lean/OAI/Computability/UniqueGames/PCP/ExpanderTables.lean

import Mathlib.Logic.Equiv.Fin.Basic
import OAI.Computability.UniqueGames.PCP.ZigzagGraphsLemmas

namespace OAI

section

namespace UniqueGamesTheorem.Foundations.PCP.ExpanderTables

open PoweringWalks SpectralReturn

def rowIndex (v d : Nat) : Fin v × Fin d ≃ Fin (v * d) := finProdFinEquiv

theorem rowIndex_val (v d : Nat) (x : Fin v × Fin d) :
    (rowIndex v d x).val = x.2.val + d * x.1.val := rfl

/-- Stored reverse-dart indices, with a proof of the finite involution law. -/
structure Table (v d : Nat) where
  rows : Vector (Fin (v * d)) (v * d)
  involutive : Function.Involutive (fun i : Fin (v * d) => rows[i])

def reverseIndex {v d : Nat} (table : Table v d) (i : Fin (v * d)) : Fin (v * d) :=
  table.rows[i]

def lookup {v d : Nat} (table : Table v d) (x : Fin v × Fin d) : Fin v × Fin d :=
  (rowIndex v d).symm (reverseIndex table (rowIndex v d x))

theorem lookup_involutive {v d : Nat} (table : Table v d) :
    Function.Involutive (lookup table) := by
  intro x
  simp only [lookup, Equiv.apply_symm_apply]
  exact (congrArg (rowIndex v d).symm (table.involutive (rowIndex v d x))).trans
    ((rowIndex v d).symm_apply_apply x)

def graph {v d : Nat} (table : Table v d) : PortGraph (Fin v) (Fin d) where
  rot := ZigzagGraphs.involutionEquiv (lookup table) (lookup_involutive table)
  rot_involutive := lookup_involutive table

@[simp] theorem graph_rot {v d : Nat} (table : Table v d) (x : Fin v × Fin d) :
    (graph table).rot x = lookup table x := rfl

def ofGraph {v d : Nat} (G : PortGraph (Fin v) (Fin d)) : Table v d where
  rows := Vector.ofFn (fun i => rowIndex v d (G.rot ((rowIndex v d).symm i)))
  involutive := by
    intro i
    simp only [Fin.getElem_fin, Vector.getElem_ofFn, Fin.eta, Equiv.symm_apply_apply]
    exact (congrArg (rowIndex v d) (G.rot_involutive ((rowIndex v d).symm i))).trans
      ((rowIndex v d).apply_symm_apply i)

@[simp] theorem lookup_ofGraph {v d : Nat} (G : PortGraph (Fin v) (Fin d))
    (x : Fin v × Fin d) : lookup (ofGraph G) x = G.rot x := by
  simp only [lookup, reverseIndex, ofGraph, Fin.getElem_fin, Vector.getElem_ofFn, Fin.eta,
    Equiv.symm_apply_apply]

private theorem graph_ext {V D : Type*} {G H : PortGraph V D}
    (h : ∀ x, G.rot x = H.rot x) : G = H := by
  have hr : G.rot = H.rot := Equiv.ext h
  cases G
  cases H
  cases hr
  rfl

@[simp] theorem graph_ofGraph {v d : Nat} (G : PortGraph (Fin v) (Fin d)) :
    graph (ofGraph G) = G := graph_ext (lookup_ofGraph G)

theorem row_count {v d : Nat} (table : Table v d) : table.rows.toList.length = v * d := by
  simp

/-- Explicit mixed-radix vertex coordinates for one square/zigzag step. -/
def vertexIndex (v q : Nat) : Fin v × (Fin q × Fin q) ≃ Fin (v * (q * q)) :=
  (Equiv.prodCongr (Equiv.refl _) (rowIndex q q)).trans (rowIndex v (q * q))

/-- The actual square/zigzag graph, expressed in the table's integer coordinates. -/
def stepGraph {v d : Nat} (G : Table v (d * d))
    (H : Table ((d * d) * (d * d)) d) :
    PortGraph (Fin (v * ((d * d) * (d * d)))) (Fin (d * d)) :=
  GraphTransport.reindex
    (ZigzagGraphs.zigzag (ZigzagGraphs.square (graph G))
      (GraphTransport.reindex (graph H) (rowIndex (d * d) (d * d)).symm (Equiv.refl _)))
    (vertexIndex v (d * d)) (rowIndex d d)

/-- Materialize every row of the next table in the declared order. -/
def step {v d : Nat} (G : Table v (d * d))
    (H : Table ((d * d) * (d * d)) d) :
    Table (v * ((d * d) * (d * d))) (d * d) := ofGraph (stepGraph G H)

/-- A row uses exactly the displayed four lookups in already stored tables. -/
def stepLookup {v d : Nat} (G : Table v (d * d))
    (H : Table ((d * d) * (d * d)) d)
    (x : Fin (v * ((d * d) * (d * d))) × Fin (d * d)) :
    Fin (v * ((d * d) * (d * d))) × Fin (d * d) :=
  let vc := (rowIndex v ((d * d) * (d * d))).symm x.1
  let ports := (rowIndex d d).symm x.2
  let first := lookup H (vc.2, ports.1)
  let squarePorts := (rowIndex (d * d) (d * d)).symm first.1
  let outerFirst := lookup G (vc.1, squarePorts.1)
  let outerSecond := lookup G (outerFirst.1, squarePorts.2)
  let last := lookup H (rowIndex (d * d) (d * d) (outerSecond.2, outerFirst.2), ports.2)
  (rowIndex v ((d * d) * (d * d)) (outerSecond.1, last.1),
    rowIndex d d (last.2, first.2))

theorem lookup_step {v d : Nat} (G : Table v (d * d))
    (H : Table ((d * d) * (d * d)) d)
    (x : Fin (v * ((d * d) * (d * d))) × Fin (d * d)) :
    lookup (step G H) x = stepLookup G H x := by
  simp [step, stepGraph, GraphTransport.reindex,
    Equiv.trans_apply, Equiv.prodCongr_apply,
    Equiv.prodCongr_symm, Equiv.refl_apply, vertexIndex, Equiv.apply_symm_apply,
    ZigzagGraphs.zigzag, ZigzagGraphs.square, ZigzagGraphs.involutionEquiv,
    ZigzagGraphs.cloudFirst, ZigzagGraphs.crossCloud, graph_rot,
    stepLookup, Prod.map, Equiv.symm_symm]
  all_goals repeat' constructor

theorem step_certificate {v d : Nat} [NeZero v] [NeZero d]
    (G : Table v (d * d)) (H : Table ((d * d) * (d * d)) d)
    (hG : SpectralCertificate (graph G) (1 / 2 : ℝ))
    (hH : SpectralCertificate (graph H) (1 / 100 : ℝ)) :
    SpectralCertificate (graph (step G H)) (1 / 2 : ℝ) := by
  rw [step, graph_ofGraph]
  apply GraphTransport.reindex_spectralCertificate
  apply ZigzagSpectral.square_zigzag_halfCertificate
  · exact hG
  · exact GraphTransport.reindex_spectralCertificate _ _ _ _ hH

def initial (d : Nat) : Table 1 (d * d) :=
  ofGraph { rot := Equiv.refl _, rot_involutive := fun _ => rfl }

theorem initial_certificate (d : Nat) :
    SpectralCertificate (graph (initial d)) (1 / 2 : ℝ) := by
  refine ⟨by norm_num, by norm_num, ?_⟩
  intro f hf
  have hfun : f = fun _ => f 0 := funext (fun x => congrArg f (Subsingleton.elim x 0))
  have hm : mean f = f 0 := (congrArg mean hfun).trans (mean_const _)
  have hz : f 0 = 0 := hm.symm.trans hf
  rw [hfun, hz]
  simp [energy, mean, averagingOperator]

def vertexCount (q : Nat) : Nat → Nat
  | 0 => 1
  | n + 1 => vertexCount q n * (q * q)

theorem vertexCount_eq (q n : Nat) : vertexCount q n = (q * q) ^ n := by
  induction n with
  | zero => rfl
  | succ n ih => simp only [vertexCount, ih, pow_succ]

theorem vertexCount_positive {q : Nat} (hq : 0 < q) (n : Nat) :
    0 < vertexCount q n := by
  rw [vertexCount_eq]
  exact Nat.pow_pos (Nat.mul_pos hq hq)

/-- A bottom-up vector of rotation rows. The recursive result is passed once
to `step`, whose row formula reads only that already materialized table. -/
def family {d : Nat} (H : Table ((d * d) * (d * d)) d) :
    (n : Nat) → Table (vertexCount (d * d) n) (d * d)
  | 0 => initial d
  | n + 1 => step (family H n) H

theorem family_certificate {d : Nat} [NeZero d]
    (H : Table ((d * d) * (d * d)) d)
    (hH : SpectralCertificate (graph H) (1 / 100 : ℝ)) (n : Nat) :
    SpectralCertificate (graph (family H n)) (1 / 2 : ℝ) := by
  induction n with
  | zero => exact initial_certificate d
  | succ n ih =>
    let : NeZero (vertexCount (d * d) n) :=
      ⟨Nat.ne_of_gt (vertexCount_positive (Nat.mul_pos (NeZero.pos d) (NeZero.pos d)) n)⟩
    exact step_certificate _ _ ih hH

theorem family_row_count {d : Nat} (H : Table ((d * d) * (d * d)) d) (n : Nat) :
    (family H n).rows.toList.length = ((d * d) * (d * d)) ^ n * (d * d) := by
  rw [row_count, vertexCount_eq]

/-- The only selected data is one fixed finite table, independent of the input. -/
theorem exists_base_table : ∃ H : Table
    ((Expanders.baseDegree * Expanders.baseDegree) *
      (Expanders.baseDegree * Expanders.baseDegree)) Expanders.baseDegree,
    SpectralCertificate (graph H) (1 / 100 : ℝ) := by
  classical
  have hv : Fintype.card ExpanderFamily.CloudPort =
      (Expanders.baseDegree * Expanders.baseDegree) *
        (Expanders.baseDegree * Expanders.baseDegree) := by
    rw [ExpanderFamily.card_cloudPort]
    unfold ExpanderFamily.growth
    ring
  let vertices := Fintype.equivFinOfCardEq hv
  let ports := Fintype.equivFinOfCardEq Expanders.card_basePort
  let G := GraphTransport.reindex ExpanderFamily.baseOnCloud vertices ports
  refine ⟨ofGraph G, ?_⟩
  rw [graph_ofGraph]
  exact GraphTransport.reindex_spectralCertificate _ _ _ _
    ExpanderFamily.baseOnCloud_certificate

end UniqueGamesTheorem.Foundations.PCP.ExpanderTables

end

end OAI