improve method _build_flow_graph

[dcoudert]

We use method is_directed_acyclic to find cycles instead of computing shortest paths. This should have been done long ago as it's faster this way.

Before:

sage: D = digraphs.TransitiveTournament(10)
sage: D.add_edges((i+1, i) for i in range(9))
sage: flow = {e: 1 for e in D.edge_iterator(labels=0)}
sage: %time F = D._build_flow_graph(flow, False)
CPU times: user 1.45 ms, sys: 320 µs, total: 1.77 ms
Wall time: 1.55 ms

After:

sage: D = digraphs.TransitiveTournament(10)
sage: D.add_edges((i+1, i) for i in range(9))
sage: flow = {e: 1 for e in D.edge_iterator(labels=0)}
sage: %time F = D._build_flow_graph(flow, False)
CPU times: user 958 µs, sys: 244 µs, total: 1.2 ms
Wall time: 1.02 ms

Component: graph theory

Author: David Coudert

Branch/Commit: 0a24c03

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/26941

[dcoudert]

New commits:

0a24c03

trac #26941: improve _build_flow_graph

[dcoudert]

Branch: public/26941_build_flow_graph

[dcoudert]

Commit: 0a24c03

[tscrim]

comment:2

This seems like a better approach (at least it makes the code more natural). So LGTM.

[tscrim]

Reviewer: Travis Scrimshaw

[vbraun]

Changed branch from public/26941_build_flow_graph to 0a24c03