fix: add unit tests to Row

### What changes were proposed in this pull request?

#75 shows that `Row` type has a bug.

I added tests to the `Row` types.

### Why are the changes needed?

To guarantee bug free implementation of `Row` type.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

The PR adds unit-tests for the `Row` type.

fixes #76

Signed-off-by: Mustafa Elbehery <melbeherredhat.com>

Closes #77 from Elbehery/76_test_row.

Authored-by: Mustafa Elbehery <melbeher@redhat.com>
Signed-off-by: Martin Grund <martin.grund@databricks.com>

spark/sql/types/row.go

@@ -17,9 +17,15 @@
 package types
 
 type Row interface {
+	// At returns field's value at the given index within a [Row].
+	// It returns nil for invalid indices.
 	At(index int) any
+	// Value returns field's value of the given column's name within a [Row].
+	// It returns nil for invalid column's name.
 	Value(name string) any
+	// Values returns values of all fields within a [Row] as a slice of any.
 	Values() []any
+	// Len returns the number of fields within a [Row]
 	Len() int
 	FieldNames() []string
 }
@@ -30,11 +36,18 @@ type rowImpl struct {
 }
 
 func (r *rowImpl) At(index int) any {
+	if index < 0 || index > len(r.values) {
+		return nil
+	}
 	return r.values[index]
 }
 
 func (r *rowImpl) Value(name string) any {
-	return r.values[r.offsets[name]]
+	idx, ok := r.offsets[name]
+	if !ok {
+		return nil
+	}
+	return r.values[idx]
 }
 
 func (r *rowImpl) Values() []any {

spark/sql/types/row_test.go

@@ -0,0 +1,110 @@
+//
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements.  See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package types
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// rowImplTest is a read-only sample [Row] to be used in all tests.
+var rowImplSample rowImpl = rowImpl{
+	values: []any{1, 2, 3, 4, 5},
+	offsets: map[string]int{
+		"one":   0,
+		"two":   1,
+		"three": 2,
+		"four":  3,
+		"five":  4,
+	},
+}
+
+func TestRowImpl_At(t *testing.T) {
+	testCases := []struct {
+		name  string
+		input int
+		exp   any
+	}{
+		{
+			name:  "index within range",
+			input: 2,
+			exp:   3,
+		},
+		{
+			name:  "index out of range",
+			input: 6,
+			exp:   nil,
+		},
+		{
+			name:  "negative index",
+			input: -1,
+			exp:   nil,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			act := rowImplSample.At(tc.input)
+			require.Equal(t, tc.exp, act)
+		})
+	}
+}
+
+func TestRowImpl_Value(t *testing.T) {
+	testCases := []struct {
+		name  string
+		input string
+		exp   any
+	}{
+		{
+			name:  "valid field name",
+			input: "two",
+			exp:   2,
+		},
+		{
+			name:  "invalid field name",
+			input: "six",
+			exp:   nil,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			act := rowImplSample.Value(tc.input)
+			require.Equal(t, tc.exp, act)
+		})
+	}
+}
+
+func TestRowImpl_Values(t *testing.T) {
+	exp := []any{1, 2, 3, 4, 5}
+	act := rowImplSample.Values()
+	require.Equal(t, exp, act)
+}
+
+func TestRowImpl_Len(t *testing.T) {
+	exp := 5
+	act := rowImplSample.Len()
+	require.Equal(t, exp, act)
+}
+
+func TestRowImpl_FieldNames(t *testing.T) {
+	exp := []string{"one", "two", "three", "four", "five"}
+	act := rowImplSample.FieldNames()
+	require.ElementsMatch(t, exp, act)
+}