndPosition> path)
         : const_iterator::ConstEntryProxy(path) {}
       ValueType &getValue() const {
         return const_cast<ValueType &>(
                                   const_iterator::ConstEntryProxy::getValue());
       }
     };

     EntryProxy operator*() const {
       return EntryProxy(this->Stack);
     }

     iterator &operator++() {
       return static_cast<iterator&>(const_iterator::operator++());
     }
     iterator operator++(int) {
       iterator copy = *this;
       operator++();
       return copy;
     }

     // Base implementations of operator== are fine.

     using value_type = EntryProxy;
     using pointer = EntryProxy;
     using reference = EntryProxy;
   };

   iterator begin() { return iterator(Root); }
   iterator end() { return iterator(); }

   const_iterator begin() const { return const_iterator(Root); }
   const_iterator end() const { return const_iterator(); }

   /// A handle to the mapping for a given key.  Only invalidated by
   /// changes that remove the mapping.
   class Handle {
     Node *Ptr;
   public:
     Handle() : Ptr(nullptr) {}
     explicit Handle(Node *ptr) : Ptr(ptr) {}

     explicit operator bool() const { return Ptr != nullptr; }
     ValueType &operator*() const {
       return Ptr->get();
     }
   };

   using KeyIterator = typename KeyType::iterator;

   /// Find the longest prefix of the given encoded sequence which has
   /// an entry in this map, and return an iterator corresponding to the
   /// end of the prefix.
   std::pair<Handle, KeyIterator>
   findPrefix(KeyType key) const {
     KeyType remainingKey;
     auto node = const_cast<PrefixMap*>(this)
                              ->getOrCreatePrefixNode(key, &remainingKey);
     assert(!node || node->HasValue);
     return { Handle(node), remainingKey.begin() };
   }

   /// Remove all entries in the map.
   void clear() {
     deleteTree(Root);
     Root = nullptr;
   }

   /// Get or create an entry in the map.
   ///
   /// \return a handle to the entry and a bool indicating (if true)
   ///   that the map was modified to insert the mapping.
   template <typename Fn>
   std::pair<Handle, bool>
   insertLazy(KeyType key, const Fn &create) {
     auto node = getOrCreatePrefixNode(key, nullptr);
     assert(node);
     if (node->HasValue) {
       return { Handle(node), false };
     } else {
       node->emplace(create());
       return { Handle(node), true };
     }
   }

   std::pair<Handle, bool>
   insert(KeyType key, ValueType &&value) {
     return insertLazy(key,
                       [&]() -> ValueType && { return std::move(value); });
   }

   std::pair<Handle, bool>
   insert(KeyType key, const ValueType &value) {
     return insertLazy(key,
                       [&]() -> const ValueType & { return value; });
   }

   /// Insert a new entry into the map, asserting that it doesn't
   /// already exist.
   template <typename Fn>
   Handle insertNewLazy(KeyType key, const Fn &create) {
     auto node = getOrCreatePrefixNode(key, nullptr);
     assert(node);
     node->emplace(create());
     return Handle(node);
   }

   Handle insertNew(KeyType key, ValueType &&value) {
     return insertNewLazy(key,
                          [&]() -> ValueType && { return std::move(value); });
   }

   Handle insertNew(KeyType key, const ValueType &value) {
     return insertNewLazy(key,
                          [&]() -> const ValueType & { return value; });
   }

   SWIFT_DEBUG_DUMP { print(llvm::errs()); }
   void print(raw_ostream &out) const {
     printOpaquePrefixMap(out, Root,
                          [](raw_ostream &out, void *_node) {
       Node *node = reinterpret_cast<Node*>(_node);
       PrefixMapKeyPrinter<KeyElementType>::print(out, node->getLocalKey());
       if (node->HasValue) {
         out << " (" << node->get() << ')';
       }
     });
   }
 };

 /// The standard implementation of a key printer:
 ///   {0,1,2,3}
 template <class KeyElementType>
 class PrefixMapKeyPrinter {
 public:
   static void print(raw_ostream &out, ArrayRef<KeyElementType> key) {
     out << '{';
     for (size_t i = 0; i != key.size(); ++i) {
       if (i != 0) out << ',';
       out << key[i];
     }
     out << '}';
   }
 };

 /// A key printer for char sequences that prints as a quoted string:
 ///   "hello, \"world\""
 template <>
 class PrefixMapKeyPrinter<char> {
 public:
   static void print(raw_ostream &out, ArrayRef<char> key);
 };

 /// A key printer for byte sequences that prints as a byte-string:
 //   '0F346E'
 template <>
 class PrefixMapKeyPrinter<unsigned char> {
 public:
   static void print(raw_ostream &out, ArrayRef<unsigned char> key);
 };

 } // end namespace swift

 #endif // SWIFT_BASIC_PREFIXMAP_H
	//===--- PrefixMap.h - A ternary tree ---------------------------- C++ --===//
	//
	// This source file is part of the Swift.org open source project
	//
	// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
	// Licensed under Apache License v2.0 with Runtime Library Exception
	//
	// See https://swift.org/LICENSE.txt for license information
	// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
	//
	//===----------------------------------------------------------------------===//
	//
	// This file defines a data structure which stores a map whose keys are
	// sequences of comparable values. Specifically, it implements the trie
	// variant known as a ternary search tree. In performance, it is similar
	// to a binary tree; however, it has two properties specific to the use of
	// homogeneous sequences as keys:
	//
	// - Individual entries do not necessarily store the entire key; instead,
	// the key data may be spread over a sequence of nodes. This causes the
	// tree to be much more space-compact when keys share common prefixes.
	// This does require an extra pointer of storage in each node.
	//
	// Unlike some traditional presentations of ternary trees, this
	// implementation allows more than one key element per node.
	//
	// - It is efficient to find entries that share a common prefix with
	// a given key.
	//
	// FIXME: The current implementation doesn't rebalance siblings.
	//
	//===----------------------------------------------------------------------===//

	#ifndef SWIFT_BASIC_PREFIXMAP_H
	#define SWIFT_BASIC_PREFIXMAP_H

	#include "swift/Basic/Debug.h"
	#include "swift/Basic/LLVM.h"
	#include "swift/Basic/type_traits.h"
	#include "llvm/ADT/ArrayRef.h"
	#include "llvm/ADT/PointerIntPair.h"
	#include "llvm/Support/raw_ostream.h"
	#include <algorithm>
	#include <iterator>

	namespace swift {

	void printOpaquePrefixMap(raw_ostream &out, void *root,
	void (printNode)(raw_ostream &, void));
	template <class KeyElementType> class PrefixMapKeyPrinter;

	/// A map whose keys are sequences of comparable values, optimized for
	/// finding a mapped value for the longest matching initial subsequence.
	template <class KeyElementType, class ValueType,
	size_t InlineKeyCapacity = std::max(
	(sizeof(void *) - 1) / sizeof(KeyElementType), size_t(1))>
	class PrefixMap {
	public:
	using KeyType = ArrayRef<KeyElementType>;

	static_assert(IsTriviallyCopyable<KeyElementType>::value,
	"key element type must be trivially copyable");
	static_assert(IsTriviallyConstructible<KeyElementType>::value,
	"key element type must be trivially default-initializable");

	private:
	template <typename T>
	union UninitializedStorage {
	alignas(T) char Storage[sizeof(T)];

	template <typename... A>
	void emplace(A && ...value) {
	::new((void*) &Storage) T(std::forward<A>(value)...);
	}

	T &get() { return reinterpret_cast<T>(Storage); }
	const T &get() const { return reinterpret_cast<T>(Storage); }
	};

	// We expect to see a lot of entries for keys like:
	// ab
	// abcdef
	// abcdefg
	// abcdefh
	// corresponding to closely-related paths and different prefixes
	// of the same path. This basically demands something like a trie.
	//
	// The rules of our data structure are:
	// - A node's complete key is the concatenation of its non-local prefix
	// and its local key. The non-local prefix is the concatenation of
	// of the local keys of the Further links followed from the root.
	// - A node's local key contains no common prefix with the local keys of
	// its left or right siblings.

	struct Node;
	struct NodeBase {
	// The initial layout of this struct is assumed in the out-of-line
	// printing code; you'll need to modify it.

	// Left and right siblings: nodes which share the same non-local
	// prefix as this one, but which share no common local prefix with it.
	Node *Left = nullptr;
	Node *Right = nullptr;

	// Further children: nodes whose non-local prefix is the concatenation
	// of the non-local prefix of this node and its local key.
	Node *Further = nullptr;

	KeyElementType Key[InlineKeyCapacity];
	unsigned char KeyLength : 7;
	unsigned char HasValue : 1;
	static_assert(InlineKeyCapacity < (1 << 7),
	"can't store inline key length in bit-field");

	NodeBase() : KeyLength(0), HasValue(false) {}

	KeyType getLocalKey() const { return { Key, KeyLength }; }
	};
	struct Node : NodeBase {
	private:
	UninitializedStorage<ValueType> Value;

	public:
	Node() { /* leave Value uninitialized */ }

	// We split NodeBase out so that we can just delegate to something that
	// copies all the other fields.
	Node(const Node &other) : NodeBase(other) {
	if (this->HasValue) {
	Value.emplace(other.Value.get());
	}
	}

	ValueType &get() {
	assert(this->HasValue);
	return Value.get();
	}
	const ValueType &get() const {
	assert(this->HasValue);
	return Value.get();
	}

	template <typename... A>
	void emplace(A && ...value) {
	assert(!this->HasValue);
	Value.emplace(std::forward<A>(value)...);
	this->HasValue = true;
	}
	};

	/// Splits a node in two. The second part must always be non-empty.
	///
	/// ref -> cur 'abcdef' -> ...
	/// =>
	/// ref -> split 'abc' -> cur 'def' -> ...
	///
	/// Returns the node that stores the common prefix as its key.
	///
	/// TODO: consider just reorganizing cur and its Further node if
	/// cur doesn't have a value or left/right children, e.g.
	/// ref -> cur 'abcdefg' -> further 'hij'
	/// =>
	/// ref -> cur 'abc' -> further 'defghij'
	static Node splitNode(Node *ref, size_t splitIndex) {
	auto cur = *ref;
	assert(splitIndex < cur->KeyLength &&
	"split index would leave second node with empty key");

	auto split = new Node();
	split->Further = cur;

	// Move the sibling links of cur onto split unless we're giving split
	// an empty local key, which is the only case where the siblings will
	// have split's key as a prefix.
	if (splitIndex != 0) {
	split->Left = cur->Left;
	split->Right = cur->Right;
	cur->Left = nullptr;
	cur->Right = nullptr;
	}

	// Initialize the key of the split node.
	split->KeyLength = splitIndex;
	memcpy(split->Key, cur->Key, splitIndex * sizeof(KeyElementType));

	// Slide cur's key if the split point wasn't the start.
	if (splitIndex != 0) {
	cur->KeyLength -= splitIndex;
	memmove(cur->Key, cur->Key + splitIndex,
	cur->KeyLength * sizeof(KeyElementType));
	}

	*ref = split;
	return split;
	}

	/// Try to find a node corresponding to a prefix of the given key.
	///
	/// If 'remainingLookupKey' is non-null, the returned node will correspond
	/// to the longest prefix that had a value set, and 'remainingLookupKey'
	/// will be set to the part of the key that was not matched. The tree
	/// will not be modified.
	///
	/// If 'remainingLookupKey' is null, the returned node will correspond
	/// to the complete lookup key. This node will be created if necessary.
	Node *getOrCreatePrefixNode(KeyType lookupKey,
	KeyType *remainingLookupKey) {
	// Invariant: 'best' contains the last node we followed the Further
	// link of that had a value. This is used only when not creating
	// an exact match.
	Node *best = nullptr;

	Node **next = &Root;
	while (Node * const cur = *next) {
	KeyType curKey = cur->getLocalKey();

	// Compare the lookup key with the stored key in the node.
	size_t len = std::min(curKey.size(), lookupKey.size());
	size_t i = 0;
	for (; i != len; ++i) {
	if (lookupKey[i] != curKey[i]) break;
	}

	// If we didn't reach the end of the common length, then we have two
	// basic cases:
	// looking up 'def' in 'abc' or 'ghi'
	// looking up 'abd' in 'abc' or 'abce'
	if (i != len) {
	assert(len != 0);
	bool goLeft = (lookupKey[i] < curKey[i]);

	// If there's no common prefix, just go to the appropriate side.
	if (i == 0) {
	next = (goLeft ? &cur->Left : &cur->Right);
	continue;
	}

	// Otherwise, there's a common prefix, but it's not cur's entire
	// key, so there's no node at that common prefix. That means that
	// there can't be an exact match. So if we don't need to return
	// one, we can just stop here.
	if (remainingLookupKey)
	return best;

	// Otherwise, we need to split cur at the appropriate place.
	Node *split = splitNode(next, i);

	// Continue from a sibling of its Further link.
	lookupKey = lookupKey.slice(i);
	next = (goLeft ? &split->Further->Left : &split->Further->Right);
	assert(*next == nullptr);
	break;
	}

	// If we did reach the end of the common length, then we have three
	// cases:
	// looking up 'abc' in 'abc'
	// looking up 'abc' in 'abcdef'
	// looking up 'abc' in 'ab'

	// We might have exhausted the node's local key.
	// (This could be empty.)
	if (len == curKey.size()) {
	lookupKey = lookupKey.slice(len);

	// Remember this as the best mapped match if it has a value.
	if (remainingLookupKey && cur->HasValue) {
	best = cur;
	*remainingLookupKey = lookupKey;
	}

	// If we've exhausted the lookup key, too, we have an exact match.
	if (lookupKey.empty()) {
	return (remainingLookupKey ? best : cur);
	}

	// Otherwise, we have a suffix match; continue along the Further
	// path.
	next = &cur->Further;
	continue;
	}

	// Otherwise, we matched a prefix of the node's key.
	assert(lookupKey.size() < curKey.size());

	// If we don't need to create an exact node, don't claim anything
	// more from the key and just use our deepest match.
	if (remainingLookupKey) {
	return best;
	}

	// Okay, we're supposed to return a node that exactly matches the key.
	// Split 'cur'.
	Node *split = splitNode(next, len);
	return split;
	}

	// Okay, we reached the end of our search. If we don't need an exact
	// match, return our best match so far.
	if (remainingLookupKey) {
	return best;
	}

	// Otherwise, create nodes until we're out of lookup key.
	// TODO: balance the subtree when creating nodes to the left or right.
	do {
	best = new Node();
	*next = best;
	next = &best->Further;

	auto len = std::min(InlineKeyCapacity, lookupKey.size());
	best->KeyLength = len;
	memcpy(best->Key, lookupKey.data(), len * sizeof(KeyElementType));
	lookupKey = lookupKey.slice(len);
	} while (!lookupKey.empty());

	return best;
	}

	/// Delete all the nodes in a subtree.
	static void deleteTree(Node *root) {
	if (!root) return;

	SmallVector<Node *, 8> stack;
	auto pushChildrenAndDelete = [&](Node *node) {
	if (node->Left) stack.push_back(node->Left);
	if (node->Right) stack.push_back(node->Right);
	if (node->Further) stack.push_back(node->Further);
	delete node;
	};
	pushChildrenAndDelete(root);
	while (!stack.empty()) {
	pushChildrenAndDelete(stack.pop_back_val());
	}
	}

	/// Copy all the nodes in a subtree.
	static Node cloneTree(Node root) {
	if (!root) return nullptr;

	SmallVector<Node **, 8> stack;
	auto copyAndPushChildren = [&](Node **ptr) {
	assert(*ptr);
	Node copy = new Node(*ptr);
	*ptr = copy;
	if (copy->Left) stack.push_back(&copy->Left);
	if (copy->Right) stack.push_back(&copy->Right);
	if (copy->Further) stack.push_back(&copy->Further);
	};
	copyAndEnqueueChildren(&root);
	while (!stack.empty()) {
	copyAndEnqueueChildren(stack.pop_back_val());
	}
	return root;
	}

	Node *Root = nullptr;

	public:
	PrefixMap() {}

	PrefixMap(const PrefixMap &other) : Root(cloneTree(other.Root)) {}
	PrefixMap(PrefixMap &&other) : Root(other.Root) {
	other.Root = nullptr;
	}
	PrefixMap &operator=(const PrefixMap &other) {
	deleteTree(Root);
	Root = cloneTree(other.Root);
	return *this;
	}
	PrefixMap &operator=(PrefixMap &&other) {
	deleteTree(Root);
	Root = other.Root;
	other.Root = nullptr;
	return *this;
	}

	~PrefixMap() {
	deleteTree(Root);
	}

	/// Are there any entries in this map?
	bool empty() const {
	// The only way to create nodes is to insert an entry, and we don't
	// yet support delete, so having any nodes means we're non-empty.
	return Root == nullptr;
	}

	/// Return the number of entries in this map.
	size_t size() const {
	if (!Root) return 0;

	size_t result = 0;
	SmallVector<Node *, 16> stack;
	auto handleNode = [&](Node *node) {
	if (node->HasValue) result++;
	if (node->Left) stack.push_back(node->Left);
	if (node->Right) stack.push_back(node->Right);
	if (node->Further) stack.push_back(node->Further);
	};
	handleNode(Root);
	while (!stack.empty())
	handleNode(stack.pop_back_val());
	return result;
	}

	/// An input iterator over the entries in the map.
	///
	/// This iterator stores a stack of the access path to the entry and
	/// is therefore expensive to copy, and its operator* returns a proxy
	/// object with a reference to its internal storage. These are both
	/// legal for C++ input iterators, but still, be careful.
	class const_iterator {
	protected:
	enum Position {
	/// We are visiting the node's left subtree.
	Left,
	/// If the node is on the top of the stack, we are visiting its value.
	/// Otherwise, we are visiting its further subtree.
	Further,
	// We remove the node from the stack when visiting its right subtree.
	};
	using NodeAndPosition = llvm::PointerIntPair<Node*, 1, Position>;
	SmallVector<NodeAndPosition, 8> Stack;
	public:
	const_iterator() {}
	explicit const_iterator(Node *node) { enter(node); }

	/// A proxy object referencing a valid entry in the map.
	class ConstEntryProxy {
	ArrayRef<NodeAndPosition> Path;
	public:
	explicit ConstEntryProxy(ArrayRef<NodeAndPosition> path) : Path(path) {
	assert(!Path.empty() && Path.back().getPointer()->HasValue);
	}

	/// Return the value of the entry. The returned reference is valid
	/// as long as the entry remains in the map.
	const ValueType &getValue() const {
	return Path.back().getPointer()->get();
	}

	/// Read the value's key into the given buffer.
	KeyType getKey(SmallVectorImpl<KeyElementType> &buffer) const {
	buffer.clear();
	for (auto &entry : Path.drop_back()) {
	if (entry.getInt() != Further) continue;
	auto key = entry.getPointer()->getLocalKey();
	buffer.append(key.begin(), key.end());
	}
	auto key = Path.back().getPointer()->getLocalKey();
	buffer.append(key.begin(), key.end());
	return buffer;
	}
	};

	/// Return a proxy value for the entry. The returned proxy is invalidated
	/// by any change to the underlying iterator.
	ConstEntryProxy operator*() const {
	assert(!Stack.empty());
	assert(Stack.back().getPointer()->HasValue);
	return ConstEntryProxy(Stack);
	}

	/// Advance the iterator.
	const_iterator &operator++() {
	assert(!Stack.empty());

	while (true) {
	if (Stack.back().getInt() == Left) {
	Stack.back().setInt(Further);
	if (Stack.back().getPointer()->HasValue) return *this;
	}
	if (enter(Stack.back().getPointer()->Further)) return *this;

	do {
	Node *top = Stack.pop_back_val().getPointer();
	if (enter(top->Right)) return *this;
	if (Stack.empty()) return *this;
	} while (Stack.back().getInt() == Further);
	}
	}
	const_iterator operator++(int) {
	const_iterator copy = *this;
	operator++();
	return copy;
	}

	bool operator==(const const_iterator &other) const {
	return Stack == other.Stack;
	}
	bool operator!=(const const_iterator &other) const {
	return !operator==(other);
	}

	using difference_type = ptrdiff_t;
	using iterator_category = std::input_iterator_tag;
	using value_type = ConstEntryProxy;
	using pointer = ConstEntryProxy;
	using reference = ConstEntryProxy;

	private:
	bool enter(Node *node) {
	if (!node) return false;
	Stack.push_back({node, Left});
	if (enter(node->Left)) return true;
	Stack.back().setInt(Further);
	if (node->HasValue) return true;
	if (enter(node->Further)) return true;
	Stack.pop_back();
	return enter(node->Right);
	}
	};

	/// An input iterator over the entries in this map.
	///
	/// This iterator stores a stack of the access path to the entry and
	/// is therefore expensive to copy, and its operator* returns a proxy
	/// object with a reference to its internal storage. These are both
	/// legal for C++ input iterators, but still, be careful.
	struct iterator : const_iterator {
	iterator() : const_iterator() {}
	explicit iterator(Node *node) : const_iterator(node) {}

	struct EntryProxy : const_iterator::ConstEntryProxy {
	explicit EntryProxy(
	ArrayRef<typename const_iterator::NodeAndPosition> path)
	: const_iterator::ConstEntryProxy(path) {}
	ValueType &getValue() const {
	return const_cast<ValueType &>(
	const_iterator::ConstEntryProxy::getValue());
	}
	};

	EntryProxy operator*() const {
	return EntryProxy(this->Stack);
	}

	iterator &operator++() {
	return static_cast<iterator&>(const_iterator::operator++());
	}
	iterator operator++(int) {
	iterator copy = *this;
	operator++();
	return copy;
	}

	// Base implementations of operator== are fine.

	using value_type = EntryProxy;
	using pointer = EntryProxy;
	using reference = EntryProxy;
	};

	iterator begin() { return iterator(Root); }
	iterator end() { return iterator(); }

	const_iterator begin() const { return const_iterator(Root); }
	const_iterator end() const { return const_iterator(); }

	/// A handle to the mapping for a given key. Only invalidated by
	/// changes that remove the mapping.
	class Handle {
	Node *Ptr;
	public:
	Handle() : Ptr(nullptr) {}
	explicit Handle(Node *ptr) : Ptr(ptr) {}

	explicit operator bool() const { return Ptr != nullptr; }
	ValueType &operator*() const {
	return Ptr->get();
	}
	};

	using KeyIterator = typename KeyType::iterator;

	/// Find the longest prefix of the given encoded sequence which has
	/// an entry in this map, and return an iterator corresponding to the
	/// end of the prefix.
	std::pair<Handle, KeyIterator>
	findPrefix(KeyType key) const {
	KeyType remainingKey;
	auto node = const_cast<PrefixMap*>(this)
	->getOrCreatePrefixNode(key, &remainingKey);
	assert(!node \|\| node->HasValue);
	return { Handle(node), remainingKey.begin() };
	}

	/// Remove all entries in the map.
	void clear() {
	deleteTree(Root);
	Root = nullptr;
	}

	/// Get or create an entry in the map.
	///
	/// \return a handle to the entry and a bool indicating (if true)
	/// that the map was modified to insert the mapping.
	template <typename Fn>
	std::pair<Handle, bool>
	insertLazy(KeyType key, const Fn &create) {
	auto node = getOrCreatePrefixNode(key, nullptr);
	assert(node);
	if (node->HasValue) {
	return { Handle(node), false };
	} else {
	node->emplace(create());
	return { Handle(node), true };
	}
	}

	std::pair<Handle, bool>
	insert(KeyType key, ValueType &&value) {
	return insertLazy(key,
	[&]() -> ValueType && { return std::move(value); });
	}

	std::pair<Handle, bool>
	insert(KeyType key, const ValueType &value) {
	return insertLazy(key,
	[&]() -> const ValueType & { return value; });
	}

	/// Insert a new entry into the map, asserting that it doesn't
	/// already exist.
	template <typename Fn>
	Handle insertNewLazy(KeyType key, const Fn &create) {
	auto node = getOrCreatePrefixNode(key, nullptr);
	assert(node);
	node->emplace(create());
	return Handle(node);
	}

	Handle insertNew(KeyType key, ValueType &&value) {
	return insertNewLazy(key,
	[&]() -> ValueType && { return std::move(value); });
	}

	Handle insertNew(KeyType key, const ValueType &value) {
	return insertNewLazy(key,
	[&]() -> const ValueType & { return value; });
	}

	SWIFT_DEBUG_DUMP { print(llvm::errs()); }
	void print(raw_ostream &out) const {
	printOpaquePrefixMap(out, Root,
	[](raw_ostream &out, void *_node) {
	Node node = reinterpret_cast<Node>(_node);
	PrefixMapKeyPrinter<KeyElementType>::print(out, node->getLocalKey());
	if (node->HasValue) {
	out << " (" << node->get() << ')';
	}
	});
	}
	};

	/// The standard implementation of a key printer:
	/// {0,1,2,3}
	template <class KeyElementType>
	class PrefixMapKeyPrinter {
	public:
	static void print(raw_ostream &out, ArrayRef<KeyElementType> key) {
	out << '{';
	for (size_t i = 0; i != key.size(); ++i) {
	if (i != 0) out << ',';
	out << key[i];
	}
	out << '}';
	}
	};

	/// A key printer for char sequences that prints as a quoted string:
	/// "hello, \"world\""
	template <>
	class PrefixMapKeyPrinter<char> {
	public:
	static void print(raw_ostream &out, ArrayRef<char> key);
	};

	/// A key printer for byte sequences that prints as a byte-string:
	// '0F346E'
	template <>
	class PrefixMapKeyPrinter<unsigned char> {
	public:
	static void print(raw_ostream &out, ArrayRef<unsigned char> key);
	};

	} // end namespace swift

	#endif // SWIFT_BASIC_PREFIXMAP_H