Adopt idiomatic Go style without breaking the public API

anniversary.go

@@ -18,38 +18,35 @@ package hdate
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
 import (
-	"errors"
-	"strconv"
+	"fmt"
 )
 
-/*
-GetYahrzeit calculates yahrzeit.
-hyear must be after original date of death.
-Returns an error when requested year precedes or is same as original year.
-
-Hebcal uses the algorithm defined in "Calendrical Calculations"
-by Edward M. Reingold and Nachum Dershowitz.
-
-The customary anniversary date of a death is more complicated and depends
-also on the character of the year in which the first anniversary occurs.
-There are several cases:
-  - If the date of death is Marcheshvan 30, the anniversary in general depends
-    on the first anniversary; if that first anniversary was not Marcheshvan 30,
-    use the day before Kislev 1.
-  - If the date of death is Kislev 30, the anniversary in general again depends
-    on the first anniversary — if that was not Kislev 30, use the day before
-    Tevet 1.
-  - If the date of death is Adar II, the anniversary is the same day in the
-    last month of the Hebrew year (Adar or Adar II).
-  - If the date of death is Adar I 30, the anniversary in a Hebrew year that
-    is not a leap year (in which Adar only has 29 days) is the last day in
-    Shevat.
-  - In all other cases, use the normal (that is, same month number) anniversary
-    of the date of death. [Calendrical Calculations p. 113]
-*/
+// GetYahrzeit calculates yahrzeit.
+// hyear must be after original date of death.
+// Returns an error when requested year precedes or is same as original year.
+//
+// Hebcal uses the algorithm defined in "Calendrical Calculations"
+// by Edward M. Reingold and Nachum Dershowitz.
+//
+// The customary anniversary date of a death is more complicated and depends
+// also on the character of the year in which the first anniversary occurs.
+// There are several cases:
+//   - If the date of death is Marcheshvan 30, the anniversary in general depends
+//     on the first anniversary; if that first anniversary was not Marcheshvan 30,
+//     use the day before Kislev 1.
+//   - If the date of death is Kislev 30, the anniversary in general again depends
+//     on the first anniversary — if that was not Kislev 30, use the day before
+//     Tevet 1.
+//   - If the date of death is Adar II, the anniversary is the same day in the
+//     last month of the Hebrew year (Adar or Adar II).
+//   - If the date of death is Adar I 30, the anniversary in a Hebrew year that
+//     is not a leap year (in which Adar only has 29 days) is the last day in
+//     Shevat.
+//   - In all other cases, use the normal (that is, same month number) anniversary
+//     of the date of death. [Calendrical Calculations p. 113]
 func GetYahrzeit(hyear int, date HDate) (HDate, error) {
 	if hyear <= date.Year() {
-		return HDate{}, errors.New("year " + strconv.Itoa(hyear) + " occurs on or before original date")
+		return HDate{}, fmt.Errorf("year %d occurs on or before original date", hyear)
 	}
 
 	if date.Month() == Cheshvan && date.Day() == 30 && !LongCheshvan(date.Year()+1) {
@@ -83,30 +80,29 @@ func GetYahrzeit(hyear int, date HDate) (HDate, error) {
 	return New(hyear, date.Month(), date.Day()), nil
 }
 
-/*
-GetBirthdayOrAnniversary calculates a birthday or anniversary (non-yahrzeit).
-hyear must be after original date of anniversary.
-Returns an error when requested year precedes or is same as original year.
-
-Hebcal uses the algorithm defined in "Calendrical Calculations"
-by Edward M. Reingold and Nachum Dershowitz.
-
-The birthday of someone born in Adar of an ordinary year or Adar II of
-a leap year is also always in the last month of the year, be that Adar
-or Adar II. The birthday in an ordinary year of someone born during the
-first 29 days of Adar I in a leap year is on the corresponding day of Adar;
-in a leap year, the birthday occurs in Adar I, as expected.
-
-Someone born on the thirtieth day of Marcheshvan, Kislev, or Adar I
-has his birthday postponed until the first of the following month in
-years where that day does not occur. [Calendrical Calculations p. 111]
-*/
+// GetBirthdayOrAnniversary calculates a birthday or anniversary (non-yahrzeit).
+// hyear must be after original date of anniversary.
+// Returns an error when requested year precedes or is same as original year.
+//
+// Hebcal uses the algorithm defined in "Calendrical Calculations"
+// by Edward M. Reingold and Nachum Dershowitz.
+//
+// The birthday of someone born in Adar of an ordinary year or Adar II of
+// a leap year is also always in the last month of the year, be that Adar
+// or Adar II. The birthday in an ordinary year of someone born during the
+// first 29 days of Adar I in a leap year is on the corresponding day of Adar;
+// in a leap year, the birthday occurs in Adar I, as expected.
+//
+// Someone born on the thirtieth day of Marcheshvan, Kislev, or Adar I
+// has his birthday postponed until the first of the following month in
+// years where that day does not occur. [Calendrical Calculations p. 111]
 func GetBirthdayOrAnniversary(hyear int, date HDate) (HDate, error) {
 	origYear := date.Year()
 	if hyear == origYear {
 		return date, nil
-	} else if hyear < origYear {
-		return HDate{}, errors.New("year " + strconv.Itoa(hyear) + " occurs before original date")
+	}
+	if hyear < origYear {
+		return HDate{}, fmt.Errorf("year %d occurs before original date", hyear)
 	}
 	isOrigLeap := IsLeapYear(origYear)
 	month := date.Month()