(** A list of moves [(src1, dst1), ..., (srcN, dstN)] can be given
  three different semantics, as environment transformers.
  The first semantics corresponds to a parallel assignment:
  [dst1, ..., dstN] are set to the initial values of [src1, ..., srcN]. *)

Fixpoint exec_par (m: moves) (e: env) {struct m}: env :=
  match m with
  | nil => e
  | (s, d) :: m' => update d (e s) (exec_par m' e)
  end.(** The second semantics corresponds to a sequence of individual
  assignments: first, [dst1] is set to the initial value of [src1];
  then, [dst2] is set to the current value of [src2] after the first
  assignment; etc. *)

Fixpoint exec_seq (m: moves) (e: env) {struct m}: env :=
  match m with
  | nil => e
  | (s, d) :: m' => exec_seq m' (update d (e s) e)
  end.(** The third semantics is also sequential, but executes the moves
  in reverse order, starting with [srcN, dstN] and finishing with
  [src1, dst1]. *)

Fixpoint exec_seq_rev (m: moves) (e: env) {struct m}: env :=
  match m with
  | nil => e
  | (s, d) :: m' =>
      let e' := exec_seq_rev m' e in
      update d (e' s) e'
  end.(** * Specification of the non-deterministic algorithm *)

(** Rideau and Serpette's parallel move compilation algorithm is first
   specified as a non-deterministic set of rewriting rules operating
   over triples [(mu, sigma, tau)] of moves.  We define these rules
   as an inductive predicate [transition] relating triples of moves,
   and its reflexive transitive closure [transitions]. *)

Inductive state: Type :=
  State (mu: moves) (sigma: moves) (tau: moves) : state.(** * Specification of the non-deterministic algorithm *)

(** Rideau and Serpette's parallel move compilation algorithm is first
   specified as a non-deterministic set of rewriting rules operating
   over triples [(mu, sigma, tau)] of moves.  We define these rules
   as an inductive predicate [transition] relating triples of moves,
   and its reflexive transitive closure [transitions]. *)

Inductive state: Type :=
  State (mu: moves) (sigma: moves) (tau: moves) : state.Definition no_read (mu: moves) (d: reg) : Prop :=
  ~In d (srcs mu).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Inductive transition: state -> state -> Prop :=
  | tr_nop: forall mu1 r mu2 sigma tau,
      transition (State (mu1 ++ (r, r) :: mu2) sigma tau)
                 (State (mu1 ++ mu2) sigma tau)
  | tr_start: forall mu1 s d mu2 tau,
      transition (State (mu1 ++ (s, d) :: mu2) nil tau)
                 (State (mu1 ++ mu2) ((s, d) :: nil) tau)
  | tr_push: forall mu1 d r mu2 s sigma tau,
      transition (State (mu1 ++ (d, r) :: mu2) ((s, d) :: sigma) tau)
                 (State (mu1 ++ mu2) ((d, r) :: (s, d) :: sigma) tau)
  | tr_loop: forall mu sigma s d tau,
      transition (State mu (sigma ++ (s, d) :: nil) tau)
                 (State mu (sigma ++ (temp s, d) :: nil) ((s, temp s) :: tau))
  | tr_pop: forall mu s0 d0 s1 d1 sigma tau,
      no_read mu d1 -> d1 <> s0 ->
      transition (State mu ((s1, d1) :: sigma ++ (s0, d0) :: nil) tau)
                 (State mu (sigma ++ (s0, d0) :: nil) ((s1, d1) :: tau))
  | tr_last: forall mu s d tau,
      no_read mu d ->
      transition (State mu ((s, d) :: nil) tau)
                 (State mu nil ((s, d) :: tau)).Definition transitions: state -> state -> Prop :=
  clos_refl_trans state transition.(** ** Well-formedness properties of states *)

(** A state [mu, sigma, tau] is well-formed if the following properties hold:

- [mu] concatenated with [sigma] is a ``mill'', that is, no destination
  appears twice (predicate [is_mill] below).
- No temporary register appears in [mu] (predicate [move_no_temp]).
- No temporary register appears in [sigma] except perhaps as the source
  of the last move in [sigma] (predicate [temp_last]).
- [sigma] is a ``path'', that is, the source of a move is the destination
  of the following move.
*)

Definition is_mill (m: moves) : Prop :=
  list_norepet (dests m).Definition is_not_temp (r: reg) : Prop :=
  forall d, r <> temp d.Definition move_no_temp (m: moves) : Prop :=
  forall s d, In (s, d) m -> is_not_temp s /\ is_not_temp d.Definition temp_last (m: moves) : Prop :=
  match List.rev m with
  | nil => True
  | (s, d) :: m' => is_not_temp d /\ move_no_temp m'
  end.Definition is_first_dest (m: moves) (d: reg) : Prop :=
  match m with
  | nil => True
  | (s0, d0) :: _ => d = d0
  end.Inductive is_path: moves -> Prop :=
  | is_path_nil:
      is_path nil
  | is_path_cons: forall s d m,
      is_first_dest m s ->
      is_path m ->
      is_path ((s, d) :: m).Inductive is_path: moves -> Prop :=
  | is_path_nil:
      is_path nil
  | is_path_cons: forall s d m,
      is_first_dest m s ->
      is_path m ->
      is_path ((s, d) :: m).Inductive is_path: moves -> Prop :=
  | is_path_nil:
      is_path nil
  | is_path_cons: forall s d m,
      is_first_dest m s ->
      is_path m ->
      is_path ((s, d) :: m).Inductive state_wf: state -> Prop :=
  | state_wf_intro:
      forall mu sigma tau,
      is_mill (mu ++ sigma) ->
      move_no_temp mu ->
      temp_last sigma ->
      is_path sigma ->
      state_wf (State mu sigma tau).Inductive state_wf: state -> Prop :=
  | state_wf_intro:
      forall mu sigma tau,
      is_mill (mu ++ sigma) ->
      move_no_temp mu ->
      temp_last sigma ->
      is_path sigma ->
      state_wf (State mu sigma tau).

Lemma srcs_decomp: forall m1 s d m2, srcs (m1 ++ (s, d) :: m2) = srcs m1 ++ s :: srcs m2.