构建大型单词到索引到单词字典的最有效数据结构是什么?

What is the most efficient data structure to build a large word-to-index-to-word dictionary?

我想索引大量字符串(将每个字符串映射到一个数值),但也能够从其数字索引中检索每个字符串。

由于内存问题,无法使用哈希表或 python 字典,因此我决定使用 radix trie 来存储字符串,我可以非常快速地检索任何字符串的索引并且处理大量字符串。

我的问题是我还需要从它们的数字索引中检索字符串,如果我维护一个 "reverse index" 列表 [string1, string2, ..., stringn] 我将失去内存优势的特里。

我认为 "reverse index" 可能是指向某种 Trie 结构的最后一个节点的指针列表,但首先,python 中没有指针,其次我是不确定我是否可以 "node-level" 访问我当前使用的 Trie 结构。

这种数据结构已经存在了吗?如果不是,您将如何在 python 中执行此操作?

根据 ,您需要两个用于键和值查找的同步数据结构,每个都保存对另一个叶节点的引用。

ID 查找的结构可以是任何具有足够效率的结构 -- 平衡树、散列 table、另一个 trie。

为了能够从叶节点引用中提取值,trie 需要允许 1) 叶节点引用本身(不一定是真正的 Python 引用,任何它的 API 可以采用); 2) 沿着 trie 树从那个引用中提取单词。

请注意,引用实际上是一个唯一的整数,因此如果您的 ID 不大于一个整数,则将某些内容重用为 ID 是有意义的——例如trie 节点引用自己。然后如果 trie API 可以验证这样的引用(即判断它是否有一个带有这样引用的已用节点)这将充当 ID 查找并且您根本不需要第二个结构!这样,ID 将是非持久性的,尽管因为参考值(有效内存地址)在进程和运行之间发生变化。

我在自言自语,因为我最终创建了自己的数据结构,它非常适合我遇到的单词到索引到单词的问题,仅使用 python3 内置-在函数中。

我试图让它简洁高效,但显然还有改进的余地,C 绑定会更好。

所以最终结果是一个 indexedtrie class,它看起来像一个 python dict(如果你用 default_factory 参数调用它,则为 defaultdict)但也可以像列表,因为自动维护了一种"reversed index"。

存储在内部基数树中的键可以是任何可下标的对象(字节、字符串、元组、列表)和您想要在其中存储任何内容的值。

indextrie class 也是可选的,您可以从关于 "prefix search" 和这类事情的基数尝试中受益!

trie 中的每个键都与一个唯一的整数索引相关联,您可以使用索引检索键或使用键检索索引,整个过程速度快且内存安全,所以我个人认为这是最好的之一世界上的数据结构,它应该集成在 python 标准库中 :).

废话不多说了,下面是代码,欢迎改编使用:

"""
A Python3 indexed trie class.
An indexed trie's key can be any subscriptable object. 
Keys of the indexed trie are stored using a "radix trie", a space-optimized data-structure which has many advantages (see https://en.wikipedia.org/wiki/Radix_tree).
Also, each key in the indexed trie is associated to a unique index which is build dynamically.

Indexed trie is used like a python dictionary (and even a collections.defaultdict if you want to) but its values can also be accessed or updated (but not created) like a list!

Example:
    >>> t = indextrie()
    >>> t["abc"] = "hello"
    >>> t[0]
    'hello'
    >>> t["abc"]
    'hello'
    >>> t.index2key(0)
    'abc'
    >>> t.key2index("abc")
    0
    >>> t[:]
    [0]
    >>> print(t)
    {(0, 'abc'): hello}
"""

__author__ = "@fbparis"

_SENTINEL = object()

class _Node(object):
    """
    A single node in the trie.
    """
    __slots__ = "_children", "_parent", "_index", "_key"

    def __init__(self, key, parent, index=None):
        self._children = set()
        self._key = key
        self._parent = parent
        self._index = index
        self._parent._children.add(self)

class IndexedtrieKey(object):
    """
    A pair (index, key) acting as an indexedtrie's key
    """
    __slots__ = "index", "key"

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

    def __repr__(self):
        return "(%d, %s)" % (self.index, self.key)

class indexedtrie(object):
    """
    The indexed trie data-structure.
    """
    __slots__ = "_children", "_indexes", "_values", "_nodescount", "_default_factory"

    def __init__(self, items=None, default_factory=_SENTINEL):
        """
        A list of items can be passed to initialize the indexed trie.
        """
        self._children = set()
        self.setdefault(default_factory)
        self._indexes = []
        self._values = []
        self._nodescount = 0 # keeping track of nodes count is purely informational
        if items is not None:
            for k, v in items:
                if isinstance(k, IndexedtrieKey):
                    self.__setitem__(k.key, v)
                else:
                    self.__setitem__(k, v)

    @classmethod
    def fromkeys(cls, keys, value=_SENTINEL, default_factory=_SENTINEL):
        """
        Build a new indexedtrie from a list of keys.
        """
        obj = cls(default_factory=default_factory)
        for key in keys:
            if value is _SENTINEL:
                if default_factory is not _SENTINEL:
                    obj[key] = obj._default_factory()
                else:
                    obj[key] = None
            else:
                obj[key] = value
        return obj

    @classmethod
    def fromsplit(cls, keys, value=_SENTINEL, default_factory=_SENTINEL):
        """
        Build a new indexedtrie from a splitable object.
        """
        obj = cls(default_factory=default_factory)
        for key in keys.split():
            if value is _SENTINEL:
                if default_factory is not _SENTINEL:
                    obj[key] = obj._default_factory()
                else:
                    obj[key] = None
            else:
                obj[key] = value
        return obj

    def setdefault(self, factory=_SENTINEL):
        """
        """
        if factory is not _SENTINEL:
            # indexed trie will act like a collections.defaultdict except in some cases because the __missing__
            # method is not implemented here (on purpose).
            # That means that simple lookups on a non existing key will return a default value without adding
            # the key, which is the more logical way to do.
            # Also means that if your default_factory is for example "list", you won't be able to create new
            # items with "append" or "extend" methods which are updating the list itself.
            # Instead you have to do something like trie["newkey"] += [...]
            try:
                _ = factory()
            except TypeError:
                # a default value is also accepted as default_factory, even "None"
                self._default_factory = lambda: factory
            else:
                self._default_factory = factory
        else:
            self._default_factory = _SENTINEL

    def copy(self):
        """
        Return a pseudo-shallow copy of the indexedtrie.
        Keys and nodes are deepcopied, but if you store some referenced objects in values, only the references will be copied.
        """
        return self.__class__(self.items(), default_factory=self._default_factory)

    def __len__(self):
        return len(self._indexes)

    def __repr__(self):
        if self._default_factory is not _SENTINEL:
            default = ", default_value=%s" % self._default_factory()
        else:
            default = ""
        return "<%s object at %s: %d items, %d nodes%s>" % (self.__class__.__name__, hex(id(self)), len(self), self._nodescount, default)

    def __str__(self):
        ret = ["%s: %s" % (k, v) for k, v in self.items()]
        return "{%s}" % ", ".join(ret)

    def __iter__(self):
        return self.keys()

    def __contains__(self, key_or_index):
        """
        Return True if the key or index exists in the indexed trie.
        """
        if isinstance(key_or_index, IndexedtrieKey):
            return key_or_index.index >= 0 and key_or_index.index < len(self)
        if isinstance(key_or_index, int):
            return key_or_index >= 0 and key_or_index < len(self)
        if self._seems_valid_key(key_or_index):
            try:
                node = self._get_node(key_or_index)
            except KeyError:
                return False
            else:
                return node._index is not None
        raise TypeError("invalid key type")

    def __getitem__(self, key_or_index):
        """
        """
        if isinstance(key_or_index, IndexedtrieKey):
            return self._values[key_or_index.index]
        if isinstance(key_or_index, int) or isinstance(key_or_index, slice):
            return self._values[key_or_index]
        if self._seems_valid_key(key_or_index):
            try:
                node = self._get_node(key_or_index)
            except KeyError:
                if self._default_factory is _SENTINEL:
                    raise
                else:
                    return self._default_factory()
            else:
                if node._index is None:
                    if self._default_factory is _SENTINEL:
                        raise KeyError
                    else:
                        return self._default_factory()
                else:
                    return self._values[node._index]            
        raise TypeError("invalid key type")

    def __setitem__(self, key_or_index, value):
        """
        """
        if isinstance(key_or_index, IndexedtrieKey):
            self._values[key_or_index.index] = value
        elif isinstance(key_or_index, int):
            self._values[key_or_index] = value
        elif isinstance(key_or_index, slice):
            raise NotImplementedError
        elif self._seems_valid_key(key_or_index):
            try:
                node = self._get_node(key_or_index)
            except KeyError:
                # create a new node
                self._add_node(key_or_index, value)
            else:
                if node._index is None:
                    # if node exists but not indexed, we index it and update the value
                    self._add_to_index(node, value)
                else:
                    # else we update its value
                    self._values[node._index] = value
        else:
            raise TypeError("invalid key type")

    def __delitem__(self, key_or_index):
        """
        """
        if isinstance(key_or_index, IndexedtrieKey):
            node = self._indexes[key_or_index.index]
        elif isinstance(key_or_index, int):
            node = self._indexes[key_or_index]
        elif isinstance(key_or_index, slice):
            raise NotImplementedError
        elif self._seems_valid_key(key_or_index):
            node = self._get_node(key_or_index)
            if node._index is None:
                raise KeyError
        else:
            raise TypeError("invalid key type")
        # switch last index with deleted index (except if deleted index is last index)
        last_node, last_value = self._indexes.pop(), self._values.pop()
        if node._index != last_node._index:
            last_node._index = node._index
            self._indexes[node._index] = last_node
            self._values[node._index] = last_value
        if len(node._children) > 1:
            #case 1: node has more than 1 child, only turn index off
            node._index = None
        elif len(node._children) == 1:
            # case 2: node has 1 child
            child = node._children.pop()
            child._key = node._key + child._key
            child._parent = node._parent
            node._parent._children.add(child)
            node._parent._children.remove(node)
            del(node)
            self._nodescount -= 1
        else:
            # case 3: node has no child, check the parent node
            parent = node._parent
            parent._children.remove(node)
            del(node)
            self._nodescount -= 1
            if hasattr(parent, "_index"):
                if parent._index is None and len(parent._children) == 1:
                    node = parent._children.pop()
                    node._key = parent._key + node._key
                    node._parent = parent._parent
                    parent._parent._children.add(node)
                    parent._parent._children.remove(parent)
                    del(parent)
                    self._nodescount -= 1

    @staticmethod
    def _seems_valid_key(key):
        """
        Return True if "key" can be a valid key (must be subscriptable).
        """
        try:
            _ = key[:0]
        except TypeError:
            return False
        return True

    def keys(self, prefix=None):
        """
        Yield keys stored in the indexedtrie where key is a IndexedtrieKey object.
        If prefix is given, yield only keys of items with key matching the prefix.
        """
        if prefix is None:
            for i, node in enumerate(self._indexes):
                yield IndexedtrieKey(i, self._get_key(node))
        else:
            if self._seems_valid_key(prefix):
                empty = prefix[:0]
                children = [(empty, prefix, child) for child in self._children]
                while len(children):
                    _children = []
                    for key, prefix, child in children:
                        if prefix == child._key[:len(prefix)]:
                            _key = key + child._key
                            _children.extend([(_key, empty, _child) for _child in child._children])
                            if child._index is not None:
                                yield IndexedtrieKey(child._index, _key)
                        elif prefix[:len(child._key)] == child._key:
                            _prefix = prefix[len(child._key):]
                            _key = key + prefix[:len(child._key)]
                            _children.extend([(_key, _prefix, _child) for _child in child._children])
                    children = _children
            else:
                raise ValueError("invalid prefix type")

    def values(self, prefix=None):
        """
        Yield values stored in the indexedtrie.
        If prefix is given, yield only values of items with key matching the prefix.
        """
        if prefix is None:
            for value in self._values:
                yield value
        else:
            for key in self.keys(prefix):
                yield self._values[key.index]

    def items(self, prefix=None):
        """
        Yield (key, value) pairs stored in the indexedtrie where key is a IndexedtrieKey object.
        If prefix is given, yield only (key, value) pairs of items with key matching the prefix.
        """
        for key in self.keys(prefix):
            yield key, self._values[key.index]

    def show_tree(self, node=None, level=0):
        """
        Pretty print the internal trie (recursive function).
        """
        if node is None:
            node = self
        for child in node._children:
            print("-" * level + "<key=%s, index=%s>" % (child._key, child._index))
            if len(child._children):
                self.show_tree(child, level + 1)

    def _get_node(self, key):
        """
        Return the node associated to key or raise a KeyError.
        """
        children = self._children
        while len(children):
            notfound = True
            for child in children:
                if key == child._key:
                    return child
                if child._key == key[:len(child._key)]:
                    children = child._children
                    key = key[len(child._key):]
                    notfound = False
                    break
            if notfound:
                break
        raise KeyError

    def _add_node(self, key, value):
        """
        Add a new key in the trie and updates indexes and values.
        """
        children = self._children
        parent = self
        moved = None
        done = len(children) == 0
        # we want to insert key="abc"
        while not done:
            done = True
            for child in children:
                # assert child._key != key # uncomment if you don't trust me
                if child._key == key[:len(child._key)]:
                    # case 1: child's key is "ab", insert "c" in child's children 
                    parent = child
                    children = child._children
                    key = key[len(child._key):]
                    done = len(children) == 0
                    break
                elif key == child._key[:len(key)]:
                    # case 2: child's key is "abcd", we insert "abc" in place of the child
                    # child's parent will be the inserted node and child's key is now "d"
                    parent = child._parent
                    moved = child
                    parent._children.remove(moved)
                    moved._key = moved._key[len(key):]
                    break
                elif type(key) is type(child._key): # don't mess it up
                    # find longest common prefix
                    prefix = key[:0]
                    for i, c in enumerate(key):
                        if child._key[i] != c:
                            prefix = key[:i]
                            break
                    if prefix:
                        # case 3: child's key is abd, we spawn a new node with key "ab"
                        # to replace child ; child's key is now "d" and child's parent is 
                        # the new created node.
                        # the new node will also be inserted as a child of this node
                        # with key "c"
                        node = _Node(prefix, child._parent)
                        self._nodescount += 1
                        child._parent._children.remove(child)
                        child._key = child._key[len(prefix):]
                        child._parent = node
                        node._children.add(child)
                        key = key[len(prefix):]
                        parent = node
                        break
        # create the new node
        node = _Node(key, parent)
        self._nodescount += 1
        if moved is not None:
            # if we have moved an existing node, update it
            moved._parent = node
            node._children.add(moved)
        self._add_to_index(node, value)

    def _get_key(self, node):
        """
        Rebuild key from a terminal node.
        """
        key = node._key
        while node._parent is not self:
            node = node._parent
            key = node._key + key
        return key

    def _add_to_index(self, node, value):
        """
        Add a new node to the index.
        Also record its value.
        """
        node._index = len(self)
        self._indexes.append(node)
        self._values.append(value)

    def key2index(self, key):
        """
        key -> index
        """
        if self._seems_valid_key(key):
            node = self._get_node(key)
            if node._index is not None:
                return node._index
            raise KeyError
        raise TypeError("invalid key type")

    def index2key(self, index):
        """
        index or IndexedtrieKey -> key.
        """
        if isinstance(index, IndexedtrieKey):
            index = index.index
        elif not isinstance(index, int):
            raise TypeError("index must be an int")
        if index < 0 or index > len(self._indexes):
            raise IndexError
        return self._get_key(self._indexes[index])