Library prosa.implementation.definitions.extrapolated_arrival_curve
From mathcomp Require Export ssreflect ssrbool eqtype ssrnat div seq path fintype bigop.
Require Export prosa.behavior.time.
Require Export prosa.util.all.
Require Export prosa.behavior.time.
Require Export prosa.util.all.
This file introduces an implementation of arrival curves via the
periodic extension of finite arrival-curve prefix. An
arrival-curve prefix is a pair comprising a horizon and a list of
steps. The horizon defines the domain of the prefix, in which no
extrapolation is necessary. The list of steps (duration × value)
describes the value changes of the corresponding arrival curve
within the domain.
For time instances outside of an arrival-curve prefix's domain,
extrapolation is necessary. Therefore, past the domain,
arrival-curve values are extrapolated assuming that the
arrival-curve prefix is repeated with a period equal to the
horizon.
Note that such a periodic extension does not necessarily give the
tightest curve, and hence it is not optimal. The advantage is
speed of calculation: periodic extension can be done in constant
time, whereas the optimal extension takes roughly quadratic time
in the number of steps.
An arrival-curve prefix is defined as a pair comprised of a
horizon and a list of steps (duration × value) that describe the
value changes of the described arrival curve.
For example, an arrival-curve prefix (5, [:: (1, 3)]) describes
an arrival sequence with job bursts of size 3 happening every
5 time instances.
Given an inter-arrival time p (or period p), the corresponding
arrival-curve prefix can be defined as (p, [:: (1, 1)]).
The first component of arrival-curve prefix ac_prefix is called horizon.
The second component of ac_prefix is called steps.
The time steps of ac_prefix are the first components of the
steps. That is, these are time instances before the horizon
where the corresponding arrival curve makes a step.
Definition step_at (ac_prefix : ArrivalCurvePrefix) (t : duration) :=
last (0, 0) [ seq step <- steps_of ac_prefix | fst step ≤ t ].
(* The function value_at returns the _value_ of the last step
(duration × value) such that duration ≤ t *)
Definition value_at (ac_prefix : ArrivalCurvePrefix) (t : duration) :=
snd (step_at ac_prefix t).
last (0, 0) [ seq step <- steps_of ac_prefix | fst step ≤ t ].
(* The function value_at returns the _value_ of the last step
(duration × value) such that duration ≤ t *)
Definition value_at (ac_prefix : ArrivalCurvePrefix) (t : duration) :=
snd (step_at ac_prefix t).
Finally, we define a function extrapolated_arrival_curve that
performs the periodic extension of the arrival-curve prefix (and
hence, defines an arrival curve).
Value of extrapolated_arrival_curve t is defined as
t %/ h × value_at horizon plus value_at (t mod horizon).
The first summand corresponds to k full repetitions of the
arrival-curve prefix inside interval
[0,t). The second summand
corresponds to the residual change inside interval [k*h, t).
Definition extrapolated_arrival_curve (ac_prefix : ArrivalCurvePrefix) (t : duration) :=
let h := horizon_of ac_prefix in
t %/ h × value_at ac_prefix h + value_at ac_prefix (t %% h).
let h := horizon_of ac_prefix in
t %/ h × value_at ac_prefix h + value_at ac_prefix (t %% h).
In the following section, we define a few validity predicates.
Horizon should be positive.
Horizon should bound time steps.
Definition large_horizon (ac_prefix : ArrivalCurvePrefix) :=
∀ s, s \in time_steps_of ac_prefix → s ≤ horizon_of ac_prefix.
∀ s, s \in time_steps_of ac_prefix → s ≤ horizon_of ac_prefix.
Steps should be strictly increasing both in time steps and values.
Definition ltn_steps a b := (fst a < fst b) && (snd a < snd b).
Definition sorted_ltn_steps (ac_prefix : ArrivalCurvePrefix) :=
sorted ltn_steps (steps_of ac_prefix).
Definition sorted_ltn_steps (ac_prefix : ArrivalCurvePrefix) :=
sorted ltn_steps (steps_of ac_prefix).
The conjunction of the 5 afore-defined properties defines a
valid arrival-curve prefix.
Definition valid_arrival_curve_prefix (ac_prefix : ArrivalCurvePrefix) :=
positive_horizon ac_prefix
∧ large_horizon ac_prefix
∧ no_inf_arrivals ac_prefix
∧ specified_bursts ac_prefix
∧ sorted_ltn_steps ac_prefix.
positive_horizon ac_prefix
∧ large_horizon ac_prefix
∧ no_inf_arrivals ac_prefix
∧ specified_bursts ac_prefix
∧ sorted_ltn_steps ac_prefix.
We also define a predicate for non-decreasing order that is
more convenient for proving some of the claims.