-- ArkLib/ProofSystem/Binius/BinaryBasefold/Prelude.lean

import ArkLib.Data.CodingTheory.BerlekampWelch.BerlekampWelch

import ArkLib.Data.CodingTheory.ReedSolomon

import ArkLib.Data.FieldTheory.AdditiveNTT.AdditiveNTT

import ArkLib.Data.MvPolynomial.Multilinear

import ArkLib.Data.Vector.Basic

import ArkLib.ProofSystem.Sumcheck.Spec.SingleRound

namespace Binius.BinaryBasefold

open OracleSpec OracleComp ProtocolSpec Finset AdditiveNTT Polynomial MvPolynomial
  Binius.BinaryBasefold

open scoped NNReal

open ReedSolomon Code BerlekampWelch

open Finset AdditiveNTT Polynomial MvPolynomial Nat Matrix

section Preliminaries

variable {L : Type} [CommRing L] (ℓ : ℕ) [NeZero ℓ]

variable (𝓑 : Fin 2 ↪ L)

end Preliminaries

noncomputable section       -- expands with 𝔽q in front

variable {r : ℕ} [NeZero r]

variable {L : Type} [Field L] [Fintype L] [DecidableEq L] [CharP L 2]

variable (𝔽q : Type) [Field 𝔽q] [Fintype 𝔽q] [DecidableEq 𝔽q]
  [h_Fq_char_prime : Fact (Nat.Prime (ringChar 𝔽q))] [hF₂ : Fact (Fintype.card 𝔽q = 2)]

variable [Algebra 𝔽q L]

variable (β : Fin r → L) [hβ_lin_indep : Fact (LinearIndependent 𝔽q β)]
  [h_β₀_eq_1 : Fact (β 0 = 1)]

variable {ℓ 𝓡 ϑ : ℕ} (γ_repetitions : ℕ) [NeZero ℓ] [NeZero 𝓡] [NeZero ϑ] -- Should we allow ℓ = 0?

variable {h_ℓ_add_R_rate : ℓ + 𝓡 < r} -- ℓ ∈ {1, ..., r-1}

variable {𝓑 : Fin 2 ↪ L}

section Essentials

noncomputable def fiber_coeff
    (i : Fin r) (steps : ℕ)
    (j : Fin (ℓ + 𝓡 - i)) (elementIdx : Fin (2 ^ steps))
    (y_coeffs : Fin (ℓ + 𝓡 - (i + steps)) →₀ 𝔽q) : 𝔽q :=
  if hj : j.val < steps then
    if Nat.getBit (k := j) (n := elementIdx) = 0 then 0 else 1
  else y_coeffs ⟨j.val - steps, by admit /- proof elided -/
    ⟩

noncomputable def qMap_total_fiber
    
      (i : Fin r) (steps : ℕ) (h_i_add_steps : i.val + steps < ℓ + 𝓡)
        (y : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i.val + steps, by admit /- proof elided -/
        ⟩)) :
    Fin (2 ^ steps) → sDomain 𝔽q β h_ℓ_add_R_rate i :=
  if h_steps : steps = 0 then by
    
    subst h_steps
    simp only [add_zero, Fin.eta] at y
    exact fun _ => y
  else by
    
    let basis_y := sDomain_basis 𝔽q β h_ℓ_add_R_rate (i := ⟨i+steps,by admit /- proof elided -/
    ⟩) (by admit /- proof elided -/
    )
    let y_coeffs : Fin (ℓ + 𝓡 - (↑i + steps)) →₀ 𝔽q := basis_y.repr y

    let basis_x := sDomain_basis 𝔽q β h_ℓ_add_R_rate ⟨i, by admit /- proof elided -/
    ⟩ (by admit /- proof elided -/
    )
    exact fun elementIdx => by admit /- proof elided -/

def pointToIterateQuotientIndex (i : Fin (ℓ + 1)) (steps : ℕ) (h_i_add_steps : i.val + steps ≤ ℓ)
    (x : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩)) : Fin (2 ^ steps) :=

-- Repository definitions used:
content: def getBit (k n : Nat) : Nat := (n >>> k) &&& 1

content: noncomputable def sDomain (i : Fin r) : Subspace 𝔽q L :=
  let W_i_norm := normalizedW 𝔽q β i
  let h_W_i_norm_is_additive : IsLinearMap 𝔽q (fun x : L => W_i_norm.eval x) :=
    AdditiveNTT.normalizedW_is_additive 𝔽q β i
  Submodule.map (polyEvalLinearMap W_i_norm h_W_i_norm_is_additive)
    (U 𝔽q β ⟨ℓ + R_rate, h_ℓ_add_R_rate⟩)

content: noncomputable def normalizedW (i : Fin r) : L[X] :=
  C (1 / (W 𝔽q β i).eval (β i)) * W 𝔽q β i

content: noncomputable def W (i : Fin r) : L[X] :=
  ∏ u : U 𝔽q β i, (X - C u.val)

content: def U (i : Fin r) : Subspace 𝔽q L := Submodule.span 𝔽q (β '' (Set.Ico 0 i))

content: noncomputable def polyEvalLinearMap {L 𝔽q : Type*} [Field L] [Field 𝔽q] [Algebra 𝔽q L]
  (p : L[X]) (hp_add : IsLinearMap 𝔽q (fun x : L => p.eval x)) : L →ₗ[𝔽q] L :=
{
  toFun    := fun x => p.eval x,
  map_add' := hp_add.map_add,
  map_smul' := hp_add.map_smul
}

content: noncomputable def sDomain_basis (i : Fin r) (h_i : i < ℓ + R_rate) :
    Basis (Fin (ℓ + R_rate - i)) 𝔽q (
      sDomain 𝔽q β h_ℓ_add_R_rate i) :=

content: def sBasis (i : Fin r) (h_i : i < ℓ + R_rate) : Fin (ℓ + R_rate - i) → L :=
  fun k => β ⟨i + k.val, by admit /- proof elided -/
  ⟩

content: def binaryFinMapToNat {n : ℕ} (m : Fin n → ℕ) (h_binary : ∀ j: Fin n, m j ≤ 1) : Fin (2^n) :=

content: def symm (eqv : Equiv pSpec pSpec') : Equiv pSpec' pSpec where
  round_eq := eqv.round_eq.symm
  dir_eq := fun i => by admit /- proof elided -/

content: @[ext]
structure Equiv {m n : ℕ} (pSpec : ProtocolSpec m) (pSpec' : ProtocolSpec n) where
  round_eq : m = n
  dir_eq : ∀ i, pSpec.dir i = pSpec'.dir (Fin.cast round_eq i)
  typeEquiv : ∀ i, pSpec.«Type» i ≃ pSpec'.«Type» (Fin.cast round_eq i)

content: @[ext]
structure ProtocolSpec (n : ℕ) where
   
  dir : Fin n → Direction
   
  «Type» : Fin n → Type
deriving Inhabited

content: inductive Direction where
  | P_to_V  
  | V_to_P 
deriving DecidableEq, Inhabited, Repr

content: noncomputable def iteratedQuotientMap (i : Fin ℓ) (k : ℕ)
    (h_bound : i.val + k ≤ ℓ) (x : (sDomain 𝔽q β
    h_ℓ_add_R_rate) ⟨i, by omega⟩) :
    (sDomain 𝔽q β h_ℓ_add_R_rate) ⟨i.val + k, by omega⟩ :=

content: noncomputable def intermediateNormVpoly
    
    (i: Fin (ℓ+1)) (k : Fin (ℓ - i + 1)) : L[X] :=
  
  Fin.foldl (n:=k) (fun acc j =>
    (qMap 𝔽q β ⟨(i : ℕ) + (j : ℕ), by admit /- proof elided -/
    ⟩).comp acc) (X)

content: noncomputable def qMap (i : Fin r) : L[X] :=
  let constMultiplier := ((W 𝔽q β i).eval (β i))^(Fintype.card 𝔽q)
    / ((W 𝔽q β (i + 1)).eval (β (i + 1)))
  C constMultiplier * ∏ c: 𝔽q, (X - C (algebraMap 𝔽q L c))

content: noncomputable def qCompositionChain (i : Fin r) : L[X] :=
  match i with
  | ⟨0, _⟩ => X
  | ⟨k + 1, h_k_add_1⟩ => (qMap 𝔽q β ⟨k, by admit /- proof elided -/
  ⟩).comp (qCompositionChain ⟨k, by admit /- proof elided -/
  ⟩)

content: @[elab_as_elim] def Fin.succRecOnSameFinType {motive : Fin r → Sort _}
    (zero : motive (0 : Fin r))
    (succ : ∀ i : Fin r, i + 1 < r → motive i → motive (i + 1)) : ∀ (i : Fin r), motive i
  | ⟨0, _⟩ => by admit /- proof elided -/
  | ⟨Nat.succ i_val, h⟩ => by admit /- proof elided -/

-- Repository lemmas used:
content: lemma Xⱼ_zero_eq_one (ℓ : ℕ) (h_ℓ : ℓ ≤ r) :
  Xⱼ 𝔽q β ℓ h_ℓ ⟨0, by exact Nat.two_pow_pos ℓ⟩ = 1

content: @[simp]
lemma lt_add_of_pos_right_of_le (a b c : ℕ) [NeZero c] (h : a ≤ b) : a < b + c

content: omit [DecidableEq 𝔽q] hF₂ in
lemma getSDomainBasisCoeff_of_iteratedQuotientMap
    [NeZero R_rate] (i : Fin ℓ) (k : ℕ)
    (h_bound : i.val + k ≤ ℓ) (x : (sDomain 𝔽q β
    h_ℓ_add_R_rate) ⟨i, by omega⟩) :
    let y

content: theorem base_intermediateNormVpoly
  (k : Fin (ℓ + 1)) :
  intermediateNormVpoly 𝔽q β h_ℓ_add_R_rate ⟨0, by
    by_contra ht
    simp only [not_lt, nonpos_iff_eq_zero] at ht
    contradiction
  ⟩ ⟨k, by simp only [tsub_zero]; omega⟩ =
  normalizedW 𝔽q β ⟨k, by omega⟩

content: lemma normalizedW_eq_qMap_composition (ℓ R_rate : ℕ) (i : Fin r) :
    normalizedW 𝔽q β i = qCompositionChain 𝔽q β (ℓ:=ℓ) (R_rate:=R_rate) i

content: lemma qMap_comp_normalizedW (i : Fin r) (h_i_add_1 : i + 1 < r) :
  (qMap 𝔽q β i).comp (normalizedW 𝔽q β i) = normalizedW 𝔽q β (i + 1)

content: lemma qCompositionChain_eq_foldl (i : Fin r) :
  qCompositionChain 𝔽q β (ℓ:=ℓ) (R_rate:=R_rate) i =
  Fin.foldl (n:=i) (fun acc j =>
    (qMap 𝔽q β ⟨j, by omega⟩).comp acc) (X)

content: omit [DecidableEq 𝔽q] hF₂ h_β₀_eq_1 in
lemma getSDomainBasisCoeff_of_sum_repr [NeZero R_rate] (i : Fin (ℓ + 1))
    (x : (sDomain 𝔽q β h_ℓ_add_R_rate) ⟨i, by omega⟩)
    (x_coeffs : Fin (ℓ + R_rate - i) → 𝔽q)
    (hx : x = ∑ j_x, (x_coeffs j_x) • (sDomain_basis 𝔽q β
      h_ℓ_add_R_rate (i := ⟨i, by omega⟩) (h_i := by
        simp only; apply Nat.lt_add_of_pos_right_of_le; omega) j_x).val) :
    ∀ (j: Fin (ℓ + R_rate - i)), ((sDomain_basis 𝔽q β
      h_ℓ_add_R_rate (i := ⟨i, by omega⟩) (h_i := by
        simp only; apply Nat.lt_add_of_pos_right_of_le; omega)).repr x) j = x_coeffs j

content: omit [DecidableEq 𝔽q] hF₂ h_β₀_eq_1 in
lemma get_sDomain_basis (i : Fin r) (h_i : i < ℓ + R_rate) :
    ∀ (k : Fin (ℓ + R_rate - i)),
    (sDomain_basis 𝔽q β h_ℓ_add_R_rate
      i (by omega)) k = eval (β ⟨i + k.val, by omega⟩) (normalizedW 𝔽q β i)

content: omit [DecidableEq L] [DecidableEq 𝔽q] h_Fq_char_prime hF₂ hβ_lin_indep h_β₀_eq_1 in
theorem intermediateNormVpoly_comp (i : Fin ℓ) (k : Fin (ℓ - i + 1))
  (l : Fin (ℓ - (i.val + k.val) + 1)) :
  intermediateNormVpoly 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩) (k:=⟨k + l, by
      simp only; omega⟩) =
    (intermediateNormVpoly 𝔽q β h_ℓ_add_R_rate (i := ⟨i+k, by omega⟩) (k:=⟨l, by
      simp only; omega⟩)).comp (
  intermediateNormVpoly 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩) (k:=⟨k, by
      simp only; omega⟩)
  )

content: omit [DecidableEq L] [DecidableEq 𝔽q] hF₂ hβ_lin_indep h_β₀_eq_1 in
lemma intermediateNormVpoly_eval_is_linear_map (i : Fin (ℓ + 1)) (k : Fin (ℓ - i + 1)) :
  IsLinearMap 𝔽q (fun x : L =>
    (intermediateNormVpoly 𝔽q β h_ℓ_add_R_rate i k).eval x)

content: theorem qMap_is_linear_map (i : Fin r) :
  IsLinearMap 𝔽q (f:=fun inner_p ↦ (qMap 𝔽q β i).comp inner_p)

content: omit h_Fq_char_prime in
lemma 𝔽q_element_eq_zero_or_eq_one : ∀ c: 𝔽q, c = 0 ∨ c = 1

content: lemma getBit_of_binaryFinMapToNat {n : ℕ} (m : Fin n → ℕ) (h_binary: ∀ j: Fin n, m j ≤ 1) :
    ∀ k: ℕ, Nat.getBit k (binaryFinMapToNat m h_binary).val
      = if h_k: k < n then m ⟨k, by omega⟩ else 0

content: lemma and_two_pow_eq_zero_of_getBit_0 {n i : ℕ} (h_getBit : getBit i n = 0)
    : n &&& (2 ^ i) = 0

content: lemma and_eq_zero_iff_and_each_getBit_eq_zero {n m : ℕ} :
    n &&& m = 0 ↔ ∀ k, Nat.getBit k n &&& Nat.getBit k m = 0

content: lemma eq_iff_eq_all_getBits {n m : ℕ} : n = m ↔ ∀ k, getBit k n = getBit k m

content: lemma shiftRight_and_one_distrib {n m k : ℕ} :
    Nat.getBit k (n &&& m) = Nat.getBit k n &&& Nat.getBit k m

content: lemma getBit_two_pow {i k : ℕ} : (getBit k (2^i) = if i == k then 1 else 0)

content: lemma getBit_of_lt_two_pow {n: ℕ} (a: Fin (2^n)) (k: ℕ) :
  getBit k a = if k < n then getBit k a else 0

content: lemma getBit_eq_testBit (k n : Nat) : getBit k n = if n.testBit k then 1 else 0

content: lemma getBit_lt_2 {k n : Nat} : getBit k n < 2

content: lemma getBit_of_xor {n m k: ℕ} : getBit k (n ^^^ m) = getBit k n ^^^ getBit k m

content: lemma getBit_zero_eq_zero {k : Nat} : getBit k 0 = 0

content: lemma sum_of_and_eq_zero_is_xor {n m : ℕ} (h_n_AND_m : n &&& m = 0) : n + m = n ^^^ m

content: lemma sum_eq_xor_plus_twice_and (n : Nat) : ∀ m : ℕ, n + m = (n ^^^ m) + 2 * (n &&& m)

content: lemma and_by_split_lowBits {n m n1 m1 bn bm : ℕ} (h_bn : bn < 2) (h_bm : bm < 2)
  (h_n : n = n1 * 2 + bn) (h_m : m = m1 * 2 + bm):
  n &&& m = (n1 &&& m1) * 2 + (bn &&& bm)

content: lemma div_2_form {nD2 b : ℕ} (h_b : b < 2):
  (nD2 * 2 + b) / 2 = nD2

content: lemma or_by_split_lowBits {n m n1 m1 bn bm : ℕ} (h_bn : bn < 2) (h_bm : bm < 2)
  (h_n : n = n1 * 2 + bn) (h_m : m = m1 * 2 + bm):
  n ||

content: lemma xor_by_split_lowBits {n m n1 m1 bn bm : ℕ} (h_bn : bn < 2) (h_bm : bm < 2)
  (h_n : n = n1 * 2 + bn) (h_m : m = m1 * 2 + bm):
  n ^^^ m = (n1 ^^^ m1) * 2 + (bn ^^^ bm)

-- Local definitions used:
content: noncomputable def fiber_coeff
    (i : Fin r) (steps : ℕ)
    (j : Fin (ℓ + 𝓡 - i)) (elementIdx : Fin (2 ^ steps))
    (y_coeffs : Fin (ℓ + 𝓡 - (i + steps)) →₀ 𝔽q) : 𝔽q :=
  if hj : j.val < steps then
    if Nat.getBit (k := j) (n := elementIdx) = 0 then 0 else 1
  else y_coeffs ⟨j.val - steps, by admit /- proof elided -/
    ⟩

content: noncomputable def qMap_total_fiber
    
      (i : Fin r) (steps : ℕ) (h_i_add_steps : i.val + steps < ℓ + 𝓡)
        (y : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i.val + steps, by admit /- proof elided -/
        ⟩)) :
    Fin (2 ^ steps) → sDomain 𝔽q β h_ℓ_add_R_rate i :=
  if h_steps : steps = 0 then by
    
    subst h_steps
    simp only [add_zero, Fin.eta] at y
    exact fun _ => y
  else by
    
    let basis_y := sDomain_basis 𝔽q β h_ℓ_add_R_rate (i := ⟨i+steps,by admit /- proof elided -/
    ⟩) (by admit /- proof elided -/
    )
    let y_coeffs : Fin (ℓ + 𝓡 - (↑i + steps)) →₀ 𝔽q := basis_y.repr y

    let basis_x := sDomain_basis 𝔽q β h_ℓ_add_R_rate ⟨i, by admit /- proof elided -/
    ⟩ (by admit /- proof elided -/
    )
    exact fun elementIdx => by admit /- proof elided -/

content: def pointToIterateQuotientIndex (i : Fin (ℓ + 1)) (steps : ℕ) (h_i_add_steps : i.val + steps ≤ ℓ)
    (x : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩)) : Fin (2 ^ steps) :=

-- Local lemmas used:
content: omit [NeZero ℓ] in
lemma fin_ℓ_steps_lt_ℓ_add_R (i : Fin ℓ) (steps : ℕ) (h : i.val + steps ≤ ℓ)
    : i.val + steps < ℓ + 𝓡

content: lemma qMap_total_fiber_repr_coeff (i : Fin ℓ) (steps : ℕ) (h_i_add_steps : i.val + steps ≤ ℓ)
    (y : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i.val + steps, by omega⟩))
    (k : Fin (2 ^ steps)) :
    let x := qMap_total_fiber 𝔽q β (i := ⟨i, by omega⟩)
      (steps := steps)
      (h_i_add_steps := by simp only; exact fin_ℓ_steps_lt_ℓ_add_R i steps h_i_add_steps) (y := y) k
    let basis_y := sDomain_basis 𝔽q β h_ℓ_add_R_rate (i := ⟨i.val + steps, by omega⟩)
      (h_i := by simp only; exact fin_ℓ_steps_lt_ℓ_add_R i steps h_i_add_steps)
    let y_coeffs := basis_y.repr y
    ∀ j, -- j refers to bit index of the fiber point x
      ((sDomain_basis 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩) (by simp only; omega)).repr x) j
      = fiber_coeff (i := i) (steps := steps) (j := j) (elementIdx := k)
        (y_coeffs := y_coeffs)

content: theorem generates_quotient_point_if_is_fiber_of_y
    (i : Fin ℓ) (steps : ℕ) (h_i_add_steps : i.val + steps ≤ ℓ)
    (x : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩))
    (y : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i.val + steps, by omega⟩))
    (hx_is_fiber : ∃ (k : Fin (2 ^ steps)), x = qMap_total_fiber 𝔽q β (i := ⟨i, by omega⟩)
      (steps := steps) (h_i_add_steps := by
        simp only; exact fin_ℓ_steps_lt_ℓ_add_R i steps h_i_add_steps) (y := y) k) :
    y = iteratedQuotientMap 𝔽q β h_ℓ_add_R_rate i (k := steps) (h_bound := h_i_add_steps) x

-- Target theorem and proof:
theorem is_fiber_iff_generates_quotient_point (i : Fin ℓ) (steps : ℕ)
    (h_i_add_steps : i.val + steps ≤ ℓ)
    (x : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i, by omega⟩))
    (y : sDomain 𝔽q β h_ℓ_add_R_rate (i := ⟨i.val + steps, by omega⟩)) :
    let qMapFiber := qMap_total_fiber 𝔽q β (i := ⟨i, by omega⟩) (steps := steps)
      (h_i_add_steps := by simp only; exact fin_ℓ_steps_lt_ℓ_add_R i steps h_i_add_steps) (y := y)
    let k := pointToIterateQuotientIndex (i := ⟨i, by omega⟩) (steps := steps)
      (h_i_add_steps := h_i_add_steps) (x := x)
    y = iteratedQuotientMap 𝔽q β h_ℓ_add_R_rate i (k := steps) (h_bound := h_i_add_steps) x ↔
    qMapFiber k = x
:= qMap_total_fiber 𝔽q β (i := ⟨i, by omega⟩) (steps := steps)
      (h_i_add_steps := by simp only; exact fin_ℓ_steps_lt_ℓ_add_R i steps h_i_add_steps) (y := y)
    let k := pointToIterateQuotientIndex (i := ⟨i, by omega⟩) (steps := steps)
      (h_i_add_steps := h_i_add_steps) (x := x)
    y = iteratedQuotientMap 𝔽q β h_ℓ_add_R_rate i (k := steps) (h_bound := h_i_add_steps) x ↔
    qMapFiber k = x := by
  let basis_x := sDomain_basis 𝔽q β h_ℓ_add_R_rate ⟨i, by omega⟩
    (by simp only; omega)
  let basis_y := sDomain_basis 𝔽q β h_ℓ_add_R_rate ⟨i.val + steps, by omega⟩
    (h_i := by apply Nat.lt_add_of_pos_right_of_le; omega)
  simp only
  set k := pointToIterateQuotientIndex (i := ⟨i, by omega⟩) (steps := steps)
    (h_i_add_steps := h_i_add_steps) (x := x)
  constructor
  · intro h_x_generates_y
    -- ⊢ qMap_total_fiber ...` ⟨↑i, ⋯⟩ steps ⋯ y k = x
    -- We prove that `qMap_total_fiber` with this `k` reconstructs `x` via basis repr
    apply basis_x.repr.injective
    ext j
    let reConstructedX := basis_x.repr (qMap_total_fiber 𝔽q β (i := ⟨i, by omega⟩)
      (steps := steps) (h_i_add_steps := by apply Nat.lt_add_of_pos_right_of_le; omega) (y := y) k)
    have h_repr_of_reConstructedX := qMap_total_fiber_repr_coeff 𝔽q β i (steps := steps)
      (h_i_add_steps := by omega) (y := y) (k := k) (j := j)
    simp only at h_repr_of_reConstructedX
    -- ⊢ repr of reConstructedX at j = repr of x at j
    rw [h_repr_of_reConstructedX]; dsimp [k, pointToIterateQuotientIndex, fiber_coeff];
    rw [getBit_of_binaryFinMapToNat]; simp only [Fin.eta, dite_eq_right_iff, ite_eq_left_iff,
      one_ne_zero, imp_false, Decidable.not_not]
    -- Now we only need to do case analysis
    by_cases h_j : j.val < steps
    · -- Case 1 : The first `steps` coefficients, determined by `k`.
      simp only [h_j, ↓reduceDIte, forall_const]
      by_cases h_coeff_j_of_x : basis_x.repr x j = 0
      · simp only [basis_x, h_coeff_j_of_x, ↓reduceIte];
      · simp only [basis_x, h_coeff_j_of_x, ↓reduceIte];
        have h_coeff := 𝔽q_element_eq_zero_or_eq_one 𝔽q (c := basis_x.repr x j)
        simp only [h_coeff_j_of_x, false_or] at h_coeff
        exact id (Eq.symm h_coeff)
    · -- Case 2 : The remaining coefficients, determined by `y`.
      simp only [h_j, ↓reduceDIte]
      simp only [basis_x]
      -- ⊢ Here we compare coeffs, not the basis elements
      simp only [h_x_generates_y]
      have h_res := getSDomainBasisCoeff_of_iteratedQuotientMap 𝔽q β h_ℓ_add_R_rate i (k := steps)
        (h_bound := by omega) x (j := ⟨j - steps, by -- TODO : make this index bound proof cleaner
          simp only; rw [←Nat.sub_sub]; -- ⊢ ↑j - steps < ℓ + 𝓡 - ↑i - steps
          apply Nat.sub_lt_sub_right;
          · exact Nat.le_of_not_lt h_j
          · exact j.isLt
        ⟩) -- ⊢ ↑j - steps < ℓ + 𝓡 - (↑i + steps)
      have h_j_sub_add_steps : j - steps + steps = j := by omega
      simp only at h_res
      simp only [h_j_sub_add_steps, Fin.eta] at h_res
      exact h_res
  · intro h_x_is_fiber_of_y
    -- y is the quotient point of x over steps steps
    apply generates_quotient_point_if_is_fiber_of_y (h_i_add_steps := h_i_add_steps)
      (x := x) (y := y) (hx_is_fiber := by use k; exact h_x_is_fiber_of_y.symm)