[win32] Hopefully improved CPU usage check

src/cpu_win32.cpp

@@ -1,11 +1,13 @@
 #include <windows.h>
 #include <thread>
+#include <string.h>
 #include "cpu.h"
+#include <iostream>
 #define SystemProcessorPerformanceInformation 0x8
 #define SystemBasicInformation    0x0
 FILETIME last_userTime, last_kernelTime, last_idleTime;
 
-uint64_t FromFileTime( const FILETIME& ft ) {
+uint64_t FileTimeToInt64( const FILETIME& ft ) {
     ULARGE_INTEGER uli = { 0 };
     uli.LowPart = ft.dwLowDateTime;
     uli.HighPart = ft.dwHighDateTime;
@@ -14,18 +16,44 @@ uint64_t FromFileTime( const FILETIME& ft ) {
 
 bool CPUStats::UpdateCPUData()
 {
-    FILETIME idleTime;
-    FILETIME kernelTime;
-    FILETIME userTime;
-    BOOL res = GetSystemTimes( &idleTime, &kernelTime, &userTime );
-    int usr = FromFileTime(userTime) - FromFileTime(last_userTime);
-    int ker = FromFileTime(kernelTime) - FromFileTime(last_kernelTime);
-    int idl = FromFileTime(idleTime) - FromFileTime(last_idleTime);
-    int sys = ker + usr;
-    m_cpuDataTotal.percent = (sys - idl) *100 / sys;
-    last_userTime = userTime;
-    last_kernelTime = kernelTime;
-    last_idleTime = idleTime;
+    #define NUMBER_OF_PROCESSORS (8)
+    #define PROCESSOR_BUFFER_SIZE (NUMBER_OF_PROCESSORS * 8)
+    static ULONG64 ProcessorIdleTimeBuffer [ PROCESSOR_BUFFER_SIZE ];
+
+    FILETIME IdleTime, KernelTime, UserTime;
+    static unsigned long long PrevTotal = 0;
+    static unsigned long long PrevIdle = 0;
+    static unsigned long long PrevUser = 0;
+    unsigned long long ThisTotal;
+    unsigned long long ThisIdle, ThisKernel, ThisUser;
+    unsigned long long TotalSinceLast, IdleSinceLast, UserSinceLast;
+
+
+    // GET THE KERNEL / USER / IDLE times.  
+    // And oh, BTW, kernel time includes idle time
+    GetSystemTimes( & IdleTime, & KernelTime, & UserTime);
+
+    ThisIdle = FileTimeToInt64(IdleTime);
+    ThisKernel = FileTimeToInt64 (KernelTime);
+    ThisUser = FileTimeToInt64 (UserTime);
+
+    ThisTotal = ThisKernel + ThisUser;
+    TotalSinceLast = ThisTotal - PrevTotal;
+    IdleSinceLast = ThisIdle - PrevIdle;
+    UserSinceLast = ThisUser - PrevUser;
+    double Headroom;
+    Headroom =  (double)IdleSinceLast / (double)TotalSinceLast ;
+    double Load;
+    Load = 1.0 - Headroom;
+    Headroom *= 100.0;  // to make it percent
+    Load *= 100.0;  // percent
+
+    PrevTotal = ThisTotal;
+    PrevIdle = ThisIdle;
+    PrevUser = ThisUser;
+
+    // print results to output window of VS when run in Debug
+    m_cpuDataTotal.percent = Load;
     return true;
 }
 CPUStats::CPUStats()