# Source code for sympy.combinatorics.graycode

from __future__ import print_function, division

from sympy.core import Basic
from sympy.core.compatibility import range

import random

[docs]class GrayCode(Basic): """ A Gray code is essentially a Hamiltonian walk on a n-dimensional cube with edge length of one. The vertices of the cube are represented by vectors whose values are binary. The Hamilton walk visits each vertex exactly once. The Gray code for a 3d cube is ['000','100','110','010','011','111','101', '001']. A Gray code solves the problem of sequentially generating all possible subsets of n objects in such a way that each subset is obtained from the previous one by either deleting or adding a single object. In the above example, 1 indicates that the object is present, and 0 indicates that its absent. Gray codes have applications in statistics as well when we want to compute various statistics related to subsets in an efficient manner. References: [1] Nijenhuis,A. and Wilf,H.S.(1978). Combinatorial Algorithms. Academic Press. [2] Knuth, D. (2011). The Art of Computer Programming, Vol 4 Addison Wesley Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3) >>> list(a.generate_gray()) ['000', '001', '011', '010', '110', '111', '101', '100'] >>> a = GrayCode(4) >>> list(a.generate_gray()) ['0000', '0001', '0011', '0010', '0110', '0111', '0101', '0100', \ '1100', '1101', '1111', '1110', '1010', '1011', '1001', '1000'] """ _skip = False _current = 0 _rank = None def __new__(cls, n, *args, **kw_args): """ Default constructor. It takes a single argument n which gives the dimension of the Gray code. The starting Gray code string (start) or the starting rank may also be given; the default is to start at rank = 0 ('0...0'). Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3) >>> a GrayCode(3) >>> a.n 3 >>> a = GrayCode(3, start='100') >>> a.current '100' >>> a = GrayCode(4, rank=4) >>> a.current '0110' >>> a.rank 4 """ if n < 1 or int(n) != n: raise ValueError( 'Gray code dimension must be a positive integer, not %i' % n) n = int(n) args = (n,) + args obj = Basic.__new__(cls, *args) if 'start' in kw_args: obj._current = kw_args["start"] if len(obj._current) > n: raise ValueError('Gray code start has length %i but ' 'should not be greater than %i' % (len(obj._current), n)) elif 'rank' in kw_args: if int(kw_args["rank"]) != kw_args["rank"]: raise ValueError('Gray code rank must be a positive integer, ' 'not %i' % kw_args["rank"]) obj._rank = int(kw_args["rank"]) % obj.selections obj._current = obj.unrank(n, obj._rank) return obj
[docs] def next(self, delta=1): """ Returns the Gray code a distance delta (default = 1) from the current value in canonical order. Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3, start='110') >>> a.next().current '111' >>> a.next(-1).current '010' """ return GrayCode(self.n, rank=(self.rank + delta) % self.selections)
@property def selections(self): """ Returns the number of bit vectors in the Gray code. Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3) >>> a.selections 8 """ return 2**self.n @property def n(self): """ Returns the dimension of the Gray code. Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(5) >>> a.n 5 """ return self.args[0]
[docs] def generate_gray(self, **hints): """ Generates the sequence of bit vectors of a Gray Code. [1] Knuth, D. (2011). The Art of Computer Programming, Vol 4, Addison Wesley Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3) >>> list(a.generate_gray()) ['000', '001', '011', '010', '110', '111', '101', '100'] >>> list(a.generate_gray(start='011')) ['011', '010', '110', '111', '101', '100'] >>> list(a.generate_gray(rank=4)) ['110', '111', '101', '100'] See Also ======== skip """ bits = self.n start = None if "start" in hints: start = hints["start"] elif "rank" in hints: start = GrayCode.unrank(self.n, hints["rank"]) if start is not None: self._current = start current = self.current graycode_bin = gray_to_bin(current) if len(graycode_bin) > self.n: raise ValueError('Gray code start has length %i but should ' 'not be greater than %i' % (len(graycode_bin), bits)) self._current = int(current, 2) graycode_int = int(''.join(graycode_bin), 2) for i in range(graycode_int, 1 << bits): if self._skip: self._skip = False else: yield self.current bbtc = (i ^ (i + 1)) gbtc = (bbtc ^ (bbtc >> 1)) self._current = (self._current ^ gbtc) self._current = 0
[docs] def skip(self): """ Skips the bit generation. Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3) >>> for i in a.generate_gray(): ... if i == '010': ... a.skip() ... print(i) ... 000 001 011 010 111 101 100 See Also ======== generate_gray """ self._skip = True
@property def rank(self): """ Ranks the Gray code. A ranking algorithm determines the position (or rank) of a combinatorial object among all the objects w.r.t. a given order. For example, the 4 bit binary reflected Gray code (BRGC) '0101' has a rank of 6 as it appears in the 6th position in the canonical ordering of the family of 4 bit Gray codes. References: [1] http://statweb.stanford.edu/~susan/courses/s208/node12.html Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> a = GrayCode(3) >>> list(a.generate_gray()) ['000', '001', '011', '010', '110', '111', '101', '100'] >>> GrayCode(3, start='100').rank 7 >>> GrayCode(3, rank=7).current '100' See Also ======== unrank """ if self._rank is None: self._rank = int(gray_to_bin(self.current), 2) return self._rank @property def current(self): """ Returns the currently referenced Gray code as a bit string. Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> GrayCode(3, start='100').current '100' """ rv = self._current or '0' if type(rv) is not str: rv = bin(rv)[2:] return rv.rjust(self.n, '0') @classmethod
[docs] def unrank(self, n, rank): """ Unranks an n-bit sized Gray code of rank k. This method exists so that a derivative GrayCode class can define its own code of a given rank. The string here is generated in reverse order to allow for tail-call optimization. Examples ======== >>> from sympy.combinatorics.graycode import GrayCode >>> GrayCode(5, rank=3).current '00010' >>> GrayCode.unrank(5, 3) '00010' See Also ======== rank """ def _unrank(k, n): if n == 1: return str(k % 2) m = 2**(n - 1) if k < m: return '0' + _unrank(k, n - 1) return '1' + _unrank(m - (k % m) - 1, n - 1) return _unrank(rank, n)
def random_bitstring(n): """ Generates a random bitlist of length n. Examples ======== >>> from sympy.combinatorics.graycode import random_bitstring >>> random_bitstring(3) # doctest: +SKIP 100 """ return ''.join([random.choice('01') for i in range(n)]) def gray_to_bin(bin_list): """ Convert from Gray coding to binary coding. We assume big endian encoding. Examples ======== >>> from sympy.combinatorics.graycode import gray_to_bin >>> gray_to_bin('100') '111' See Also ======== bin_to_gray """ b = [bin_list[0]] for i in range(1, len(bin_list)): b += str(int(b[i - 1] != bin_list[i])) return ''.join(b) def bin_to_gray(bin_list): """ Convert from binary coding to gray coding. We assume big endian encoding. Examples ======== >>> from sympy.combinatorics.graycode import bin_to_gray >>> bin_to_gray('111') '100' See Also ======== gray_to_bin """ b = [bin_list[0]] for i in range(0, len(bin_list) - 1): b += str(int(bin_list[i]) ^ int(b[i - 1])) return ''.join(b) def get_subset_from_bitstring(super_set, bitstring): """ Gets the subset defined by the bitstring. Examples ======== >>> from sympy.combinatorics.graycode import get_subset_from_bitstring >>> get_subset_from_bitstring(['a', 'b', 'c', 'd'], '0011') ['c', 'd'] >>> get_subset_from_bitstring(['c', 'a', 'c', 'c'], '1100') ['c', 'a'] See Also ======== graycode_subsets """ if len(super_set) != len(bitstring): raise ValueError("The sizes of the lists are not equal") return [super_set[i] for i, j in enumerate(bitstring) if bitstring[i] == '1'] def graycode_subsets(gray_code_set): """ Generates the subsets as enumerated by a Gray code. Examples ======== >>> from sympy.combinatorics.graycode import graycode_subsets >>> list(graycode_subsets(['a', 'b', 'c'])) [[], ['c'], ['b', 'c'], ['b'], ['a', 'b'], ['a', 'b', 'c'], \ ['a', 'c'], ['a']] >>> list(graycode_subsets(['a', 'b', 'c', 'c'])) [[], ['c'], ['c', 'c'], ['c'], ['b', 'c'], ['b', 'c', 'c'], \ ['b', 'c'], ['b'], ['a', 'b'], ['a', 'b', 'c'], ['a', 'b', 'c', 'c'], \ ['a', 'b', 'c'], ['a', 'c'], ['a', 'c', 'c'], ['a', 'c'], ['a']] See Also ======== get_subset_from_bitstring """ for bitstring in list(GrayCode(len(gray_code_set)).generate_gray()): yield get_subset_from_bitstring(gray_code_set, bitstring)