Source code for sympy.combinatorics.prufer

from sympy.core import Basic
from sympy.core.compatibility import iterable, as_int
from sympy.utilities.iterables import flatten

from collections import defaultdict

[docs]class Prufer(Basic): """ The Prufer correspondence is an algorithm that describes the bijection between labeled trees and the Prufer code. A Prufer code of a labeled tree is unique up to isomorphism and has a length of n - 2. Prufer sequences were first used by Heinz Prufer to give a proof of Cayley's formula. References ========== .. [1] http://mathworld.wolfram.com/LabeledTree.html """ _prufer_repr = None _tree_repr = None _nodes = None _rank = None @property
[docs] def prufer_repr(self): """Returns Prufer sequence for the Prufer object. This sequence is found by removing the highest numbered vertex, recording the node it was attached to, and continuuing until only two verices remain. The Prufer sequence is the list of recorded nodes. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> Prufer([[0, 3], [1, 3], [2, 3], [3, 4], [4, 5]]).prufer_repr [3, 3, 3, 4] >>> Prufer([1, 0, 0]).prufer_repr [1, 0, 0] See Also ======== to_prufer """ if self._prufer_repr is None: self._prufer_repr = self.to_prufer(self._tree_repr[:], self.nodes) return self._prufer_repr
@property
[docs] def tree_repr(self): """Returns the tree representation of the Prufer object. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> Prufer([[0, 3], [1, 3], [2, 3], [3, 4], [4, 5]]).tree_repr [[0, 3], [1, 3], [2, 3], [3, 4], [4, 5]] >>> Prufer([1, 0, 0]).tree_repr [[1, 2], [0, 1], [0, 3], [0, 4]] See Also ======== to_tree """ if self._tree_repr is None: self._tree_repr = self.to_tree(self._prufer_repr[:]) return self._tree_repr
@property
[docs] def nodes(self): """Returns the number of nodes in the tree. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> Prufer([[0, 3], [1, 3], [2, 3], [3, 4], [4, 5]]).nodes 6 >>> Prufer([1, 0, 0]).nodes 5 """ return self._nodes
@property
[docs] def rank(self): """Returns the rank of the Prufer sequence. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> p = Prufer([[0, 3], [1, 3], [2, 3], [3, 4], [4, 5]]) >>> p.rank 778 >>> p.next(1).rank 779 >>> p.prev().rank 777 See Also ======== prufer_rank, next, prev, size """ if self._rank is None: self._rank = self.prufer_rank() return self._rank
@property
[docs] def size(self): """Return the number of possible trees of this Prufer object. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> Prufer([0]*4).size == Prufer([6]*4).size == 1296 True See Also ======== prufer_rank, rank, next, prev """ return self.prev(self.rank).prev().rank + 1
@staticmethod
[docs] def to_prufer(tree, n): """Return the Prufer sequence for a tree given as a list of edges where ``n`` is the number of nodes in the tree. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> a = Prufer([[0, 1], [0, 2], [0, 3]]) >>> a.prufer_repr [0, 0] >>> Prufer.to_prufer([[0, 1], [0, 2], [0, 3]], 4) [0, 0] See Also ======== prufer_repr: returns Prufer sequence of a Prufer object. """ d = defaultdict(int) L = [] for edge in tree: # Increment the value of the corresponding # node in the degree list as we encounter an # edge involving it. d[edge[0]] += 1 d[edge[1]] += 1 for i in xrange(n - 2): # find the smallest leaf for x in xrange(n): if d[x] == 1: break # find the node it was connected to y = None for edge in tree: if x == edge[0]: y = edge[1] elif x == edge[1]: y = edge[0] if y is not None: break # record and update L.append(y) for j in (x, y): d[j] -= 1 if not d[j]: d.pop(j) tree.remove(edge) return L
@staticmethod
[docs] def to_tree(prufer): """Return the tree (as a list of edges) of the given Prufer sequence. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> a = Prufer([0, 2], 4) >>> a.tree_repr [[0, 1], [0, 2], [2, 3]] >>> Prufer.to_tree([0, 2]) [[0, 1], [0, 2], [2, 3]] References ========== - http://hamberg.no/erlend/2010/11/06/prufer-sequence/ See Also ======== tree_repr: returns tree representation of a Prufer object. """ tree = [] last = [] n = len(prufer) + 2 d = defaultdict(lambda: 1) for p in prufer: d[p] += 1 for i in prufer: for j in xrange(n): # find the smallest leaf (degree = 1) if d[j] == 1: break # (i, j) is the new edge that we append to the tree # and remove from the degree dictionary d[i] -= 1 d[j] -= 1 tree.append(sorted([i, j])) last = [i for i in xrange(n) if d[i] == 1] or [0, 1] tree.append(last) return tree
@staticmethod
[docs] def edges(*runs): """Return a list of edges and the number of nodes from the given runs that connect nodes in an integer-labelled tree. All node numbers will be shifted so that the minimum node is 0. It is not a problem if edges are repeated in the runs; only unique edges are returned. There is no assumption made about what the range of the node labels should be, but all nodes from the smallest through the largest must be present. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> Prufer.edges([1, 2, 3], [2, 4, 5]) # a T ([[0, 1], [3, 4], [1, 2], [1, 3]], 5) Duplicate edges are removed: >>> Prufer.edges([0, 1, 2, 3], [1, 4, 5], [1, 4, 6]) # a K ([[0, 1], [1, 2], [4, 6], [4, 5], [1, 4], [2, 3]], 7) """ e = set() nmin = runs[0][0] for r in runs: for i in range(len(r)-1): a, b = r[i: i + 2] if b < a: a, b = b, a e.add((a, b)) rv = [] got = set() nmin = nmax = None while e: ei = e.pop() for i in ei: got.add(i) nmin = min(ei[0], nmin) if nmin != None else ei[0] nmax = max(ei[1], nmax) if nmax != None else ei[1] rv.append(list(ei)) missing = set(range(nmin, nmax + 1)) - got if missing: missing = [i + nmin for i in missing] if len(missing) == 1: msg = 'Node %s is missing.' % missing.pop() else: msg = 'Nodes %s are missing.' % list(sorted(missing)) raise ValueError(msg) if nmin != 0: for i, ei in enumerate(rv): rv[i] = [n - nmin for n in ei] nmax -= nmin return rv, nmax + 1
[docs] def prufer_rank(self): """Computes the rank of a Prufer sequence. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> a = Prufer([[0, 1], [0, 2], [0, 3]]) >>> a.prufer_rank() 0 See Also ======== rank, next, prev, size """ r = 0 p = 1 for i in xrange(self.nodes - 3, -1, -1): r += p*self.prufer_repr[i] p *= self.nodes return r
@classmethod
[docs] def unrank(self, rank, n): """Finds the unranked Prufer sequence. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> Prufer.unrank(0, 4) Prufer([0, 0]) """ n, rank = as_int(n), as_int(rank) L = defaultdict(int) for i in xrange(n - 3, -1, -1): L[i] = rank % n rank = (rank - L[i])//n return Prufer([L[i] for i in xrange(len(L))])
def __new__(cls, *args, **kw_args): """The constructor for the Prufer object. Examples ======== >>> from sympy.combinatorics.prufer import Prufer A Prufer object can be constructed from a list of edges: >>> a = Prufer([[0, 1], [0, 2], [0, 3]]) >>> a.prufer_repr [0, 0] If the number of nodes is given, no checking of the nodes will be performed; it will be assumed that nodes 0 through n - 1 are present: >>> Prufer([[0, 1], [0, 2], [0, 3]], 4) Prufer([[0, 1], [0, 2], [0, 3]], 4) A Prufer object can be constructed from a Prufer sequence: >>> b = Prufer([1, 3]) >>> b.tree_repr [[0, 1], [1, 3], [2, 3]] """ ret_obj = Basic.__new__(cls, *args, **kw_args) args = [list(args[0])] if args[0] and iterable(args[0][0]): if not args[0][0]: raise ValueError('Prufer expects at least one edge in the tree.') if len(args) > 1: nnodes = args[1] else: nodes = set(flatten(args[0])) nnodes = max(nodes) + 1 if nnodes != len(nodes): missing = set(range(nnodes)) - nodes if len(missing) == 1: msg = 'Node %s is missing.' % missing.pop() else: msg = 'Nodes %s are missing.' % list(sorted(missing)) raise ValueError(msg) ret_obj._tree_repr = [list(i) for i in args[0]] ret_obj._nodes = nnodes else: ret_obj._prufer_repr = args[0] ret_obj._nodes = len(ret_obj._prufer_repr) + 2 return ret_obj
[docs] def next(self, delta=1): """Generates the Prufer sequence that is delta beyond the current one. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> a = Prufer([[0, 1], [0, 2], [0, 3]]) >>> b = a.next(1) # == a.next() >>> b.tree_repr [[0, 2], [0, 1], [1, 3]] >>> b.rank 1 See Also ======== prufer_rank, rank, prev, size """ return Prufer.unrank(self.rank + delta, self.nodes)
[docs] def prev(self, delta=1): """Generates the Prufer sequence that is -delta before the current one. Examples ======== >>> from sympy.combinatorics.prufer import Prufer >>> a = Prufer([[0, 1], [1, 2], [2, 3], [1, 4]]) >>> a.rank 36 >>> b = a.prev() >>> b Prufer([1, 2, 0]) >>> b.rank 35 See Also ======== prufer_rank, rank, next, size """ return Prufer.unrank(self.rank - delta, self.nodes)