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Editing: _plist.py
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)
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