VM: Add tests for stack item

pkg/vm/stack/stackitem_test.go

@@ -0,0 +1,68 @@
+package stack
+
+import (
+	"math/big"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// A simple test to ensure that by embedding the abstract interface
+// we immediately become a stack item, with the default values set to nil
+func TestInterfaceEmbedding(t *testing.T) {
+
+	// Create an anonymous struct that embeds the abstractItem
+	a := struct {
+		*abstractItem
+	}{
+		&abstractItem{},
+	}
+
+	// Since interface checking can be done at compile time.
+	// If he abstractItem did not implement all methods of our interface `Item`
+	// Then any struct which embeds it, will also not implement the Item interface.
+	// This test would then give errors, at compile time.
+	var Items []Item
+	Items = append(Items, a)
+
+	// Default methods should give errors
+	// Here we just need to test against one of the methods in the interface
+	for _, element := range Items {
+		x, err := element.Integer()
+		assert.Nil(t, x)
+		assert.NotNil(t, err, nil)
+	}
+
+}
+
+// TestIntCasting is a simple test to test that the Integer method is overwritten
+// from the abstractItem
+func TestIntMethodOverride(t *testing.T) {
+
+	testValues := []int64{0, 10, 200, 30, 90}
+	var Items []Item
+
+	// Convert a range of int64s into Stack Integers
+	// Adding them into an array of StackItems
+	for _, num := range testValues {
+		stackInteger, err := NewInt(big.NewInt(num))
+		if err != nil {
+			t.Fail()
+		}
+		Items = append(Items, stackInteger)
+	}
+
+	// For each item, call the Integer method on the interface
+	// Which should return an integer and no error
+	// as the stack integer struct overrides that method
+	for i, element := range Items {
+		k, err := element.Integer()
+		if err != nil {
+			t.Fail()
+		}
+		if k.val.Cmp(big.NewInt(testValues[i])) != 0 {
+			t.Fail()
+		}
+	}
+
+}