Add consistent monotonic day counter - get_day_count()

I've written several experimental bits of code that revolve around the
need for a consistent calendar, but implementing one is extremely hard
in mods due to time changes and mods overriding core.get_timeofday(),
which will conflict.

The second part of the problem is that doing this from a mod requires
constant maintenance of a settings file.

An implementation in core is trivial, however, and solves all of
these problems at virtually no cost: No extra branches in server
steps, and a single branch when minetest.set_time_of_day(), which is
entirely reasonable.

We store the day_count value in env_meta.txt.

The use case is obvious: This change allows mods to create an actual
virtual calendar, or properly account for seasonal changes, etc..

We add a "/days" chatcommand that displays the current day count. No
permissions are needed. It can only retrieve the day count, not
modify it.

6 files changed, 46 insertions, 1 deletions

diff --git a/builtin/game/chatcommands.lua b/builtin/game/chatcommands.lua
index 2c7a0b532..0b1976640 100644
--- a/builtin/game/chatcommands.lua
+++ b/builtin/game/chatcommands.lua
@@ -785,6 +785,13 @@ core.register_chatcommand("time", {
 	end,
 })
 
+core.register_chatcommand("days", {
+	description = "Display day count",
+	func = function(name, param)
+		return true, "Current day is " .. core.get_day_count()
+	end
+})
+
 core.register_chatcommand("shutdown", {
 	description = "shutdown server",
 	privs = {server=true},
diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index 65af51578..e2ab65ddc 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -1998,6 +1998,8 @@ and `minetest.auth_reload` call the authetification handler.
     * `val` is between `0` and `1`; `0` for midnight, `0.5` for midday
 * `minetest.get_timeofday()`
 * `minetest.get_gametime()`: returns the time, in seconds, since the world was created
+* `minetest.get_day_count()`: returns number days elapsed since world was created,
+    * accounting for time changes.
 * `minetest.find_node_near(pos, radius, nodenames)`: returns pos or `nil`
     * `radius`: using a maximum metric
     * `nodenames`: e.g. `{"ignore", "group:tree"}` or `"default:dirt"`
diff --git a/src/environment.cpp b/src/environment.cpp
index 6da376a15..0d00ed170 100644
--- a/src/environment.cpp
+++ b/src/environment.cpp
@@ -208,6 +208,8 @@ void Environment::setDayNightRatioOverride(bool enable, u32 value)
 void Environment::setTimeOfDay(u32 time)
 {
 	MutexAutoLock lock(this->m_time_lock);
+	if (m_time_of_day > time)
+		m_day_count++;
 	m_time_of_day = time;
 	m_time_of_day_f = (float)time / 24000.0;
 }
@@ -238,8 +240,10 @@ void Environment::stepTimeOfDay(float dtime)
 	bool sync_f = false;
 	if (units > 0) {
 		// Sync at overflow
-		if (m_time_of_day + units >= 24000)
+		if (m_time_of_day + units >= 24000) {
 			sync_f = true;
+			m_day_count++;
+		}
 		m_time_of_day = (m_time_of_day + units) % 24000;
 		if (sync_f)
 			m_time_of_day_f = (float)m_time_of_day / 24000.0;
@@ -256,6 +260,13 @@ void Environment::stepTimeOfDay(float dtime)
 	}
 }
 
+u32 Environment::getDayCount()
+{
+	// Atomic<u32> counter
+	return m_day_count;
+}
+
+
 /*
 	ABMWithState
 */
@@ -727,6 +738,7 @@ void ServerEnvironment::saveMeta()
 	args.setU64("lbm_introduction_times_version", 1);
 	args.set("lbm_introduction_times",
 		m_lbm_mgr.createIntroductionTimesString());
+	args.setU64("day_count", m_day_count);
 	args.writeLines(ss);
 	ss<<"EnvArgsEnd\n";
 
@@ -792,6 +804,12 @@ void ServerEnvironment::loadMeta()
 	}
 	m_lbm_mgr.loadIntroductionTimes(lbm_introduction_times, m_gamedef, m_game_time);
 
+	try {
+		m_day_count = args.getU64("day_count");
+	} catch (SettingNotFoundException &e) {
+		// If missing, start the day counter
+		m_day_count = 0;
+	}
 }
 
 void ServerEnvironment::loadDefaultMeta()
diff --git a/src/environment.h b/src/environment.h
index 448ed70e6..9b91a0928 100644
--- a/src/environment.h
+++ b/src/environment.h
@@ -95,6 +95,8 @@ public:
 
 	void setDayNightRatioOverride(bool enable, u32 value);
 
+	u32 getDayCount();
+
 	// counter used internally when triggering ABMs
 	u32 m_added_objects;
 
@@ -117,6 +119,9 @@ protected:
 	// Overriding the day-night ratio is useful for custom sky visuals
 	bool m_enable_day_night_ratio_override;
 	u32 m_day_night_ratio_override;
+	// Days from the server start, accounts for time shift
+	// in game (e.g. /time or bed usage)
+	Atomic<u32> m_day_count;
 	/*
 	 * Above: values managed by m_time_lock
 	*/
diff --git a/src/script/lua_api/l_env.cpp b/src/script/lua_api/l_env.cpp
index f4ddc2afc..af89da9a1 100644
--- a/src/script/lua_api/l_env.cpp
+++ b/src/script/lua_api/l_env.cpp
@@ -561,6 +561,15 @@ int ModApiEnvMod::l_get_timeofday(lua_State *L)
 	return 1;
 }
 
+// get_day_count() -> int
+int ModApiEnvMod::l_get_day_count(lua_State *L)
+{
+	GET_ENV_PTR;
+
+	lua_pushnumber(L, env->getDayCount());
+	return 1;
+}
+
 // get_gametime()
 int ModApiEnvMod::l_get_gametime(lua_State *L)
 {
@@ -1055,6 +1064,7 @@ void ModApiEnvMod::Initialize(lua_State *L, int top)
 	API_FCT(set_timeofday);
 	API_FCT(get_timeofday);
 	API_FCT(get_gametime);
+	API_FCT(get_day_count);
 	API_FCT(find_node_near);
 	API_FCT(find_nodes_in_area);
 	API_FCT(find_nodes_in_area_under_air);
diff --git a/src/script/lua_api/l_env.h b/src/script/lua_api/l_env.h
index 0e385ffef..89dd7978f 100644
--- a/src/script/lua_api/l_env.h
+++ b/src/script/lua_api/l_env.h
@@ -113,6 +113,9 @@ private:
 	// get_gametime()
 	static int l_get_gametime(lua_State *L);
 
+	// get_day_count() -> int
+	static int l_get_day_count(lua_State *L);
+
 	// find_node_near(pos, radius, nodenames) -> pos or nil
 	// nodenames: eg. {"ignore", "group:tree"} or "default:dirt"
 	static int l_find_node_near(lua_State *L);