Init.Data.Nat.Dvd

@[simp] protected theorem dvd_refl (a : Nat) : a ∣ a := ⟨1, by simp⟩

@[simp] protected theorem dvd_zero (a : Nat) : a ∣ 0 := ⟨0, by simp⟩

protected theorem dvd_mul_left (a b : Nat) : a ∣ b * a := ⟨b, Nat.mul_comm b a⟩

protected theorem dvd_mul_right (a b : Nat) : a ∣ a * b := ⟨b, rfl⟩

protected theorem dvd_trans {a b c : Nat} (h₁ : a ∣ b) (h₂ : b ∣ c) : a ∣ c :=
  match h₁, h₂ with
  | ⟨d, (h₃ : b = a * d)⟩, ⟨e, (h₄ : c = b * e)⟩ =>
    ⟨d * e, show c = a * (d * e) by simp[h₃,h₄, Nat.mul_assoc]⟩

protected theorem dvd_mul_left_of_dvd {a b : Nat} (h : a ∣ b) (c : Nat) : a ∣ c * b :=
  Nat.dvd_trans h (Nat.dvd_mul_left _ _)

protected theorem dvd_mul_right_of_dvd {a b : Nat} (h : a ∣ b) (c : Nat) : a ∣ b * c :=
  Nat.dvd_trans h (Nat.dvd_mul_right _ _)

@[simp] protected theorem zero_dvd {n : Nat} : 0 ∣ n0 ∣ n ↔ n = 0 :=
  ⟨Nat.eq_zero_of_zero_dvd, fun h => h.symm ▸ Nat.dvd_zero 0⟩

protected theorem dvd_add {a b c : Nat} (h₁ : a ∣ b) (h₂ : a ∣ c) : a ∣ b + c :=
  let ⟨d, hd⟩ := h₁; let ⟨e, he⟩ := h₂; ⟨d + e, by simp [Nat.left_distrib, hd, he]⟩

protected theorem dvd_add_iff_right {k m n : Nat} (h : k ∣ m) : k ∣ nk ∣ n ↔ k ∣ m + n :=
  ⟨Nat.dvd_add h,
    match m, h with
    | _, ⟨d, rfl⟩ => fun ⟨e, he⟩ =>
      ⟨e - d, by rw [Nat.mul_sub_left_distrib, ← he, Nat.add_sub_cancel_left]⟩⟩

protected theorem dvd_add_iff_left {k m n : Nat} (h : k ∣ n) : k ∣ mk ∣ m ↔ k ∣ m + n := by
  rw [Nat.add_comm]; exact Nat.dvd_add_iff_right h

theorem dvd_mod_iff {k m n : Nat} (h: k ∣ n) : k ∣ m % nk ∣ m % n ↔ k ∣ m := by
  have := Nat.dvd_add_iff_left (m := m % n) <| Nat.dvd_trans h <| Nat.dvd_mul_right n (m / n)
  rwa [mod_add_div] at this

theorem le_of_dvd {m n : Nat} (h : 0 < n) : m ∣ n → m ≤ n
  | ⟨k, e⟩ => by
    revert h
    rw [e]
    match k with
    | 0 => intro hn; simp at hn
    | pk+1 =>
      intro
      have := Nat.mul_le_mul_left m (succ_pos pk)
      rwa [Nat.mul_one] at this

protected theorem dvd_antisymm : ∀ {m n : Nat}, m ∣ n → n ∣ m → m = n
  | _, 0, _, h₂ => Nat.eq_zero_of_zero_dvd h₂
  | 0, _, h₁, _ => (Nat.eq_zero_of_zero_dvd h₁).symm
  | _+1, _+1, h₁, h₂ => Nat.le_antisymm (le_of_dvd (succ_pos _) h₁) (le_of_dvd (succ_pos _) h₂)

theorem pos_of_dvd_of_pos {m n : Nat} (H1 : m ∣ n) (H2 : 0 < n) : 0 < m :=
  Nat.pos_of_ne_zero fun m0 => Nat.ne_of_gt H2 <| Nat.eq_zero_of_zero_dvd (m0 ▸ H1)

@[simp] protected theorem one_dvd (n : Nat) : 1 ∣ n := ⟨n, n.one_mul.symm⟩

theorem mod_eq_zero_of_dvd {m n : Nat} (H : m ∣ n) : n % m = 0 := by
  let ⟨z, H⟩ := H; rw [H, mul_mod_right]

theorem dvd_of_mod_eq_zero {m n : Nat} (H : n % m = 0) : m ∣ n := by
  exists n / m
  have := (mod_add_div n m).symm
  rwa [H, Nat.zero_add] at this

theorem dvd_iff_mod_eq_zero {m n : Nat} : m ∣ nm ∣ n ↔ n % m = 0 :=
  ⟨mod_eq_zero_of_dvd, dvd_of_mod_eq_zero⟩

instance decidable_dvd : @DecidableRel Nat Nat (·∣·) :=
  fun _ _ => decidable_of_decidable_of_iff dvd_iff_mod_eq_zero.symm

theorem emod_pos_of_not_dvd {a b : Nat} (h : ¬ a ∣ b) : 0 < b % a := by
  rw [dvd_iff_mod_eq_zero] at h
  exact Nat.pos_of_ne_zero h

protected theorem mul_div_cancel' {n m : Nat} (H : n ∣ m) : n * (m / n) = m := by
  have := mod_add_div m n
  rwa [mod_eq_zero_of_dvd H, Nat.zero_add] at this

protected theorem div_mul_cancel {n m : Nat} (H : n ∣ m) : m / n * n = m := by
  rw [Nat.mul_comm, Nat.mul_div_cancel' H]

@[simp] theorem mod_mod_of_dvd (a : Nat) (h : c ∣ b) : a % b % c = a % c := by
  rw (occs := [2]) [← mod_add_div a b]
  have ⟨x, h⟩ := h
  subst h
  rw [Nat.mul_assoc, add_mul_mod_self_left]

protected theorem dvd_of_mul_dvd_mul_left
    (kpos : 0 < k) (H : k * m ∣ k * n) : m ∣ n := by
  let ⟨l, H⟩ := H
  rw [Nat.mul_assoc] at H
  exact ⟨_, Nat.eq_of_mul_eq_mul_left kpos H⟩

protected theorem dvd_of_mul_dvd_mul_right (kpos : 0 < k) (H : m * k ∣ n * k) : m ∣ n := by
  rw [Nat.mul_comm m k, Nat.mul_comm n k] at H; exact Nat.dvd_of_mul_dvd_mul_left kpos H

theorem dvd_sub {k m n : Nat} (h₁ : k ∣ m) (h₂ : k ∣ n) : k ∣ m - n :=
  if H : n ≤ m then
    (Nat.dvd_add_iff_left h₂).2 <| by rwa [Nat.sub_add_cancel H]
  else
    Nat.sub_eq_zero_of_le (Nat.le_of_not_le H) ▸ Nat.dvd_zero k

theorem dvd_sub_iff_right {m n k : Nat} (hkn : k ≤ n) (h : m ∣ n) : m ∣ n - km ∣ n - k ↔ m ∣ k := by
  refine ⟨?_, dvd_sub h⟩
  let ⟨x, hx⟩ := h
  cases hx
  intro hy
  let ⟨y, hy⟩ := hy
  have hk : k = m * (x - y) := by
    rw [Nat.sub_eq_iff_eq_add hkn] at hy
    rw [Nat.mul_sub, hy, Nat.add_comm, Nat.add_sub_cancel]
  exact hk ▸ Nat.dvd_mul_right _ _

theorem dvd_sub_iff_left {m n k : Nat} (hkn : k ≤ n) (h : m ∣ k) : m ∣ n - km ∣ n - k ↔ m ∣ n := by
  rw (occs := [2]) [← Nat.sub_add_cancel hkn]
  exact Nat.dvd_add_iff_left h

protected theorem mul_dvd_mul {a b c d : Nat} : a ∣ b → c ∣ d → a * c ∣ b * d
  | ⟨e, he⟩, ⟨f, hf⟩ =>
    ⟨e * f, by simp [he, hf, Nat.mul_assoc, Nat.mul_left_comm, Nat.mul_comm]⟩

protected theorem mul_dvd_mul_left (a : Nat) (h : b ∣ c) : a * b ∣ a * c :=
  Nat.mul_dvd_mul (Nat.dvd_refl a) h

protected theorem mul_dvd_mul_right (h: a ∣ b) (c : Nat) : a * c ∣ b * c :=
  Nat.mul_dvd_mul h (Nat.dvd_refl c)

@[simp] theorem dvd_one {n : Nat} : n ∣ 1n ∣ 1 ↔ n = 1 :=
  ⟨eq_one_of_dvd_one, fun h => h.symm ▸ Nat.dvd_refl _⟩

protected theorem mul_div_assoc (m : Nat) (H : k ∣ n) : m * n / k = m * (n / k) := by
  match Nat.eq_zero_or_pos k with
  | .inl h0 => rw [h0, Nat.div_zero, Nat.div_zero, Nat.mul_zero]
  | .inr hpos =>
    have h1 : m * n / k = m * (n / k * k) / k := by rw [Nat.div_mul_cancel H]
    rw [h1, ← Nat.mul_assoc, Nat.mul_div_cancel _ hpos]

protected theorem dvd_eq_true_of_mod_eq_zero {m n : Nat} (h : n % m == 0) : (m ∣ n) = True := by
  simp [Nat.dvd_of_mod_eq_zero, eq_of_beq h]

protected theorem dvd_eq_false_of_mod_ne_zero {m n : Nat} (h : n % m != 0) : (m ∣ n) = False := by
  simp [eq_of_beq] at h
  simp [dvd_iff_mod_eq_zero, h]