Prevent NPE in CSV diff rendering when column removed (#17018)

Fixes #16837 if a column is deleted.

We were clobbering the columns that were added by looping through the aline (base) and then when bline (head) was looped through, it clobbered what was in the "cells" array that is show in the diff, and then left a nil cell because nothing was shifted.

This fix properly shifts the cells, and properly puts the b cell either at its location or after, according to what the aline placed in the cells.

This includes test, adding a new test function since adding/removing cells works best with three columns, not two, which results in 4 columns of the resulting cells because it has a deleted column and an added column. If you try this locally, you can try those cases and others, such as adding a column.

There was no need to do anything special for the rows when `aline == 0 || bline == 0` so that was removed. This allows the same code to be used for removed or added lines, with the bcell text always being the RightCell, acell text being the LeftCell.

I still added the patch zeripath gave at https://github.com/go-gitea/gitea/issues/16837#issuecomment-913007382 so that just in case for some reason a cell is nil (which shouldn't happen now) it doesn't throw a 500 error, so the user can at least view the raw diff.

Also fixes in the [view.go](https://github.com/go-gitea/gitea/pull/17018/files#diff-43a7f4747c7ba8bff888c9be11affaafd595fd55d27f3333840eb19df9fad393L521) file how if a CSV file is empty (either created empty or if you edit it and remove all contents) it throws a huge 500 error when you then save it (when you view the file). Since we allow creating, saving and pushing empty files, we shouldn't throw an error on an empty CSV file, but just show its empty contents. This doesn't happen if it is a Markdown file or other type of file that is empty.
EDIT: Now handled in the markup/csv renderer code

2 files changed, 322 insertions, 132 deletions

diff --git a/services/gitdiff/csv.go b/services/gitdiff/csv.go
index f4310d8772..099660b172 100644
--- a/services/gitdiff/csv.go
+++ b/services/gitdiff/csv.go
@@ -21,10 +21,12 @@ type TableDiffCellType uint8
 
 // TableDiffCellType possible values.
 const (
-	TableDiffCellEqual TableDiffCellType = iota + 1
+	TableDiffCellUnchanged TableDiffCellType = iota + 1
 	TableDiffCellChanged
 	TableDiffCellAdd
 	TableDiffCellDel
+	TableDiffCellMovedUnchanged
+	TableDiffCellMovedChanged
 )
 
 // TableDiffCell represents a cell of a TableDiffRow
@@ -53,6 +55,9 @@ type csvReader struct {
 	eof    bool
 }
 
+// ErrorUndefinedCell is for when a row, column coordinates do not exist in the CSV
+var ErrorUndefinedCell = errors.New("undefined cell")
+
 // createCsvReader creates a csvReader and fills the buffer
 func createCsvReader(reader *csv.Reader, bufferRowCount int) (*csvReader, error) {
 	csv := &csvReader{reader: reader}
@@ -70,7 +75,7 @@ func createCsvReader(reader *csv.Reader, bufferRowCount int) (*csvReader, error)
 
 // GetRow gets a row from the buffer if present or advances the reader to the requested row. On the end of the file only nil gets returned.
 func (csv *csvReader) GetRow(row int) ([]string, error) {
-	if row < len(csv.buffer) {
+	if row < len(csv.buffer) && row >= 0 {
 		return csv.buffer[row], nil
 	}
 	if csv.eof {
@@ -131,7 +136,11 @@ func createCsvDiffSingle(reader *csv.Reader, celltype TableDiffCellType) ([]*Tab
 		}
 		cells := make([]*TableDiffCell, len(row))
 		for j := 0; j < len(row); j++ {
-			cells[j] = &TableDiffCell{LeftCell: row[j], Type: celltype}
+			if celltype == TableDiffCellDel {
+				cells[j] = &TableDiffCell{LeftCell: row[j], Type: celltype}
+			} else {
+				cells[j] = &TableDiffCell{RightCell: row[j], Type: celltype}
+			}
 		}
 		rows = append(rows, &TableDiffRow{RowIdx: i, Cells: cells})
 		i++
@@ -141,185 +150,267 @@ func createCsvDiffSingle(reader *csv.Reader, celltype TableDiffCellType) ([]*Tab
 }
 
 func createCsvDiff(diffFile *DiffFile, baseReader *csv.Reader, headReader *csv.Reader) ([]*TableDiffSection, error) {
-	a, err := createCsvReader(baseReader, maxRowsToInspect)
+	// Given the baseReader and headReader, we are going to create CSV Reader for each, baseCSVReader and b respectively
+	baseCSVReader, err := createCsvReader(baseReader, maxRowsToInspect)
 	if err != nil {
 		return nil, err
 	}
-
-	b, err := createCsvReader(headReader, maxRowsToInspect)
+	headCSVReader, err := createCsvReader(headReader, maxRowsToInspect)
 	if err != nil {
 		return nil, err
 	}
 
-	a2b, b2a := getColumnMapping(a, b)
+	// Initializing the mappings of base to head (a2bColMap) and head to base (b2aColMap) columns
+	a2bColMap, b2aColMap := getColumnMapping(baseCSVReader, headCSVReader)
 
-	columns := len(a2b) + countUnmappedColumns(b2a)
-	if len(a2b) < len(b2a) {
-		columns = len(b2a) + countUnmappedColumns(a2b)
+	// Determines how many cols there will be in the diff table, which includes deleted columns from base and added columns to base
+	numDiffTableCols := len(a2bColMap) + countUnmappedColumns(b2aColMap)
+	if len(a2bColMap) < len(b2aColMap) {
+		numDiffTableCols = len(b2aColMap) + countUnmappedColumns(a2bColMap)
 	}
 
-	createDiffRow := func(aline int, bline int) (*TableDiffRow, error) {
-		cells := make([]*TableDiffCell, columns)
-
-		if aline == 0 || bline == 0 {
-			var (
-				row      []string
-				celltype TableDiffCellType
-				err      error
-			)
-			if bline == 0 {
-				row, err = a.GetRow(aline - 1)
-				celltype = TableDiffCellDel
-			} else {
-				row, err = b.GetRow(bline - 1)
-				celltype = TableDiffCellAdd
-			}
+	// createDiffTableRow takes the row # of the `a` line and `b` line of a diff (starting from 1), 0 if the line doesn't exist (undefined)
+	// in the base or head respectively.
+	// Returns a TableDiffRow which has the row index
+	createDiffTableRow := func(aLineNum int, bLineNum int) (*TableDiffRow, error) {
+		// diffTableCells is a row of the diff table. It will have a cells for added, deleted, changed, and unchanged content, thus either
+		// the same size as the head table or bigger
+		diffTableCells := make([]*TableDiffCell, numDiffTableCols)
+		var bRow *[]string
+		if bLineNum > 0 {
+			row, err := headCSVReader.GetRow(bLineNum - 1)
 			if err != nil {
 				return nil, err
 			}
-			if row == nil {
-				return nil, nil
-			}
-			for i := 0; i < len(row); i++ {
-				cells[i] = &TableDiffCell{LeftCell: row[i], Type: celltype}
-			}
-			return &TableDiffRow{RowIdx: bline, Cells: cells}, nil
+			bRow = &row
 		}
-
-		arow, err := a.GetRow(aline - 1)
-		if err != nil {
-			return nil, err
-		}
-		brow, err := b.GetRow(bline - 1)
-		if err != nil {
-			return nil, err
+		var aRow *[]string
+		if aLineNum > 0 {
+			row, err := baseCSVReader.GetRow(aLineNum - 1)
+			if err != nil {
+				return nil, err
+			}
+			aRow = &row
 		}
-		if len(arow) == 0 && len(brow) == 0 {
+		if aRow == nil && bRow == nil {
+			// No content
 			return nil, nil
 		}
 
-		for i := 0; i < len(a2b); i++ {
-			acell, _ := getCell(arow, i)
-			if a2b[i] == unmappedColumn {
-				cells[i] = &TableDiffCell{LeftCell: acell, Type: TableDiffCellDel}
-			} else {
-				bcell, _ := getCell(brow, a2b[i])
-
-				celltype := TableDiffCellChanged
-				if acell == bcell {
-					celltype = TableDiffCellEqual
+		aIndex := 0      // tracks where we are in the a2bColMap
+		bIndex := 0      // tracks where we are in the b2aColMap
+		colsAdded := 0   // incremented whenever we found a column was added
+		colsDeleted := 0 // incrememted whenever a column was deleted
+
+		// We loop until both the aIndex and bIndex are greater than their col map, which then we are done
+		for aIndex < len(a2bColMap) || bIndex < len(b2aColMap) {
+			// Starting from where aIndex is currently pointing, we see if the map is -1 (dleeted) and if is, create column to note that, increment, and look at the next aIndex
+			for aIndex < len(a2bColMap) && a2bColMap[aIndex] == -1 && (bIndex >= len(b2aColMap) || aIndex <= bIndex) {
+				var aCell string
+				if aRow != nil {
+					if cell, err := getCell(*aRow, aIndex); err != nil {
+						if err != ErrorUndefinedCell {
+							return nil, err
+						}
+					} else {
+						aCell = cell
+					}
 				}
+				diffTableCells[bIndex+colsDeleted] = &TableDiffCell{LeftCell: aCell, Type: TableDiffCellDel}
+				aIndex++
+				colsDeleted++
+			}
 
-				cells[i] = &TableDiffCell{LeftCell: acell, RightCell: bcell, Type: celltype}
+			// aIndex is now pointing to a column that also exists in b, or is at the end of a2bColMap. If the former,
+			// we can just increment aIndex until it points to a -1 column or one greater than the current bIndex
+			for aIndex < len(a2bColMap) && a2bColMap[aIndex] != -1 {
+				aIndex++
 			}
-		}
-		for i := 0; i < len(b2a); i++ {
-			if b2a[i] == unmappedColumn {
-				bcell, _ := getCell(brow, i)
-				cells[i] = &TableDiffCell{LeftCell: bcell, Type: TableDiffCellAdd}
+
+			// Starting from where bIndex is currently pointing, we see if the map is -1 (added) and if is, create column to note that, increment, and look at the next aIndex
+			for bIndex < len(b2aColMap) && b2aColMap[bIndex] == -1 && (aIndex >= len(a2bColMap) || bIndex < aIndex) {
+				var bCell string
+				cellType := TableDiffCellAdd
+				if bRow != nil {
+					if cell, err := getCell(*bRow, bIndex); err != nil {
+						if err != ErrorUndefinedCell {
+							return nil, err
+						}
+					} else {
+						bCell = cell
+					}
+				} else {
+					cellType = TableDiffCellDel
+				}
+				diffTableCells[bIndex+colsDeleted] = &TableDiffCell{RightCell: bCell, Type: cellType}
+				bIndex++
+				colsAdded++
+			}
+
+			// aIndex is now pointing to a column that also exists in a, or is at the end of b2aColMap. If the former,
+			// we get the a col and b col values (if they exist), figure out if they are the same or not, and if the column moved, and add it to the diff table
+			for bIndex < len(b2aColMap) && b2aColMap[bIndex] != -1 && (aIndex >= len(a2bColMap) || bIndex < aIndex) {
+				var diffTableCell TableDiffCell
+
+				var aCell *string
+				// get the aCell value if the aRow exists
+				if aRow != nil {
+					if cell, err := getCell(*aRow, b2aColMap[bIndex]); err != nil {
+						if err != ErrorUndefinedCell {
+							return nil, err
+						}
+					} else {
+						aCell = &cell
+						diffTableCell.LeftCell = cell
+					}
+				} else {
+					diffTableCell.Type = TableDiffCellAdd
+				}
+
+				var bCell *string
+				// get the bCell value if the bRow exists
+				if bRow != nil {
+					if cell, err := getCell(*bRow, bIndex); err != nil {
+						if err != ErrorUndefinedCell {
+							return nil, err
+						}
+					} else {
+						bCell = &cell
+						diffTableCell.RightCell = cell
+					}
+				} else {
+					diffTableCell.Type = TableDiffCellDel
+				}
+
+				// if both a and b have a row that exists, compare the value and determine if the row has moved
+				if aCell != nil && bCell != nil {
+					moved := ((bIndex + colsDeleted) != (b2aColMap[bIndex] + colsAdded))
+					if *aCell != *bCell {
+						if moved {
+							diffTableCell.Type = TableDiffCellMovedChanged
+						} else {
+							diffTableCell.Type = TableDiffCellChanged
+						}
+					} else {
+						if moved {
+							diffTableCell.Type = TableDiffCellMovedUnchanged
+						} else {
+							diffTableCell.Type = TableDiffCellUnchanged
+						}
+						diffTableCell.LeftCell = ""
+					}
+				}
+
+				// Add the diff column to the diff row
+				diffTableCells[bIndex+colsDeleted] = &diffTableCell
+				bIndex++
 			}
 		}
 
-		return &TableDiffRow{RowIdx: bline, Cells: cells}, nil
+		return &TableDiffRow{RowIdx: bLineNum, Cells: diffTableCells}, nil
 	}
 
-	var sections []*TableDiffSection
+	// diffTableSections are TableDiffSections which represent the diffTableSections we get when doing a diff, each will be its own table in the view
+	var diffTableSections []*TableDiffSection
 
 	for i, section := range diffFile.Sections {
-		var rows []*TableDiffRow
+		// Each section has multiple diffTableRows
+		var diffTableRows []*TableDiffRow
 		lines := tryMergeLines(section.Lines)
+		// Loop through the merged lines to get each row of the CSV diff table for this section
 		for j, line := range lines {
 			if i == 0 && j == 0 && (line[0] != 1 || line[1] != 1) {
-				diffRow, err := createDiffRow(1, 1)
+				diffTableRow, err := createDiffTableRow(1, 1)
 				if err != nil {
 					return nil, err
 				}
-				if diffRow != nil {
-					rows = append(rows, diffRow)
+				if diffTableRow != nil {
+					diffTableRows = append(diffTableRows, diffTableRow)
 				}
 			}
-			diffRow, err := createDiffRow(line[0], line[1])
+			diffTableRow, err := createDiffTableRow(line[0], line[1])
 			if err != nil {
 				return nil, err
 			}
-			if diffRow != nil {
-				rows = append(rows, diffRow)
+			if diffTableRow != nil {
+				diffTableRows = append(diffTableRows, diffTableRow)
 			}
 		}
 
-		if len(rows) > 0 {
-			sections = append(sections, &TableDiffSection{Rows: rows})
+		if len(diffTableRows) > 0 {
+			diffTableSections = append(diffTableSections, &TableDiffSection{Rows: diffTableRows})
 		}
 	}
 
-	return sections, nil
+	return diffTableSections, nil
 }
 
 // getColumnMapping creates a mapping of columns between a and b
-func getColumnMapping(a *csvReader, b *csvReader) ([]int, []int) {
-	arow, _ := a.GetRow(0)
-	brow, _ := b.GetRow(0)
+func getColumnMapping(baseCSVReader *csvReader, headCSVReader *csvReader) ([]int, []int) {
+	baseRow, _ := baseCSVReader.GetRow(0)
+	headRow, _ := headCSVReader.GetRow(0)
 
-	a2b := []int{}
-	b2a := []int{}
+	base2HeadColMap := []int{}
+	head2BaseColMap := []int{}
 
-	if arow != nil {
-		a2b = make([]int, len(arow))
+	if baseRow != nil {
+		base2HeadColMap = make([]int, len(baseRow))
 	}
-	if brow != nil {
-		b2a = make([]int, len(brow))
+	if headRow != nil {
+		head2BaseColMap = make([]int, len(headRow))
 	}
 
-	for i := 0; i < len(b2a); i++ {
-		b2a[i] = unmappedColumn
+	// Initializes all head2base mappings to be unmappedColumn (-1)
+	for i := 0; i < len(head2BaseColMap); i++ {
+		head2BaseColMap[i] = unmappedColumn
 	}
 
-	bcol := 0
-	for i := 0; i < len(a2b); i++ {
-		a2b[i] = unmappedColumn
-
-		acell, ea := getCell(arow, i)
-		if ea == nil {
-			for j := bcol; j < len(b2a); j++ {
-				bcell, eb := getCell(brow, j)
-				if eb == nil && acell == bcell {
-					a2b[i] = j
-					b2a[j] = i
-					bcol = j + 1
-					break
+	// Loops through the baseRow and see if there is a match in the head row
+	for i := 0; i < len(baseRow); i++ {
+		base2HeadColMap[i] = unmappedColumn
+		baseCell, err := getCell(baseRow, i)
+		if err == nil {
+			for j := 0; j < len(headRow); j++ {
+				if head2BaseColMap[j] == -1 {
+					headCell, err := getCell(headRow, j)
+					if err == nil && baseCell == headCell {
+						base2HeadColMap[i] = j
+						head2BaseColMap[j] = i
+						break
+					}
 				}
 			}
 		}
 	}
 
-	tryMapColumnsByContent(a, a2b, b, b2a)
-	tryMapColumnsByContent(b, b2a, a, a2b)
+	tryMapColumnsByContent(baseCSVReader, base2HeadColMap, headCSVReader, head2BaseColMap)
+	tryMapColumnsByContent(headCSVReader, head2BaseColMap, baseCSVReader, base2HeadColMap)
 
-	return a2b, b2a
+	return base2HeadColMap, head2BaseColMap
 }
 
 // tryMapColumnsByContent tries to map missing columns by the content of the first lines.
-func tryMapColumnsByContent(a *csvReader, a2b []int, b *csvReader, b2a []int) {
-	start := 0
-	for i := 0; i < len(a2b); i++ {
-		if a2b[i] == unmappedColumn {
-			if b2a[start] == unmappedColumn {
-				rows := util.Min(maxRowsToInspect, util.Max(0, util.Min(len(a.buffer), len(b.buffer))-1))
+func tryMapColumnsByContent(baseCSVReader *csvReader, base2HeadColMap []int, headCSVReader *csvReader, head2BaseColMap []int) {
+	for i := 0; i < len(base2HeadColMap); i++ {
+		headStart := 0
+		for base2HeadColMap[i] == unmappedColumn && headStart < len(head2BaseColMap) {
+			if head2BaseColMap[headStart] == unmappedColumn {
+				rows := util.Min(maxRowsToInspect, util.Max(0, util.Min(len(baseCSVReader.buffer), len(headCSVReader.buffer))-1))
 				same := 0
 				for j := 1; j <= rows; j++ {
-					acell, ea := getCell(a.buffer[j], i)
-					bcell, eb := getCell(b.buffer[j], start+1)
-					if ea == nil && eb == nil && acell == bcell {
+					baseCell, baseErr := getCell(baseCSVReader.buffer[j], i)
+					headCell, headErr := getCell(headCSVReader.buffer[j], headStart)
+					if baseErr == nil && headErr == nil && baseCell == headCell {
 						same++
 					}
 				}
 				if (float32(same) / float32(rows)) > minRatioToMatch {
-					a2b[i] = start + 1
-					b2a[start+1] = i
+					base2HeadColMap[i] = headStart
+					head2BaseColMap[headStart] = i
 				}
 			}
+			headStart++
 		}
-		start = a2b[i]
 	}
 }
 
@@ -328,7 +419,7 @@ func getCell(row []string, column int) (string, error) {
 	if column < len(row) {
 		return row[column], nil
 	}
-	return "", errors.New("Undefined column")
+	return "", ErrorUndefinedCell
 }
 
 // countUnmappedColumns returns the count of unmapped columns.
diff --git a/services/gitdiff/csv_test.go b/services/gitdiff/csv_test.go
index fb84d6ed06..1710e91c58 100644
--- a/services/gitdiff/csv_test.go
+++ b/services/gitdiff/csv_test.go
@@ -19,9 +19,9 @@ func TestCSVDiff(t *testing.T) {
 		diff  string
 		base  string
 		head  string
-		cells [][2]TableDiffCellType
+		cells [][]TableDiffCellType
 	}{
-		// case 0
+		// case 0 - initial commit of a csv
 		{
 			diff: `diff --git a/unittest.csv b/unittest.csv
 --- a/unittest.csv
@@ -29,11 +29,14 @@ func TestCSVDiff(t *testing.T) {
 @@ -0,0 +1,2 @@
 +col1,col2
 +a,a`,
-			base:  "",
-			head:  "col1,col2\na,a",
-			cells: [][2]TableDiffCellType{{TableDiffCellAdd, TableDiffCellAdd}, {TableDiffCellAdd, TableDiffCellAdd}},
+			base: "",
+			head: `col1,col2
+a,a`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellAdd, TableDiffCellAdd},
+				{TableDiffCellAdd, TableDiffCellAdd}},
 		},
-		// case 1
+		// case 1 - adding 1 row at end
 		{
 			diff: `diff --git a/unittest.csv b/unittest.csv
 --- a/unittest.csv
@@ -43,11 +46,17 @@ func TestCSVDiff(t *testing.T) {
 -a,a
 +a,a
 +b,b`,
-			base:  "col1,col2\na,a",
-			head:  "col1,col2\na,a\nb,b",
-			cells: [][2]TableDiffCellType{{TableDiffCellEqual, TableDiffCellEqual}, {TableDiffCellEqual, TableDiffCellEqual}, {TableDiffCellAdd, TableDiffCellAdd}},
+			base: `col1,col2
+a,a`,
+			head: `col1,col2
+a,a
+b,b`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellUnchanged, TableDiffCellUnchanged}, {TableDiffCellUnchanged, TableDiffCellUnchanged},
+				{TableDiffCellAdd, TableDiffCellAdd},
+			},
 		},
-		// case 2
+		// case 2 - row deleted
 		{
 			diff: `diff --git a/unittest.csv b/unittest.csv
 --- a/unittest.csv
@@ -56,11 +65,17 @@ func TestCSVDiff(t *testing.T) {
  col1,col2
 -a,a
  b,b`,
-			base:  "col1,col2\na,a\nb,b",
-			head:  "col1,col2\nb,b",
-			cells: [][2]TableDiffCellType{{TableDiffCellEqual, TableDiffCellEqual}, {TableDiffCellDel, TableDiffCellDel}, {TableDiffCellEqual, TableDiffCellEqual}},
+			base: `col1,col2
+a,a
+b,b`,
+			head: `col1,col2
+b,b`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellUnchanged, TableDiffCellUnchanged}, {TableDiffCellDel, TableDiffCellDel},
+				{TableDiffCellUnchanged, TableDiffCellUnchanged},
+			},
 		},
-		// case 3
+		// case 3 - row changed
 		{
 			diff: `diff --git a/unittest.csv b/unittest.csv
 --- a/unittest.csv
@@ -69,11 +84,16 @@ func TestCSVDiff(t *testing.T) {
  col1,col2
 -b,b
 +b,c`,
-			base:  "col1,col2\nb,b",
-			head:  "col1,col2\nb,c",
-			cells: [][2]TableDiffCellType{{TableDiffCellEqual, TableDiffCellEqual}, {TableDiffCellEqual, TableDiffCellChanged}},
+			base: `col1,col2
+b,b`,
+			head: `col1,col2
+b,c`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellUnchanged, TableDiffCellUnchanged},
+				{TableDiffCellUnchanged, TableDiffCellChanged},
+			},
 		},
-		// case 4
+		// case 4 - all deleted
 		{
 			diff: `diff --git a/unittest.csv b/unittest.csv
 --- a/unittest.csv
@@ -81,9 +101,88 @@ func TestCSVDiff(t *testing.T) {
 @@ -1,2 +0,0 @@
 -col1,col2
 -b,c`,
-			base:  "col1,col2\nb,c",
-			head:  "",
-			cells: [][2]TableDiffCellType{{TableDiffCellDel, TableDiffCellDel}, {TableDiffCellDel, TableDiffCellDel}},
+			base: `col1,col2
+b,c`,
+			head: "",
+			cells: [][]TableDiffCellType{
+				{TableDiffCellDel, TableDiffCellDel},
+				{TableDiffCellDel, TableDiffCellDel},
+			},
+		},
+		// case 5 - renames first column
+		{
+			diff: `diff --git a/unittest.csv b/unittest.csv
+--- a/unittest.csv
++++ b/unittest.csv
+@@ -1,3 +1,3 @@
+-col1,col2,col3
++cola,col2,col3
+ a,b,c`,
+			base: `col1,col2,col3
+a,b,c`,
+			head: `cola,col2,col3
+a,b,c`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellDel, TableDiffCellAdd, TableDiffCellUnchanged, TableDiffCellUnchanged},
+				{TableDiffCellDel, TableDiffCellAdd, TableDiffCellUnchanged, TableDiffCellUnchanged},
+			},
+		},
+		// case 6 - inserts a column after first, deletes last column
+		{
+			diff: `diff --git a/unittest.csv b/unittest.csv
+--- a/unittest.csv
++++ b/unittest.csv
+@@ -1,2 +1,2 @@
+-col1,col2,col3
+-a,b,c
++col1,col1a,col2
++a,d,b`,
+			base: `col1,col2,col3
+a,b,c`,
+			head: `col1,col1a,col2
+a,d,b`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellUnchanged, TableDiffCellAdd, TableDiffCellDel, TableDiffCellMovedUnchanged},
+				{TableDiffCellUnchanged, TableDiffCellAdd, TableDiffCellDel, TableDiffCellMovedUnchanged},
+			},
+		},
+		// case 7 - deletes first column, inserts column after last
+		{
+			diff: `diff --git a/unittest.csv b/unittest.csv
+--- a/unittest.csv
++++ b/unittest.csv
+@@ -1,2 +1,2 @@
+-col1,col2,col3
+-a,b,c
++col2,col3,col4
++b,c,d`,
+			base: `col1,col2,col3
+a,b,c`,
+			head: `col2,col3,col4
+b,c,d`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellDel, TableDiffCellUnchanged, TableDiffCellUnchanged, TableDiffCellAdd},
+				{TableDiffCellDel, TableDiffCellUnchanged, TableDiffCellUnchanged, TableDiffCellAdd},
+			},
+		},
+		// case 8 - two columns deleted, 2 added
+		{
+			diff: `diff --git a/unittest.csv b/unittest.csv
+--- a/unittest.csv
++++ b/unittest.csv
+@@ -1,2 +1,2 @@
+-col1,col2,col
+-a,b,c
++col3,col4,col5
++c,d,e`,
+			base: `col1,col2,col3
+a,b,c`,
+			head: `col3,col4,col5
+c,d,e`,
+			cells: [][]TableDiffCellType{
+				{TableDiffCellDel, TableDiffCellMovedUnchanged, TableDiffCellDel, TableDiffCellAdd, TableDiffCellAdd},
+				{TableDiffCellDel, TableDiffCellMovedUnchanged, TableDiffCellDel, TableDiffCellAdd, TableDiffCellAdd},
+			},
 		},
 	}
 
@@ -116,7 +215,7 @@ func TestCSVDiff(t *testing.T) {
 		assert.Len(t, section.Rows, len(c.cells), "case %d: should be %d rows", n, len(c.cells))
 
 		for i, row := range section.Rows {
-			assert.Len(t, row.Cells, 2, "case %d: row %d should have two cells", n, i)
+			assert.Len(t, row.Cells, len(c.cells[i]), "case %d: row %d should have %d cells", n, i, len(c.cells[i]))
 			for j, cell := range row.Cells {
 				assert.Equal(t, c.cells[i][j], cell.Type, "case %d: row %d cell %d should be equal", n, i, j)
 			}