spreadsheet: support inserting rows within a sheet

Fixes unidoc#203

_examples/spreadsheet/insert-rows/main.go

@@ -0,0 +1,36 @@
+// Copyright 2017 Baliance. All rights reserved.
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"baliance.com/gooxml/spreadsheet"
+)
+
+func main() {
+	ss := spreadsheet.New()
+	// add a single sheet
+	sheet := ss.AddSheet()
+
+	// rows
+	for r := 0; r < 5; r++ {
+		row := sheet.AddRow()
+		// and cells
+		for c := 0; c < 5; c++ {
+			cell := row.AddCell()
+			cell.SetString(fmt.Sprintf("row %d cell %d", r, c))
+		}
+	}
+
+	// no insert some rows after row 2
+	for i := 0; i < 4; i++ {
+		sheet.InsertRow(2).AddCell().SetString(fmt.Sprintf("inserted at 2, iter %d", i))
+	}
+
+	if err := ss.Validate(); err != nil {
+		log.Fatalf("error validating sheet: %s", err)
+	}
+
+	ss.SaveToFile("insert-rows.xlsx")
+}

spreadsheet/sheet.go

@@ -116,6 +116,30 @@ func (s Sheet) AddRow() Row {
 	return s.AddNumberedRow(maxRowID + 1)
 }
 
+// InsertRow inserts a new row into a spreadsheet at a particular row number.  This
+// row will now be the row number specified, and any rows after it will be renumbed.
+func (s Sheet) InsertRow(rowNum int) Row {
+	rIdx := uint32(rowNum)
+
+	// Renumber every row after the row we're inserting
+	for _, r := range s.Rows() {
+		if r.x.RAttr != nil && *r.x.RAttr >= rIdx {
+			*r.x.RAttr++
+			// and we also need to recompute cell references
+			for _, c := range r.Cells() {
+				cref, err := reference.ParseCellReference(c.Reference())
+				if err != nil {
+					continue
+				}
+				cref.RowIdx++
+				c.x.RAttr = gooxml.String(cref.String())
+			}
+		}
+	}
+	// finally AddNumberedRow will add and re-sort rows
+	return s.AddNumberedRow(rIdx)
+}
+
 // Name returns the sheet name
 func (s Sheet) Name() string {
 	return s.cts.NameAttr