Using binary search instead of sequential search for cell (qax-os#2129)

- Update unit tests

Author: shcabin

cell.go

@@ -17,6 +17,7 @@ import (
 	"math"
 	"os"
 	"reflect"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -1498,23 +1499,30 @@ func (f *File) getCellStringFunc(sheet, cell string, fn func(x *xlsxWorksheet, c
 		return "", nil
 	}
 
-	for rowIdx := range ws.SheetData.Row {
-		rowData := &ws.SheetData.Row[rowIdx]
-		if rowData.R != row {
-			continue
+	idx, found := sort.Find(len(ws.SheetData.Row), func(i int) int {
+		if ws.SheetData.Row[i].R == row {
+			return 0
 		}
-		for colIdx := range rowData.C {
-			colData := &rowData.C[colIdx]
-			if cell != colData.R {
-				continue
-			}
+		if ws.SheetData.Row[i].R > row {
+			return -1
+		}
+		return 1
+	})
+	if !found {
+		return "", nil
+	}
+	rowData := ws.SheetData.Row[idx]
+	for colIdx := range rowData.C {
+		colData := &rowData.C[colIdx]
+		if cell == colData.R {
 			val, ok, err := fn(ws, colData)
 			if err != nil {
 				return "", err
 			}
 			if ok {
 				return val, nil
 			}
+			break
 		}
 	}
 	return "", nil

cell_test.go

@@ -1265,3 +1265,14 @@ func TestSetCellIntFunc(t *testing.T) {
 func TestSIString(t *testing.T) {
 	assert.Empty(t, xlsxSI{}.String())
 }
+
+func TestGetCellStringFunc(t *testing.T) {
+	f := NewFile()
+	ws, ok := f.Sheet.Load("xl/worksheets/sheet1.xml")
+	assert.True(t, ok)
+	ws.(*xlsxWorksheet).SheetData.Row = []xlsxRow{{R: 2}}
+	val, err := f.GetCellValue("Sheet1", "A1")
+	assert.Empty(t, val)
+	assert.NoError(t, err)
+	assert.NoError(t, f.Close())
+}