Refactor date conversion logic and update tests

Replaces previous Solar Hijri conversion logic with a more accurate algorithm, introducing new helper functions and error handling. Updates Kurdish calendar test cases to match corrected conversion results and fixes a typo in Sorani month names. Adds a new main.go example for converting Gregorian dates to Kurdish calendar dates.

Author: rojcode

cmd/main.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/rojcode/kurdical"
+)
+
+func main() {
+	// Convert 24 Dec 2025 to Kurdish
+	date := time.Date(2025, 10, 11, 0, 0, 0, 0, time.UTC)
+	kurdish := kurdical.GregorianToKurdish(date, kurdical.Sorani, kurdical.MedianKingdom)
+
+	fmt.Printf("Gregorian: %s\n", date.Format("2006-01-02"))
+	fmt.Printf("Kurdish (Median Kingdom, Sorani): %d-%d-%d %s\n", kurdish.Year, kurdish.Month, kurdish.Day, kurdish.MonthName)
+
+	// Also show Fall of Nineveh epoch
+	kurdish2 := kurdical.GregorianToKurdish(date, kurdical.Kurmanji, kurdical.FallOfNineveh)
+	fmt.Printf("Kurdish (Fall of Nineveh, Kurmanji): %d-%d-%d %s\n", kurdish2.Year, kurdish2.Month, kurdish2.Day, kurdish2.MonthName)
+}

conversions.go

@@ -1,83 +1,182 @@
 package kurdical
 
-// julianDayNumber calculates the Julian Day Number for a Gregorian date.
-func julianDayNumber(year, month, day int) int {
-	a := (14 - month) / 12
-	y := year + 4800 - a
-	m := month + 12*a - 3
-	return day + (153*m+2)/5 + 365*y + y/4 - y/100 + y/400 - 32045
+import (
+	"fmt"
+	"time"
+)
+
+var (
+	breaks = [...]int{-61, 9, 38, 199, 426, 686, 756, 818, 1111, 1181, 1210,
+		1635, 2060, 2097, 2192, 2262, 2324, 2394, 2456, 3178}
+)
+
+// gregorianToSolarHijri converts Gregorian date to Solar Hijri.
+func gregorianToSolarHijri(gYear, gMonth, gDay int) (sYear, sMonth, sDay int) {
+	jy, jm, jd, err := toJalaali(gYear, time.Month(gMonth), gDay)
+	if err != nil {
+		// For invalid dates, return 0
+		return 0, 0, 0
+	}
+	return jy, int(jm), jd
 }
 
-// jdnToGregorian converts Julian Day Number to Gregorian date.
-func jdnToGregorian(jdn int) (year, month, day int) {
-	l := jdn + 68569
-	n := (4 * l) / 146097
-	l = l - (146097*n+3)/4
-	i := (4000 * (l + 1)) / 1461001
-	l = l - (1461*i)/4 + 31
-	j := (80 * l) / 2447
-	day = l - (2447*j)/80
-	l = j / 11
-	month = j + 2 - 12*l
-	year = 100*(n-49) + i + l
-	return
+// solarHijriToGregorian converts Solar Hijri date to Gregorian.
+func solarHijriToGregorian(sYear, sMonth, sDay int) (gYear, gMonth, gDay int) {
+	gy, gm, gd, err := toGregorian(sYear, Month(sMonth), sDay)
+	if err != nil {
+		return 0, 0, 0
+	}
+	return gy, int(gm), gd
 }
 
-// isSolarHijriLeap determines if a Solar Hijri year is leap.
-func isSolarHijriLeap(year int) bool {
-	return (year+1)%4 == 0
+// Month represents a month of the year.
+type Month int
+
+const (
+	Farvardin Month = 1 + iota
+	Ordibehesht
+	Khordad
+	Tir
+	Mordad
+	Shahrivar
+	Mehr
+	Aban
+	Azar
+	Dey
+	Bahman
+	Esfand
+)
+
+// toJalaali converts Gregorian to Jalaali date.
+func toJalaali(gregorianYear int, gregorianMonth time.Month, gregorianDay int) (int, Month, int, error) {
+	jy, jm, jd, err := d2j(g2d(gregorianYear, int(gregorianMonth), gregorianDay))
+	return jy, Month(jm), jd, err
 }
 
-// gregorianToSolarHijri converts Gregorian date to Solar Hijri.
-func gregorianToSolarHijri(gYear, gMonth, gDay int) (sYear, sMonth, sDay int) {
-	gJDN := julianDayNumber(gYear, gMonth, gDay)
-	sJDN := gJDN - 1948087
-	sYear = 1
-	for {
-		daysInYear := 365
-		if isSolarHijriLeap(sYear) {
-			daysInYear = 366
+// toGregorian converts Jalaali to Gregorian date.
+func toGregorian(jalaaliYear int, jalaaliMonth Month, jalaaliDay int) (int, time.Month, int, error) {
+	jdn, err := j2d(jalaaliYear, int(jalaaliMonth), jalaaliDay)
+	if err != nil {
+		return 0, 0, 0, err
+	}
+	gy, gm, gd := d2g(jdn)
+	return gy, time.Month(gm), gd, nil
+}
+
+func j2d(jy, jm, jd int) (jdn int, err error) {
+	_, gy, march, err := jalCal(jy)
+	if err != nil {
+		return 0, err
+	}
+	return g2d(gy, 3, march) + (jm-1)*31 - div(jm, 7)*(jm-7) + jd - 1, nil
+}
+
+func d2j(jdn int) (int, int, int, error) {
+	gy, _, _ := d2g(jdn)
+	jy := gy - 621
+	leap, _, march, err := jalCal(jy)
+	jdn1f := g2d(gy, 3, march)
+
+	if err != nil {
+		return 0, 0, 0, err
+	}
+
+	k := jdn - jdn1f
+	if k >= 0 {
+		if k <= 185 {
+			jm := 1 + div(k, 31)
+			jd := mod(k, 31) + 1
+			return jy, jm, jd, nil
 		}
-		if sJDN <= daysInYear {
-			break
+		k -= 186
+	} else {
+		jy--
+		k += 179
+		if leap == 1 {
+			k++
 		}
-		sJDN -= daysInYear
-		sYear++
 	}
-	monthDays := []int{31, 31, 31, 31, 31, 31, 30, 30, 30, 30, 30, 29}
-	if isSolarHijriLeap(sYear) {
-		monthDays[11] = 30
+	jm := 7 + div(k, 30)
+	jd := mod(k, 30) + 1
+	return jy, jm, jd, nil
+}
+
+func jalCal(jy int) (int, int, int, error) {
+	bl, gy, leapJ, jp := len(breaks), jy+621, -14, breaks[0]
+	jump := 0
+
+	if jy < jp || jy >= breaks[bl-1] {
+		return 0, 0, 0, &ErrorInvalidYear{jy}
 	}
-	sMonth = 1
-	for sMonth <= 12 {
-		if sJDN <= monthDays[sMonth-1] {
-			sDay = sJDN
+
+	for i := 1; i < bl; i++ {
+		jm := breaks[i]
+		jump = jm - jp
+		if jy < jm {
 			break
 		}
-		sJDN -= monthDays[sMonth-1]
-		sMonth++
+		leapJ += div(jump, 33)*8 + div(mod(jump, 33), 4)
+		jp = jm
 	}
-	return
-}
+	n := jy - jp
 
-// solarHijriToGregorian converts Solar Hijri date to Gregorian.
-func solarHijriToGregorian(sYear, sMonth, sDay int) (gYear, gMonth, gDay int) {
-	days := 0
-	for y := 1; y < sYear; y++ {
-		days += 365
-		if isSolarHijriLeap(y) {
-			days++
-		}
+	leapJ += div(n, 33)*8 + div(mod(n, 33)+3, 4)
+	if mod(jump, 33) == 4 && jump-n == 4 {
+		leapJ++
 	}
-	monthDays := []int{31, 31, 31, 31, 31, 31, 30, 30, 30, 30, 30, 29}
-	if isSolarHijriLeap(sYear) {
-		monthDays[11] = 30
+
+	leapG := div(gy, 4) - div((div(gy, 100)+1)*3, 4) - 150
+
+	march := 20 + leapJ - leapG
+
+	if jump-n < 6 {
+		n -= jump + div(jump+4, 33)*33
 	}
-	for m := 1; m < sMonth; m++ {
-		days += monthDays[m-1]
+	leap := mod(mod(n+1, 33)-1, 4)
+	if leap == -1 {
+		leap = 4
 	}
-	days += sDay - 1
-	jdn := 1948087 + days
-	gYear, gMonth, gDay = jdnToGregorian(jdn)
-	return
+
+	return leap, gy, march, nil
+}
+
+func g2d(gy, gm, gd int) int {
+	d := div((gy+div(gm-8, 6)+100100)*1461, 4) +
+		div(153*mod(gm+9, 12)+2, 5) +
+		gd - 34840408
+	d = d - div(div(gy+100100+div(gm-8, 6), 100)*3, 4) + 752
+	return d
+}
+
+func d2g(jdn int) (int, int, int) {
+	j := 4*jdn + 139361631
+	j = j + div(div(4*jdn+183187720, 146097)*3, 4)*4 - 3908
+	i := div(mod(j, 1461), 4)*5 + 308
+	gd := div(mod(i, 153), 5) + 1
+	gm := mod(div(i, 153), 12) + 1
+	gy := div(j, 1461) - 100100 + div(8-gm, 6)
+	return gy, gm, gd
+}
+
+func div(a, b int) int {
+	return a / b
+}
+
+func mod(a, b int) int {
+	return a % b
+}
+
+// ErrorInvalidYear represents an error for invalid year.
+type ErrorInvalidYear struct {
+	Year int
+}
+
+func (e *ErrorInvalidYear) Error() string {
+	return fmt.Sprintf("invalid year: %d", e.Year)
+}
+
+// isSolarHijriLeap determines if a Solar Hijri year is leap.
+func isSolarHijriLeap(year int) bool {
+	leap, _, _, err := jalCal(year)
+	return err == nil && leap == 1
 }

dialects.go

@@ -49,7 +49,7 @@ var monthNames = map[Dialect][]string{
 		"گولان",
 		"زه‌ردان",
 		"په‌رپه‌ر",
-		"گ‌لاویژ",
+		"گەلاویژ",
 		"نوخشان",
 		"به‌ران",
 		"خه‌زان",