From 10450963cb238d2289c4626dc393b8655d622d91 Mon Sep 17 00:00:00 2001
From: Federico Hernandez <ultrafredde@gmail.com>
Date: Tue, 23 Mar 2010 02:13:35 +0100
Subject: [PATCH] Eeaster algorithm

---
 src/Date.cpp         | 26 ++++++++++++++++++++++++++
 src/Date.h           |  2 ++
 src/tests/date.t.cpp | 20 +++++++++++++++++++-
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/src/Date.cpp b/src/Date.cpp
index 8a010f28d..4482bfc63 100644
--- a/src/Date.cpp
+++ b/src/Date.cpp
@@ -964,3 +964,29 @@ bool Date::isRelativeDate (const std::string& input)
 }
 
 ////////////////////////////////////////////////////////////////////////////////
+time_t easter (int year)
+{
+  int Y = year;
+  int a = Y % 19;
+  int b = Y / 100;
+  int c = Y % 100;
+  int d = b / 4;
+  int e = b % 4;
+  int f = (b + 8) / 25;
+  int g = (b - f + 1) / 3;
+  int h = (19 * a + b - d - g + 15) % 30;
+  int i = c / 4;
+  int k = c % 4;
+  int L = (32 + 2 * e + 2 * i - h - k) % 7;
+  int m = (a + 11 * h + 22 * L) / 451;
+  int month = (h + L - 7 * m + 114) / 31;
+  int day = ((h + L - 7 * m + 114) % 31) + 1;
+  struct tm t = {0};
+  t.tm_isdst = -1;   // Requests that mktime determine summer time effect.
+  t.tm_mday  = day;
+  t.tm_mon   = month - 1;
+  t.tm_year  = year - 1900;
+  return mktime (&t);
+}
+
+////////////////////////////////////////////////////////////////////////////////
diff --git a/src/Date.h b/src/Date.h
index b5c5f2822..72bb9f1c8 100644
--- a/src/Date.h
+++ b/src/Date.h
@@ -30,6 +30,8 @@
 #include <stdio.h>
 #include <string>
 
+time_t easter (int year);
+
 class Date;
 
 class Date
diff --git a/src/tests/date.t.cpp b/src/tests/date.t.cpp
index 9898be661..671b3c567 100644
--- a/src/tests/date.t.cpp
+++ b/src/tests/date.t.cpp
@@ -34,7 +34,7 @@ Context context;
 ////////////////////////////////////////////////////////////////////////////////
 int main (int argc, char** argv)
 {
-  UnitTest t (135);
+  UnitTest t (143);
 
   try
   {
@@ -225,6 +225,24 @@ int main (int argc, char** argv)
     t.is (fromString13.minute (),   34, "ctor (std::string) -> N");
     t.is (fromString13.second (),   56, "ctor (std::string) -> S");
 
+    // Easter
+    Date e1 (easter(1980));
+    t.is (e1.toString (), "4/6/1980", "Easter 4/6/1980");
+    Date e2 (easter(1995));
+    t.is (e2.toString (), "4/16/1995", "Easter 4/16/1995");
+    Date e3 (easter(2000));
+    t.is (e3.toString (), "4/23/2000", "Easter 4/23/2000");
+    Date e4 (easter(2009));
+    t.is (e4.toString (), "4/12/2009", "Easter 4/12/2009");
+    Date e5 (easter(2010));
+    t.is (e5.toString (), "4/4/2010", "Easter 4/4/2010");
+    Date e6 (easter(2011));
+    t.is (e6.toString (), "4/24/2011", "Easter 4/24/2011");
+    Date e7 (easter(2012));
+    t.is (e7.toString (), "4/8/2012", "Easter 4/8/2012");
+    Date e8 (easter(2020));
+    t.is (e8.toString (), "4/12/2020", "Easter 4/12/2020");
+
     // Relative dates.
     Date r1 ("today");
     t.ok (r1.sameDay (now), "today = now");