-- Mathlib/NumberTheory/LegendreSymbol/JacobiSymbol.lean
theorem fastJacobiSymAux.eq_jacobiSym {a b : ℕ} {flip : Bool} {ha0 : a > 0}
    (hb2 : b % 2 = 1) (hb1 : b > 1) :
    fastJacobiSymAux a b flip ha0 = if flip then -J(a | b) else J(a | b)

Proof state:
case ind.isFalse.isTrue
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : a % 2 = 0
⊢ (if (decide (b % 8 = 3 ∨ b % 8 = 5) ^^ flip) = true then -J(↑a / 2 | b) else J(↑a / 2 | b)) =
    if flip = true then -if b % 8 = 3 ∨ b % 8 = 5 then -J(↑a / 2 | b) else J(↑a / 2 | b)
    else if b % 8 = 3 ∨ b % 8 = 5 then -J(↑a / 2 | b) else J(↑a / 2 | b)

Tactic:
by_cases h : b % 8 = 3 ∨ b % 8 = 5 <;> simp [h]

Result:
case pos
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : a % 2 = 0
h : b % 8 = 3 ∨ b % 8 = 5
⊢ (if flip = false then -J(↑a / 2 | b) else J(↑a / 2 | b)) = if flip = true then J(↑a / 2 | b) else -J(↑a / 2 | b)

Proof state:
case ind.isTrue
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : a % 4 = 0
⊢ (if flip = true then -J(↑(a / 4) | b) else J(↑(a / 4) | b)) = if flip = true then -J(↑a | b) else J(↑a | b)

Tactic:
simp only [Int.ofNat_ediv, Nat.cast_ofNat, div_four_left (a := a) (mod_cast ha4) hb2]

Result:
no goals

Proof state:
case ind.isFalse.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
hba : ¬b % a = 0
⊢ (if (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) = true then -J(↑b | a) else J(↑b | a)) =
    if flip = true then -J(↑a | b) else J(↑a | b)

Tactic:
rw [← quadratic_reciprocity_if (Nat.mod_two_ne_zero.mp ha2) hb2]

Result:
case ind.isFalse.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
hba : ¬b % a = 0
⊢ (if (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) = true then -J(↑b | a) else J(↑b | a)) =
    if flip = true then -if a % 4 = 3 ∧ b % 4 = 3 then -J(↑b | a) else J(↑b | a)
    else if a % 4 = 3 ∧ b % 4 = 3 then -J(↑b | a) else J(↑b | a)

Proof state:
case ind.isFalse.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
hba : ¬b % a = 0
⊢ (if (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) = true then -J(↑(b % a) | a) else J(↑(b % a) | a)) =
    if flip = true then -J(↑a | b) else J(↑a | b)

Tactic:
simp only [Int.natCast_mod, ← mod_left]

Result:
case ind.isFalse.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
hba : ¬b % a = 0
⊢ (if (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) = true then -J(↑b | a) else J(↑b | a)) =
    if flip = true then -J(↑a | b) else J(↑a | b)

Proof state:
case ind.isFalse.isFalse.isTrue
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : a = 1
⊢ (if flip = true then -1 else 1) = if flip = true then -J(↑a | b) else J(↑a | b)

Tactic:
subst ha1

Result:
case ind.isFalse.isFalse.isTrue
b : ℕ
flip : Bool
hb2 : b % 2 = 1
hb1 : b > 1
IH :
  ∀ m < 1,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
ha0 : 1 > 0
ha4 : ¬1 % 4 = 0
ha2 : ¬1 % 2 = 0
⊢ (if flip = true then -1 else 1) = if flip = true then -J(↑1 | b) else J(↑1 | b)

Proof state:
case ind.isFalse.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
hba : ¬b % a = 0
⊢ fastJacobiSymAux (b % a) a (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) ⋯ = if flip = true then -J(↑a | b) else J(↑a | b)

Tactic:
rw [IH (b % a) (b.mod_lt ha0) (Nat.mod_two_ne_zero.mp ha2) (lt_of_le_of_ne ha0 (Ne.symm ha1))]

Result:
case ind.isFalse.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
hba : ¬b % a = 0
⊢ (if (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) = true then -J(↑(b % a) | a) else J(↑(b % a) | a)) =
    if flip = true then -J(↑a | b) else J(↑a | b)

Proof state:
case ind.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
⊢ (if ha1 : a = 1 then if flip = true then -1 else 1
    else if hba : b % a = 0 then 0 else fastJacobiSymAux (b % a) a (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) ⋯) =
    if flip = true then -J(↑a | b) else J(↑a | b)

Tactic:
split <;> rename_i ha1

Result:
case ind.isFalse.isFalse.isTrue
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : a = 1
⊢ (if flip = true then -1 else 1) = if flip = true then -J(↑a | b) else J(↑a | b)

case ind.isFalse.isFalse.isFalse
a : ℕ
IH :
  ∀ m < a,
    ∀ {b : ℕ} {flip : Bool} {ha0 : m > 0},
      b % 2 = 1 → b > 1 → fastJacobiSymAux m b flip ha0 = if flip = true then -J(↑m | b) else J(↑m | b)
b : ℕ
flip : Bool
ha0 : a > 0
hb2 : b % 2 = 1
hb1 : b > 1
ha4 : ¬a % 4 = 0
ha2 : ¬a % 2 = 0
ha1 : ¬a = 1
⊢ (if hba : b % a = 0 then 0 else fastJacobiSymAux (b % a) a (decide (a % 4 = 3 ∧ b % 4 = 3) ^^ flip) ⋯) =
    if flip = true then -J(↑a | b) else J(↑a | b)