SuffixTreeNode.h

Go to the documentation of this file.

//===- llvm/ADT/SuffixTreeNode.h - Nodes for SuffixTrees --------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file defines nodes for use within a SuffixTree.
//
// Each node has either no children or at least two children, with the root
// being a exception in the empty tree.
//
// Children are represented as a map between unsigned integers and nodes. If
// a node N has a child M on unsigned integer k, then the mapping represented
// by N is a proper prefix of the mapping represented by M. Note that this,
// although similar to a trie is somewhat different: each node stores a full
// substring of the full mapping rather than a single character state.
//
// Each internal node contains a pointer to the internal node representing
// the same string, but with the first character chopped off. This is stored
// in \p Link. Each leaf node stores the start index of its respective
// suffix in \p SuffixIdx.
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_SUPPORT_SUFFIXTREE_NODE_H
#define LLVM_SUPPORT_SUFFIXTREE_NODE_H
#include "llvm/ADT/DenseMap.h"
 
namespace llvm {
 
/// A node in a suffix tree which represents a substring or suffix.
struct SuffixTreeNode {
public:
  /// Represents an undefined index in the suffix tree.
  static const unsigned EmptyIdx = -1;
  enum class NodeKind { ST_Leaf, ST_Internal };
 
private:
  const NodeKind Kind;
 
  /// The start index of this node's substring in the main string.
  unsigned StartIdx = EmptyIdx;
 
  /// The length of the string formed by concatenating the edge labels from
  /// the root to this node.
  unsigned ConcatLen = 0;
 
  /// These two indices give a range of indices for its leaf descendants.
  /// Imagine drawing a tree on paper and assigning a unique index to each leaf
  /// node in monotonically increasing order from left to right. This way of
  /// numbering the leaf nodes allows us to associate a continuous range of
  /// indices with each internal node. For example, if a node has leaf
  /// descendants with indices i, i+1, ..., j, then its LeftLeafIdx is i and
  /// its RightLeafIdx is j. These indices are for LeafNodes in the SuffixTree
  /// class, which is constructed using post-order depth-first traversal.
  unsigned LeftLeafIdx = EmptyIdx;
  unsigned RightLeafIdx = EmptyIdx;
 
public:
  // LLVM RTTI boilerplate.
  NodeKind getKind() const { return Kind; }
 
  /// \return the start index of this node's substring in the entire string.
  unsigned getStartIdx() const;
 
  /// \returns the end index of this node.
  virtual unsigned getEndIdx() const = 0;
 
  /// \return the index of this node's left most leaf node.
  unsigned getLeftLeafIdx() const;
 
  /// \return the index of this node's right most leaf node.
  unsigned getRightLeafIdx() const;
 
  /// Set the index of the left most leaf node of this node to \p Idx.
  void setLeftLeafIdx(unsigned Idx);
 
  /// Set the index of the right most leaf node of this node to \p Idx.
  void setRightLeafIdx(unsigned Idx);
 
  /// Advance this node's StartIdx by \p Inc.
  void incrementStartIdx(unsigned Inc);
 
  /// Set the length of the string from the root to this node to \p Len.
  void setConcatLen(unsigned Len);
 
  /// \returns the length of the string from the root to this node.
  unsigned getConcatLen() const;
 
  SuffixTreeNode(NodeKind Kind, unsigned StartIdx)
      : Kind(Kind), StartIdx(StartIdx) {}
  virtual ~SuffixTreeNode() = default;
};
 
// A node with two or more children, or the root.
struct SuffixTreeInternalNode : SuffixTreeNode {
private:
  /// The end index of this node's substring in the main string.
  ///
  /// Every leaf node must have its \p EndIdx incremented at the end of every
  /// step in the construction algorithm. To avoid having to update O(N)
  /// nodes individually at the end of every step, the end index is stored
  /// as a pointer.
  unsigned EndIdx = EmptyIdx;
 
  /// A pointer to the internal node representing the same sequence with the
  /// first character chopped off.
  ///
  /// This acts as a shortcut in Ukkonen's algorithm. One of the things that
  /// Ukkonen's algorithm does to achieve linear-time construction is
  /// keep track of which node the next insert should be at. This makes each
  /// insert O(1), and there are a total of O(N) inserts. The suffix link
  /// helps with inserting children of internal nodes.
  ///
  /// Say we add a child to an internal node with associated mapping S. The
  /// next insertion must be at the node representing S - its first character.
  /// This is given by the way that we iteratively build the tree in Ukkonen's
  /// algorithm. The main idea is to look at the suffixes of each prefix in the
  /// string, starting with the longest suffix of the prefix, and ending with
  /// the shortest. Therefore, if we keep pointers between such nodes, we can
  /// move to the next insertion point in O(1) time. If we don't, then we'd
  /// have to query from the root, which takes O(N) time. This would make the
  /// construction algorithm O(N^2) rather than O(N).
  SuffixTreeInternalNode *Link = nullptr;
 
public:
  // LLVM RTTI boilerplate.
  static bool classof(const SuffixTreeNode *N) {
    return N->getKind() == NodeKind::ST_Internal;
  }
 
  /// \returns true if this node is the root of its owning \p SuffixTree.
  bool isRoot() const;
 
  /// \returns the end index of this node's substring in the entire string.
  unsigned getEndIdx() const override;
 
  /// Sets \p Link to \p L. Assumes \p L is not null.
  void setLink(SuffixTreeInternalNode *L);
 
  /// \returns the pointer to the Link node.
  SuffixTreeInternalNode *getLink() const;
 
  /// The children of this node.
  ///
  /// A child existing on an unsigned integer implies that from the mapping
  /// represented by the current node, there is a way to reach another
  /// mapping by tacking that character on the end of the current string.
  DenseMap<unsigned, SuffixTreeNode *> Children;
 
  SuffixTreeInternalNode(unsigned StartIdx, unsigned EndIdx,
                         SuffixTreeInternalNode *Link)
      : SuffixTreeNode(NodeKind::ST_Internal, StartIdx), EndIdx(EndIdx),
        Link(Link) {}
 
  virtual ~SuffixTreeInternalNode() = default;
};
 
// A node representing a suffix.
struct SuffixTreeLeafNode : SuffixTreeNode {
private:
  /// The start index of the suffix represented by this leaf.
  unsigned SuffixIdx = EmptyIdx;
 
  /// The end index of this node's substring in the main string.
  ///
  /// Every leaf node must have its \p EndIdx incremented at the end of every
  /// step in the construction algorithm. To avoid having to update O(N)
  /// nodes individually at the end of every step, the end index is stored
  /// as a pointer.
  unsigned *EndIdx = nullptr;
 
public:
  // LLVM RTTI boilerplate.
  static bool classof(const SuffixTreeNode *N) {
    return N->getKind() == NodeKind::ST_Leaf;
  }
 
  /// \returns the end index of this node's substring in the entire string.
  unsigned getEndIdx() const override;
 
  /// \returns the start index of the suffix represented by this leaf.
  unsigned getSuffixIdx() const;
 
  /// Sets the start index of the suffix represented by this leaf to \p Idx.
  void setSuffixIdx(unsigned Idx);
  SuffixTreeLeafNode(unsigned StartIdx, unsigned *EndIdx)
      : SuffixTreeNode(NodeKind::ST_Leaf, StartIdx), EndIdx(EndIdx) {}
 
  virtual ~SuffixTreeLeafNode() = default;
};
} // namespace llvm
#endif // LLVM_SUPPORT_SUFFIXTREE_NODE_H