(************************************************************************)
(*         *      The Rocq Prover / The Rocq Development Team           *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
(* <O___,, * (see version control and CREDITS file for authors & dates) *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
(************************************************************************)

From Stdlib Require Import Rbase.
From Stdlib Require Import Rfunctions.
From Stdlib Require Import SeqSeries.
From Stdlib Require Import Rtrigo1.
From Stdlib Require Import R_sqrt.
#[local] Open Scope R_scope.

Lemma tan_PI : tan PI = 0.
Proof.
  unfold tan; rewrite sin_PI; rewrite cos_PI; unfold Rdiv;
    apply Rmult_0_l.
Qed.

Lemma sin_3PI2 : sin (3 * (PI / 2)) = -1.
Proof.
  replace (3 * (PI / 2)) with (PI + PI / 2).
  - rewrite sin_plus; rewrite sin_PI; rewrite cos_PI; rewrite sin_PI2; ring.
  - pattern PI at 1; rewrite <-(Rplus_half_diag PI); ring.
Qed.

Lemma tan_2PI : tan (2 * PI) = 0.
Proof.
  unfold tan; rewrite sin_2PI; unfold Rdiv; apply Rmult_0_l.
Qed.

Lemma sin_cos_PI4 : sin (PI / 4) = cos (PI / 4).
Proof.
  rewrite cos_sin.
  replace (PI / 2 + PI / 4) with (- (PI / 4) + PI) by field.
  rewrite neg_sin, sin_neg; ring.
Qed.

Lemma sin_PI3_cos_PI6 : sin (PI / 3) = cos (PI / 6).
Proof.
  replace (PI / 6) with (PI / 2 - PI / 3) by field.
  now rewrite cos_shift.
Qed.

Lemma sin_PI6_cos_PI3 : cos (PI / 3) = sin (PI / 6).
Proof.
  replace (PI / 6) with (PI / 2 - PI / 3) by field.
  now rewrite sin_shift.
Qed.

Lemma PI6_RGT_0 : 0 < PI / 6.
Proof.
  unfold Rdiv; apply Rmult_lt_0_compat;
    [ apply PI_RGT_0 | apply Rinv_0_lt_compat; prove_sup0 ].
Qed.

Lemma PI6_RLT_PI2 : PI / 6 < PI / 2.
Proof.
  unfold Rdiv; apply Rmult_lt_compat_l.
  - apply PI_RGT_0.
  - apply Rinv_lt_contravar; prove_sup.
Qed.

Lemma sin_PI6 : sin (PI / 6) = 1 / 2.
Proof.
  apply Rmult_eq_reg_l with (2 * cos (PI / 6)).
  - replace (2 * cos (PI / 6) * sin (PI / 6)) with
      (2 * sin (PI / 6) * cos (PI / 6)) by ring.
    rewrite <- sin_2a; replace (2 * (PI / 6)) with (PI / 3) by field.
    rewrite sin_PI3_cos_PI6.
    field.
  - apply prod_neq_R0.
    + discrR.
    + cut (0 < cos (PI / 6));
        [ intro H1; auto with real
        | apply cos_gt_0;
          [ apply (Rlt_trans (- (PI / 2)) 0 (PI / 6) _PI2_RLT_0 PI6_RGT_0)
          | apply PI6_RLT_PI2 ] ].
Qed.

Lemma sqrt2_neq_0 : sqrt 2 <> 0.
Proof.
  assert (Hyp : 0 < 2);
    [ prove_sup0
      | generalize (Rlt_le 0 2 Hyp); intro H1; red; intro H2;
        generalize (sqrt_eq_0 2 H1 H2); intro H; absurd (2 = 0);
          [ discrR | assumption ] ].
Qed.

Lemma R1_sqrt2_neq_0 : 1 / sqrt 2 <> 0.
Proof.
  generalize (Rinv_neq_0_compat (sqrt 2) sqrt2_neq_0); intro H;
    generalize (prod_neq_R0 1 (/ sqrt 2) R1_neq_R0 H);
      intro H0; assumption.
Qed.

Lemma sqrt3_2_neq_0 : 2 * sqrt 3 <> 0.
Proof.
  apply prod_neq_R0;
    [ discrR
      | assert (Hyp : 0 < 3);
        [ prove_sup0
          | generalize (Rlt_le 0 3 Hyp); intro H1; red; intro H2;
            generalize (sqrt_eq_0 3 H1 H2); intro H; absurd (3 = 0);
              [ discrR | assumption ] ] ].
Qed.

Lemma Rlt_sqrt2_0 : 0 < sqrt 2.
Proof.
  assert (Hyp : 0 < 2);
    [ prove_sup0
      | generalize (sqrt_positivity 2 (Rlt_le 0 2 Hyp)); intro H1; elim H1;
        intro H2;
          [ assumption
            | absurd (0 = sqrt 2);
              [ apply (not_eq_sym (A:=R)); apply sqrt2_neq_0 | assumption ] ] ].
Qed.

Lemma Rlt_sqrt3_0 : 0 < sqrt 3.
Proof.
  cut (0%nat <> 1%nat);
    [ intro H0; assert (Hyp : 0 < 2);
      [ prove_sup0
        | generalize (Rlt_le 0 2 Hyp); intro H1; assert (Hyp2 : 0 < 3);
          [ prove_sup0
            | generalize (Rlt_le 0 3 Hyp2); intro H2;
              generalize (lt_INR_0 1 (proj1 (Nat.neq_0_lt_0 1) (Nat.neq_sym 0 1 H0)));
                unfold INR; intro H3;
                  generalize (Rplus_lt_compat_l 2 0 1 H3);
                    rewrite Rplus_comm; rewrite Rplus_0_l; replace (2 + 1) with 3;
                      [ intro H4; generalize (sqrt_lt_1 2 3 H1 H2 H4); clear H3; intro H3;
                        apply (Rlt_trans 0 (sqrt 2) (sqrt 3) Rlt_sqrt2_0 H3)
                        | ring ] ] ]
      | discriminate ].
Qed.

Lemma PI4_RGT_0 : 0 < PI / 4.
Proof.
  unfold Rdiv; apply Rmult_lt_0_compat;
    [ apply PI_RGT_0 | apply Rinv_0_lt_compat; prove_sup0 ].
Qed.

Lemma cos_PI4 : cos (PI / 4) = 1 / sqrt 2.
Proof.
  apply Rsqr_inj; trivial.
  - apply cos_ge_0; trivial.
    + left; apply (Rlt_trans (- (PI / 2)) 0 (PI / 4) _PI2_RLT_0 PI4_RGT_0); trivial.
    + left; apply PI4_RLT_PI2; trivial.
  - left; apply (Rmult_lt_0_compat 1 (/ sqrt 2)); trivial.
    + prove_sup; trivial.
    + apply Rinv_0_lt_compat; apply Rlt_sqrt2_0; trivial.
  - rewrite Rsqr_div'.
    rewrite Rsqr_1; rewrite Rsqr_sqrt; trivial.
    + unfold Rsqr; pattern (cos (PI / 4)) at 1;
        rewrite <- sin_cos_PI4;
        replace (sin (PI / 4) * cos (PI / 4)) with
        (1 / 2 * (2 * sin (PI / 4) * cos (PI / 4))) by field.
      rewrite <- sin_2a; replace (2 * (PI / 4)) with (PI / 2) by field.
      rewrite sin_PI2; trivial.
      field.
    + left; prove_sup; trivial.
Qed.

Lemma sin_PI4 : sin (PI / 4) = 1 / sqrt 2.
Proof.
  rewrite sin_cos_PI4; apply cos_PI4.
Qed.

Lemma tan_PI4 : tan (PI / 4) = 1.
Proof.
  unfold tan; rewrite sin_cos_PI4.
  unfold Rdiv; apply Rinv_r.
  change (cos (PI / 4) <> 0); rewrite cos_PI4; apply R1_sqrt2_neq_0.
Qed.

Lemma cos_3PI4 : cos (3 * (PI / 4)) = -1 / sqrt 2.
Proof.
  replace (3 * (PI / 4)) with (PI / 2 - - (PI / 4)) by field.
  rewrite cos_shift; rewrite sin_neg; rewrite sin_PI4.
  unfold Rdiv.
  ring.
Qed.

Lemma sin_3PI4 : sin (3 * (PI / 4)) = 1 / sqrt 2.
Proof.
  replace (3 * (PI / 4)) with (PI / 2 - - (PI / 4)) by field.
  now rewrite sin_shift, cos_neg, cos_PI4.
Qed.

Lemma cos_PI6 : cos (PI / 6) = sqrt 3 / 2.
Proof.
  apply Rsqr_inj; trivial.
  - apply cos_ge_0; trivial.
    + left; apply (Rlt_trans (- (PI / 2)) 0 (PI / 6) _PI2_RLT_0 PI6_RGT_0); trivial.
    + left; apply PI6_RLT_PI2; trivial.
  - left; apply (Rmult_lt_0_compat (sqrt 3) (/ 2)); trivial.
    + apply Rlt_sqrt3_0; trivial.
    + apply Rinv_0_lt_compat; prove_sup0; trivial.
  - rewrite Rsqr_div'.
    rewrite cos2; unfold Rsqr; rewrite sin_PI6; rewrite sqrt_def; trivial.
    + field.
    + left ; prove_sup0.
Qed.

Lemma tan_PI6 : tan (PI / 6) = 1 / sqrt 3.
Proof.
  unfold tan; rewrite sin_PI6; rewrite cos_PI6; unfold Rdiv;
    repeat rewrite Rmult_1_l; rewrite Rinv_mult.
  rewrite Rinv_inv.
  rewrite (Rmult_comm (/ 2)); rewrite Rmult_assoc; rewrite Rinv_r.
  - apply Rmult_1_r.
  - discrR.
Qed.

Lemma sin_PI3 : sin (PI / 3) = sqrt 3 / 2.
Proof.
  rewrite sin_PI3_cos_PI6; apply cos_PI6.
Qed.

Lemma cos_PI3 : cos (PI / 3) = 1 / 2.
Proof.
  rewrite sin_PI6_cos_PI3; apply sin_PI6.
Qed.

Lemma tan_PI3 : tan (PI / 3) = sqrt 3.
Proof.
  unfold tan; rewrite sin_PI3; rewrite cos_PI3; unfold Rdiv;
    rewrite Rmult_1_l; rewrite Rinv_inv.
  rewrite Rmult_assoc; rewrite Rinv_l.
  - apply Rmult_1_r.
  - discrR.
Qed.

Lemma sin_2PI3 : sin (2 * (PI / 3)) = sqrt 3 / 2.
Proof.
  rewrite <-Rplus_diag; rewrite sin_plus; rewrite sin_PI3; rewrite cos_PI3;
    unfold Rdiv; repeat rewrite Rmult_1_l; rewrite (Rmult_comm (/ 2));
      repeat rewrite <- Rmult_assoc; rewrite <-Rplus_half_diag;
        reflexivity.
Qed.

Lemma cos_2PI3 : cos (2 * (PI / 3)) = -1 / 2.
Proof.
  rewrite cos_2a, sin_PI3, cos_PI3.
  replace (sqrt 3 / 2 * (sqrt 3 / 2)) with ((sqrt 3 * sqrt 3) / 4) by field.
  rewrite sqrt_sqrt.
  - field.
  - left ; prove_sup0.
Qed.

Lemma tan_2PI3 : tan (2 * (PI / 3)) = - sqrt 3.
Proof.
  unfold tan; rewrite sin_2PI3, cos_2PI3.
  field.
Qed.

Lemma cos_5PI4 : cos (5 * (PI / 4)) = -1 / sqrt 2.
Proof.
  replace (5 * (PI / 4)) with (PI / 4 + PI) by field.
  rewrite neg_cos; rewrite cos_PI4; unfold Rdiv.
  ring.
Qed.

Lemma sin_5PI4 : sin (5 * (PI / 4)) = -1 / sqrt 2.
Proof.
  replace (5 * (PI / 4)) with (PI / 4 + PI) by field.
  rewrite neg_sin; rewrite sin_PI4; unfold Rdiv.
  ring.
Qed.

Lemma sin_cos5PI4 : cos (5 * (PI / 4)) = sin (5 * (PI / 4)).
Proof.
  rewrite cos_5PI4; rewrite sin_5PI4; reflexivity.
Qed.

Lemma Rgt_3PI2_0 : 0 < 3 * (PI / 2).
Proof.
  apply Rmult_lt_0_compat;
    [ prove_sup0
      | unfold Rdiv; apply Rmult_lt_0_compat;
        [ apply PI_RGT_0 | apply Rinv_0_lt_compat; prove_sup0 ] ].
Qed.

Lemma Rgt_2PI_0 : 0 < 2 * PI.
Proof.
  apply Rmult_lt_0_compat; [ prove_sup0 | apply PI_RGT_0 ].
Qed.

Lemma Rlt_PI_3PI2 : PI < 3 * (PI / 2).
Proof.
  generalize PI2_RGT_0; intro H1;
    generalize (Rplus_lt_compat_l PI 0 (PI / 2) H1);
      replace (PI + PI / 2) with (3 * (PI / 2)).
  - rewrite Rplus_0_r; intro H2; assumption.
  - pattern PI at 2; rewrite <-Rplus_half_diag; ring.
Qed.

Lemma Rlt_3PI2_2PI : 3 * (PI / 2) < 2 * PI.
Proof.
  generalize PI2_RGT_0; intro H1;
    generalize (Rplus_lt_compat_l (3 * (PI / 2)) 0 (PI / 2) H1);
      replace (3 * (PI / 2) + PI / 2) with (2 * PI).
  - rewrite Rplus_0_r; intro H2; assumption.
  - rewrite <-Rplus_diag; pattern PI at 1 2; rewrite <-Rplus_half_diag; ring.
Qed.

(***************************************************************)
(** Radian -> Degree | Degree -> Radian                        *)
(***************************************************************)

Definition plat : R := 180.
Definition toRad (x:R) : R := x * PI * / plat.
Definition toDeg (x:R) : R := x * plat * / PI.

Lemma rad_deg : forall x:R, toRad (toDeg x) = x.
Proof.
  intro; unfold toRad, toDeg;
    replace (x * plat * / PI * PI * / plat) with
    (x * (plat * / plat) * (PI * / PI)); [ idtac | ring ].
  repeat rewrite Rinv_r.
  - ring.
  - apply PI_neq0.
  - unfold plat; discrR.
Qed.

Lemma toRad_inj : forall x y:R, toRad x = toRad y -> x = y.
Proof.
  intros; unfold toRad in H; apply Rmult_eq_reg_l with PI.
  - rewrite <- (Rmult_comm x); rewrite <- (Rmult_comm y).
    apply Rmult_eq_reg_l with (/ plat).
    + rewrite <- (Rmult_comm (x * PI)); rewrite <- (Rmult_comm (y * PI));
        assumption.
    + apply Rinv_neq_0_compat; unfold plat; discrR.
  - apply PI_neq0.
Qed.

Lemma deg_rad : forall x:R, toDeg (toRad x) = x.
Proof.
  intro x; apply toRad_inj; rewrite (rad_deg (toRad x)); reflexivity.
Qed.

Definition sind (x:R) : R := sin (toRad x).
Definition cosd (x:R) : R := cos (toRad x).
Definition tand (x:R) : R := tan (toRad x).

Lemma Rsqr_sin_cos_d_one : forall x:R, Rsqr (sind x) + Rsqr (cosd x) = 1.
Proof.
  intro x; unfold sind; unfold cosd; apply sin2_cos2.
Qed.

(***************************************************)
(**               Other properties                 *)
(***************************************************)

Lemma sin_lb_ge_0 : forall a:R, 0 <= a -> a <= PI / 2 -> 0 <= sin_lb a.
Proof.
  intros; case (Rtotal_order 0 a); intro.
  - left; apply sin_lb_gt_0; assumption.
  - elim H1; intro.
    + rewrite <- H2; unfold sin_lb; unfold sin_approx;
        unfold sum_f_R0; unfold sin_term;
        repeat rewrite pow_ne_zero.
      * unfold Rdiv; repeat rewrite Rmult_0_l; repeat rewrite Rmult_0_r;
          repeat rewrite Rplus_0_r; right; reflexivity.
      * discriminate.
      * discriminate.
      * discriminate.
      * discriminate.
    + elim (Rlt_irrefl 0 (Rle_lt_trans 0 a 0 H H2)).
Qed.
