Created
March 27, 2012 02:25
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Decompose a permutation into disjoint cycles
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def cycles(perm): | |
remain = set(perm) | |
result = [] | |
while len(remain) > 0: | |
n = remain.pop() | |
cycle = [n] | |
while True: | |
n = perm[n] | |
if n not in remain: | |
break | |
remain.remove(n) | |
cycle.append(n) | |
result.append(cycle) | |
return result |
Hi,
Below how I did it in the repo LABelsToolkit on github.
Not sure it is more efficient that the previous solution.
I tried (unsuccessfully) to implement it recursively, than I tried (again unsuccessfully) to make only the bottleneck
merge_decoupled_permutation
recursive.
# Auxiliaries:
def is_valid_permutation(in_perm):
"""
A permutation is a list of 2 lists of same size:
a = [[1,2,3], [2,3,1]]
means permute 1 with 2, 2 with 3, 3 with 1.
:param in_perm: input permutation.
"""
if not len(in_perm) == 2:
return False
if not len(in_perm[0]) == len(in_perm[1]):
return False
if not all(isinstance(n, int) for n in in_perm[0]):
return False
if not all(isinstance(n, int) for n in in_perm[1]):
return False
if not set(in_perm[0]) == set(in_perm[1]):
return False
return True
def lift_list(input_list):
"""
List of nested lists becomes a list with the element exposed in the main list.
:param input_list: a list of lists.
:return: eliminates the first nesting levels of lists.
E.G.
>> lift_list([1, 2, [1,2,3], [1,2], [4,5, 6], [3,4]])
[1, 2, 1, 2, 3, 1, 2, 4, 5, 6, 3, 4]
"""
if input_list == []:
return []
else:
return lift_list(input_list[0]) + (lift_list(input_list[1:]) if len(input_list) > 1 else []) \
if type(input_list) is list else [input_list]
def decouple_permutation(perm):
"""
from [[1, 2, 3, 4, 5], [3, 4, 5, 2, 1]]
to [[1,3], [2,4], [3,5], [4,2], [5,1]]
"""
return [a for a in [list(a) for a in zip(perm[0], perm[1]) if perm[0] != perm[1]] if a[0] != a[1]]
def merge_decoupled_permutation(decoupled):
"""
From [[1,3], [2,4], [3,5], [4,2], [5,1]]
to [[1, 3, 5], [2, 4]]
"""
ans = []
while len(decoupled):
index_next = [k[0] for k in decoupled[1:]].index(decoupled[0][-1]) + 1
decoupled[0].append(decoupled[index_next][1])
decoupled.pop(index_next)
if decoupled[0][0] == decoupled[0][-1]:
ans.append(decoupled[0][:-1])
decoupled.pop(0)
return ans
# Main methods:
def from_permutation_to_disjoints_cycles(perm):
"""
from [[1, 2, 3, 4, 5], [3, 4, 5, 2, 1]]
to [[1, 3, 5], [2, 4]]
"""
if not is_valid_permutation(perm):
raise IOError('Input permutation is not valid')
return merge_decoupled_permutation(decouple_permutation(perm))
def from_disjoint_cycles_to_permutation(dc):
"""
from [[1, 3, 5], [2, 4]]
to [[1, 2, 3, 4, 5], [3, 4, 5, 2, 1]]
"""
perm = [0, ] * max(lift_list(dc))
for cycle in dc:
for i, c in enumerate(cycle):
perm[c-1] = cycle[(i + 1) % len(cycle)]
return [list(range(1, len(perm) + 1)), perm]
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little mistake line 8 : n = perm[n-1]