add ch 8, easter

doc.go

@@ -86,6 +86,7 @@
 //	3. Interpolation                                     interp
 //	4. Curve Fitting                                     fit
 //	7. Julian Day                                        julian
+//	8. Date of Easter                                    easter
 //	10. Dynamical Time and Universal Time                deltat
 //	11. The Earth's Globe                                globe
 //	12. Sidereal Time at Greenwich                       sidereal

easter/easter.go

@@ -0,0 +1,33 @@
+// Copyright 2013 Sonia Keys
+// License MIT: http://www.opensource.org/licenses/MIT
+
+// Easter: Chapter 8, Date of Easter
+package easter
+
+// Gregorian returns month and day of Easter in the Gregorian calendar.
+func Gregorian(y int) (m, d int) {
+	a := y % 19
+	b, c := y/100, y%100
+	d, e := b/4, b%4
+	f := (b + 8) / 25
+	g := (b - f + 1) / 3
+	h := (19*a + b - d - g + 15) % 30
+	i, k := c/4, c%4
+	l := (32 + 2*e + 2*i - h - k) % 7
+	m = (a + 11*h + 22*l) / 451
+	n := h + l - 7*m + 114
+	n, p := n/31, n%31
+	return n, p + 1
+}
+
+// Julian returns month and day of Easter in the Julian calendar.
+func Julian(y int) (m, d int) {
+	a := y % 4
+	b := y % 7
+	c := y % 19
+	d = (19*c + 15) % 30
+	e := (2*a + 4*b - d + 34) % 7
+	f := d + e + 114
+	f, g := f/31, f%31
+	return f, g + 1
+}

easter/easter_test.go

@@ -0,0 +1,32 @@
+package easter_test
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/soniakeys/meeus/easter"
+)
+
+func ExampleGregorian() {
+	// Example values from p. 68.
+	for _, y := range []int{1991, 1992, 1993, 1954, 2000, 1818} {
+		m, d := easter.Gregorian(y)
+		fmt.Println(y, ":", time.Month(m), d)
+	}
+	// Output:
+	// 1991 : March 31
+	// 1992 : April 19
+	// 1993 : April 11
+	// 1954 : April 18
+	// 2000 : April 23
+	// 1818 : March 22
+}
+
+func ExampleJulian() {
+	// Example value from p. 69.
+	y := 1243
+	m, d := easter.Julian(y)
+	fmt.Println(y, ":", time.Month(m), d)
+	// Output:
+	// 1243 : April 12
+}

readme.md

@@ -17,6 +17,7 @@ Cross reference from chapters to package names
     <tr><td>3.  Interpolation</td><td>interp</td><td>under renovation</td></tr>
     <tr><td>4.  Curve Fitting</td><td>fit</td><td>complete</td></tr>
     <tr><td>7.  Julian Day</td><td>julian</td><td>complete</td></tr>
+    <tr><td>8.  Date of Easter</td><td>easter</td><td>complete</td></tr>
     <tr><td>10. Dynamical Time and Universal Time</td><td>deltat</td><td>complete</td></tr>
     <tr><td>11. The Earth's Globe</td><td>globe</td><td>complete</td></tr>
     <tr><td>12. Sidereal Time at Greenwich</td><td>sidereal</td><td>complete</td></tr>