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

(define get-nanosecs)
(let ((%factor 0))

  (define %calc-freq (foreign-lambda* unsigned-integer64 ()
    "LARGE_INTEGER li;"
    "if( !QueryPerformanceFrequency( &li ) ) return( 0 );"
    "return( li.QuadPart );"))

  (define %calc-time (foreign-lambda* unsigned-integer64 ()
    "LARGE_INTEGER li;"
    "if( !QueryPerformanceCounter( &li ) ) return( 0 );"
    "return( li.QuadPart );"))

  (define (%calc-factor) (/ (%calc-freq) #e1e9))

  (set! get-nanosecs (lambda () (floor (* (%calc-time) %factor))))

  ;we immediatly determine the factor
  ;it should not change as long as the machine runs
  (set! %factor (%calc-factor))
  (unless (positive? %factor)
    (error 'get-nanosecs "cannot determine scale factor" %factor) ) )