def bellman_ford(G, s):
"""
Bellman-Ford single-source shortest-paths solution in the general case.
Algorithm relaxes edges, making `|V|-1` passes over the edges of the graph,
decreasing an estimate on the path from starting vertex `s`, gradually spreading
the frontier of relaxed edges beyond those already estimated. In that sense,
Bellman-Ford is also commonly regarded as a dynamic programming algorithm.
After `|V|-1` passes of every edge in a graph, it will deterministically
completely relax all of its edges, unless there's a negative-weight cycle.
Second loop checks for cycles by verifying that none of the edges can be
further relaxed otherwise reporting that negative-weight cycle exists.
Complexity: O(VE) for main loop. Initialization O(V) + O(E) for cycle check.
:param Graph G: Adjacency list weighted directed graph representation
:param Vertex s: Starting vertex
:return bool: Returns True iff the graph contains no negative-weight cycles
that are reachable from the starting vertex, False otherwise. Graph vertices
are updated.
"""
n = len(G.V) # Total number of vertices in a graph
initialize_single_source(G, s)
for i in range(0, n - 1):
# Relax every edge `|V|-1` times
for u, v in G.E():
relax(u, v)
for u, v in G.E():
# Check if edge cannot be further relaxed
if v.d > u.d + weight(u, v):
return False
return True
def relax(u, v):
"""
Relaxes edge (u, v).
Term "relaxation" actually means tightening an upper bound. The process
consists of testing whether we can improve the shortest path to `v` found
so far by going through `u` and if so, updating both the estimate and
parent relation.
Once upper bound property (∂) is achieved, it never changes. Relaxed edges
also follow triangle inequality: `∂(s, v) ≤ ∂(s, u) + w(u, v)`.
Longest path can be calculated using negated weights.
Complexity: O(1)
:param Vertex u: Source vertex
:param Vertex v: Target vertex
:return None: Vertex `v` is updated
"""
w = weight(u, v)
if v.d > u.d + w:
v.d = u.d + w
v.p = u
