tools/splaytree.py

# Copyright 2008 the V8 project authors. All rights reserved.
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# modification, are permitted provided that the following conditions are
# met:
#
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#       with the distribution.
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#       from this software without specific prior written permission.
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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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class Node(object):
  """Nodes in the splay tree."""

  def __init__(self, key, value):
    self.key = key
    self.value = value
    self.left = None
    self.right = None


class KeyNotFoundError(Exception):
  """KeyNotFoundError is raised when removing a non-existing node."""

  def __init__(self, key):
    self.key = key


class SplayTree(object):
  """The splay tree itself is just a reference to the root of the tree."""

  def __init__(self):
    """Create a new SplayTree."""
    self.root = None

  def IsEmpty(self):
    """Is the SplayTree empty?"""
    return not self.root

  def Insert(self, key, value):
    """Insert a new node in the SplayTree."""
    # If the tree is empty, insert the new node.
    if self.IsEmpty():
      self.root = Node(key, value)
      return
    # Splay on the key to move the last node on the search path for
    # the key to the root of the tree.
    self.Splay(key)
    # Ignore repeated insertions with the same key.
    if self.root.key == key:
      return
    # Insert the new node.
    node = Node(key, value)
    if key > self.root.key:
      node.left = self.root
      node.right = self.root.right
      self.root.right = None
    else:
      node.right = self.root
      node.left = self.root.left
      self.root.left = None
    self.root = node

  def Remove(self, key):
    """Remove the node with the given key from the SplayTree."""
    # Raise exception for key that is not found if the tree is empty.
    if self.IsEmpty():
      raise KeyNotFoundError(key)
    # Splay on the key to move the node with the given key to the top.
    self.Splay(key)
    # Raise exception for key that is not found.
    if self.root.key != key:
      raise KeyNotFoundError(key)
    removed = self.root
    # Link out the root node.
    if not self.root.left:
      # No left child, so the new tree is just the right child.
      self.root = self.root.right
    else:
      # Left child exists.
      right = self.root.right
      # Make the original left child the new root.
      self.root = self.root.left
      # Splay to make sure that the new root has an empty right child.
      self.Splay(key)
      # Insert the original right child as the right child of the new
      # root.
      self.root.right = right
    return removed

  def Find(self, key):
    """Returns the node with the given key or None if no such node exists."""
    if self.IsEmpty():
      return None
    self.Splay(key)
    if self.root.key == key:
      return self.root
    return None

  def FindMax(self):
    """Returns the node with the largest key value."""
    if self.IsEmpty():
      return None
    current = self.root
    while current.right != None:
      current = current.right
    return current

  # Returns the node with the smallest key value.
  def FindMin(self):
    if self.IsEmpty():
      return None
    current = self.root
    while current.left != None:
      current = current.left
    return current

  def FindGreatestsLessThan(self, key):
    """Returns node with greatest key less than or equal to the given key."""
    if self.IsEmpty():
      return None
    # Splay on the key to move the node with the given key or the last
    # node on the search path to the top of the tree.
    self.Splay(key)
    # Now the result is either the root node or the greatest node in
    # the left subtree.
    if self.root.key <= key:
      return self.root
    else:
      tmp = self.root
      self.root = self.root.left
      result = self.FindMax()
      self.root = tmp
      return result

  def ExportValueList(self):
    """Returns a list containing all the values of the nodes in the tree."""
    result = []
    nodes_to_visit = [self.root]
    while len(nodes_to_visit) > 0:
      node = nodes_to_visit.pop()
      if not node:
        continue
      result.append(node.value)
      nodes_to_visit.append(node.left)
      nodes_to_visit.append(node.right)
    return result

  def Splay(self, key):
    """Perform splay operation.

    Perform the splay operation for the given key. Moves the node with
    the given key to the top of the tree.  If no node has the given
    key, the last node on the search path is moved to the top of the
    tree.

    This uses the simplified top-down splaying algorithm from:

    "Self-adjusting Binary Search Trees" by Sleator and Tarjan

    """
    if self.IsEmpty():
      return
    # Create a dummy node.  The use of the dummy node is a bit
    # counter-intuitive: The right child of the dummy node will hold
    # the L tree of the algorithm.  The left child of the dummy node
    # will hold the R tree of the algorithm.  Using a dummy node, left
    # and right will always be nodes and we avoid special cases.
    dummy = left = right = Node(None, None)
    current = self.root
    while True:
      if key < current.key:
        if not current.left:
          break
        if key < current.left.key:
          # Rotate right.
          tmp = current.left
          current.left = tmp.right
          tmp.right = current
          current = tmp
          if not current.left:
            break
        # Link right.
        right.left = current
        right = current
        current = current.left
      elif key > current.key:
        if not current.right:
          break
        if key > current.right.key:
          # Rotate left.
          tmp = current.right
          current.right = tmp.left
          tmp.left = current
          current = tmp
          if not current.right:
            break
        # Link left.
        left.right = current
        left = current
        current = current.right
      else:
        break
    # Assemble.
    left.right = current.left
    right.left = current.right
    current.left = dummy.right
    current.right = dummy.left
    self.root = current