Add event dates to time_season: easter, halloween, xmas

Add easter, halloween and xmas to `enum ETimeSeason` and `time_season` function. This unifies the logic for detecting these events and seasons in one function. This allows adding support for special easter, halloween and xmas theme maps in the future. The `localtime` function is not used outside `system.c` anymore. The code for detecting easter is taken from upstream.

src/base/system.cpp

@@ -2669,18 +2669,58 @@ int time_houroftheday()
 	return time_info->tm_hour;
 }
 
-int time_season()
+static bool time_iseasterday(time_t time_data, struct tm *time_info)
+{
+	// compute Easter day (Sunday) using https://en.wikipedia.org/w/index.php?title=Computus&oldid=890710285#Anonymous_Gregorian_algorithm
+	int Y = time_info->tm_year + 1900;
+	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;
+
+	// (now-1d ≤ easter ≤ now+2d) <=> (easter-2d ≤ now ≤ easter+1d) <=> (Good Friday ≤ now ≤ Easter Monday)
+	for(int day_offset = -1; day_offset <= 2; day_offset++)
+	{
+		time_data = time_data + day_offset * 60 * 60 * 24;
+		time_info = localtime(&time_data);
+		if(time_info->tm_mon == month - 1 && time_info->tm_mday == day)
+			return true;
+	}
+	return false;
+}
+
+ETimeSeason time_season()
 {
 	time_t time_data;
-	struct tm *time_info;
-
 	time(&time_data);
-	time_info = localtime(&time_data);
+	struct tm *time_info = localtime(&time_data);
 
 	if((time_info->tm_mon == 11 && time_info->tm_mday == 31) || (time_info->tm_mon == 0 && time_info->tm_mday == 1))
 	{
 		return SEASON_NEWYEAR;
 	}
+	else if(time_info->tm_mon == 11 && time_info->tm_mday >= 24 && time_info->tm_mday <= 26)
+	{
+		return SEASON_XMAS;
+	}
+	else if((time_info->tm_mon == 9 && time_info->tm_mday == 31) || (time_info->tm_mon == 10 && time_info->tm_mday == 1))
+	{
+		return SEASON_HALLOWEEN;
+	}
+	else if(time_iseasterday(time_data, time_info))
+	{
+		return SEASON_EASTER;
+	}
 
 	switch(time_info->tm_mon)
 	{
@@ -2700,8 +2740,10 @@ int time_season()
 	case 9:
 	case 10:
 		return SEASON_AUTUMN;
+	default:
+		dbg_assert(false, "Invalid month");
+		dbg_break();
 	}
-	return SEASON_SPRING; // should never happen
 }
 
 void str_append(char *dst, const char *src, int dst_size)

src/base/system.h

@@ -803,12 +803,15 @@ int time_houroftheday();
 /**
  * @ingroup Time
  */
-enum
+enum ETimeSeason
 {
 	SEASON_SPRING = 0,
 	SEASON_SUMMER,
 	SEASON_AUTUMN,
 	SEASON_WINTER,
+	SEASON_EASTER,
+	SEASON_HALLOWEEN,
+	SEASON_XMAS,
 	SEASON_NEWYEAR
 };
 
@@ -821,7 +824,7 @@ enum
  *
  * @see SEASON_SPRING
  */
-int time_season();
+ETimeSeason time_season();
 
 /**
  * @defgroup Network-General

src/game/client/components/menu_background.cpp

@@ -212,15 +212,18 @@ void CMenuBackground::LoadMenuBackground(bool HasDayHint, bool HasNightHint)
 			switch(time_season())
 			{
 			case SEASON_SPRING:
+			case SEASON_EASTER:
 				pMenuMap = "heavens";
 				break;
 			case SEASON_SUMMER:
 				pMenuMap = "jungle";
 				break;
 			case SEASON_AUTUMN:
+			case SEASON_HALLOWEEN:
 				pMenuMap = "autumn";
 				break;
 			case SEASON_WINTER:
+			case SEASON_XMAS:
 				pMenuMap = "winter";
 				break;
 			case SEASON_NEWYEAR:

src/game/client/components/skins.cpp

@@ -3,7 +3,6 @@
 
 #include <base/math.h>
 #include <base/system.h>
-#include <ctime>
 
 #include <engine/engine.h>
 #include <engine/graphics.h>
@@ -299,12 +298,8 @@ void CSkins::OnInit()
 
 	if(g_Config.m_Events)
 	{
-		time_t RawTime;
-		struct tm *pTimeInfo;
-		std::time(&RawTime);
-		pTimeInfo = localtime(&RawTime);
-		if(pTimeInfo->tm_mon == 11 && pTimeInfo->tm_mday >= 24 && pTimeInfo->tm_mday <= 26)
-		{ // Christmas
+		if(time_season() == SEASON_XMAS)
+		{
 			str_copy(m_aEventSkinPrefix, "santa");
 		}
 	}

src/game/server/player.cpp

@@ -87,16 +87,13 @@ void CPlayer::Reset()
 
 	if(g_Config.m_Events)
 	{
-		time_t RawTime;
-		struct tm *pTimeInfo;
-		time(&RawTime);
-		pTimeInfo = localtime(&RawTime);
-		if((pTimeInfo->tm_mon == 11 && pTimeInfo->tm_mday == 31) || (pTimeInfo->tm_mon == 0 && pTimeInfo->tm_mday == 1))
-		{ // New Year
+		const ETimeSeason Season = time_season();
+		if(Season == SEASON_NEWYEAR)
+		{
 			m_DefEmote = EMOTE_HAPPY;
 		}
-		else if((pTimeInfo->tm_mon == 9 && pTimeInfo->tm_mday == 31) || (pTimeInfo->tm_mon == 10 && pTimeInfo->tm_mday == 1))
-		{ // Halloween
+		else if(Season == SEASON_HALLOWEEN)
+		{
 			m_DefEmote = EMOTE_ANGRY;
 			m_Halloween = true;
 		}