Added internal hi-resolution timer to physac...

... and now physac thread creation is done in InitPhysics() and it is
destroyed in ClosePhysics(). User just need to call these functions to
use physac module.

src/physac.h

@@ -177,6 +177,18 @@ PHYSACDEF Rectangle TransformToRectangle(Transform transform);
 #endif
 
 #include <math.h>           // Required for: cos(), sin(), abs(), fminf()
+#include <stdint.h>         // Required for typedef unsigned long long int uint64_t, used by hi-res timer
+#include <pthread.h>        // Required for: pthread_create()
+#include "utils.h"          // Required for: TraceLog()
+
+#if defined(PLATFORM_DESKTOP)
+    // Functions required to query time on Windows
+    int __stdcall QueryPerformanceCounter(unsigned long long int *lpPerformanceCount);
+    int __stdcall QueryPerformanceFrequency(unsigned long long int *lpFrequency);
+#elif defined(PLATFORM_ANDROID) || defined(PLATFORM_RPI)
+    #include <sys/time.h>       // Required for: timespec
+    #include <time.h>           // Required for: clock_gettime()
+#endif
 
 //----------------------------------------------------------------------------------
 // Defines and Macros
@@ -195,6 +207,9 @@ PHYSACDEF Rectangle TransformToRectangle(Transform transform);
 //----------------------------------------------------------------------------------
 // Global Variables Definition
 //----------------------------------------------------------------------------------
+static bool physicsThreadEnabled = false;                           // Physics calculations thread exit control
+static uint64_t baseTime;                                           // Base time measure for hi-res timer
+static double currentTime, previousTime;                            // Used to track timmings
 static PhysicBody physicBodies[MAX_PHYSIC_BODIES];                  // Physic bodies pool
 static int physicBodiesCount;                                       // Counts current enabled physic bodies
 static Vector2 gravityForce;                                        // Gravity force
@@ -202,6 +217,9 @@ static Vector2 gravityForce;                                        // Gravity f
 //----------------------------------------------------------------------------------
 // Module specific Functions Declaration
 //----------------------------------------------------------------------------------
+static void* PhysicsThread(void *arg);                              // Physics calculations thread function
+static void InitTimer(void);                                        // Initialize hi-resolution timer
+static double GetCurrentTime(void);                                 // Time measure returned are microseconds
 static float Vector2DotProduct(Vector2 v1, Vector2 v2);             // Returns the dot product of two Vector2
 static float Vector2Length(Vector2 v);                              // Returns the length of a Vector2
 
@@ -215,6 +233,10 @@ PHYSACDEF void InitPhysics(Vector2 gravity)
     // Initialize physics variables
     physicBodiesCount = 0;
     gravityForce = gravity;
+    
+    // Create physics thread
+    pthread_t tid;
+    pthread_create(&tid, NULL, &PhysicsThread, NULL);
 }
 
 // Update physic objects, calculating physic behaviours and collisions detection
@@ -592,6 +614,9 @@ PHYSACDEF void UpdatePhysics(double deltaTime)
 // Unitialize all physic objects and empty the objects pool
 PHYSACDEF void ClosePhysics()
 {
+    // Exit physics thread loop
+    physicsThreadEnabled = false;
+    
     // Free all dynamic memory allocations
     for (int i = 0; i < physicBodiesCount; i++) PHYSAC_FREE(physicBodies[i]);
     
@@ -710,6 +735,65 @@ PHYSACDEF Rectangle TransformToRectangle(Transform transform)
 //----------------------------------------------------------------------------------
 // Module specific Functions Definition
 //----------------------------------------------------------------------------------
+// Physics calculations thread function
+static void* PhysicsThread(void *arg)
+{
+    // Initialize thread loop state
+    physicsThreadEnabled = true;
+    
+    // Initialize hi-resolution timer
+    InitTimer();
+    
+    // Physics update loop
+    while (physicsThreadEnabled) 
+    {
+        currentTime = GetCurrentTime();
+        double deltaTime = (double)(currentTime - previousTime);
+        previousTime = currentTime;
+
+        // Delta time value needs to be inverse multiplied by physics time step value (1/target fps)
+        UpdatePhysics(deltaTime/PHYSICS_TIMESTEP);
+    }
+    
+    return NULL;
+}
+
+// Initialize hi-resolution timer
+static void InitTimer(void)
+{
+#if defined(PLATFORM_ANDROID) || defined(PLATFORM_RPI)
+    struct timespec now;
+
+    if (clock_gettime(CLOCK_MONOTONIC, &now) == 0)  // Success
+    {
+        baseTime = (uint64_t)now.tv_sec*1000000000LLU + (uint64_t)now.tv_nsec;
+    }
+    else TraceLog(WARNING, "No hi-resolution timer available");
+#endif
+
+    previousTime = GetCurrentTime();       // Get time as double
+}
+
+// Time measure returned are microseconds
+static double GetCurrentTime(void)
+{
+#if defined(PLATFORM_DESKTOP)
+    unsigned long long int clockFrequency, currentTime;
+    
+    QueryPerformanceFrequency(&clockFrequency);
+    QueryPerformanceCounter(&currentTime);
+
+    return (double)(currentTime/clockFrequency);
+#endif
+
+#if defined(PLATFORM_ANDROID) || defined(PLATFORM_RPI)
+    struct timespec ts;
+    clock_gettime(CLOCK_MONOTONIC, &ts);
+    uint64_t time = (uint64_t)ts.tv_sec*1000000000LLU + (uint64_t)ts.tv_nsec;
+
+    return (double)(time - baseTime)*1e-9;
+#endif
+}
 
 // Returns the dot product of two Vector2
 static float Vector2DotProduct(Vector2 v1, Vector2 v2)