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from collections.abc import Sequence, Hashable
from numbers import Integral
from functools import reduce
class _PListBuilder(object):
"""
Helper class to allow construction of a list without
having to reverse it in the end.
"""
__slots__ = ('_head', '_tail')
def __init__(self):
self._head = _EMPTY_PLIST
self._tail = _EMPTY_PLIST
def _append(self, elem, constructor):
if not self._tail:
self._head = constructor(elem)
self._tail = self._head
else:
self._tail.rest = constructor(elem)
self._tail = self._tail.rest
return self._head
def append_elem(self, elem):
return self._append(elem, lambda e: PList(e, _EMPTY_PLIST))
def append_plist(self, pl):
return self._append(pl, lambda l: l)
def build(self):
return self._head
class _PListBase(object):
__slots__ = ('__weakref__',)
# Selected implementations can be taken straight from the Sequence
# class, other are less suitable. Especially those that work with
# index lookups.
count = Sequence.count
index = Sequence.index
def __reduce__(self):
# Pickling support
return plist, (list(self),)
def __len__(self):
"""
Return the length of the list, computed by traversing it.
This is obviously O(n) but with the current implementation
where a list is also a node the overhead of storing the length
in every node would be quite significant.
"""
return sum(1 for _ in self)
def __repr__(self):
return "plist({0})".format(list(self))
__str__ = __repr__
def cons(self, elem):
"""
Return a new list with elem inserted as new head.
>>> plist([1, 2]).cons(3)
plist([3, 1, 2])
"""
return PList(elem, self)
def mcons(self, iterable):
"""
Return a new list with all elements of iterable repeatedly cons:ed to the current list.
NB! The elements will be inserted in the reverse order of the iterable.
Runs in O(len(iterable)).
>>> plist([1, 2]).mcons([3, 4])
plist([4, 3, 1, 2])
"""
head = self
for elem in iterable:
head = head.cons(elem)
return head
def reverse(self):
"""
Return a reversed version of list. Runs in O(n) where n is the length of the list.
>>> plist([1, 2, 3]).reverse()
plist([3, 2, 1])
Also supports the standard reversed function.
>>> reversed(plist([1, 2, 3]))
plist([3, 2, 1])
"""
result = plist()
head = self
while head:
result = result.cons(head.first)
head = head.rest
return result
__reversed__ = reverse
def split(self, index):
"""
Spilt the list at position specified by index. Returns a tuple containing the
list up until index and the list after the index. Runs in O(index).
>>> plist([1, 2, 3, 4]).split(2)
(plist([1, 2]), plist([3, 4]))
"""
lb = _PListBuilder()
right_list = self
i = 0
while right_list and i < index:
lb.append_elem(right_list.first)
right_list = right_list.rest
i += 1
if not right_list:
# Just a small optimization in the cases where no split occurred
return self, _EMPTY_PLIST
return lb.build(), right_list
def __iter__(self):
li = self
while li:
yield li.first
li = li.rest
def __lt__(self, other):
if not isinstance(other, _PListBase):
return NotImplemented
return tuple(self) < tuple(other)
def __eq__(self, other):
"""
Traverses the lists, checking equality of elements.
This is an O(n) operation, but preserves the standard semantics of list equality.
"""
if not isinstance(other, _PListBase):
return NotImplemented
self_head = self
other_head = other
while self_head and other_head:
if not self_head.first == other_head.first:
return False
self_head = self_head.rest
other_head = other_head.rest
return not self_head and not other_head
def __getitem__(self, index):
# Don't use this this data structure if you plan to do a lot of indexing, it is
# very inefficient! Use a PVector instead!
if isinstance(index, slice):
if index.start is not None and index.stop is None and (index.step is None or index.step == 1):
return self._drop(index.start)
# Take the easy way out for all other slicing cases, not much structural reuse possible anyway
return plist(tuple(self)[index])
if not isinstance(index, Integral):
raise TypeError("'%s' object cannot be interpreted as an index" % type(index).__name__)
if index < 0:
# NB: O(n)!
index += len(self)
try:
return self._drop(index).first
except AttributeError as e:
raise IndexError("PList index out of range") from e
def _drop(self, count):
if count < 0:
raise IndexError("PList index out of range")
head = self
while count > 0:
head = head.rest
count -= 1
return head
def __hash__(self):
return hash(tuple(self))
def remove(self, elem):
"""
Return new list with first element equal to elem removed. O(k) where k is the position
of the element that is removed.
Raises ValueError if no matching element is found.
>>> plist([1, 2, 1]).remove(1)
plist([2, 1])
"""
builder = _PListBuilder()
head = self
while head:
if head.first == elem:
return builder.append_plist(head.rest)
builder.append_elem(head.first)
head = head.rest
raise ValueError('{0} not found in PList'.format(elem))
class PList(_PListBase):
"""
Classical Lisp style singly linked list. Adding elements to the head using cons is O(1).
Element access is O(k) where k is the position of the element in the list. Taking the
length of the list is O(n).
Fully supports the Sequence and Hashable protocols including indexing and slicing but
if you need fast random access go for the PVector instead.
Do not instantiate directly, instead use the factory functions :py:func:`l` or :py:func:`plist` to
create an instance.
Some examples:
>>> x = plist([1, 2])
>>> y = x.cons(3)
>>> x
plist([1, 2])
>>> y
plist([3, 1, 2])
>>> y.first
3
>>> y.rest == x
True
>>> y[:2]
plist([3, 1])
"""
__slots__ = ('first', 'rest')
def __new__(cls, first, rest):
instance = super(PList, cls).__new__(cls)
instance.first = first
instance.rest = rest
return instance
def __bool__(self):
return True
__nonzero__ = __bool__
Sequence.register(PList)
Hashable.register(PList)
class _EmptyPList(_PListBase):
__slots__ = ()
def __bool__(self):
return False
__nonzero__ = __bool__
@property
def first(self):
raise AttributeError("Empty PList has no first")
@property
def rest(self):
return self
Sequence.register(_EmptyPList)
Hashable.register(_EmptyPList)
_EMPTY_PLIST = _EmptyPList()
def plist(iterable=(), reverse=False):
"""
Creates a new persistent list containing all elements of iterable.
Optional parameter reverse specifies if the elements should be inserted in
reverse order or not.
>>> plist([1, 2, 3])
plist([1, 2, 3])
>>> plist([1, 2, 3], reverse=True)
plist([3, 2, 1])
"""
if not reverse:
iterable = list(iterable)
iterable.reverse()
return reduce(lambda pl, elem: pl.cons(elem), iterable, _EMPTY_PLIST)
def l(*elements):
"""
Creates a new persistent list containing all arguments.
>>> l(1, 2, 3)
plist([1, 2, 3])
"""
return plist(elements)