Section ListHybrid.
Variable A B State : Set.
Variable defA : A.
Variable c1 c2 : A -> Comp B.
Hypothesis c1_wf : forall a, well_formed_comp (c1 a).
Hypothesis c2_wf : forall a, well_formed_comp (c2 a).
Hypothesis A_EqDec : EqDec A.
Hypothesis B_EqDec : EqDec B.
Hypothesis State_EqDec : EqDec State.
Variable A1 : Comp ((list A) * State).
Variable A2 : State -> list B -> Comp bool.
Variable maxA : nat.
Hypothesis maxA_correct :
forall ls s_A,
In (ls, s_A) (getSupport A1) ->
(length ls <= maxA)%nat.
Definition ListHybrid_G (c : A -> Comp B) :=
[lsA, s_A] <-$2 A1;
lsB <-$ compMap _ c lsA;
A2 s_A lsB.
Definition ListHybrid_Advantage :=
| Pr[ListHybrid_G c1] - Pr[ListHybrid_G c2] |.
Definition G_hybrid i :=
[lsA, s_A] <-$2 A1;
lsA1 <- firstn i lsA;
lsA2 <- skipn i lsA;
lsB1 <-$ compMap _ c1 lsA1;
lsB2 <-$ compMap _ c2 lsA2;
A2 s_A (lsB1 ++ lsB2).
unfold ListHybrid_G, G_hybrid.
comp_skip.
comp_simp.
eapply comp_spec_eq_impl_eq.
simpl.
comp_simp.
simpl.
eapply comp_spec_eq_refl.
Qed.
induction ls; destruct n; intuition; simpl in *.
omega.
eapply IHls.
omega.
Qed.
unfold ListHybrid_G, G_hybrid.
comp_skip.
comp_simp.
rewrite firstn_ge_all.
comp_skip.
rewrite skipn_ge_nil.
eapply comp_spec_eq_impl_eq.
simpl.
comp_simp.
rewrite app_nil_r.
eapply comp_spec_eq_refl.
eauto.
eauto.
Qed.
Definition B1 i :=
[lsA, s_A] <-$2 A1;
lsA1 <- firstn i lsA;
a <- nth i lsA defA;
lsA2 <- skipn (S i) lsA;
incla <- if (lt_dec i (length lsA)) then true else false;
ret (a, (incla, lsA1, lsA2, s_A)).
Definition B2 (e : bool * _ * _ * _) b :=
let '(incla, lsA1, lsA2, s_A) := e in
lsB1 <-$ compMap _ c1 lsA1;
lsB2 <-$ compMap _ c2 lsA2;
lsB <- if incla then (lsB1 ++ (b :: lsB2)) else lsB1;
A2 s_A lsB.
Local Open Scope list_scope.
induction ls; intuition; simpl in *.
comp_skip.
Qed.

Theorem skipn_lt_nil_false :
forall (A : Type)(ls : list A)(n : nat),
n < length ls ->
skipn n ls = nil ->
False.