AdventOfCode-2020/day8.v

99 lines
3.7 KiB
Coq
Raw Normal View History

2020-12-11 11:32:54 -08:00
Require Import Coq.ZArith.Int.
Require Import Coq.Lists.ListSet.
Require Import Coq.Vectors.VectorDef.
Require Import Coq.Vectors.Fin.
Module DayEight (Import M:Int).
(* We need to coerce natural numbers into integers to add them. *)
Parameter nat_to_t : nat -> t.
(* We need a way to convert integers back into finite sets. *)
Parameter clamp : forall {n}, t -> option (Fin.t n).
Definition fin := Fin.t.
(* The opcode of our instructions. *)
Inductive opcode : Type :=
| add
| nop
| jmp.
(* The result of running a program is either the accumulator
or an infinite loop error. In the latter case, we return the
set of instructions that we tried. *)
Inductive run_result {n : nat} : Type :=
| Ok : t -> run_result
| Fail : set (fin n) -> run_result.
Definition state n : Type := (fin (S n) * set (fin n) * t).
(* An instruction is a pair of an opcode and an argument. *)
Definition inst : Type := (opcode * t).
(* An input is a bounded list of instructions. *)
Definition input (n : nat) := VectorDef.t inst n.
(* 'indices' represents the list of instruction
addresses, which are used for calculating jumps. *)
Definition indices (n : nat) := VectorDef.t (fin n) n.
(* Compute the destination jump index, an integer. *)
Definition jump_t {n} (pc : fin n) (off : t) : t :=
M.add (nat_to_t (proj1_sig (to_nat pc))) off.
(* Compute a destination index that's valid.
Not all inputs are valid, so this may fail. *)
Definition valid_jump_t {n} (pc : fin n) (off : t) : option (fin (S n)) := @clamp (S n) (jump_t pc off).
Definition weaken_one {n} (f : fin n) : fin (S n).
Proof.
apply (@cast (n + 1)).
+ apply L. apply f.
+ rewrite <- plus_n_Sm. rewrite <- plus_n_O. reflexivity.
Defined.
Inductive step_noswap {n} : input n -> state n -> state n -> Prop :=
| step_noswap_acc : forall inp pc' v acc t,
nth inp pc' = (add, t) ->
~ set_mem Fin.eq_dec pc' v = true ->
step_noswap inp (weaken_one pc', v, acc) (FS pc', set_add Fin.eq_dec pc' v, M.add acc t)
| step_noswap_nop : forall inp pc' v acc t,
nth inp pc' = (nop, t) ->
~ set_mem Fin.eq_dec pc' v = true ->
step_noswap inp (weaken_one pc', v, acc) (FS pc', set_add Fin.eq_dec pc' v, acc)
| step_noswap_jmp : forall inp pc' pc'' v acc t,
nth inp pc' = (jmp, t) ->
~ set_mem Fin.eq_dec pc' v = true ->
valid_jump_t pc' t = Some pc'' ->
step_noswap inp (weaken_one pc', v, acc) (pc'', set_add Fin.eq_dec pc' v, acc).
Fixpoint nat_to_fin (n : nat) : fin (S n) :=
match n with
| O => F1
| S n' => FS (nat_to_fin n')
end.
Inductive run_noswap {n} : input n -> state n -> state n -> Prop :=
| run_noswap_ok : forall inp v acc,
run_noswap inp (nat_to_fin n, v, acc) (nat_to_fin n, v, acc)
| run_noswap_fail : forall inp pc' v acc,
set_mem Fin.eq_dec pc' v = true ->
run_noswap inp (weaken_one pc', v, acc) (weaken_one pc', v, acc)
| run_noswap_trans : forall inp st st' st'',
step_noswap inp st st' -> run_noswap inp st' st'' -> run_noswap inp st st''.
Inductive valid_inst {n} : inst -> fin n -> Prop :=
| valid_inst_add : forall t f, valid_inst (add, t) f
| valid_inst_nop : forall t f f',
valid_jump_t f t = Some f' -> valid_inst (nop, t) f
| valid_inst_jmp : forall t f f',
valid_jump_t f t = Some f' -> valid_inst (jmp, t) f.
(* An input is valid if all its instructions are valid. *)
Definition valid_input {n} (inp : input n) : Prop := forall (pc : fin n),
valid_inst (nth inp pc) pc.
Theorem valid_input_terminates : forall n (inp : input n) st,
valid_input inp -> exists st', run_noswap inp st st'.
Proof.
(* Stoppped here. *)
Admitted.
End DayEight.