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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Abortable Timed Action
Date: Sat, 9 Jan 2016 10:10:08 +0100
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Date: 2016-01-09T10:10:08+01:00
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On 2016-01-09 09:45, Simon Wright wrote:
> "T.G." <anon@anon.org> writes:
>
>> delay until is an interesting idea. I'm assuming that time drift would
>> be an issue in periodic actions that repeat at a certain interval, but
>> in that case delay until could also have issues if it loses some
>> accuracy on each iteration.

Yes, if the period is not a multiple of the clock and system timer 
interrupt source. Though there is no way to eliminate this type of 
jitter, so why worry?

The resolution of Time (not accuracy) can be a problem, e.g. in the 
suggested schema below, which will accumulate error. The error can be 
compensated by computing Next as

    Count := Count + 1;
    Next :=
       Start + Duration (Long_Float (Count) * Long_Float (Interval));

Though, it is difficult to imagine a scenario where that would be really 
needed.

> The common solution is something like
>
>     with Ada.Calendar;
>     with Ada.Text_IO; use Ada.Text_IO;
>     procedure Delay_Until is
>        task T is
>           entry Exec_After (T : Duration);
>        end T;
>        task body T is
>           Start : Ada.Calendar.Time;
>           Next : Ada.Calendar.Time;
>           Interval : Duration;
>           use type Ada.Calendar.Time;
>        begin
>           accept Exec_After (T : Duration) do
>              Start := Ada.Calendar.Clock;
>              Next := Start + T;
>              Interval := T;
>           end Exec_After;
>           loop
>              delay until Next;
>              Next := Next + Interval;  -- *not* Ada.Calendar.Clock + Interval

An missing ticks detection and compensation:

    if Next < Clock then
       Next := Clock;
       Put_Line ("Ticks missed!");
    end if;


-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de