Source code for sympy.utilities.enumerative

from __future__ import print_function, division
from sympy.core.compatibility import range

"""
Algorithms and classes to support enumerative combinatorics.

Currently just multiset partitions, but more could be added.

Terminology (following Knuth, algorithm 7.1.2.5M TAOCP)
*multiset* aaabbcccc has a *partition* aaabc | bccc

The submultisets, aaabc and bccc of the partition are called
*parts*, or sometimes *vectors*.  (Knuth notes that multiset
partitions can be thought of as partitions of vectors of integers,
where the ith element of the vector gives the multiplicity of
element i.)

The values a, b and c are *components* of the multiset.  These
correspond to elements of a set, but in a multiset can be present
with a multiplicity greater than 1.

The algorithm deserves some explanation.

Think of the part aaabc from the multiset above.  If we impose an
ordering on the components of the multiset, we can represent a part
with a vector, in which the value of the first element of the vector
corresponds to the multiplicity of the first component in that
part. Thus, aaabc can be represented by the vector [3, 1, 1].  We
can also define an ordering on parts, based on the lexicographic
ordering of the vector (leftmost vector element, i.e., the element
with the smallest component number, is the most significant), so
that [3, 1, 1] > [3, 1, 0] and [3, 1, 1] > [2, 1, 4].  The ordering
on parts can be extended to an ordering on partitions: First, sort
the parts in each partition, left-to-right in decreasing order. Then
partition A is greater than partition B if A's leftmost/greatest
part is greater than B's leftmost part.  If the leftmost parts are
equal, compare the second parts, and so on.

In this ordering, the greatest partition of a given multiset has only
one part.  The least partition is the one in which the components
are spread out, one per part.

The enumeration algorithms in this file yield the partitions of the
argument multiset in decreasing order.  The main data structure is a
stack of parts, corresponding to the current partition.  An
important invariant is that the parts on the stack are themselves in
decreasing order.  This data structure is decremented to find the
next smaller partition.  Most often, decrementing the partition will
only involve adjustments to the smallest parts at the top of the
stack, much as adjacent integers *usually* differ only in their last
few digits.

Knuth's algorithm uses two main operations on parts:

Decrement - change the part so that it is smaller in the
  (vector) lexicographic order, but reduced by the smallest amount possible.
  For example, if the multiset has vector [5,
  3, 1], and the bottom/greatest part is [4, 2, 1], this part would
  decrement to [4, 2, 0], while [4, 0, 0] would decrement to [3, 3,
  1].  A singleton part is never decremented -- [1, 0, 0] is not
  decremented to [0, 3, 1].  Instead, the decrement operator needs
  to fail for this case.  In Knuth's pseudocode, the decrement
  operator is step m5.

Spread unallocated multiplicity - Once a part has been decremented,
  it cannot be the rightmost part in the partition.  There is some
  multiplicity that has not been allocated, and new parts must be
  created above it in the stack to use up this multiplicity.  To
  maintain the invariant that the parts on the stack are in
  decreasing order, these new parts must be less than or equal to
  the decremented part.
  For example, if the multiset is [5, 3, 1], and its most
  significant part has just been decremented to [5, 3, 0], the
  spread operation will add a new part so that the stack becomes
  [[5, 3, 0], [0, 0, 1]].  If the most significant part (for the
  same multiset) has been decremented to [2, 0, 0] the stack becomes
  [[2, 0, 0], [2, 0, 0], [1, 3, 1]].  In the pseudocode, the spread
  operation for one part is step m2.  The complete spread operation
  is a loop of steps m2 and m3.

In order to facilitate the spread operation, Knuth stores, for each
component of each part, not just the multiplicity of that component
in the part, but also the total multiplicity available for this
component in this part or any lesser part above it on the stack.

One added twist is that Knuth does not represent the part vectors as
arrays. Instead, he uses a sparse representation, in which a
component of a part is represented as a component number (c), plus
the multiplicity of the component in that part (v) as well as the
total multiplicity available for that component (u).  This saves
time that would be spent skipping over zeros.

"""

class PartComponent(object):
    """Internal class used in support of the multiset partitions
    enumerators and the associated visitor functions.

    Represents one component of one part of the current partition.

    A stack of these, plus an auxiliary frame array, f, represents a
    partition of the multiset.

    Knuth's pseudocode makes c, u, and v separate arrays.
    """

    __slots__ = ('c', 'u', 'v')

    def __init__(self):
        self.c = 0   # Component number
        self.u = 0   # The as yet unpartitioned amount in component c
                     # *before* it is allocated by this triple
        self.v = 0   # Amount of c component in the current part
                     # (v<=u).  An invariant of the representation is
                     # that the next higher triple for this component
                     # (if there is one) will have a value of u-v in
                     # its u attribute.

    def __repr__(self):
        "for debug/algorithm animation purposes"
        return 'c:%d u:%d v:%d' % (self.c, self.u, self.v)

    def __eq__(self, other):
        """Define  value oriented equality, which is useful for testers"""
        return (isinstance(other, self.__class__) and
                self.c == other.c and
                self.u == other.u and
                self.v == other.v)

    def __ne__(self, other):
        """Defined for consistency with __eq__"""
        return not self == other


# This function tries to be a faithful implementation of algorithm
# 7.1.2.5M in Volume 4A, Combinatoral Algorithms, Part 1, of The Art
# of Computer Programming, by Donald Knuth.  This includes using
# (mostly) the same variable names, etc.  This makes for rather
# low-level Python.

# Changes from Knuth's pseudocode include
# - use PartComponent struct/object instead of 3 arrays
# - make the function a generator
# - map (with some difficulty) the GOTOs to Python control structures.
# - Knuth uses 1-based numbering for components, this code is 0-based
# - renamed variable l to lpart.
# - flag variable x takes on values True/False instead of 1/0
#
[docs]def multiset_partitions_taocp(multiplicities): """Enumerates partitions of a multiset. Parameters ========== multiplicities list of integer multiplicities of the components of the multiset. Yields ====== state Internal data structure which encodes a particular partition. This output is then usually processed by a vistor function which combines the information from this data structure with the components themselves to produce an actual partition. Unless they wish to create their own visitor function, users will have little need to look inside this data structure. But, for reference, it is a 3-element list with components: f is a frame array, which is used to divide pstack into parts. lpart points to the base of the topmost part. pstack is an array of PartComponent objects. The ``state`` output offers a peek into the internal data structures of the enumeration function. The client should treat this as read-only; any modification of the data structure will cause unpredictable (and almost certainly incorrect) results. Also, the components of ``state`` are modified in place at each iteration. Hence, the visitor must be called at each loop iteration. Accumulating the ``state`` instances and processing them later will not work. Examples ======== >>> from sympy.utilities.enumerative import list_visitor >>> from sympy.utilities.enumerative import multiset_partitions_taocp >>> # variables components and multiplicities represent the multiset 'abb' >>> components = 'ab' >>> multiplicities = [1, 2] >>> states = multiset_partitions_taocp(multiplicities) >>> list(list_visitor(state, components) for state in states) [[['a', 'b', 'b']], [['a', 'b'], ['b']], [['a'], ['b', 'b']], [['a'], ['b'], ['b']]] See Also ======== sympy.utilities.iterables.multiset_partitions: Takes a multiset as input and directly yields multiset partitions. It dispatches to a number of functions, including this one, for implementation. Most users will find it more convenient to use than multiset_partitions_taocp. """ # Important variables. # m is the number of components, i.e., number of distinct elements m = len(multiplicities) # n is the cardinality, total number of elements whether or not distinct n = sum(multiplicities) # The main data structure, f segments pstack into parts. See # list_visitor() for example code indicating how this internal # state corresponds to a partition. # Note: allocation of space for stack is conservative. Knuth's # exercise 7.2.1.5.68 gives some indication of how to tighten this # bound, but this is not implemented. pstack = [PartComponent() for i in range(n * m + 1)] f = [0] * (n + 1) # Step M1 in Knuth (Initialize) # Initial state - entire multiset in one part. for j in range(m): ps = pstack[j] ps.c = j ps.u = multiplicities[j] ps.v = multiplicities[j] # Other variables f[0] = 0 a = 0 lpart = 0 f[1] = m b = m # in general, current stack frame is from a to b - 1 while True: while True: # Step M2 (Subtract v from u) j = a k = b x = False while j < b: pstack[k].u = pstack[j].u - pstack[j].v if pstack[k].u == 0: x = True elif not x: pstack[k].c = pstack[j].c pstack[k].v = min(pstack[j].v, pstack[k].u) x = pstack[k].u < pstack[j].v k = k + 1 else: # x is True pstack[k].c = pstack[j].c pstack[k].v = pstack[k].u k = k + 1 j = j + 1 # Note: x is True iff v has changed # Step M3 (Push if nonzero.) if k > b: a = b b = k lpart = lpart + 1 f[lpart + 1] = b # Return to M2 else: break # Continue to M4 # M4 Visit a partition state = [f, lpart, pstack] yield state # M5 (Decrease v) while True: j = b-1 while (pstack[j].v == 0): j = j - 1 if j == a and pstack[j].v == 1: # M6 (Backtrack) if lpart == 0: return lpart = lpart - 1 b = a a = f[lpart] # Return to M5 else: pstack[j].v = pstack[j].v - 1 for k in range(j + 1, b): pstack[k].v = pstack[k].u break # GOTO M2
# --------------- Visitor functions for multiset partitions --------------- # A visitor takes the partition state generated by # multiset_partitions_taocp or other enumerator, and produces useful # output (such as the actual partition).
[docs]def factoring_visitor(state, primes): """Use with multiset_partitions_taocp to enumerate the ways a number can be expressed as a product of factors. For this usage, the exponents of the prime factors of a number are arguments to the partition enumerator, while the corresponding prime factors are input here. Examples ======== To enumerate the factorings of a number we can think of the elements of the partition as being the prime factors and the multiplicities as being their exponents. >>> from sympy.utilities.enumerative import factoring_visitor >>> from sympy.utilities.enumerative import multiset_partitions_taocp >>> from sympy import factorint >>> primes, multiplicities = zip(*factorint(24).items()) >>> primes (2, 3) >>> multiplicities (3, 1) >>> states = multiset_partitions_taocp(multiplicities) >>> list(factoring_visitor(state, primes) for state in states) [[24], [8, 3], [12, 2], [4, 6], [4, 2, 3], [6, 2, 2], [2, 2, 2, 3]] """ f, lpart, pstack = state factoring = [] for i in range(lpart + 1): factor = 1 for ps in pstack[f[i]: f[i + 1]]: if ps.v > 0: factor *= primes[ps.c] ** ps.v factoring.append(factor) return factoring
[docs]def list_visitor(state, components): """Return a list of lists to represent the partition. Examples ======== >>> from sympy.utilities.enumerative import list_visitor >>> from sympy.utilities.enumerative import multiset_partitions_taocp >>> states = multiset_partitions_taocp([1, 2, 1]) >>> s = next(states) >>> list_visitor(s, 'abc') # for multiset 'a b b c' [['a', 'b', 'b', 'c']] >>> s = next(states) >>> list_visitor(s, [1, 2, 3]) # for multiset '1 2 2 3 [[1, 2, 2], [3]] """ f, lpart, pstack = state partition = [] for i in range(lpart+1): part = [] for ps in pstack[f[i]:f[i+1]]: if ps.v > 0: part.extend([components[ps.c]] * ps.v) partition.append(part) return partition
[docs]class MultisetPartitionTraverser(): """ Has methods to ``enumerate`` and ``count`` the partitions of a multiset. This implements a refactored and extended version of Knuth's algorithm 7.1.2.5M [AOCP]_." The enumeration methods of this class are generators and return data structures which can be interpreted by the same visitor functions used for the output of ``multiset_partitions_taocp``. See Also ======== multiset_partitions_taocp sympy.utilities.iterables.multiset_partititions Examples ======== >>> from sympy.utilities.enumerative import MultisetPartitionTraverser >>> m = MultisetPartitionTraverser() >>> m.count_partitions([4,4,4,2]) 127750 >>> m.count_partitions([3,3,3]) 686 References ========== .. [AOCP] Algorithm 7.1.2.5M in Volume 4A, Combinatoral Algorithms, Part 1, of The Art of Computer Programming, by Donald Knuth. .. [Factorisatio] On a Problem of Oppenheim concerning "Factorisatio Numerorum" E. R. Canfield, Paul Erdos, Carl Pomerance, JOURNAL OF NUMBER THEORY, Vol. 17, No. 1. August 1983. See section 7 for a description of an algorithm similar to Knuth's. .. [Yorgey] Generating Multiset Partitions, Brent Yorgey, The Monad.Reader, Issue 8, September 2007. """ def __init__(self): self.debug = False # TRACING variables. These are useful for gathering # statistics on the algorithm itself, but have no particular # benefit to a user of the code. self.k1 = 0 self.k2 = 0 self.p1 = 0 def db_trace(self, msg): """Useful for usderstanding/debugging the algorithms. Not generally activated in end-user code.""" if self.debug: letters = 'abcdefghijklmnopqrstuvwxyz' state = [self.f, self.lpart, self.pstack] print("DBG:", msg, ["".join(part) for part in list_visitor(state, letters)], animation_visitor(state)) # # Helper methods for enumeration # def _initialize_enumeration(self, multiplicities): """Allocates and initializes the partition stack. This is called from the enumeration/counting routines, so there is no need to call it separately.""" num_components = len(multiplicities) # cardinality is the total number of elements, whether or not distinct cardinality = sum(multiplicities) # pstack is the partition stack, which is segmented by # f into parts. self.pstack = [PartComponent() for i in range(num_components * cardinality + 1)] self.f = [0] * (cardinality + 1) # Initial state - entire multiset in one part. for j in range(num_components): ps = self.pstack[j] ps.c = j ps.u = multiplicities[j] ps.v = multiplicities[j] self.f[0] = 0 self.f[1] = num_components self.lpart = 0 # The decrement_part() method corresponds to step M5 in Knuth's # algorithm. This is the base version for enum_all(). Modified # versions of this method are needed if we want to restrict # sizes of the partitions produced. def decrement_part(self, part): """Decrements part (a subrange of pstack), if possible, returning True iff the part was successfully decremented. If you think of the v values in the part as a multi-digit integer (least significant digit on the right) this is basically decrementing that integer, but with the extra constraint that the leftmost digit cannot be decremented to 0. Parameters ========== part The part, represented as a list of PartComponent objects, which is to be decremented. """ plen = len(part) for j in range(plen - 1, -1, -1): if (j == 0 and part[j].v > 1) or (j > 0 and part[j].v > 0): # found val to decrement part[j].v -= 1 # Reset trailing parts back to maximum for k in range(j + 1, plen): part[k].v = part[k].u return True return False # Version to allow number of parts to be bounded from above. # Corresponds to (a modified) step M5. def decrement_part_small(self, part, ub): """Decrements part (a subrange of pstack), if possible, returning True iff the part was successfully decremented. Parameters ========== part part to be decremented (topmost part on the stack) ub the maximum number of parts allowed in a partition returned by the calling traversal. Notes ===== The goal of this modification of the ordinary decrement method is to fail (meaning that the subtree rooted at this part is to be skipped) when it can be proved that this part can only have child partitions which are larger than allowed by ``ub``. If a decision is made to fail, it must be accurate, otherwise the enumeration will miss some partitions. But, it is OK not to capture all the possible failures -- if a part is passed that shouldn't be, the resulting too-large partitions are filtered by the enumeration one level up. However, as is usual in constrained enumerations, failing early is advantageous. The tests used by this method catch the most common cases, although this implementation is by no means the last word on this problem. The tests include: 1) ``lpart`` must be less than ``ub`` by at least 2. This is because once a part has been decremented, the partition will gain at least one child in the spread step. 2) If the leading component of the part is about to be decremented, check for how many parts will be added in order to use up the unallocated multiplicity in that leading component, and fail if this number is greater than allowed by ``ub``. (See code for the exact expression.) This test is given in the answer to Knuth's problem 7.2.1.5.69. 3) If there is *exactly* enough room to expand the leading component by the above test, check the next component (if it exists) once decrementing has finished. If this has ``v == 0``, this next component will push the expansion over the limit by 1, so fail. """ if self.lpart >= ub - 1: self.p1 += 1 # increment to keep track of usefulness of tests return False plen = len(part) for j in range(plen - 1, -1, -1): # Knuth's mod, (answer to problem 7.2.1.5.69) if (j == 0) and (part[0].v - 1)*(ub - self.lpart) < part[0].u: self.k1 += 1 return False if (j == 0 and part[j].v > 1) or (j > 0 and part[j].v > 0): # found val to decrement part[j].v -= 1 # Reset trailing parts back to maximum for k in range(j + 1, plen): part[k].v = part[k].u # Have now decremented part, but are we doomed to # failure when it is expanded? Check one oddball case # that turns out to be surprisingly common - exactly # enough room to expand the leading component, but no # room for the second component, which has v=0. if (plen > 1 and (part[1].v == 0) and (part[0].u - part[0].v) == ((ub - self.lpart - 1) * part[0].v)): self.k2 += 1 self.db_trace("Decrement fails test 3") return False return True return False def decrement_part_large(self, part, amt, lb): """Decrements part, while respecting size constraint. A part can have no children which are of sufficient size (as indicated by ``lb``) unless that part has sufficient unallocated multiplicity. When enforcing the size constraint, this method will decrement the part (if necessary) by an amount needed to ensure sufficient unallocated multiplicity. Returns True iff the part was successfully decremented. Parameters ========== part part to be decremented (topmost part on the stack) amt Can only take values 0 or 1. A value of 1 means that the part must be decremented, and then the size constraint is enforced. A value of 0 means just to enforce the ``lb`` size constraint. lb The partitions produced by the calling enumeration must have more parts than this value. """ if amt == 1: # In this case we always need to increment, *before* # enforcing the "sufficient unallocated multiplicity" # constraint. Easiest for this is just to call the # regular decrement method. if not self.decrement_part(part): return False # Next, perform any needed additional decrementing to respect # "sufficient unallocated multiplicity" (or fail if this is # not possible). min_unalloc = lb - self.lpart if min_unalloc <= 0: return True total_mult = sum(pc.u for pc in part) total_alloc = sum(pc.v for pc in part) if total_mult <= min_unalloc: return False deficit = min_unalloc - (total_mult - total_alloc) if deficit <= 0: return True for i in range(len(part) - 1, -1, -1): if i == 0: if part[0].v > deficit: part[0].v -= deficit return True else: return False # This shouldn't happen, due to above check else: if part[i].v >= deficit: part[i].v -= deficit return True else: deficit -= part[i].v part[i].v = 0 def decrement_part_range(self, part, lb, ub): """Decrements part (a subrange of pstack), if possible, returning True iff the part was successfully decremented. Parameters ========== part part to be decremented (topmost part on the stack) ub the maximum number of parts allowed in a partition returned by the calling traversal. lb The partitions produced by the calling enumeration must have more parts than this value. Notes ===== Combines the constraints of _small and _large decrement methods. If returns success, part has been decremented at least once, but perhaps by quite a bit more if needed to meet the lb constraint. """ # Constraint in the range case is just enforcing both the # constraints from _small and _large cases. Note the 0 as the # second argument to the _large call -- this is the signal to # decrement only as needed to for constraint enforcement. The # short circuiting and left-to-right order of the 'and' # operator is important for this to work correctly. return self.decrement_part_small(part, ub) and \ self.decrement_part_large(part, 0, lb) def spread_part_multiplicity(self): """Returns True if a new part has been created, and adjusts pstack, f and lpart as needed. Notes ===== Spreads unallocated multiplicity from the current top part into a new part created above the current on the stack. This new part is constrained to be less than or equal to the old in terms of the part ordering. This call does nothing (and returns False) if the current top part has no unallocated multiplicity. """ j = self.f[self.lpart] # base of current top part k = self.f[self.lpart + 1] # ub of current; potential base of next base = k # save for later comparison changed = False # Set to true when the new part (so far) is # strictly less than (as opposed to less than # or equal) to the old. for j in range(self.f[self.lpart], self.f[self.lpart + 1]): self.pstack[k].u = self.pstack[j].u - self.pstack[j].v if self.pstack[k].u == 0: changed = True else: self.pstack[k].c = self.pstack[j].c if changed: # Put all available multiplicity in this part self.pstack[k].v = self.pstack[k].u else: # Still maintaining ordering constraint if self.pstack[k].u < self.pstack[j].v: self.pstack[k].v = self.pstack[k].u changed = True else: self.pstack[k].v = self.pstack[j].v k = k + 1 if k > base: # Adjust for the new part on stack self.lpart = self.lpart + 1 self.f[self.lpart + 1] = k return True return False def top_part(self): """Return current top part on the stack, as a slice of pstack. """ return self.pstack[self.f[self.lpart]:self.f[self.lpart + 1]] # Same interface and funtionality as multiset_partitions_taocp(), # but some might find this refactored version easier to follow.
[docs] def enum_all(self, multiplicities): """Enumerate the partitions of a multiset. Examples ======== >>> from sympy.utilities.enumerative import list_visitor >>> from sympy.utilities.enumerative import MultisetPartitionTraverser >>> m = MultisetPartitionTraverser() >>> states = m.enum_all([2,2]) >>> list(list_visitor(state, 'ab') for state in states) [[['a', 'a', 'b', 'b']], [['a', 'a', 'b'], ['b']], [['a', 'a'], ['b', 'b']], [['a', 'a'], ['b'], ['b']], [['a', 'b', 'b'], ['a']], [['a', 'b'], ['a', 'b']], [['a', 'b'], ['a'], ['b']], [['a'], ['a'], ['b', 'b']], [['a'], ['a'], ['b'], ['b']]] See also ======== multiset_partitions_taocp(): which provides the same result as this method, but is about twice as fast. Hence, enum_all is primarily useful for testing. Also see the function for a discussion of states and visitors. """ self._initialize_enumeration(multiplicities) while True: while self.spread_part_multiplicity(): pass # M4 Visit a partition state = [self.f, self.lpart, self.pstack] yield state # M5 (Decrease v) while not self.decrement_part(self.top_part()): # M6 (Backtrack) if self.lpart == 0: return self.lpart -= 1
[docs] def enum_small(self, multiplicities, ub): """Enumerate multiset partitions with no more than ``ub`` parts. Equivalent to enum_range(multiplicities, 0, ub) See also ======== enum_all, enum_large, enum_range Parameters ========== multiplicities list of multiplicities of the components of the multiset. ub Maximum number of parts Examples ======== >>> from sympy.utilities.enumerative import list_visitor >>> from sympy.utilities.enumerative import MultisetPartitionTraverser >>> m = MultisetPartitionTraverser() >>> states = m.enum_small([2,2], 2) >>> list(list_visitor(state, 'ab') for state in states) [[['a', 'a', 'b', 'b']], [['a', 'a', 'b'], ['b']], [['a', 'a'], ['b', 'b']], [['a', 'b', 'b'], ['a']], [['a', 'b'], ['a', 'b']]] The implementation is based, in part, on the answer given to exercise 69, in Knuth [AOCP]_. """ # Keep track of iterations which do not yield a partition. # Clearly, we would like to keep this number small. self.discarded = 0 if ub <= 0: return self._initialize_enumeration(multiplicities) while True: good_partition = True while self.spread_part_multiplicity(): self.db_trace("spread 1") if self.lpart >= ub: self.discarded += 1 good_partition = False self.db_trace(" Discarding") self.lpart = ub - 2 break # M4 Visit a partition if good_partition: state = [self.f, self.lpart, self.pstack] yield state # M5 (Decrease v) while not self.decrement_part_small(self.top_part(), ub): self.db_trace("Failed decrement, going to backtrack") # M6 (Backtrack) if self.lpart == 0: return self.lpart -= 1 self.db_trace("Backtracked to") self.db_trace("decrement ok, about to expand")
[docs] def enum_large(self, multiplicities, lb): """Enumerate the partitions of a multiset with lb < num(parts) Equivalent to enum_range(multiplicities, lb, sum(multiplicities)) See also ======== enum_all, enum_small, enum_range Parameters ========== multiplicities list of multiplicities of the components of the multiset. lb Number of parts in the partition must be greater than this lower bound. Examples ======== >>> from sympy.utilities.enumerative import list_visitor >>> from sympy.utilities.enumerative import MultisetPartitionTraverser >>> m = MultisetPartitionTraverser() >>> states = m.enum_large([2,2], 2) >>> list(list_visitor(state, 'ab') for state in states) [[['a', 'a'], ['b'], ['b']], [['a', 'b'], ['a'], ['b']], [['a'], ['a'], ['b', 'b']], [['a'], ['a'], ['b'], ['b']]] """ self.discarded = 0 if lb >= sum(multiplicities): return self._initialize_enumeration(multiplicities) self.decrement_part_large(self.top_part(), 0, lb) while True: good_partition = True while self.spread_part_multiplicity(): if not self.decrement_part_large(self.top_part(), 0, lb): # Failure here should be rare/impossible self.discarded += 1 good_partition = False break # M4 Visit a partition if good_partition: state = [self.f, self.lpart, self.pstack] yield state # M5 (Decrease v) while not self.decrement_part_large(self.top_part(), 1, lb): # M6 (Backtrack) if self.lpart == 0: return self.lpart -= 1
[docs] def enum_range(self, multiplicities, lb, ub): """Enumerate the partitions of a multiset with ``lb < num(parts) <= ub``. In particular, if partitions with exactly ``k`` parts are desired, call with ``(multiplicities, k - 1, k)``. This method generalizes enum_all, enum_small, and enum_large. Examples ======== >>> from sympy.utilities.enumerative import list_visitor >>> from sympy.utilities.enumerative import MultisetPartitionTraverser >>> m = MultisetPartitionTraverser() >>> states = m.enum_range([2,2], 1, 2) >>> list(list_visitor(state, 'ab') for state in states) [[['a', 'a', 'b'], ['b']], [['a', 'a'], ['b', 'b']], [['a', 'b', 'b'], ['a']], [['a', 'b'], ['a', 'b']]] """ # combine the constraints of the _large and _small # enumerations. self.discarded = 0 if ub <= 0 or lb >= sum(multiplicities): return self._initialize_enumeration(multiplicities) self.decrement_part_large(self.top_part(), 0, lb) while True: good_partition = True while self.spread_part_multiplicity(): self.db_trace("spread 1") if not self.decrement_part_large(self.top_part(), 0, lb): # Failure here - possible in range case? self.db_trace(" Discarding (large cons)") self.discarded += 1 good_partition = False break elif self.lpart >= ub: self.discarded += 1 good_partition = False self.db_trace(" Discarding small cons") self.lpart = ub - 2 break # M4 Visit a partition if good_partition: state = [self.f, self.lpart, self.pstack] yield state # M5 (Decrease v) while not self.decrement_part_range(self.top_part(), lb, ub): self.db_trace("Failed decrement, going to backtrack") # M6 (Backtrack) if self.lpart == 0: return self.lpart -= 1 self.db_trace("Backtracked to") self.db_trace("decrement ok, about to expand")
def count_partitions_slow(self, multiplicities): """Returns the number of partitions of a multiset whose elements have the multiplicities given in ``multiplicities``. Primarily for comparison purposes. It follows the same path as enumerate, and counts, rather than generates, the partitions. See Also ======== count_partitions Has the same calling interface, but is much faster. """ # number of partitions so far in the enumeration self.pcount = 0 self._initialize_enumeration(multiplicities) while True: while self.spread_part_multiplicity(): pass # M4 Visit (count) a partition self.pcount += 1 # M5 (Decrease v) while not self.decrement_part(self.top_part()): # M6 (Backtrack) if self.lpart == 0: return self.pcount self.lpart -= 1
[docs] def count_partitions(self, multiplicities): """Returns the number of partitions of a multiset whose components have the multiplicities given in ``multiplicities``. For larger counts, this method is much faster than calling one of the enumerators and counting the result. Uses dynamic programming to cut down on the number of nodes actually explored. The dictionary used in order to accelerate the counting process is stored in the ``MultisetPartitionTraverser`` object and persists across calls. If the the user does not expect to call ``count_partitions`` for any additional multisets, the object should be cleared to save memory. On the other hand, the cache built up from one count run can significantly speed up subsequent calls to ``count_partitions``, so it may be advantageous not to clear the object. Examples ======== >>> from sympy.utilities.enumerative import MultisetPartitionTraverser >>> m = MultisetPartitionTraverser() >>> m.count_partitions([9,8,2]) 288716 >>> m.count_partitions([2,2]) 9 >>> del m Notes ===== If one looks at the workings of Knuth's algorithm M [AOCP]_, it can be viewed as a traversal of a binary tree of parts. A part has (up to) two children, the left child resulting from the spread operation, and the right child from the decrement operation. The ordinary enumeration of multiset partitions is an in-order traversal of this tree, and with the partitions corresponding to paths from the root to the leaves. The mapping from paths to partitions is a little complicated, since the partition would contain only those parts which are leaves or the parents of a spread link, not those which are parents of a decrement link. For counting purposes, it is sufficient to count leaves, and this can be done with a recursive in-order traversal. The number of leaves of a subtree rooted at a particular part is a function only of that part itself, so memoizing has the potential to speed up the counting dramatically. This method follows a computational approach which is similar to the hypothetical memoized recursive function, but with two differences: 1) This method is iterative, borrowing its structure from the other enumerations and maintaining an explicit stack of parts which are in the process of being counted. (There may be multisets which can be counted reasonably quickly by this implementation, but which would overflow the default Python recursion limit with a recursive implementation.) 2) Instead of using the part data structure directly, a more compact key is constructed. This saves space, but more importantly coalesces some parts which would remain separate with physical keys. Unlike the enumeration functions, there is currently no _range version of count_partitions. If someone wants to stretch their brain, it should be possible to construct one by memoizing with a histogram of counts rather than a single count, and combining the histograms. """ # number of partitions so far in the enumeration self.pcount = 0 # dp_stack is list of lists of (part_key, start_count) pairs self.dp_stack = [] # dp_map is map part_key-> count, where count represents the # number of multiset which are descendants of a part with this # key, **or any of its decrements** # Thus, when we find a part in the map, we add its count # value to the running total, cut off the enumeration, and # backtrack if not hasattr(self, 'dp_map'): self.dp_map = {} self._initialize_enumeration(multiplicities) pkey = part_key(self.top_part()) self.dp_stack.append([(pkey, 0), ]) while True: while self.spread_part_multiplicity(): pkey = part_key(self.top_part()) if pkey in self.dp_map: # Already have a cached value for the count of the # subtree rooted at this part. Add it to the # running counter, and break out of the spread # loop. The -1 below is to compensate for the # leaf that this code path would otherwise find, # and which gets incremented for below. self.pcount += (self.dp_map[pkey] - 1) self.lpart -= 1 break else: self.dp_stack.append([(pkey, self.pcount), ]) # M4 count a leaf partition self.pcount += 1 # M5 (Decrease v) while not self.decrement_part(self.top_part()): # M6 (Backtrack) for key, oldcount in self.dp_stack.pop(): self.dp_map[key] = self.pcount - oldcount if self.lpart == 0: return self.pcount self.lpart -= 1 # At this point have successfully decremented the part on # the stack and it does not appear in the cache. It needs # to be added to the list at the top of dp_stack pkey = part_key(self.top_part()) self.dp_stack[-1].append((pkey, self.pcount),)
def part_key(part): """Helper for MultisetPartitionTraverser.count_partitions that creates a key for ``part``, that only includes information which can affect the count for that part. (Any irrelevant information just reduces the effectiveness of dynamic programming.) Notes ===== This member function is a candidate for future exploration. There are likely symmetries that can be exploited to coalesce some ``part_key`` values, and thereby save space and improve performance. """ # The component number is irrelevant for counting partitions, so # leave it out of the memo key. rval = [] for ps in part: rval.append(ps.u) rval.append(ps.v) return tuple(rval)