win: Let CalculateIdleState() deal better with GetTickCount() wrapping around.

chrome/browser/idle_win.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/idle.h"
 
 #include <limits.h>
 #include <windows.h>
 
 static bool IsScreensaverRunning();
 static bool IsWorkstationLocked();
 
 IdleState CalculateIdleState(unsigned int idle_threshold) {
   if (IsScreensaverRunning() || IsWorkstationLocked())
     return IDLE_STATE_LOCKED;
 
   LASTINPUTINFO last_input_info = {0};
   last_input_info.cbSize = sizeof(LASTINPUTINFO);
   DWORD current_idle_time = 0;
   if (::GetLastInputInfo(&last_input_info)) {
-    current_idle_time = ::GetTickCount() - last_input_info.dwTime;
-    // Will go -ve if we have been idle for a long time (2gb seconds).
-    if (current_idle_time < 0)
-      current_idle_time = INT_MAX;
+    DWORD now = ::GetTickCount();
+    if (now < last_input_info.dwTime) {
+      // GetTickCount() wraps around every 49.7 days -- assume it wrapped just
+      // once.
+      const DWORD kMaxDWORD = static_cast<DWORD>(-1);
+      DWORD time_before_wrap = kMaxDWORD - last_input_info.dwTime;
+      DWORD time_after_wrap = now;
+      // The sum is always smaller than kMaxDWORD.
+      current_idle_time = time_before_wrap + time_after_wrap;
+    } else {
+      current_idle_time = now - last_input_info.dwTime;
+    }
+
     // Convert from ms to seconds.
     current_idle_time /= 1000;
   }
 
   if (current_idle_time >= idle_threshold)
     return IDLE_STATE_IDLE;
   return IDLE_STATE_ACTIVE;
 }
 
 bool IsScreensaverRunning() {
@@ skipping 13 matching lines @@
       UOI_NAME,
       name,
       sizeof(name),
       &needed)) {
         is_locked = lstrcmpi(name, L"default") != 0;
     }
     ::CloseDesktop(input_desk);
   }
   return is_locked;
 }