improve has_multiple_edges

[sagetrac-ylchapuy]

This patch cuts down by 30% the time for sage -t graph.py on my machine... (and doctests of course still pass)

CC: @nathanncohen

Component: graph theory

Author: Yann Laigle-Chapuy

Reviewer: Robert Miller

Merged: sage-4.3.alpha1

Issue created by migration from https://trac.sagemath.org/ticket/7547

[sagetrac-ylchapuy]

Description changed:

--- 
+++ 
@@ -1,2 +1 @@
-It seems to be the main bottleneck in graph plotting!
-This patch cuts down by 30% the time for `sage -t graph.py` on my machine... (and doctests of course still pass)
+This patch cuts down by 30% the time for sage -t graph.py on my machine... (and doctests of course still pass) 

[sagetrac-ylchapuy]

comment:1

For the record:

sage: P = graphs.PetersenGraph()
sage: D = graphs.DodecahedralGraph()
sage: L = D.lexicographic_product(P) 
sage: L.allow_multiple_edges(True)
sage: time L.has_multiple_edges()

Before: Wall time: 39.56 s

After: Wall time: 19.32 s

I hope I did no mistake, it's 4 a.m here...

[rlmill]

comment:2

Nice work!

[nathanncohen]

comment:3

Well done !! :-)

Nathann

[sagetrac-ylchapuy]

comment:4

Sorry to give myself a "needs work" but my ideas are much clearer this morning.
New patch to come in a few minutes!