# Library Coq.PArith.Pnat

Require Import BinPos PeanoNat.

Properties of the injection from binary positive numbers to Peano natural numbers
Original development by Pierre Crégut, CNET, Lannion, France

Local Open Scope positive_scope.
Local Open Scope nat_scope.

Module Pos2Nat.
Import Pos.

Pos.to_nat is a morphism for successor, addition, multiplication

Lemma inj_succ p : to_nat (succ p) = S (to_nat p).

Theorem inj_add p q : to_nat (p + q) = to_nat p + to_nat q.

Theorem inj_mul p q : to_nat (p * q) = to_nat p * to_nat q.

Mapping of xH, xO and xI through Pos.to_nat

Lemma inj_1 : to_nat 1 = 1.

Lemma inj_xO p : to_nat (xO p) = 2 * to_nat p.

Lemma inj_xI p : to_nat (xI p) = S (2 * to_nat p).

Pos.to_nat maps to the strictly positive subset of nat

Lemma is_succ : forall p, exists n, to_nat p = S n.

Pos.to_nat is strictly positive

Lemma is_pos p : 0 < to_nat p.

Pos.to_nat is a bijection between positive and non-zero nat, with Pos.of_nat as reciprocal. See Nat2Pos.id below for the dual equation.

Theorem id p : of_nat (to_nat p) = p.

Pos.to_nat is hence injective

Lemma inj p q : to_nat p = to_nat q -> p = q.

Lemma inj_iff p q : to_nat p = to_nat q <-> p = q.

Pos.to_nat is a morphism for comparison

Lemma inj_compare p q : (p ?= q)%positive = (to_nat p ?= to_nat q).

Pos.to_nat is a morphism for lt, le, etc

Lemma inj_lt p q : (p < q)%positive <-> to_nat p < to_nat q.

Lemma inj_le p q : (p <= q)%positive <-> to_nat p <= to_nat q.

Lemma inj_gt p q : (p > q)%positive <-> to_nat p > to_nat q.

Lemma inj_ge p q : (p >= q)%positive <-> to_nat p >= to_nat q.

Pos.to_nat is a morphism for subtraction

Theorem inj_sub p q : (q < p)%positive ->
to_nat (p - q) = to_nat p - to_nat q.

Theorem inj_sub_max p q :
to_nat (p - q) = Nat.max 1 (to_nat p - to_nat q).

Theorem inj_pred p : (1 < p)%positive ->
to_nat (pred p) = Nat.pred (to_nat p).

Theorem inj_pred_max p :
to_nat (pred p) = Nat.max 1 (Peano.pred (to_nat p)).

Pos.to_nat and other operations

Lemma inj_min p q :
to_nat (min p q) = Nat.min (to_nat p) (to_nat q).

Lemma inj_max p q :
to_nat (max p q) = Nat.max (to_nat p) (to_nat q).

Theorem inj_iter :
forall p {A} (f:A->A) (x:A),
Pos.iter f x p = nat_rect _ x (fun _ => f) (to_nat p).

End Pos2Nat.

Module Nat2Pos.

Pos.of_nat is a bijection between non-zero nat and positive, with Pos.to_nat as reciprocal. See Pos2Nat.id above for the dual equation.

Theorem id (n:nat) : n<>0 -> Pos.to_nat (Pos.of_nat n) = n.

Theorem id_max (n:nat) : Pos.to_nat (Pos.of_nat n) = max 1 n.

Pos.of_nat is hence injective for non-zero numbers

Lemma inj (n m : nat) : n<>0 -> m<>0 -> Pos.of_nat n = Pos.of_nat m -> n = m.

Lemma inj_iff (n m : nat) : n<>0 -> m<>0 ->
(Pos.of_nat n = Pos.of_nat m <-> n = m).

Usual operations are morphisms with respect to Pos.of_nat for non-zero numbers.

Lemma inj_succ (n:nat) : n<>0 -> Pos.of_nat (S n) = Pos.succ (Pos.of_nat n).

Lemma inj_pred (n:nat) : Pos.of_nat (pred n) = Pos.pred (Pos.of_nat n).

Lemma inj_add (n m : nat) : n<>0 -> m<>0 ->
Pos.of_nat (n+m) = (Pos.of_nat n + Pos.of_nat m)%positive.

Lemma inj_mul (n m : nat) : n<>0 -> m<>0 ->
Pos.of_nat (n*m) = (Pos.of_nat n * Pos.of_nat m)%positive.

Lemma inj_compare (n m : nat) : n<>0 -> m<>0 ->
(n ?= m) = (Pos.of_nat n ?= Pos.of_nat m)%positive.

Lemma inj_sub (n m : nat) : m<>0 ->
Pos.of_nat (n-m) = (Pos.of_nat n - Pos.of_nat m)%positive.

Lemma inj_min (n m : nat) :
Pos.of_nat (min n m) = Pos.min (Pos.of_nat n) (Pos.of_nat m).

Lemma inj_max (n m : nat) :
Pos.of_nat (max n m) = Pos.max (Pos.of_nat n) (Pos.of_nat m).

End Nat2Pos.

Properties of the shifted injection from Peano natural numbers to binary positive numbers

Module Pos2SuccNat.

Composition of Pos.to_nat and Pos.of_succ_nat is successor on positive
Composition of Pos.to_nat, Pos.of_succ_nat and Pos.pred is identity on positive
Composition of Pos.of_succ_nat and Pos.to_nat is successor on nat
Pos.of_succ_nat is hence injective
Another formulation
Successor and comparison are morphisms with respect to Pos.of_succ_nat
Other operations, for instance Pos.add and plus aren't directly related this way (we would need to compensate for the successor hidden in Pos.of_succ_nat
For compatibility, old names and old-style lemmas

Notation Psucc_S := Pos2Nat.inj_succ (only parsing).
Notation Pplus_plus := Pos2Nat.inj_add (only parsing).
Notation Pmult_mult := Pos2Nat.inj_mul (only parsing).
Notation Pcompare_nat_compare := Pos2Nat.inj_compare (only parsing).
Notation nat_of_P_xH := Pos2Nat.inj_1 (only parsing).
Notation nat_of_P_xO := Pos2Nat.inj_xO (only parsing).
Notation nat_of_P_xI := Pos2Nat.inj_xI (only parsing).
Notation nat_of_P_is_S := Pos2Nat.is_succ (only parsing).
Notation nat_of_P_pos := Pos2Nat.is_pos (only parsing).
Notation nat_of_P_inj_iff := Pos2Nat.inj_iff (only parsing).
Notation nat_of_P_inj := Pos2Nat.inj (only parsing).
Notation Plt_lt := Pos2Nat.inj_lt (only parsing).
Notation Pgt_gt := Pos2Nat.inj_gt (only parsing).
Notation Ple_le := Pos2Nat.inj_le (only parsing).
Notation Pge_ge := Pos2Nat.inj_ge (only parsing).
Notation Pminus_minus := Pos2Nat.inj_sub (only parsing).
Notation iter_nat_of_P := @Pos2Nat.inj_iter (only parsing).

Notation nat_of_P_of_succ_nat := SuccNat2Pos.id_succ (only parsing).
Notation P_of_succ_nat_of_P := Pos2SuccNat.id_succ (only parsing).

Notation nat_of_P_succ_morphism := Pos2Nat.inj_succ (only parsing).
Notation nat_of_P_plus_morphism := Pos2Nat.inj_add (only parsing).
Notation nat_of_P_mult_morphism := Pos2Nat.inj_mul (only parsing).
Notation nat_of_P_compare_morphism := Pos2Nat.inj_compare (only parsing).
Notation lt_O_nat_of_P := Pos2Nat.is_pos (only parsing).
Notation ZL4 := Pos2Nat.is_succ (only parsing).
Notation nat_of_P_o_P_of_succ_nat_eq_succ := SuccNat2Pos.id_succ (only parsing).
Notation P_of_succ_nat_o_nat_of_P_eq_succ := Pos2SuccNat.id_succ (only parsing).
Notation pred_o_P_of_succ_nat_o_nat_of_P_eq_id := Pos2SuccNat.pred_id (only parsing).

Lemma nat_of_P_minus_morphism p q :
Pos.compare_cont Eq p q = Gt ->
Pos.to_nat (p - q) = Pos.to_nat p - Pos.to_nat q.

Lemma nat_of_P_lt_Lt_compare_morphism p q :
Pos.compare_cont Eq p q = Lt -> Pos.to_nat p < Pos.to_nat q.

Lemma nat_of_P_gt_Gt_compare_morphism p q :
Pos.compare_cont Eq p q = Gt -> Pos.to_nat p > Pos.to_nat q.

Lemma nat_of_P_lt_Lt_compare_complement_morphism p q :
Pos.to_nat p < Pos.to_nat q -> Pos.compare_cont Eq p q = Lt.

Definition nat_of_P_gt_Gt_compare_complement_morphism p q :
Pos.to_nat p > Pos.to_nat q -> Pos.compare_cont Eq p q = Gt.
Proof (proj2 (Pos2Nat.inj_gt p q)).