Active object
You are encouraged to solve this task according to the task description, using any language you may know.
In object-oriented programming an object is active when its state depends on clock. Usually an active object encapsulates a task that updates the object's state. To the outer world the object looks like a normal object with methods that can be called from outside. Implementation of such methods must have a certain synchronization mechanism with the encapsulated task in order to prevent object's state corruption.
A typical instance of an active object is an animation widget. The widget state changes with the time, while as an object it has all properties of a normal widget.
The task
Implement an active integrator object. The object has an input and output. The input can be set using the method Input. The input is a function of time. The output can be queried using the method Output. The object integrates its input over the time and the result becomes the object's output. So if the input is K(t) and the output is S, the object state S is changed to S + (K(t1) + K(t0)) * (t1 - t0) / 2, i.e. it integrates K using the trapeze method. Initially K is constant 0 and S is 0.
In order to test the object:
- set its input to sin (2π f t), where the frequency f=0.5Hz. The phase is irrelevant.
- wait 2s
- set the input to constant 0
- wait 0.5s
Verify that now the object's output is approximately 0 (the sine has the period of 2s). The accuracy of the result will depend on the OS scheduler time slicing and the accuracy of the clock.
Ada
<lang ada> with Ada.Calendar; use Ada.Calendar; with Ada.Numerics; use Ada.Numerics; with Ada.Numerics.Elementary_Functions; use Ada.Numerics.Elementary_Functions; with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Integrator is
type Func is access function (T : Time) return Float;
function Zero (T : Time) return Float is
begin
return 0.0;
end Zero;
Epoch : constant Time := Clock;
function Sine (T : Time) return Float is
begin
return Sin (Pi * Float (T - Epoch));
end Sine;
task type Integrator is
entry Input (Value : Func);
entry Output (Value : out Float);
entry Shut_Down;
end Integrator;
task body Integrator is
K : Func := Zero'Access;
S : Float := 0.0;
F0 : Float := 0.0;
F1 : Float;
T0 : Time := Clock;
T1 : Time;
begin
loop
select
accept Input (Value : Func) do
K := Value;
end Input;
or accept Output (Value : out Float) do
Value := S;
end Output;
or accept Shut_Down;
exit;
else
T1 := Clock;
F1 := K (T1);
S := S + 0.5 * (F1 + F0) * Float (T1 - T0);
T0 := T1;
F0 := F1;
end select;
end loop;
end Integrator;
I : Integrator;
S : Float;
begin
I.Input (Sine'Access);
delay 2.0;
I.Input (Zero'Access);
delay 0.5;
I.Output (S);
Put_Line ("Integrated" & Float'Image (S) & "s");
I.Shut_Down;
end Test_Integrator; </lang> Sample output:
Integrated-5.34100E-05s
C
<lang c>#include <stdio.h>
- include <stdlib.h>
- include <unistd.h>
- include <math.h>
- include <pthread.h>
double nullfunc(double t) {
return 0.0;
}
double func1(double t) {
return sin(2.0*M_PI*0.5*t);
}
typedef double (*objfunc)(double); struct objhandle_s {
int terminate; pthread_mutex_t access_S; pthread_mutex_t access_I; pthread_mutex_t access_t; objfunc I; pthread_t th; double S; double t; double freq;
}; typedef struct objhandle_s objhandle_t; typedef struct objhandle_s *objhandle;
void *the_integrator(void *n) {
objhandle oh = (objhandle)n;
double ts = 1.0/oh->freq;
while(oh->terminate == 0)
{
usleep((useconds_t)(ts*1000000.0));
pthread_mutex_lock(&oh->access_S);
pthread_mutex_lock(&oh->access_t);
pthread_mutex_lock(&oh->access_I);
oh->S += ( oh->I(oh->t+ts) - oh->I(oh->t) )*ts/2.0;
oh->t += ts;
pthread_mutex_unlock(&oh->access_I);
pthread_mutex_unlock(&oh->access_t);
pthread_mutex_unlock(&oh->access_S);
}
}
void destroy_integrator(objhandle integrator) {
integrator->terminate = 1; pthread_join(integrator->th, NULL); free(integrator);
}
double integrator_getsum(objhandle integrator) {
return integrator->S;
}
void integrator_setinput(objhandle integrator, objfunc nif) {
pthread_mutex_lock(&integrator->access_I); integrator->I = nif; pthread_mutex_unlock(&integrator->access_I);
}
objhandle create_integrator(double freq) {
objhandle r;
r = malloc(sizeof(objhandle_t));
if ( r != NULL )
{
pthread_mutex_init(&r->access_S, NULL);
pthread_mutex_init(&r->access_I, NULL);
pthread_mutex_init(&r->access_t, NULL);
r->terminate = 0;
r->t = 0.0;
r->S = 0.0;
r->I = nullfunc;
r->freq = freq;
pthread_create(&r->th, NULL, the_integrator, (void *)r);
}
return r;
}
int main()
{
objhandle integrator;
double sint;
integrator = create_integrator(10.0);
integrator_setinput(integrator, func1);
sleep(2);
integrator_setinput(integrator, nullfunc);
usleep(500000);
sint = integrator_getsum(integrator);
printf("S = %lf\n", sint);
destroy_integrator(integrator);
return 0;
}</lang>
Mutex for S and t (time) exists in case one wants to implement functions to reset/set S (sum) and time (t) i.e. to reset or set the integrator to a particular state.
Output:
S = -0.015451
By increasing the precision of the integrator by increasing the "sampling frequency" you can take a better result; e.g. with create_integrator(100.0) I've obtained S = -0.000157.
E
def makeIntegrator() {
var value := 0.0
var input := fn { 0.0 }
var input1 := input()
var t1 := timer.now()
def update() {
def t2 := timer.now()
def input2 :float64 := input()
def dt := (t2 - t1) / 1000
value += (input1 + input2) * dt / 2
t1 := t2
input1 := input2
}
var task() {
update <- ()
task <- ()
}
task()
def integrator {
to input(new) :void { input := new }
to output() :float64 { return value }
to shutdown() { task := fn {} }
}
return integrator
}
def test() {
def result
def pi := (-1.0).acos()
def freq := pi / 1000
def base := timer.now()
def i := makeIntegrator()
i.input(fn { (freq * timer.now()).sin() })
timer.whenPast(base + 2000, fn {
i.input(fn {0})
})
timer.whenPast(base + 2500, fn {
bind result := i.output()
i.shutdown()
})
return result
}
Haskell
<lang haskell>module Integrator (
newIntegrator, input, output, stop, Time, timeInterval
) where import Control.Concurrent (forkIO, threadDelay) import Control.Concurrent.MVar (MVar, newMVar, modifyMVar_, modifyMVar, readMVar) import Control.Exception (evaluate) import Data.Time (UTCTime) import Data.Time.Clock (getCurrentTime, diffUTCTime)
-- RC task main = do let f = 0.5 {- Hz -}
t0 <- getCurrentTime
i <- newIntegrator
input i (\t -> sin(2*pi * f * timeInterval t0 t)) -- task step 1
threadDelay 2000000 {- µs -} -- task step 2
input i (const 0) -- task step 3
threadDelay 500000 {- µs -} -- task step 4
result <- output i
stop i
print result
Implementation ------------------------------------------------------
-- Utilities for working with the time type type Time = UTCTime type Func a = Time -> a timeInterval t0 t1 = fromRational $ toRational $ diffUTCTime t1 t0
-- Type signatures of the module's interface newIntegrator :: Fractional a => IO (Integrator a) -- Create an integrator input :: Integrator a -> Func a -> IO () -- Set the input function output :: Integrator a -> IO a -- Get the current value stop :: Integrator a -> IO () -- Stop integration, don't waste CPU
-- Data structures data Integrator a = Integrator (MVar (IntState a)) -- MVar is a thread-safe mutable cell
deriving Eq
data IntState a = IntState { func :: Func a, -- The current function
run :: Bool, -- Whether to keep going
value :: a, -- The current accumulated value
time :: Time } -- The time of the previous update
newIntegrator = do
now <- getCurrentTime
state <- newMVar $ IntState { func = const 0,
run = True,
value = 0,
time = now }
thread <- forkIO (intThread state) -- The state variable is shared between the thread
return (Integrator state) -- and the client interface object.
input (Integrator stv) f = modifyMVar_ stv (\st -> return st { func = f }) output (Integrator stv) = fmap value $ readMVar stv stop (Integrator stv) = modifyMVar_ stv (\st -> return st { run = False })
-- modifyMVar_ takes an MVar and replaces its contents according to the provided function.
-- a { b = c } is record-update syntax: "the record a, except with field b changed to c"
-- Integration thread intThread :: Fractional a => MVar (IntState a) -> IO () intThread stv = whileM $ modifyMVar stv updateAndCheckRun
-- modifyMVar is like modifyMVar_ but the function returns a tuple of the new value
-- and an arbitrary extra value, which in this case ends up telling whileM whether
-- to keep looping.
where updateAndCheckRun st = do
now <- getCurrentTime
let value' = integrate (func st) (value st) (time st) now
evaluate value' -- avoid undesired laziness
return (st { value = value', time = now }, -- updated state
run st) -- whether to continue
integrate :: Fractional a => Func a -> a -> Time -> Time -> a integrate f value t0 t1 = value + (f t0 + f t1)/2 * dt
where dt = timeInterval t0 t1
-- Execute 'action' until it returns false. whileM action = do b <- action; if b then whileM action else return ()</lang>
JavaScript
<lang javascript>function Integrator(sampleIntervalMS) {
var inputF = function () { return 0.0 };
var sum = 0.0;
var t1 = new Date().getTime();
var input1 = inputF(t1 / 1000);
function update() {
var t2 = new Date().getTime();
var input2 = inputF(t2 / 1000);
var dt = (t2 - t1) / 1000;
sum += (input1 + input2) * dt / 2;
t1 = t2;
input1 = input2;
}
var updater = setInterval(update, sampleIntervalMS);
return ({
input: function (newF) { inputF = newF },
output: function () { return sum },
shutdown: function () { clearInterval(updater) },
});
}</lang>
Test program as a HTML fragment:
<lang html>
Test running... -
<script type="text/javascript">
var f = 0.5;
var i = new Integrator(1);
var displayer = setInterval(function () { document.getElementById("b").firstChild.data = i.output() }, 100)
setTimeout(function () {
i.input(function (t) { return Math.sin(2*Math.PI*f*t) }); // test step 1
setTimeout(function () { // test step 2
i.input(function (t) { return 0 }); // test step 3
setTimeout(function () { // test step 3
i.shutdown();
clearInterval(displayer);
document.getElementById("a").firstChild.data = "Done, should be about 0: "
}, 500);
}, 2000);
}, 1)
</script></lang>
Python
<lang python> from time import time, sleep
class ActiveIntegrator():
def __init__(self, interval=1e-5):
self.interval = interval
self.t0 = time() # seconds
self.inputK = lambda t: 0.0
self.k0 = 0.0
self.outputS = 0.0
def inp(self, K=lambda t: 0.0):
t1 = time()
interval = self.interval
tp0, tp1 = self.t0, self.t0+interval
kp0 = self.k0
Kp0 = self.inputK
S = self.outputS
while tp1 < t1:
kp1 = K(tp1)
S += (kp0 + kp1)*(tp1 - tp0)/2.0
kp0 = kp1
tp0, tp1 = tp1, tp1+interval
self.outputS = S
self.t0 = t1
self.inputK = K
self.k0 = self.inputK(t1)
def outp(self):
self.inp(self.inputK)
return self.outputS
if __name__ == '__main__':
from math import sin, pi ai = ActiveIntegrator() f = 0.5 ai.inp(lambda t: sin(2*pi*f*t)) sleep(2) ai.inp() print (ai.outp()) sleep(0.5) print (ai.outp())</lang>
Tcl
and the TclOO package
This implementation Tcl 8.6 for object support (for the active integrator object) and coroutine support (for the controller task). It could be rewritten to only use 8.5 plus the TclOO library. <lang Tcl>package require Tcl 8.6 oo::class create integrator {
variable e sum delay tBase t0 k0 aid constructor Template:Interval 1 {
set delay $interval set tBase [clock microseconds] set t0 0 set e { 0.0 } set k0 0.0 set sum 0.0 set aid [after $delay [namespace code {my Step}]]
}
destructor {
after cancel $aid
}
method input expression {
set e $expression
}
method output {} {
return $sum
}
method Eval t {
expr $e
}
method Step {} {
set aid [after $delay [namespace code {my Step}]] set t [expr {([clock microseconds] - $tBase) / 1e6}] set k1 [my Eval $t] set sum [expr {$sum + ($k1 + $k0) * ($t - $t0) / 2.}] set t0 $t set k0 $k1
}
}
set pi 3.14159265 proc pause {time} {
yield [after [expr {int($time * 1000)}] [info coroutine]]
} proc task {script} {
coroutine task_ apply [list {} "$script;set ::done ok"]
vwait done
} task {
integrator create i
i input {sin(2*$::pi * 0.5 * $t)}
pause 2
i input { 0.0 }
pause 0.5
puts [format %.15f [i output]]
}</lang> Sample output:
-0.000000168952702
Visual Basic .NET
Since this object is CPU intensive, shutting it down when done is crucial. To facilitate this, the IDisposable pattern was used.
<lang vbnet>Module Module1
Sub Main()
Using active As New Integrator
active.Operation = Function(t As Double) Math.Sin(2 * Math.PI * 0.5 * t)
Threading.Thread.Sleep(TimeSpan.FromSeconds(2))
Console.WriteLine(active.Value)
active.Operation = Function(t As Double) 0
Threading.Thread.Sleep(TimeSpan.FromSeconds(0.5))
Console.WriteLine(active.Value)
End Using
Console.ReadLine()
End Sub
End Module
Class Integrator
Implements IDisposable
Private m_Operation As Func(Of Double, Double)
Private m_Disposed As Boolean
Private m_SyncRoot As New Object
Private m_Value As Double
Public Sub New()
m_Operation = Function(void) 0.0
Dim t As New Threading.Thread(AddressOf MainLoop)
t.Start()
End Sub
Private Sub MainLoop()
Dim epoch = Now
Dim t0 = 0.0
Do
SyncLock m_SyncRoot
Dim t1 = (Now - epoch).TotalSeconds
m_Value = m_Value + (Operation(t1) + Operation(t0)) * (t1 - t0) / 2
t0 = t1
End SyncLock
Threading.Thread.Sleep(10)
Loop Until m_Disposed
End Sub
Public Property Operation() As Func(Of Double, Double)
Get
SyncLock m_SyncRoot
Return m_Operation
End SyncLock
End Get
Set(ByVal value As Func(Of Double, Double))
SyncLock m_SyncRoot
m_Operation = value
End SyncLock
End Set
End Property
Public ReadOnly Property Value() As Double
Get
SyncLock m_SyncRoot
Return m_Value
End SyncLock
End Get
End Property
Protected Overridable Sub Dispose(ByVal disposing As Boolean)
m_Disposed = True
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
Dispose(True)
GC.SuppressFinalize(Me)
End Sub
End Class</lang>
Output: 0.000241446762282308