From 385dfc44f766f39d006e3224aa373700b91e488b Mon Sep 17 00:00:00 2001
From: Roman Fomin <rfomin@gmail.com>
Date: Tue, 30 Nov 2021 14:52:46 +0700
Subject: [PATCH] fix scaled time interpolation (#363)

* fix scaled time interpolation

* remove float
---
 Source/doomtype.h |  3 +++
 Source/i_system.c | 32 +++++++++++++++++++++++++++++++-
 Source/i_system.h |  2 ++
 Source/i_video.c  |  5 ++++-
 4 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/Source/doomtype.h b/Source/doomtype.h
index 2e99ad45..bdb94f38 100644
--- a/Source/doomtype.h
+++ b/Source/doomtype.h
@@ -76,6 +76,9 @@ typedef int64_t Long64;
 #ifndef MAX
  #define MAX(a,b) (((a)>(b))?(a):(b))
 #endif
+#ifndef BETWEEN
+ #define BETWEEN(l,u,x) ((l)>(x)?(l):(x)>(u)?(u):(x))
+#endif
 
 #if defined(_MSC_VER) && !defined(__cplusplus)
 #define inline __inline
diff --git a/Source/i_system.c b/Source/i_system.c
index e490e5ce..18676c81 100644
--- a/Source/i_system.c
+++ b/Source/i_system.c
@@ -98,6 +98,7 @@ int I_GetTimeMS(void)
 
 // killough 4/13/98: Make clock rate adjustable by scale factor
 int realtic_clock_rate = 100;
+static int clock_rate;
 static Long64 I_GetTime_Scale = 1<<24;
 int I_GetTime_Scaled(void)
 {
@@ -119,6 +120,26 @@ static int I_GetTime_Error()
 
 int (*I_GetTime)() = I_GetTime_Error;                           // killough
 
+// During a fast demo, no time elapses in between ticks
+static int I_TickElapsedTimeFastDemo(void)
+{
+  return 0;
+}
+
+static int I_TickElapsedRealTime(void)
+{
+  return I_GetTimeMS() - I_GetTime() * 1000 / TICRATE;
+}
+
+static int I_TickElapsedScaledTime(void)
+{
+  int scaled_time = I_GetTimeMS() * clock_rate / 100;
+
+  return scaled_time - I_GetTime() * 1000 / TICRATE;
+}
+
+int (*I_TickElapsedTime)(void) = I_TickElapsedRealTime;
+
 int controllerpresent;                                         // phares 4/3/98
 
 int leds_always_off;         // Tells it not to update LEDs
@@ -227,7 +248,9 @@ extern boolean nomusicparm, nosfxparm;
 
 void I_Init(void)
 {
-   int clock_rate = realtic_clock_rate, p;
+   int p;
+
+   clock_rate = realtic_clock_rate;
    
    if((p = M_CheckParm("-speed")) && p < myargc-1 &&
       (p = atoi(myargv[p+1])) >= 10 && p <= 1000)
@@ -238,15 +261,22 @@ void I_Init(void)
 
    // killough 4/14/98: Adjustable speedup based on realtic_clock_rate
    if(fastdemo)
+   {
       I_GetTime = I_GetTime_FastDemo;
+      I_TickElapsedTime = I_TickElapsedTimeFastDemo;
+   }
    else
       if(clock_rate != 100)
       {
          I_GetTime_Scale = ((Long64) clock_rate << 24) / 100;
          I_GetTime = I_GetTime_Scaled;
+         I_TickElapsedTime = I_TickElapsedScaledTime;
       }
       else
+      {
          I_GetTime = I_GetTime_RealTime;
+         I_TickElapsedTime = I_TickElapsedRealTime;
+      }
 
    I_InitJoystick();
 
diff --git a/Source/i_system.h b/Source/i_system.h
index 3ab3db22..8f94fd56 100644
--- a/Source/i_system.h
+++ b/Source/i_system.h
@@ -44,6 +44,8 @@ int I_GetTime_RealTime();     // killough
 int I_GetTime_Adaptive(void); // killough 4/10/98
 extern int GetTime_Scale;
 
+extern int (*I_TickElapsedTime)(void);
+
 // [FG] Same as I_GetTime, but returns time in milliseconds
 int I_GetTimeMS();
 // [FG] toggle demo warp mode
diff --git a/Source/i_video.c b/Source/i_video.c
index 307d29e9..a9ccb858 100644
--- a/Source/i_video.c
+++ b/Source/i_video.c
@@ -939,7 +939,10 @@ void I_FinishUpdate(void)
    // [AM] Figure out how far into the current tic we're in as a fixed_t.
    if (uncapped)
    {
-	fractionaltic = I_GetTimeMS() * TICRATE % 1000 * FRACUNIT / 1000;
+        int tic_time = I_TickElapsedTime();
+
+        fractionaltic = tic_time * FRACUNIT * TICRATE / 1000;
+        fractionaltic = BETWEEN(0, FRACUNIT, fractionaltic);
    }
 
    I_RestoreDiskBackground();