diff --git a/pkg/sudoku/core_test.go b/pkg/sudoku/core_test.go
new file mode 100644
index 0000000..d734b07
--- /dev/null
+++ b/pkg/sudoku/core_test.go
@@ -0,0 +1,138 @@
+package sudoku
+
+import (
+	"testing"
+)
+
+var testCore *Core
+
+func TestGenerate(t *testing.T) {
+	testCore = NewCore()
+	// before generation
+	if testCore.Grid[0][0] != 0 {
+		t.Errorf("NewCore() FAILED. Should return empty Core.Grid filled by zero values")
+	} else {
+		t.Logf("NewCore() PASSED")
+	}
+
+	testCore.Generate()
+	// after generation
+	if !(testCore.Grid[0][0] > 0) {
+		t.Errorf("Core.Generate() FAILED. Should return Core.Grid filled by non-zero values")
+	} else {
+		t.Logf("Core.Generate() PASSED")
+	}
+}
+
+func TestPrepare(t *testing.T) {
+	// before prepare
+	if testCore.Task[0][0] > 0 {
+		t.Errorf("NewCore() FAILED. Should return empty Core.Task filled by zero values")
+	} else {
+		t.Logf("NewCore() PASSED")
+	}
+
+	// after preparation
+	testCore.Prepare(50)
+	isFilled := false
+	for _, row := range testCore.Task {
+		if isFilled {
+			break
+		}
+		for _, ceil := range row {
+			isFilled = ceil > 0
+			if isFilled {
+				break
+			}
+		}
+	}
+	if !isFilled {
+		t.Errorf("testCore.Prepare(%d) FAILED. Should return Core.Task filled by some non-zero values, but got all zero values", 50)
+	} else {
+		t.Logf("testCore.Prepare(%d) PASSED", 50)
+	}
+
+	// 0% fill
+	testValues := []int{0, -1, -100}
+
+	for _, tv := range testValues {
+		testCore.Prepare(tv)
+		isFilled := false
+		for _, row := range testCore.Task {
+			if isFilled {
+				break
+			}
+			for _, ceil := range row {
+				isFilled = ceil > 0
+				if isFilled {
+					break
+				}
+			}
+		}
+
+		if isFilled {
+			t.Errorf("testCore.Prepare(%d) FAILED. Should return Core.Task filled by zero values, but got non-zero value", tv)
+		} else {
+			t.Logf("testCore.Prepare(%d) PASSED", tv)
+		}
+	}
+
+	// 100% fill
+	testValues = []int{100, 101, 200}
+
+	for _, tv := range testValues {
+		testCore.Prepare(tv)
+		isFilled := true
+
+		for _, row := range testCore.Task {
+			if !isFilled {
+				break
+			}
+			for _, ceil := range row {
+				isFilled = ceil > 0
+				if !isFilled {
+					break
+				}
+			}
+		}
+
+		if !isFilled {
+			t.Errorf("testCore.Prepare(%d) FAILED. Should return Core.Task filled non-zero values, but got zero value", tv)
+		} else {
+			t.Logf("testCore.Prepare(%d) PASSED", tv)
+		}
+	}
+}
+
+func TestVerify(t *testing.T) {
+	// check 100% filled task
+	result := testCore.Verify()
+
+	if result.IsError {
+		t.Errorf("testCore.Verify() FAILED. Should be verified, but got error at position %+v", result.Position)
+	} else {
+		t.Logf("testCore.Verify() PASSED")
+	}
+
+	// check with error
+	testCore.Task[0][0] = 1
+	testCore.Task[0][1] = 1
+	result = testCore.Verify()
+
+	if !result.IsError {
+		t.Errorf("testCore.Verify() FAILED. Should be an error, but got verified")
+	} else {
+		t.Logf("testCore.Verify() PASSED")
+	}
+
+	testCore.Prepare(100)
+	testCore.Task[0][0] = 1
+	testCore.Task[1][0] = 1
+	result = testCore.Verify()
+
+	if !result.IsError {
+		t.Errorf("testCore.Verify() FAILED. Should be an error, but got verified")
+	} else {
+		t.Logf("testCore.Verify() PASSED")
+	}
+}