Add row and column offset

cellwalker.go

@@ -23,6 +23,10 @@ type CellWalker struct {
 func newCellWalker(col int, row int) *CellWalker {
 	if col > ColumnsLimit {
 		col = ColumnsLimit
+	} else {
+		if col < 1 {
+			col = 1
+		}
 	}
 	if row < 1 {
 		row = 1
@@ -152,3 +156,13 @@ func (c *CellWalker) Column(colName string) *CellWalker {
 func (c *CellWalker) Row(row int) *CellWalker {
 	return newCellWalker(c.column, row)
 }
+
+// ColumnOffset return a cell with a given offset distance to column
+func (c *CellWalker) ColumnOffset(offset int) *CellWalker {
+	return newCellWalker(c.column+offset, c.row)
+}
+
+// RowOffset return a cell with a given offset distance to row
+func (c *CellWalker) RowOffset(offset int) *CellWalker {
+	return newCellWalker(c.column, c.row+offset)
+}

cellwalker_test.go

@@ -191,3 +191,51 @@ func TestJumpToAnyRowInTheSameColumn(t *testing.T) {
 
 	assert.Equal(t, "A778", result.String())
 }
+
+func TestPositiveColumnOffset(t *testing.T) {
+	result := At("A1").ColumnOffset(4)
+
+	assert.Equal(t, "E1", result.String())
+}
+
+func TestNegativeColumnOffset(t *testing.T) {
+	result := At("AB1").ColumnOffset(-4)
+
+	assert.Equal(t, "X1", result.String())
+}
+
+func TestVeryLargePositiveColumnOffset(t *testing.T) {
+	result := At("A1").ColumnOffset(40000000)
+
+	assert.Equal(t, "XFD1", result.String())
+}
+
+func TestVeryLargeNegativeColumnOffset(t *testing.T) {
+	result := At("XFD1").ColumnOffset(-40000000)
+
+	assert.Equal(t, "A1", result.String())
+}
+
+func TestPositiveRowOffset(t *testing.T) {
+	result := At("A1").RowOffset(4)
+
+	assert.Equal(t, "A5", result.String())
+}
+
+func TestNegativeRowOffset(t *testing.T) {
+	result := At("AB10").RowOffset(-4)
+
+	assert.Equal(t, "AB6", result.String())
+}
+
+func TestVeryLargePositiveRowOffset(t *testing.T) {
+	result := At("A1").RowOffset(40000000)
+
+	assert.Equal(t, "A1048576", result.String())
+}
+
+func TestVeryLargeNegativeRowOffset(t *testing.T) {
+	result := At("A1048576").RowOffset(-40000000)
+
+	assert.Equal(t, "A1", result.String())
+}