From a94ec5dc3371b2c6b1547903dd35e67216b98872 Mon Sep 17 00:00:00 2001
From: Nico de Poel <ndepoel@gmail.com>
Date: Fri, 13 Jan 2023 13:02:58 +0100
Subject: [PATCH] Implemented real-time clock, allowing for precise timekeeping
 and framerate-independent game updates.

---
 display.c |  2 +-
 main.c    |  3 ++-
 time.c    | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
 time.h    | 35 ++++++++++++++++++++++++++++-
 4 files changed, 102 insertions(+), 5 deletions(-)

diff --git a/display.c b/display.c
index d16daad..86c057f 100644
--- a/display.c
+++ b/display.c
@@ -55,7 +55,7 @@ void display_init()
 	// ISR subsystem to the kernel
 	ResetGraph(0);
 
-	SetVideoMode(MODE_NTSC);
+	//SetVideoMode(MODE_NTSC);
 	
 	// Define display environments, first on top and second on bottom
 	SetDefDispEnv(&disp[0], 0, 0, SCREENWIDTH, SCREENHEIGHT);
diff --git a/main.c b/main.c
index 2613dc3..051f846 100644
--- a/main.c
+++ b/main.c
@@ -24,9 +24,9 @@ world_t world;
 // Init function
 void init(void)
 {
-	time_init();
 	input_init();
 	display_init();
+	time_init();
 
 	//asset_loadTexture(tim_e1m1, NULL);
 
@@ -49,6 +49,7 @@ int main(int argc, const char *argv[])
 		// Draw stuff
 		world_draw(&world);
 	
+		FntPrint(-1, "Time: %d, ticks = %d, delta time = %d, fps = %d\n", time_getRealTime() >> 12, time_getFrameNumber(), time_getDeltaTime(), time_getFrameRate() >> 8);
 		FntPrint(-1, "Camera pos = (%d, %d, %d) rot = (%d, %d, %d)\n", cam_pos.vx, cam_pos.vy, cam_pos.vz, cam_rot.vx, cam_rot.vy, cam_rot.vz);
 		FntFlush(-1);
 		
diff --git a/time.c b/time.c
index a3aebcb..a11ab59 100644
--- a/time.c
+++ b/time.c
@@ -1,19 +1,82 @@
 #include "common.h"
 #include "time.h"
 
-static int frameNumber;
+#include <hwregs_c.h>
+
+static volatile u_long realTime;    // Real time in 20,12 format (4096 ticks per second)
+static u_long frameNumber;          // Number of game ticks
+static u_long frameStartTime;       // Real time at which the current frame began
+static u_short frameDeltaTime;      // Real time since last game tick in 4,12 format
+
+static u_short avgFrameRate;        // Average framerate over the last 16 frames in 8,8 format
+static u_long measureStartTime;
+
+static void timer_callback()
+{
+    realTime += 4;
+}
 
 void time_init()
 {
+    realTime = 0;
     frameNumber = 0;
+    frameStartTime = 0;
+    frameDeltaTime = 1; // Prevent division by zero for any code using this number on the first frame
+    
+    avgFrameRate = 0;
+    measureStartTime = 0;
+
+    // Set the timer such that we get exactly 1024 interrupts per second on both PAL and NTSC (roughly once per millisecond)
+    // This allows the game speed to be completely framerate and video mode independent
+    const int counter = 4096;
+
+    EnterCriticalSection();
+	SetRCnt(RCntCNT2, counter, RCntMdINTR);
+	TIMER_CTRL(2) = 0x1E58;
+	InterruptCallback(6, timer_callback);
+	StartRCnt(RCntCNT2);
+	ChangeClearRCnt(2, 0);
+	ExitCriticalSection();
 }
 
 void time_tick()
 {
     ++frameNumber;
+
+    u_long currTime = realTime;
+    if ((frameNumber & 0xF) == 0)
+    {
+        avgFrameRate = (u_short)((0x10 << 20) / (currTime - measureStartTime));
+        measureStartTime = currTime;
+    }
+
+    frameDeltaTime = (u_short)(currTime - frameStartTime);
+    
+    // Start the next frame
+    frameStartTime = currTime;
 }
 
-int time_getFrameNumber()
+u_long time_getRealTime()
+{
+    return realTime;
+}
+
+u_long time_getFrameNumber()
 {
     return frameNumber;
 }
+
+u_long time_getFrameTime()
+{
+    return frameStartTime;
+}
+
+u_short time_getDeltaTime()
+{
+    return frameDeltaTime;
+}
+
+u_short time_getFrameRate()
+{
+    return avgFrameRate;
+}
diff --git a/time.h b/time.h
index 98a15b1..5891661 100644
--- a/time.h
+++ b/time.h
@@ -4,6 +4,39 @@
 void time_init();
 void time_tick();
 
-int time_getFrameNumber();
+/**
+ * @brief Get the real-time clock value.
+ * Note that this value will constantly update and change, so don't expect this value to remain consistent during a single frame.
+ * @return The real-time clock value in 20,12 format.
+ */
+u_long time_getRealTime();
+
+/**
+ * @brief Get the current frame count.
+ * This is the number of frames that were counted, i.e. the number of main loop iterations since application start.
+ * @return The current frame's number in 32,0 format.
+ */
+u_long time_getFrameNumber();
+
+/**
+ * @brief Get the current frame start time.
+ * The real-time value at which the current frame was started. Unlike time_getRealTime(), this value remains consistent for the entire frame.
+ * @return The current frame's start time in 20,12 format.
+ */
+u_long time_getFrameTime();
+
+/**
+ * @brief Get the delta time between frames.
+ * The amount of real time passed between the last frame and the current frame. Use this for framerate-independent time stepping.
+ * @return The frame delta time in 4,12 format.
+ */
+u_short time_getDeltaTime();
+
+/**
+ * @brief Get the average frame rate.
+ * The average framerate calculated over 16 frames. This gets updated periodically so isn't always entirely current.
+ * @return The average frame rate in 8,8 format.
+ */
+u_short time_getFrameRate();
 
 #endif  // __TIME_H__