(foreign-declare "#include <windows.h>")

(define realtime-microsecs)
(let ((%factor (the (or float boolean) #f)))
  ;
  (define %calc-time (foreign-lambda* double ()
    "LARGE_INTEGER li;
    if( !QueryPerformanceCounter( &li ) )
      return( -1.0 );
    return( ((double) li.QuadPart) );"))
  ;
  (define %calc-factor (foreign-lambda* double ()
    "LARGE_INTEGER li;
    if( !QueryPerformanceFrequency( &li ) )
      return( -1.0 );
    return( ((double) li.QuadPart) / 1000000.0 );"))
  ;
  (set! realtime-microsecs (lambda () (* (%calc-time) %factor)))
  ;
  ;we immediatly determine the factor
  ;it should not change as long as the machine runs
  (set! %factor (%calc-factor))
  (when (< %factor 0.0)
    (error 'windows-realtime-microsecs "cannot determine scale factor") ) )