Library prosa.analysis.facts.periodic.arrival_times

Require Export prosa.model.task.arrival.periodic_as_sporadic.
Require Export prosa.analysis.facts.periodic.max_inter_arrival.
Require Export prosa.analysis.facts.model.offset.

In this module, we'll prove the known arrival times of jobs that stem from periodic tasks.
Section PeriodicArrivalTimes.

Consider periodic tasks with an offset ...
  Context {Task : TaskType}.
  Context `{TaskOffset Task}.
  Context `{PeriodicModel Task}.

... and any type of jobs associated with these tasks.
  Context {Job : JobType}.
  Context `{JobTask Job Task}.
  Context `{JobArrival Job}.

Consider any unique arrival sequence with consistent arrivals ...
  Variable arr_seq : arrival_sequence Job.
  Hypothesis H_valid_arrival_sequence : valid_arrival_sequence arr_seq.
... and any periodic task tsk with a valid offset and period.
    Variable tsk : Task.
    Hypothesis H_valid_offset: valid_offset arr_seq tsk.
    Hypothesis H_valid_period: valid_period tsk.
    Hypothesis H_task_respects_periodic_model: respects_periodic_task_model arr_seq tsk.

We show that the nth job j of task tsk arrives at the instant task_offset tsk + n × task_period tsk.
  Lemma periodic_arrival_times:
    ∀ n (j : Job),
      arrives_in arr_seq j →
      job_task j = tsk →
      job_index arr_seq j = n →
      job_arrival j = task_offset tsk + n × task_period tsk.
  Proof.
    induction n.
    { intros j ARR TSK_IN ZINDEX.
      rewrite mul0n addn0.
      exact: first_job_arrival ZINDEX.
    }
    { intros j ARR TSK_IN JB_INDEX.
      move : (H_task_respects_periodic_model j ARR) ⇒ PREV_JOB.
      feed_n 2 PREV_JOB ⇒ //; first by lia.
      move : PREV_JOB ⇒ [pj [ARR' [IND [TSK ARRIVAL]]]].
      specialize (IHn pj); feed_n 3 IHn ⇒ //; first by rewrite IND JB_INDEX; lia.
      rewrite ARRIVAL IHn; lia.
    }
  Qed.

We show that for every job j of task tsk there exists a number n such that j arrives at the instant task_offset tsk + n × task_period tsk.
  Lemma job_arrival_times:
    ∀ j,
      arrives_in arr_seq j →
      job_task j = tsk →
      ∃ n, job_arrival j = task_offset tsk + n × task_period tsk.
  Proof.
    intros × ARR TSK.
    ∃ (job_index arr_seq j).
    specialize (periodic_arrival_times (job_index arr_seq j) j) ⇒ J_ARR.
    by feed_n 3 J_ARR ⇒ //.
  Qed.

If a job j of task tsk arrives at task_offset tsk + n × task_period tsk then the job_index of j is equal to n.
  Lemma job_arr_index:
    ∀ n j,
      arrives_in arr_seq j →
      job_task j = tsk →
      job_arrival j = task_offset tsk + n × task_period tsk →
      job_index arr_seq j = n.
  Proof.
    have F : task_period tsk > 0 by auto.
    induction n.
    + intros × ARR_J TSK ARR.
      destruct (PeanoNat.Nat.zero_or_succ (job_index arr_seq j)) as [Z | [m SUCC]] ⇒ //.
      by apply periodic_arrival_times in SUCC ⇒ //; lia.
    + intros × ARR_J TSK ARR.
      specialize (H_task_respects_periodic_model j); feed_n 3 H_task_respects_periodic_model ⇒ //.
      { rewrite lt0n; apply /eqP; intro EQ.
        apply (first_job_arrival _ H_valid_arrival_sequence tsk) in EQ ⇒ //.
        by rewrite EQ in ARR; lia.
      }
      move : H_task_respects_periodic_model ⇒ [j' [ARR' [IND' [TSK' ARRIVAL']]]].
      specialize (IHn j'); feed_n 3 IHn ⇒ //; first by rewrite ARR in ARRIVAL'; lia.
      rewrite IHn in IND'.
      destruct (PeanoNat.Nat.zero_or_succ (job_index arr_seq j)) as [Z | [m SUCC]]; last by lia.
      rewrite (first_job_arrival arr_seq _ tsk)// in ARR.
      by lia.
  Qed.

End PeriodicArrivalTimes.