a pure-python B tree and B+ tree implementation

btree.py

import bisect
import itertools
import operator


class _BNode(object):
    __slots__ = ["tree", "contents", "children"]

    def __init__(self, tree, contents=None, children=None):
        self.tree = tree
        self.contents = contents or []
        self.children = children or []
        if self.children:
            assert len(self.contents) + 1 == len(self.children), \
                    "one more child than data item required"

    def __repr__(self):
        name = getattr(self, "children", 0) and "Branch" or "Leaf"
        return "<%s %s>" % (name, ", ".join(map(str, self.contents)))

    def lateral(self, parent, parent_index, dest, dest_index):
        if parent_index > dest_index:
            dest.contents.append(parent.contents[dest_index])
            parent.contents[dest_index] = self.contents.pop(0)
            if self.children:
                dest.children.append(self.children.pop(0))
        else:
            dest.contents.insert(0, parent.contents[parent_index])
            parent.contents[parent_index] = self.contents.pop()
            if self.children:
                dest.children.insert(0, self.children.pop())

    def shrink(self, ancestors):
        parent = None

        if ancestors:
            parent, parent_index = ancestors.pop()
            # try to lend to the left neighboring sibling
            if parent_index:
                left_sib = parent.children[parent_index - 1]
                if len(left_sib.contents) < self.tree.order:
                    self.lateral(
                            parent, parent_index, left_sib, parent_index - 1)
                    return

            # try the right neighbor
            if parent_index + 1 < len(parent.children):
                right_sib = parent.children[parent_index + 1]
                if len(right_sib.contents) < self.tree.order:
                    self.lateral(
                            parent, parent_index, right_sib, parent_index + 1)
                    return

        center = len(self.contents) // 2
        sibling, push = self.split()

        if not parent:
            parent, parent_index = self.tree.BRANCH(
                    self.tree, children=[self]), 0
            self.tree._root = parent

        # pass the median up to the parent
        parent.contents.insert(parent_index, push)
        parent.children.insert(parent_index + 1, sibling)
        if len(parent.contents) > parent.tree.order:
            parent.shrink(ancestors)

    def grow(self, ancestors):
        parent, parent_index = ancestors.pop()

        minimum = self.tree.order // 2
        left_sib = right_sib = None

        # try to borrow from the right sibling
        if parent_index + 1 < len(parent.children):
            right_sib = parent.children[parent_index + 1]
            if len(right_sib.contents) > minimum:
                right_sib.lateral(parent, parent_index + 1, self, parent_index)
                return

        # try to borrow from the left sibling
        if parent_index:
            left_sib = parent.children[parent_index - 1]
            if len(left_sib.contents) > minimum:
                left_sib.lateral(parent, parent_index - 1, self, parent_index)
                return

        # consolidate with a sibling - try left first
        if left_sib:
            left_sib.contents.append(parent.contents[parent_index - 1])
            left_sib.contents.extend(self.contents)
            if self.children:
                left_sib.children.extend(self.children)
            parent.contents.pop(parent_index - 1)
            parent.children.pop(parent_index)
        else:
            self.contents.append(parent.contents[parent_index])
            self.contents.extend(right_sib.contents)
            if self.children:
                self.children.extend(right_sib.children)
            parent.contents.pop(parent_index)
            parent.children.pop(parent_index + 1)

        if len(parent.contents) < minimum:
            if ancestors:
                # parent is not the root
                parent.grow(ancestors)
            elif not parent.contents:
                # parent is root, and its now empty
                self.tree._root = left_sib or self

    def split(self):
        center = len(self.contents) // 2
        median = self.contents[center]
        sibling = type(self)(
                self.tree,
                self.contents[center + 1:],
                self.children[center + 1:])
        self.contents = self.contents[:center]
        self.children = self.children[:center + 1]
        return sibling, median

    def insert(self, index, item, ancestors):
        self.contents.insert(index, item)
        if len(self.contents) > self.tree.order:
            self.shrink(ancestors)

    def remove(self, index, ancestors):
        minimum = self.tree.order // 2

        if self.children:
            # try promoting from the right subtree first,
            # but only if it won't have to resize
            additional_ancestors = [(self, index + 1)]
            descendent = self.children[index + 1]
            while descendent.children:
                additional_ancestors.append((descendent, 0))
                descendent = descendent.children[0]
            if len(descendent.contents) > minimum:
                ancestors.extend(additional_ancestors)
                self.contents[index] = descendent.contents[0]
                descendent.remove(0, ancestors)
                return

            # fall back to the left child
            additional_ancestors = [(self, index)]
            descendent = self.children[index]
            while descendent.children:
                additional_ancestors.append(
                        (descendent, len(descendent.children) - 1))
                descendent = descendent.children[-1]
            ancestors.extend(additional_ancestors)
            self.contents[index] = descendent.contents[-1]
            descendent.remove(len(descendent.children) - 1, ancestors)
        else:
            self.contents.pop(index)
            if len(self.contents) < minimum and ancestors:
                self.grow(ancestors)


class _BPlusLeaf(_BNode):
    __slots__ = ["tree", "contents", "data", "next"]

    def __init__(self, tree, contents=None, data=None, next=None):
        self.tree = tree
        self.contents = contents or []
        self.data = data or []
        self.next = next
        assert len(self.contents) == len(self.data), "one data per key"

    def insert(self, index, key, data, ancestors):
        self.contents.insert(index, key)
        self.data.insert(index, data)

        if len(self.contents) > self.tree.order:
            self.shrink(ancestors)

    def lateral(self, parent, parent_index, dest, dest_index):
        if parent_index > dest_index:
            dest.contents.append(self.contents.pop(0))
            dest.data.append(self.data.pop(0))
            parent.contents[dest_index] = self.contents[0]
        else:
            dest.contents.insert(0, self.contents.pop())
            dest.data.insert(0, self.data.pop())
            parent.contents[parent_index] = dest.contents[0]

    def split(self):
        center = len(self.contents) // 2
        median = self.contents[center - 1]
        sibling = type(self)(
                self.tree,
                self.contents[center:],
                self.data[center:],
                self.next)
        self.contents = self.contents[:center]
        self.data = self.data[:center]
        self.next = sibling
        return sibling, sibling.contents[0]

    def remove(self, index, ancestors):
        minimum = self.tree.order // 2
        if index >= len(self.contents):
            self, index = self.next, 0

        key = self.contents[index]

        # if any leaf that could accept the key can do so
        # without any rebalancing necessary, then go that route
        current = self
        while current is not None and current.contents[0] == key:
            if len(current.contents) > minimum:
                if current.contents[0] == key:
                    index = 0
                else:
                    index = bisect.bisect_left(current.contents, key)
                current.contents.pop(index)
                current.data.pop(index)
                return
            current = current.next

        self.grow(ancestors)

    def grow(self, ancestors):
        minimum = self.tree.order // 2
        parent, parent_index = ancestors.pop()
        left_sib = right_sib = None

        # try borrowing from a neighbor - try right first
        if parent_index + 1 < len(parent.children):
            right_sib = parent.children[parent_index + 1]
            if len(right_sib.contents) > minimum:
                right_sib.lateral(parent, parent_index + 1, self, parent_index)
                return

        # fallback to left
        if parent_index:
            left_sib = parent.children[parent_index - 1]
            if len(left_sib.contents) > minimum:
                left_sib.lateral(parent, parent_index - 1, self, parent_index)
                return

        # join with a neighbor - try left first
        if left_sib:
            left_sib.contents.extend(self.contents)
            left_sib.data.extend(self.data)
            parent.remove(parent_index - 1, ancestors)
            return

        # fallback to right
        self.contents.extend(right_sib.contents)
        self.data.extend(right_sib.data)
        parent.remove(parent_index, ancestors)


class BTree(object):
    BRANCH = LEAF = _BNode

    def __init__(self, order):
        self.order = order
        self._root = self._bottom = self.LEAF(self)

    def _path_to(self, item):
        current = self._root
        ancestry = []

        while getattr(current, "children", None):
            index = bisect.bisect_left(current.contents, item)
            ancestry.append((current, index))
            if index < len(current.contents) \
                    and current.contents[index] == item:
                return ancestry
            current = current.children[index]

        index = bisect.bisect_left(current.contents, item)
        ancestry.append((current, index))
        present = index < len(current.contents)
        present = present and current.contents[index] == item

        return ancestry

    def _present(self, item, ancestors):
        last, index = ancestors[-1]
        return index < len(last.contents) and last.contents[index] == item

    def insert(self, item):
        current = self._root
        ancestors = self._path_to(item)
        node, index = ancestors[-1]
        while getattr(node, "children", None):
            node = node.children[index]
            index = bisect.bisect_left(node.contents, item)
            ancestors.append((node, index))
        node, index = ancestors.pop()
        node.insert(index, item, ancestors)

    def remove(self, item):
        current = self._root
        ancestors = self._path_to(item)

        if self._present(item, ancestors):
            node, index = ancestors.pop()
            node.remove(index, ancestors)
        else:
            raise ValueError("%r not in %s" % (item, self.__class__.__name__))

    def __contains__(self, item):
        return self._present(item, self._path_to(item))

    def __iter__(self):
        def _recurse(node):
            if node.children:
                for child, item in zip(node.children, node.contents):
                    for child_item in _recurse(child):
                        yield child_item
                    yield item
                for child_item in _recurse(node.children[-1]):
                    yield child_item
            else:
                for item in node.contents:
                    yield item

        for item in _recurse(self._root):
            yield item

    def __repr__(self):
        def recurse(node, accum, depth):
            accum.append(("  " * depth) + repr(node))
            for node in getattr(node, "children", []):
                recurse(node, accum, depth + 1)

        accum = []
        recurse(self._root, accum, 0)
        return "\n".join(accum)

    @classmethod
    def bulkload(cls, items, order):
        tree = object.__new__(cls)
        tree.order = order

        leaves = tree._build_bulkloaded_leaves(items)
        tree._build_bulkloaded_branches(leaves)

        return tree

    def _build_bulkloaded_leaves(self, items):
        minimum = self.order // 2
        leaves, seps = [[]], []

        for item in items:
            if len(leaves[-1]) < self.order:
                leaves[-1].append(item)
            else:
                seps.append(item)
                leaves.append([])

        if len(leaves[-1]) < minimum and seps:
            last_two = leaves[-2] + [seps.pop()] + leaves[-1]
            leaves[-2] = last_two[:minimum]
            leaves[-1] = last_two[minimum + 1:]
            seps.append(last_two[minimum])

        return [self.LEAF(self, contents=node) for node in leaves], seps

    def _build_bulkloaded_branches(self, (leaves, seps)):
        minimum = self.order // 2
        levels = [leaves]

        while len(seps) > self.order + 1:
            items, nodes, seps = seps, [[]], []

            for item in items:
                if len(nodes[-1]) < self.order:
                    nodes[-1].append(item)
                else:
                    seps.append(item)
                    nodes.append([])

            if len(nodes[-1]) < minimum and seps:
                last_two = nodes[-2] + [seps.pop()] + nodes[-1]
                nodes[-2] = last_two[:minimum]
                nodes[-1] = last_two[minimum + 1:]
                seps.append(last_two[minimum])

            offset = 0
            for i, node in enumerate(nodes):
                children = levels[-1][offset:offset + len(node) + 1]
                nodes[i] = self.BRANCH(self, contents=node, children=children)
                offset += len(node) + 1

            levels.append(nodes)

        self._root = self.BRANCH(self, contents=seps, children=levels[-1])


class BPlusTree(BTree):
    LEAF = _BPlusLeaf

    def _get(self, key):
        node, index = self._path_to(key)[-1]

        if index == len(node.contents):
            if node.next:
                node, index = node.next, 0
            else:
                return

        while node.contents[index] == key:
            yield node.data[index]
            index += 1
            if index == len(node.contents):
                if node.next:
                    node, index = node.next, 0
                else:
                    return

    def _path_to(self, item):
        path = super(BPlusTree, self)._path_to(item)
        node, index = path[-1]
        while hasattr(node, "children"):
            node = node.children[index]
            index = bisect.bisect_left(node.contents, item)
            path.append((node, index))
        return path

    def get(self, key, default=None):
        try:
            return self._get(key).next()
        except StopIteration:
            return default

    def getlist(self, key):
        return list(self._get(key))

    def insert(self, key, data):
        path = self._path_to(key)
        node, index = path.pop()
        node.insert(index, key, data, path)

    def remove(self, key):
        path = self._path_to(key)
        node, index = path.pop()
        node.remove(index, path)

    __getitem__ = get
    __setitem__ = insert
    __delitem__ = remove

    def __contains__(self, key):
        for item in self._get(key):
            return True
        return False

    def iteritems(self):
        node = self._root
        while hasattr(node, "children"):
            node = node.children[0]

        while node:
            for pair in itertools.izip(node.contents, node.data):
                yield pair
            node = node.next

    def iterkeys(self):
        return itertools.imap(operator.itemgetter(0), self.iteritems())

    def itervalues(self):
        return itertools.imap(operator.itemgetter(1), self.iteritems())

    __iter__ = iterkeys

    def items(self):
        return list(self.iteritems())

    def keys(self):
        return list(self.iterkeys())

    def values(self):
        return list(self.itervalues())

    def _build_bulkloaded_leaves(self, items):
        minimum = self.order // 2
        leaves, seps = [[]], []

        for item in items:
            if len(leaves[-1]) >= self.order:
                seps.append(item)
                leaves.append([])
            leaves[-1].append(item)

        if len(leaves[-1]) < minimum and seps:
            last_two = leaves[-2] + leaves[-1]
            leaves[-2] = last_two[:minimum]
            leaves[-1] = last_two[minimum:]
            seps.append(last_two[minimum])

        leaves = [self.LEAF(
                self,
                contents=[p[0] for p in pairs],
                data=[p[1] for p in pairs])
            for pairs in leaves]

        for i in xrange(len(leaves) - 1):
            leaves[i].next = leaves[i + 1]

        return leaves, [s[0] for s in seps]


import random
import unittest


class BTreeTests(unittest.TestCase):
    def test_additions(self):
        bt = BTree(20)
        l = range(2000)
        for i, item in enumerate(l):
            bt.insert(item)
            self.assertEqual(list(bt), l[:i + 1])

    def test_bulkloads(self):
        bt = BTree.bulkload(range(2000), 20)
        self.assertEqual(list(bt), range(2000))

    def test_removals(self):
        bt = BTree(20)
        l = range(2000)
        map(bt.insert, l)
        rand = l[:]
        random.shuffle(rand)
        while l:
            self.assertEqual(list(bt), l)
            rem = rand.pop()
            l.remove(rem)
            bt.remove(rem)
        self.assertEqual(list(bt), l)

    def test_insert_regression(self):
        bt = BTree.bulkload(range(2000), 50)

        for i in xrange(100000):
            bt.insert(random.randrange(2000))


class BPlusTreeTests(unittest.TestCase):
    def test_additions_sorted(self):
        bt = BPlusTree(20)
        l = range(2000)

        for item in l:
            bt.insert(item, str(item))

        for item in l:
            self.assertEqual(str(item), bt[item])

        self.assertEqual(l, list(bt))

    def test_additions_random(self):
        bt = BPlusTree(20)
        l = range(2000)
        random.shuffle(l)

        for item in l:
            bt.insert(item, str(item))

        for item in l:
            self.assertEqual(str(item), bt[item])

        self.assertEqual(range(2000), list(bt))

    def test_bulkload(self):
        bt = BPlusTree.bulkload(zip(range(2000), map(str, range(2000))), 20)

        self.assertEqual(list(bt), range(2000))

        self.assertEqual(
                list(bt.iteritems()),
                zip(range(2000), map(str, range(2000))))


if __name__ == '__main__':
    unittest.main()

Hi guys,

when i use the BPlusTree, and i try to delete one element, I find an error. For example,

in

def test_additions_sorted(self):
bt = BPlusTree(20)
l = range(2000)

    for item in l:
        bt.insert(item, str(item))

    for item in l:
        self.assertEqual(str(item), bt[item])

    for item in l:
        bt.remove(item)

    self.assertEqual(l, list(bt))

I have added the list removal and executing i get:

ERROR: test_additions_sorted (main.BPlusTreeTests)

Traceback (most recent call last):
File "btree.py", line 557, in test_additions_sorted
bt.remove(item)
File "btree.py", line 443, in remove
node.remove(index, path)
File "btree.py", line 222, in remove
self.grow(ancestors)
File "btree.py", line 253, in grow
parent.remove(parent_index, ancestors)
File "btree.py", line 136, in remove
while descendent.children:
AttributeError: children

What can be happening?

Thanks in advance.

The problem seems to be that internally, _BNode.remove() is being called instead of _BPlusLeaf.remove().

Hi,
I see that the "next" is implemented at the linked list leaves-level of the tree. What about the "previous"? How could that be implemented with a double linked list?

@teepark was thinking of merging some parts of the module to my work here which im working on here:
https://github.com/codecakes/BST ; still needs debugging but feel free to try out and maybe I would merge the best features from here and mine for a cleaner design.

Hi all,

Anyone else having problems when removing from a tree with order > 1 and 1 item? Any one knows how to fix this?

def test_delete(self):
    """
    This breaks remove function...
    """
    bt = BPlusTree(2)
    bt.insert(1000, "Test")
    bt.remove(1000)
    print(bt)

The problem seems to be at line 224 (grow function) which I am not sure if it should be called in this case...

UPDATE:

Changed line 212 to:

        if len(current.contents) > minimum or len(ancestors) == 0:

and seems working...

hey guys, I found a bug, from line 148 to line 154:

while descendent.children:
                additional_ancestors.append(
                        (descendent, len(descendent.children) - 1))
                descendent = descendent.children[-1]
ancestors.extend(additional_ancestors)
self.contents[index] = descendent.contents[-1]
descendent.remove(len(descendent.children) - 1, ancestors)

when while break, descendent must be a BPlusLeaf, whitch has no attribute children.