From 270c7701b4c43faa59b642be5774a0a867a0f1ed Mon Sep 17 00:00:00 2001
From: Hannu Hanhi <hhanhipublic@gmail.com>
Date: Mon, 8 Nov 2021 01:17:30 +0200
Subject: [PATCH 1/2] Timestamp function for Lua

---
 src/i_system.h     | 2 +-
 src/lua_baselib.c  | 9 +++++++++
 src/sdl/i_system.c | 8 +++++++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/i_system.h b/src/i_system.h
index a3790dca3..a2dd81cca 100644
--- a/src/i_system.h
+++ b/src/i_system.h
@@ -50,7 +50,7 @@ tic_t I_GetTime(void);
   */
 precise_t I_GetPreciseTime(void);
 
-/**	\brief	Returns the difference between precise times as microseconds.
+/**	\brief	Converts a precise_t to microseconds and casts it to a 32 bit integer.
   */
 int I_PreciseToMicros(precise_t);
 
diff --git a/src/lua_baselib.c b/src/lua_baselib.c
index 350c1585f..61f0e43cb 100644
--- a/src/lua_baselib.c
+++ b/src/lua_baselib.c
@@ -31,6 +31,7 @@
 #include "m_misc.h" // M_MapNumber
 #include "b_bot.h" // B_UpdateBotleader
 #include "d_clisrv.h" // CL_RemovePlayer
+#include "i_system.h" // I_GetPreciseTime, I_PreciseToMicros
 
 #include "lua_script.h"
 #include "lua_libs.h"
@@ -3876,6 +3877,12 @@ static int lib_gTicsToMilliseconds(lua_State *L)
 	return 1;
 }
 
+static int lib_iGetTimestamp(lua_State *L)
+{
+	lua_pushinteger(L, I_PreciseToMicros(I_GetPreciseTime()));
+	return 1;
+}
+
 static luaL_Reg lib[] = {
 	{"print", lib_print},
 	{"chatprint", lib_chatprint},
@@ -4150,6 +4157,8 @@ static luaL_Reg lib[] = {
 	{"G_TicsToCentiseconds",lib_gTicsToCentiseconds},
 	{"G_TicsToMilliseconds",lib_gTicsToMilliseconds},
 
+	{"I_GetTimestamp",lib_iGetTimestamp},
+
 	{NULL, NULL}
 };
 
diff --git a/src/sdl/i_system.c b/src/sdl/i_system.c
index 2ec28ebc8..ccec37093 100644
--- a/src/sdl/i_system.c
+++ b/src/sdl/i_system.c
@@ -2163,7 +2163,13 @@ precise_t I_GetPreciseTime(void)
 
 int I_PreciseToMicros(precise_t d)
 {
-	return (int)(d / (timer_frequency / 1000000.0));
+	// d is going to be converted into a double. So remove the highest bits
+	// to avoid loss of precision in the lower bits, for the (probably rare) case
+	// that the higher bits are actually used.
+	d &= ((precise_t)1 << 53) - 1; // The mantissa of a double can handle 53 bits at most.
+	// The resulting double from the calculation is converted first to UINT64 to avoid overflow,
+	// which is undefined behaviour when converting floating point values to integers.
+	return (int)(UINT64)(d / (timer_frequency / 1000000.0));
 }
 
 //

From 56c5a887c858aaf9e1def23327e56d09c546f333 Mon Sep 17 00:00:00 2001
From: Hannu Hanhi <hhanhipublic@gmail.com>
Date: Mon, 8 Nov 2021 20:28:35 +0200
Subject: [PATCH 2/2] Call the Lua timestamp function getTimeMicros

---
 src/lua_baselib.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/lua_baselib.c b/src/lua_baselib.c
index 61f0e43cb..29de7a05f 100644
--- a/src/lua_baselib.c
+++ b/src/lua_baselib.c
@@ -3877,7 +3877,7 @@ static int lib_gTicsToMilliseconds(lua_State *L)
 	return 1;
 }
 
-static int lib_iGetTimestamp(lua_State *L)
+static int lib_getTimeMicros(lua_State *L)
 {
 	lua_pushinteger(L, I_PreciseToMicros(I_GetPreciseTime()));
 	return 1;
@@ -4157,7 +4157,7 @@ static luaL_Reg lib[] = {
 	{"G_TicsToCentiseconds",lib_gTicsToCentiseconds},
 	{"G_TicsToMilliseconds",lib_gTicsToMilliseconds},
 
-	{"I_GetTimestamp",lib_iGetTimestamp},
+	{"getTimeMicros",lib_getTimeMicros},
 
 	{NULL, NULL}
 };