# Source code for tryalgo.eulerian_tour

```
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""\
Eulerian cycle
jill-jênn vie et christoph dürr - 2015-2023
"""
import random
from tryalgo.graph import write_graph
# snip{ eulerian_tour_undirected
[docs]
def eulerian_tour_undirected(graph):
"""Eulerian tour on an undirected graph
:param graph: directed graph in listlist format, cannot be listdict
:assumes: graph is eulerian
:returns: eulerian cycle as a vertex list
:complexity: `O(|V|+|E|)`
"""
P = [] # resulting tour
Q = [0] # vertices to be explored, start at 0
R = [] # path from start node
next_ = [0] * len(graph) # initialize next_ to 0 for each node
seen = [set() for _ in graph] # mark backward arcs
while Q:
start = Q.pop() # explore a cycle from start node
node = start # current node on cycle
while next_[node] < len(graph[node]): # visit all allowable arcs
neighbor = graph[node][next_[node]] # traverse an arc
next_[node] += 1 # mark arc traversed
if neighbor not in seen[node]: # not yet traversed
seen[neighbor].add(node) # mark backward arc
R.append(neighbor) # append to path from start
node = neighbor # move on
while R:
Q.append(R.pop()) # add to Q the discovered cycle R
P.append(start) # resulting path P is extended
return P
# snip}
# snip{ eulerian_tour_directed
[docs]
def eulerian_tour_directed(graph):
"""Eulerian tour on a directed graph
:param graph: directed graph in listlist format, cannot be listdict
:assumes: graph is eulerian
:returns: eulerian cycle as a vertex list
:complexity: `O(|V|+|E|)`
"""
P = [] # resulting tour
Q = [0] # vertices to be explored, start at 0
R = [] # path from start node
next_ = [0] * len(graph) # initialize next_ to 0 for each node
while Q:
start = Q.pop() # explore a cycle from start node
node = start # current node on cycle
while next_[node] < len(graph[node]): # visit all allowable arcs
neighbor = graph[node][next_[node]] # traverse an arc
next_[node] += 1 # mark arc traversed
R.append(neighbor) # append to path from start
node = neighbor # move on
while R:
Q.append(R.pop()) # add to Q the discovered cycle R
P.append(start) # resulting path P is extended
return P
# snip}
[docs]
def write_cycle(filename, graph, cycle, directed):
"""Write an eulerian tour in DOT format
:param filename: the file to be written in DOT format
:param graph: graph in listlist format, cannot be listdict
:param bool directed: describes the graph
:param cycle: tour as a vertex list
:returns: nothing
:complexity: `O(|V|^2 + |E|)`
"""
n = len(graph)
weight = [[float('inf')] * n for _ in range(n)]
for r in range(1, len(cycle)):
weight[cycle[r-1]][cycle[r]] = r
if not directed:
weight[cycle[r]][cycle[r-1]] = r
write_graph(filename, graph, arc_label=weight, directed=directed)
[docs]
def random_eulerien_graph(n):
"""Generates some random eulerian graph
:param int n: number of vertices
:returns: undirected graph in listlist representation
:complexity: linear
"""
graphe = [[] for _ in range(n)]
for v in range(n - 1):
if len(graphe[v]) % 2 == 1:
nb_min_neighbors = 1
elif n - v == 2:
nb_min_neighbors = 0
else:
nb_min_neighbors = 2
noeuds = random.sample(range(v + 1, n), random.choice(
range(nb_min_neighbors, (n - v), 2)))
graphe[v].extend(noeuds)
for w in graphe[v]:
if w > v:
graphe[w].append(v)
return graphe
[docs]
def is_eulerian_tour_directed(graph, tour):
"""Test if a given tour is Eulerian for a directed graph
:param graph: directed graph in listlist format, cannot be listdict
:param tour: vertex list
:returns: True if tour is eulerian
:complexity: `O(|V|*|E|)` under the assumption that set membership is in constant time
"""
m = len(tour) - 1 # nb of arcs in the tour
arcs = set((tour[i], tour[i+1]) for i in range(m)) # all arcs in the tour
required = sum(len(Gu) for Gu in graph) # nb of arcs in graph
if (m != len(arcs) or # no arc is visited twice
m != required): # there are not more arcs in tour than in graph
return False
# implicitly check that every arc in tour exists in the graph
for u, Gu in enumerate(graph): # loop over all arcs in the graph
for v in Gu:
if (u, v) not in arcs: # check tour visits every arc
return False
return True
[docs]
def is_eulerian_tour_undirected(graph, tour):
"""Test if a given tour is Eulerian for an undirected graph
:param graph: undirected graph in listlist format, cannot be listdict
:param tour: vertex list
:returns: True if tour is eulerian
:complexity: `O(|V|*|E|)` under the assumption that set membership is in constant time
"""
def normalize(u, v): # normalize so (3,13) or (13,3) are the same edge
return (min(u, v), max(u, v))
m = len(tour) - 1 # nb of edges in the tour
arcs = set(normalize(tour[i], tour[i+1]) for i in range(m)) # all edges in tour
if len(arcs) != m:
return False
# check that tour contains only existing arcs (by counting)
required = sum(len(Gu) for Gu in graph) // 2 # nb of edges in graph
if (m != len(arcs) or # no edge is visited twice
m != required): # there are not more edges in tour than in graph
return False
# implicitly check that every arc in tour exists in the graph
for u, Gu in enumerate(graph): # loop over all edges in the graph
for v in Gu:
if normalize(u, v) not in arcs: # check tour visits every edges
return False
return True
```