Add simple population mutation

Author: petercoles

src/Population.php

@@ -41,6 +41,16 @@ public function select($number)
         return collect($this->solutions)->sortByDesc('fitness')->take($number);
     }
 
+    public function mutate()
+    {
+        $this->solutions = collect($this->solutions)->map(function ($solution) {
+            if (mt_rand(1, 7) == 1) {
+                $solution->mutate();
+            }
+            return $solution;
+        })->toArray();
+    }
+
     protected function initialise()
     {
         for ($i = 0; $i < $this->size; $i++) {

src/Solution.php

@@ -14,12 +14,25 @@ abstract public function evaluate($data);
 
     public function initialise()
     {
-        $this->chromosomes = collect($this->genome())->map(function ($chromosome) {
-            $randomiser = 'random' . ucfirst($chromosome[0]);
-            return $this->$randomiser($chromosome);
+        $this->chromosomes = collect($this->genome())->map(function ($definition) {
+            return $this->chromosome($definition);
         })->toArray();
     }
 
+    public function chromosome($definition)
+    {
+        $randomiser = 'random' . ucfirst($definition[0]);
+        return $this->$randomiser($definition);
+    }
+
+    public function mutate()
+    {
+        $definition = $this->genome();
+        $chromosomePosition = mt_rand(0, sizeof($definition) - 1);
+        $this->chromosomes[$chromosomePosition] = $this->chromosome($definition[$chromosomePosition]);
+        return $this;
+    }
+
     public function chromosomes()
     {
         return $this->chromosomes;

tests/PopulationTest.php

@@ -63,4 +63,14 @@ public function apply_selection_strategy()
         $this->assertCount(6, $selections);
         $this->assertEquals([1052, 771, 758, 683, 641, 607], $selections->pluck('fitness')->toArray());
     }
+
+    /** @test */
+    public function apply_mutation_strategy()
+    {
+        $population = new Population(Integers::class, 12);
+        $this->assertEquals([90, -52, -1], $population->solutions()[5]->chromosomes());
+
+        $population->mutate();
+        $this->assertEquals([90, -83, -1], $population->solutions()[5]->chromosomes());
+    }
 }