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Fix sanity checks for Eulerian trail #397

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Sep 5, 2024
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Fix sanity checks for Eulerian trail #397

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2 changes: 1 addition & 1 deletion src/traversals/eulerian.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -95,7 +95,7 @@ function _check_eulerian_input(g, u)
)
end
else # isodd(du) # trail: start (u) != stop (v) - all nodes, except u and v, must have even degree
if count(x -> iseven(degree(g, x)), vertices(g)) != 2
if count(x -> isodd(degree(g, x)), vertices(g)) != 2
error(
"starting vertex has odd degree but the total number of vertices of odd degree is not equal to 2: a eulerian trail does not exist",
)
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15 changes: 12 additions & 3 deletions test/traversals/eulerian.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -5,11 +5,20 @@
@test last(eulerian(g0, 1)) == 1 # a cycle

# a tour (different start/end)
g1 = GenericGraph(SimpleGraph([Edge(1, 2), Edge(2, 3), Edge(3, 4)]))
@test eulerian(g1, 1) == [1, 2, 3, 4]
g1_3 = GenericGraph(SimpleGraph([Edge(1, 2), Edge(2, 3)]))
@test eulerian(g1_3, 3) == [3, 2, 1]
g1_4 = GenericGraph(SimpleGraph([Edge(1, 2), Edge(2, 3), Edge(3, 4)]))
@test eulerian(g1_4, 1) == [1, 2, 3, 4]
# in a graph with an Eulerian trail but not cycle, you have to start at an odd degree vertex
@test_throws ErrorException(
"starting vertex has even degree but there are other vertices with odd degree: a eulerian cycle does not exist",
) eulerian(g1, 2)
) eulerian(g1_3, 2)
# too many odd degree vertices; Eulerian cycle does not exist
# 3-pointed star with vertex 1 at the center
g_star = GenericGraph(SimpleGraph([Edge(1, 2), Edge(1, 3), Edge(1, 4)]))
@test_throws ErrorException(
"starting vertex has odd degree but the total number of vertices of odd degree is not equal to 2: a eulerian trail does not exist",
) eulerian(g_star, 1)

# a cycle with a node (vertex 2) with multiple neighbors
g2 = GenericGraph(
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