ref #65: new formula function PRICE

- fix COUPPCD result accuracy issue
- update close spreadsheet example in documentation and README

11 files changed, 178 insertions, 54 deletions

diff --git a/README.md b/README.md
index 6b87469..1245c62 100644
--- a/README.md
+++ b/README.md
@@ -77,6 +77,12 @@ func main() {
         fmt.Println(err)
         return
     }
+    defer func() {
+        // Close the spreadsheet.
+        if err := f.Close(); err != nil {
+            fmt.Println(err)
+        }
+    }()
     // Get value from cell by given worksheet name and axis.
     cell, err := f.GetCellValue("Sheet1", "B2")
     if err != nil {
@@ -96,10 +102,6 @@ func main() {
         }
         fmt.Println()
     }
-    // Close the spreadsheet.
-    if err = f.Close(); err != nil {
-        fmt.Println(err)
-    }
 }
 ```
 
@@ -184,6 +186,12 @@ func main() {
         fmt.Println(err)
         return
     }
+    defer func() {
+        // Close the spreadsheet.
+        if err := f.Close(); err != nil {
+            fmt.Println(err)
+        }
+    }()
     // Insert a picture.
     if err := f.AddPicture("Sheet1", "A2", "image.png", ""); err != nil {
         fmt.Println(err)
@@ -207,10 +215,6 @@ func main() {
     if err = f.Save(); err != nil {
         fmt.Println(err)
     }
-    // Close the spreadsheet.
-    if err = f.Close(); err != nil {
-        fmt.Println(err)
-    }
 }
 ```
 
diff --git a/README_zh.md b/README_zh.md
index 3b90eec..b946bb3 100644
--- a/README_zh.md
+++ b/README_zh.md
@@ -77,6 +77,12 @@ func main() {
         fmt.Println(err)
         return
     }
+    defer func() {
+        // 关闭工作簿
+        if err := f.Close(); err != nil {
+            fmt.Println(err)
+        }
+    }()
     // 获取工作表中指定单元格的值
     cell, err := f.GetCellValue("Sheet1", "B2")
     if err != nil {
@@ -96,10 +102,6 @@ func main() {
         }
         fmt.Println()
     }
-    // 关闭工作簿
-    if err = f.Close(); err != nil {
-        fmt.Println(err)
-    }
 }
 ```
 
@@ -184,6 +186,12 @@ func main() {
         fmt.Println(err)
         return
     }
+    defer func() {
+        // 关闭工作簿
+        if err := f.Close(); err != nil {
+            fmt.Println(err)
+        }
+    }()
     // 插入图片
     if err := f.AddPicture("Sheet1", "A2", "image.png", ""); err != nil {
         fmt.Println(err)
@@ -207,10 +215,6 @@ func main() {
     if err = f.Save(); err != nil {
         fmt.Println(err)
     }
-    // 关闭工作簿
-    if err = f.Close(); err != nil {
-        fmt.Println(err)
-    }
 }
 ```
 
diff --git a/calc.go b/calc.go
index 59131d8..1357ad4 100644
--- a/calc.go
+++ b/calc.go
@@ -516,6 +516,7 @@ type formulaFuncs struct {
 //    POISSON
 //    POWER
 //    PPMT
+//    PRICE
 //    PRICEDISC
 //    PRICEMAT
 //    PRODUCT
@@ -9723,6 +9724,40 @@ func (fn *formulaFuncs) COUPDAYSNC(argsList *list.List) formulaArg {
 	return newNumberFormulaArg(coupdays(settlement, ncd, basis))
 }
 
+// coupons is an implementation of the formula function COUPNCD and COUPPCD.
+func (fn *formulaFuncs) coupons(name string, arg formulaArg) formulaArg {
+	settlement := timeFromExcelTime(arg.List[0].Number, false)
+	maturity := timeFromExcelTime(arg.List[1].Number, false)
+	maturityDays := (maturity.Year()-settlement.Year())*12 + (int(maturity.Month()) - int(settlement.Month()))
+	coupon := 12 / int(arg.List[2].Number)
+	mod := maturityDays % coupon
+	year := settlement.Year()
+	month := int(settlement.Month())
+	if mod == 0 && settlement.Day() >= maturity.Day() {
+		month += coupon
+	} else {
+		month += mod
+	}
+	if name != "COUPNCD" {
+		month -= coupon
+	}
+	if month > 11 {
+		year += 1
+		month -= 12
+	} else if month < 0 {
+		year -= 1
+		month += 12
+	}
+	day, lastDay := maturity.Day(), time.Date(year, time.Month(month), 1, 0, 0, 0, 0, time.UTC)
+	days := getDaysInMonth(lastDay.Year(), int(lastDay.Month()))
+	if getDaysInMonth(maturity.Year(), int(maturity.Month())) == maturity.Day() {
+		day = days
+	} else if day > 27 && day > days {
+		day = days
+	}
+	return newNumberFormulaArg(daysBetween(excelMinTime1900.Unix(), makeDate(year, time.Month(month), day)) + 1)
+}
+
 // COUPNCD function calculates the number of coupons payable, between a
 // security's settlement date and maturity date, rounded up to the nearest
 // whole coupon. The syntax of the function is:
@@ -9734,16 +9769,7 @@ func (fn *formulaFuncs) COUPNCD(argsList *list.List) formulaArg {
 	if args.Type != ArgList {
 		return args
 	}
-	settlement := timeFromExcelTime(args.List[0].Number, false)
-	maturity := timeFromExcelTime(args.List[1].Number, false)
-	ncd := time.Date(settlement.Year(), maturity.Month(), maturity.Day(), 0, 0, 0, 0, time.UTC)
-	if ncd.After(settlement) {
-		ncd = ncd.AddDate(-1, 0, 0)
-	}
-	for !ncd.After(settlement) {
-		ncd = ncd.AddDate(0, 12/int(args.List[2].Number), 0)
-	}
-	return newNumberFormulaArg(daysBetween(excelMinTime1900.Unix(), makeDate(ncd.Year(), ncd.Month(), ncd.Day())) + 1)
+	return fn.coupons("COUPNCD", args)
 }
 
 // COUPNUM function calculates the number of coupons payable, between a
@@ -9773,18 +9799,7 @@ func (fn *formulaFuncs) COUPPCD(argsList *list.List) formulaArg {
 	if args.Type != ArgList {
 		return args
 	}
-	settlement := timeFromExcelTime(args.List[0].Number, false)
-	maturity := timeFromExcelTime(args.List[1].Number, false)
-	date, years := maturity, settlement.Year()-maturity.Year()
-	date = date.AddDate(years, 0, 0)
-	if settlement.After(date) {
-		date = date.AddDate(1, 0, 0)
-	}
-	month := -12 / args.List[2].Number
-	for date.After(settlement) {
-		date = date.AddDate(0, int(month), 0)
-	}
-	return newNumberFormulaArg(daysBetween(excelMinTime1900.Unix(), makeDate(date.Year(), date.Month(), date.Day())) + 1)
+	return fn.coupons("COUPPCD", args)
 }
 
 // CUMIPMT function calculates the cumulative interest paid on a loan or
@@ -10643,6 +10658,81 @@ func (fn *formulaFuncs) PPMT(argsList *list.List) formulaArg {
 	return fn.ipmt("PPMT", argsList)
 }
 
+// price is an implementation of the formula function PRICE.
+func (fn *formulaFuncs) price(settlement, maturity, rate, yld, redemption, frequency, basis formulaArg) formulaArg {
+	if basis.Number < 0 || basis.Number > 4 {
+		return newErrorFormulaArg(formulaErrorNUM, "invalid basis")
+	}
+	argsList := list.New().Init()
+	argsList.PushBack(settlement)
+	argsList.PushBack(maturity)
+	argsList.PushBack(frequency)
+	argsList.PushBack(basis)
+	e := fn.COUPDAYS(argsList)
+	dsc := fn.COUPDAYSNC(argsList).Number / e.Number
+	n := fn.COUPNUM(argsList)
+	a := fn.COUPDAYBS(argsList)
+	ret := redemption.Number / math.Pow(1+yld.Number/frequency.Number, n.Number-1+dsc)
+	ret -= 100 * rate.Number / frequency.Number * a.Number / e.Number
+	t1 := 100 * rate.Number / frequency.Number
+	t2 := 1 + yld.Number/frequency.Number
+	for k := 0.0; k < n.Number; k++ {
+		ret += t1 / math.Pow(t2, k+dsc)
+	}
+	return newNumberFormulaArg(ret)
+}
+
+// PRICE function calculates the price, per $100 face value of a security that
+// pays periodic interest. The syntax of the function is:
+//
+//    PRICE(settlement,maturity,rate,yld,redemption,frequency,[basis])
+//
+func (fn *formulaFuncs) PRICE(argsList *list.List) formulaArg {
+	if argsList.Len() != 6 && argsList.Len() != 7 {
+		return newErrorFormulaArg(formulaErrorVALUE, "PRICE requires 6 or 7 arguments")
+	}
+	args := fn.prepareDataValueArgs(2, argsList)
+	if args.Type != ArgList {
+		return args
+	}
+	settlement, maturity := args.List[0], args.List[1]
+	rate := argsList.Front().Next().Next().Value.(formulaArg).ToNumber()
+	if rate.Type != ArgNumber {
+		return rate
+	}
+	if rate.Number < 0 {
+		return newErrorFormulaArg(formulaErrorNUM, "PRICE requires rate >= 0")
+	}
+	yld := argsList.Front().Next().Next().Next().Value.(formulaArg).ToNumber()
+	if yld.Type != ArgNumber {
+		return yld
+	}
+	if yld.Number < 0 {
+		return newErrorFormulaArg(formulaErrorNUM, "PRICE requires yld >= 0")
+	}
+	redemption := argsList.Front().Next().Next().Next().Next().Value.(formulaArg).ToNumber()
+	if redemption.Type != ArgNumber {
+		return redemption
+	}
+	if redemption.Number <= 0 {
+		return newErrorFormulaArg(formulaErrorNUM, "PRICE requires redemption > 0")
+	}
+	frequency := argsList.Front().Next().Next().Next().Next().Next().Value.(formulaArg).ToNumber()
+	if frequency.Type != ArgNumber {
+		return frequency
+	}
+	if !validateFrequency(frequency.Number) {
+		return newErrorFormulaArg(formulaErrorNUM, formulaErrorNUM)
+	}
+	basis := newNumberFormulaArg(0)
+	if argsList.Len() == 7 {
+		if basis = argsList.Back().Value.(formulaArg).ToNumber(); basis.Type != ArgNumber {
+			return newErrorFormulaArg(formulaErrorNUM, formulaErrorNUM)
+		}
+	}
+	return fn.price(settlement, maturity, rate, yld, redemption, frequency, basis)
+}
+
 // PRICEDISC function calculates the price, per $100 face value of a
 // discounted security. The syntax of the function is:
 //
diff --git a/calc_test.go b/calc_test.go
index 8402ab1..894b154 100644
--- a/calc_test.go
+++ b/calc_test.go
@@ -1429,10 +1429,13 @@ func TestCalcCellValue(t *testing.T) {
 		"=COUPDAYS(\"01/01/2011\",\"10/25/2012\",4,1)": "92",
 		// COUPDAYSNC
 		"=COUPDAYSNC(\"01/01/2011\",\"10/25/2012\",4)": "24",
+		"=COUPDAYSNC(\"04/01/2012\",\"03/31/2020\",2)": "179",
 		// COUPNCD
 		"=COUPNCD(\"01/01/2011\",\"10/25/2012\",4)":   "40568",
 		"=COUPNCD(\"01/01/2011\",\"10/25/2012\",4,0)": "40568",
 		"=COUPNCD(\"10/25/2011\",\"01/01/2012\",4)":   "40909",
+		"=COUPNCD(\"04/01/2012\",\"03/31/2020\",2)":   "41182",
+		"=COUPNCD(\"01/01/2000\",\"08/30/2001\",2)":   "36585",
 		// COUPNUM
 		"=COUPNUM(\"01/01/2011\",\"10/25/2012\",4)":   "8",
 		"=COUPNUM(\"01/01/2011\",\"10/25/2012\",4,0)": "8",
@@ -1497,6 +1500,10 @@ func TestCalcCellValue(t *testing.T) {
 		// PMT
 		"=PMT(0,8,0,5000,1)":       "-625",
 		"=PMT(0.035/4,8,0,5000,1)": "-600.8520271804658",
+		// PRICE
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,100,2)":   "110.65510517844305",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,100,2,4)": "110.65510517844305",
+		"=PRICE(\"04/01/2012\",\"03/31/2020\",12%,10%,100,2)":   "110.83448359321572",
 		// PPMT
 		"=PPMT(0.05/12,2,60,50000)":   "-738.2918003208238",
 		"=PPMT(0.035/4,2,8,0,5000,1)": "-606.1094824182949",
@@ -2951,6 +2958,20 @@ func TestCalcCellValue(t *testing.T) {
 		"=PMT(0,0,\"\",0,0)": "strconv.ParseFloat: parsing \"\": invalid syntax",
 		"=PMT(0,0,0,\"\",0)": "strconv.ParseFloat: parsing \"\": invalid syntax",
 		"=PMT(0,0,0,0,\"\")": "strconv.ParseFloat: parsing \"\": invalid syntax",
+		// PRICE
+		"=PRICE()": "PRICE requires 6 or 7 arguments",
+		"=PRICE(\"\",\"02/01/2020\",12%,10%,100,2,4)":              "#VALUE!",
+		"=PRICE(\"04/01/2012\",\"\",12%,10%,100,2,4)":              "#VALUE!",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",\"\",10%,100,2,4)":   "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,\"\",100,2,4)":   "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,\"\",2,4)":   "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,100,\"\",4)": "strconv.ParseFloat: parsing \"\": invalid syntax",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",-1,10%,100,2,4)":     "PRICE requires rate >= 0",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,-1,100,2,4)":     "PRICE requires yld >= 0",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,0,2,4)":      "PRICE requires redemption > 0",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,100,2,\"\")": "#NUM!",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,100,3,4)":    "#NUM!",
+		"=PRICE(\"04/01/2012\",\"02/01/2020\",12%,10%,100,2,5)":    "invalid basis",
 		// PPMT
 		"=PPMT()":               "PPMT requires at least 4 arguments",
 		"=PPMT(0,0,0,0,0,0,0)":  "PPMT allows at most 6 arguments",
diff --git a/cell.go b/cell.go
index 41a9517..e1c7803 100644
--- a/cell.go
+++ b/cell.go
@@ -354,7 +354,7 @@ func (f *File) SetCellStr(sheet, axis, value string) error {
 // table.
 func (f *File) setCellString(value string) (t string, v string) {
 	if len(value) > TotalCellChars {
-		value = value[0:TotalCellChars]
+		value = value[:TotalCellChars]
 	}
 	t = "s"
 	v = strconv.Itoa(f.setSharedString(value))
@@ -381,7 +381,7 @@ func (f *File) setSharedString(val string) int {
 // setCellStr provides a function to set string type to cell.
 func setCellStr(value string) (t string, v string, ns xml.Attr) {
 	if len(value) > TotalCellChars {
-		value = value[0:TotalCellChars]
+		value = value[:TotalCellChars]
 	}
 	if len(value) > 0 {
 		prefix, suffix := value[0], value[len(value)-1]
diff --git a/crypt.go b/crypt.go
index 24ac7ec..ae39bba 100644
--- a/crypt.go
+++ b/crypt.go
@@ -297,7 +297,7 @@ func encryptionMechanism(buffer []byte) (mechanism string, err error) {
 		err = ErrUnknownEncryptMechanism
 		return
 	}
-	versionMajor, versionMinor := binary.LittleEndian.Uint16(buffer[0:2]), binary.LittleEndian.Uint16(buffer[2:4])
+	versionMajor, versionMinor := binary.LittleEndian.Uint16(buffer[:2]), binary.LittleEndian.Uint16(buffer[2:4])
 	if versionMajor == 4 && versionMinor == 4 {
 		mechanism = "agile"
 		return
@@ -600,7 +600,7 @@ func createIV(blockKey interface{}, encryption Encryption) ([]byte, error) {
 		tmp := make([]byte, 0x36)
 		iv = append(iv, tmp...)
 	} else if len(iv) > encryptedKey.BlockSize {
-		iv = iv[0:encryptedKey.BlockSize]
+		iv = iv[:encryptedKey.BlockSize]
 	}
 	return iv, nil
 }
diff --git a/lib.go b/lib.go
index 535161a..ccb09ac 100644
--- a/lib.go
+++ b/lib.go
@@ -526,7 +526,7 @@ func bytesReplace(s, old, new []byte, n int) []byte {
 	}
 
 	w += copy(s[w:], s[i:])
-	return s[0:w]
+	return s[:w]
 }
 
 // genSheetPasswd provides a method to generate password for worksheet
diff --git a/picture.go b/picture.go
index 332c639..2956ff1 100644
--- a/picture.go
+++ b/picture.go
@@ -481,14 +481,20 @@ func (f *File) getSheetRelationshipsTargetByID(sheet, rID string) string {
 }
 
 // GetPicture provides a function to get picture base name and raw content
-// embed in XLSX by given worksheet and cell name. This function returns the
-// file name in XLSX and file contents as []byte data types. For example:
+// embed in spreadsheet by given worksheet and cell name. This function
+// returns the file name in spreadsheet and file contents as []byte data
+// types. For example:
 //
 //    f, err := excelize.OpenFile("Book1.xlsx")
 //    if err != nil {
 //        fmt.Println(err)
 //        return
 //    }
+//    defer func() {
+//        if err := f.Close(); err != nil {
+//            fmt.Println(err)
+//        }
+//    }()
 //    file, raw, err := f.GetPicture("Sheet1", "A2")
 //    if err != nil {
 //        fmt.Println(err)
@@ -497,9 +503,6 @@ func (f *File) getSheetRelationshipsTargetByID(sheet, rID string) string {
 //    if err := ioutil.WriteFile(file, raw, 0644); err != nil {
 //        fmt.Println(err)
 //    }
-//    if err = f.Close(); err != nil {
-//        fmt.Println(err)
-//    }
 //
 func (f *File) GetPicture(sheet, cell string) (string, []byte, error) {
 	col, row, err := CellNameToCoordinates(cell)
diff --git a/sheet.go b/sheet.go
index 5738ced..1aa378b 100644
--- a/sheet.go
+++ b/sheet.go
@@ -234,7 +234,7 @@ func trimCell(column []xlsxC) []xlsxC {
 			i++
 		}
 	}
-	return col[0:i]
+	return col[:i]
 }
 
 // setContentTypes provides a function to read and update property of contents
@@ -452,12 +452,14 @@ func (f *File) GetSheetIndex(name string) int {
 //    if err != nil {
 //        return
 //    }
+//    defer func() {
+//        if err := f.Close(); err != nil {
+//            fmt.Println(err)
+//        }
+//    }()
 //    for index, name := range f.GetSheetMap() {
 //        fmt.Println(index, name)
 //    }
-//    if err = f.Close(); err != nil {
-//        fmt.Println(err)
-//    }
 //
 func (f *File) GetSheetMap() map[int]string {
 	wb := f.workbookReader()
diff --git a/styles.go b/styles.go
index 0ae9e51..183211b 100644
--- a/styles.go
+++ b/styles.go
@@ -3137,7 +3137,7 @@ func ThemeColor(baseColor string, tint float64) string {
 	if tint == 0 {
 		return "FF" + baseColor
 	}
-	r, _ := strconv.ParseUint(baseColor[0:2], 16, 64)
+	r, _ := strconv.ParseUint(baseColor[:2], 16, 64)
 	g, _ := strconv.ParseUint(baseColor[2:4], 16, 64)
 	b, _ := strconv.ParseUint(baseColor[4:6], 16, 64)
 	var h, s, l float64
diff --git a/table.go b/table.go
index 620cf20..a6959a4 100644
--- a/table.go
+++ b/table.go
@@ -446,7 +446,7 @@ func (f *File) parseFilterExpression(expression string, tokens []string) ([]int,
 		if re {
 			conditional = 1
 		}
-		expression1, token1, err := f.parseFilterTokens(expression, tokens[0:3])
+		expression1, token1, err := f.parseFilterTokens(expression, tokens[:3])
 		if err != nil {
 			return expressions, t, err
 		}