Fix apriori generates an empty array as a part of the frequent item s…

…ets (#224)

src/Phpml/Association/Apriori.php

@@ -86,12 +86,11 @@ public function getRules(): array
     public function apriori(): array
     {
         $L = [];
-        $L[1] = $this->items();
-        $L[1] = $this->frequent($L[1]);
 
-        for ($k = 2; !empty($L[$k - 1]); ++$k) {
-            $L[$k] = $this->candidates($L[$k - 1]);
-            $L[$k] = $this->frequent($L[$k]);
+        $items = $this->frequent($this->items());
+        for ($k = 1; !empty($items); ++$k) {
+            $L[$k] = $items;
+            $items = $this->frequent($this->candidates($items));
         }
 
         return $L;

tests/Phpml/Association/AprioriTest.php

@@ -64,7 +64,6 @@ public function testApriori(): void
 
         $L = $apriori->apriori();
 
-        $this->assertCount(0, $L[3]);
         $this->assertCount(4, $L[2]);
         $this->assertTrue($this->invoke($apriori, 'contains', [$L[2], [1, 2]]));
         $this->assertFalse($this->invoke($apriori, 'contains', [$L[2], [1, 3]]));
@@ -204,4 +203,29 @@ public function testSaveAndRestore(): void
         $this->assertEquals($classifier, $restoredClassifier);
         $this->assertEquals($predicted, $restoredClassifier->predict($testSamples));
     }
+
+    public function testAprioriEmpty(): void
+    {
+        $sample = [];
+
+        $apriori = new Apriori(0, 0);
+        $apriori->train($sample, []);
+
+        $L = $apriori->apriori();
+
+        $this->assertEmpty($L);
+    }
+
+    public function testAprioriSingleItem(): void
+    {
+        $sample = [['a']];
+
+        $apriori = new Apriori(0, 0);
+        $apriori->train($sample, []);
+
+        $L = $apriori->apriori();
+
+        $this->assertEquals([1], array_keys($L));
+        $this->assertEquals([['a']], $L[1]);
+    }
 }