Swap 2 random genes in swap_mutation, as documented

pygad/utils/mutation.py

@@ -362,9 +362,8 @@ def polynomial_mutation(self, offspring):
 
     def swap_mutation(self, offspring):
         """
-        Swap the values of two genes inside each offspring. One gene is
-        picked at random from the first half of the chromosome; the
-        other is its mirror in the second half.
+        Swap the values of two genes inside each offspring. The two
+        genes are 2 different genes picked at random.
 
         Parameters
         ----------
@@ -378,8 +377,7 @@ def swap_mutation(self, offspring):
         """
 
         for idx in range(offspring.shape[0]):
-            mutation_gene1 = numpy.random.randint(low=0, high=offspring.shape[1]/2, size=1)[0]
-            mutation_gene2 = mutation_gene1 + int(offspring.shape[1]/2)
+            mutation_gene1, mutation_gene2 = numpy.random.choice(offspring.shape[1], size=2, replace=False)
 
             temp = offspring[idx, mutation_gene1]
             offspring[idx, mutation_gene1] = offspring[idx, mutation_gene2]

tests/test_crossover_mutation.py

@@ -241,6 +241,26 @@ def test_random_mutation_manual_call4():
     for value in comp_sorted:
         assert value in value_space
 
+def test_swap_mutation_manual_call():
+    # Any 2 different genes can be swapped.
+    num_genes = 6
+    result, ga_instance = output_crossover_mutation(gene_type=int,
+                                                    num_genes=num_genes,
+                                                    mutation_type="swap")
+
+    temp_offspring = numpy.array([list(range(num_genes))] * 1000)
+    offspring = ga_instance.swap_mutation(offspring=temp_offspring.copy())
+
+    swapped_pairs = set()
+    for solution in offspring:
+        changed = numpy.flatnonzero(solution != numpy.arange(num_genes))
+        # Exactly 2 genes exchange their values.
+        assert len(changed) == 2
+        assert solution[changed[0]] == changed[1] and solution[changed[1]] == changed[0]
+        swapped_pairs.add(tuple(changed))
+
+    assert len(swapped_pairs) == num_genes * (num_genes - 1) // 2
+
 if __name__ == "__main__":
     #### Single-objective
     print()
@@ -285,3 +305,6 @@ def test_random_mutation_manual_call4():
 
     test_random_mutation_manual_call4()
     print()
+
+    test_swap_mutation_manual_call()
+    print()