sagemathgh-40014: set correct default algorithm for the diameter of (un)weighted graphs
    
Fixes sagemath#40013.

We now ensure that the default algorithm is `'DHV'` for weighted graphs
and `'iFUB'` for unweighted graphs.

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URL: sagemath#40014
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/graphs/graph.py

@@ -5022,6 +5022,9 @@ def diameter(self, by_weight=False, algorithm=None, weight_function=None,
         The diameter is defined to be the maximum distance between two vertices.
         It is infinite if the graph is not connected.
 
+        The default algorithm is ``'iFUB'`` [CGILM2010]_ for unweighted graphs
+        and ``'DHV'`` [Dragan2018]_ for weighted graphs.
+
         For more information and examples on how to use input variables, see
         :meth:`~GenericGraph.shortest_paths` and
         :meth:`~Graph.eccentricity`
@@ -5112,6 +5115,12 @@ def diameter(self, by_weight=False, algorithm=None, weight_function=None,
             Traceback (most recent call last):
             ...
             ValueError: algorithm 'iFUB' does not work on weighted graphs
+
+        Check that :issue:`40013` is fixed::
+
+            sage: G = graphs.PetersenGraph()
+            sage: G.diameter(by_weight=True)
+            2.0
         """
         if not self.order():
             raise ValueError("diameter is not defined for the empty graph")
@@ -5124,10 +5133,7 @@ def diameter(self, by_weight=False, algorithm=None, weight_function=None,
             weight_function = None
 
         if algorithm is None:
-            if by_weight:
-                algorithm = 'iFUB'
-            else:
-                algorithm = 'DHV'
+            algorithm = 'DHV' if by_weight else 'iFUB'
         elif algorithm == 'BFS':
             algorithm = 'standard'