Library prosa.analysis.transform.edf_trans

In this file we define the EDF transformation of a schedule, which turns a (finite prefix of a) schedule into an EDF schedule. This operation is at the core of the EDF optimality proof.
Consider any given type of jobs...
  Context {Job : JobType} `{JobCost Job} `{JobDeadline Job} `{JobArrival Job}.

... and ideal uni-processor schedules.
We say that a state s1 "has an earlier or equal deadline" than a state s2 if the job scheduled in state s1 has has an earlier or equal deadline than the job scheduled in state s2. This function is never used on idle states, so the default values are irrelevant.
  Definition earlier_deadline (s1 s2: PState) :=
    (oapp job_deadline 0 s1) (oapp job_deadline 0 s2).

We say that a state is relevant (for the purpose of the transformation) if it is not idle and if the job scheduled in it has arrived prior to some given reference time.
  Definition relevant_pstate (reference_time: instant) (s: PState) :=
    match s with
    | Nonefalse
    | Some j'job_arrival j' reference_time

Next, we define a central element of the EDF transformation procedure: given a job scheduled at a time t1, find a later time t2 before the job's deadline at which a job with an earlier-or-equal deadline is scheduled. In other words, find a job that causes the EDF_at property to be violated at time t1.
  Definition find_swap_candidate (sched: SchedType) (t1: instant) (j: Job) :=
    if search_arg sched (relevant_pstate t1) earlier_deadline t1 (job_deadline j) is Some t
    then t
    else 0.

The point-wise EDF transformation procedure: given a schedule and a time t1, ensure that the schedule satisfies EDF_at at time t1.
  Definition make_edf_at (sched: SchedType) (t1: instant): SchedType :=
    match sched t1 with
    | Nonesched (* leave idle instants alone *)
    | Some j
        t2 := find_swap_candidate sched t1 j
      in swapped sched t1 t2

To transform a finite prefix of a given reference schedule, apply make_edf_at to every point up to the given finite horizon.
  Definition edf_transform_prefix (sched: SchedType) (horizon: instant): SchedType :=
    prefix_map sched make_edf_at horizon.

Finally, a full EDF schedule (i.e., one that satisfies EDF_at at any time) is obtained by first computing an EDF prefix up to and including the requested time t, and by then looking at the last point of the prefix.
  Definition edf_transform (sched: SchedType) (t: instant): ideal.processor_state Job :=
      edf_prefix := edf_transform_prefix sched t.+1
    in edf_prefix t.

End EDFTransformation.