import OAI.Computability.UniqueGames.Machines.MachineBinaryNameMachine
import OAI.Computability.UniqueGames.Machines.MachineCompositionLemmas
import OAI.Computability.UniqueGames.Machines.MachineCopy

namespace OAI

/-!
Compare one framed binary name with an already extracted reversed key.
The candidate stream loses precisely that name frame. The key and every other
non-work stack are preserved; the three work stacks are empty again on exit.
The scanner, copy loops, and head-by-head comparison all execute actual TM2
instructions. Whole words occur only in the correctness specification.
-/

namespace UniqueGamesTheorem.BinaryNameCompare

open Turing
open UniqueGamesTheorem.Foundations.Complexity
open MachineComposition

variable {K Λ A : Type} [DecidableEq K]

abbrev Alphabet (_ : K) := Bool
abbrev State (A : Type) := (A × Bool × Option Bool) × Option Bool

def clean (ambient : A) : State A := ((ambient, false, none), none)

def scanStateEquiv (A : Type) : BinaryNameMachine.State (A × Bool) ≃ State A where
  toFun s := ((s.1.1, s.1.2, s.2.1), s.2.2)
  invFun s := ((s.1.1, s.1.2.1), s.1.2.2, s.2)
  left_inv := by rintro ⟨⟨a, b⟩, c, d⟩; rfl
  right_inv := by rintro ⟨⟨a, b, c⟩, d⟩; rfl

def compareStateEquiv (A : Type) : MachineCompare.State A ≃ State A where
  toFun s := ((s.1, s.2.1, s.2.2.1), s.2.2.2)
  invFun s := (s.1.1, s.1.2.1, s.1.2.2, s.2)
  left_inv := by rintro ⟨a, b, c, d⟩; rfl
  right_inv := by rintro ⟨⟨a, b, c⟩, d⟩; rfl

inductive Label
  | scan | copyOut | copyBack | compare
  deriving DecidableEq

protected abbrev Label.enumList : List Label := [.scan, .copyOut, .copyBack, .compare]

protected theorem Label.enumList_getElem?_ctorIdx_eq (x : Label) :
    Label.enumList[x.ctorIdx]? = some x := by
  cases x <;> rfl

protected theorem Label.enumList_nodup : Label.enumList.Nodup := by decide

instance : Fintype Label where
  elems := ⟨Label.enumList, Label.enumList_nodup⟩
  complete x := by cases x <;> decide

/-- Tapes: candidate stream, preserved key, candidate payload, key copy, scratch. -/
def instruction (tape : Fin 5 → K) (labels : Label → Λ)
    (equalExit differentExit malformedExit : Option Λ) :
    Label → TM2.Stmt (Alphabet (K := K)) Λ (State A)
  | .scan => MachineStateEquiv.statement (scanStateEquiv A)
      (BinaryNameMachine.scan (tape 0) (tape 2) (labels .scan)
        (some (labels .copyOut)) malformedExit)
  | .copyOut => UniqueGamesTheorem.Reduction.MachineTransfer.loopAt (tape 1) (tape 4) id false
      (labels .copyOut) (some (labels .copyBack))
  | .copyBack => MachineCopy.forkLoop (tape 4) (tape 1) (tape 3) false
      (labels .copyBack) (some (labels .compare))
  | .compare => MachineStateEquiv.statement (compareStateEquiv A)
      (MachineCompare.loop (tape 2) (tape 3) (labels .compare) equalExit differentExit)

def steps (payloadLength keyLength : Nat) : Nat :=
  (payloadLength + 1) + 2 * (keyLength + 1) + (max payloadLength keyLength + 1)

theorem steps_le (payloadLength keyLength : Nat) :
    steps payloadLength keyLength ≤ 2 * payloadLength + 3 * keyLength + 4 := by
  unfold steps
  omega

private theorem joinTrace {X : Type*} {f : X → X} {a b c : X} {n m : Nat}
    (first : f^[n] a = b) (second : f^[m] b = c) : f^[n + m] a = c := by
  rw [Nat.add_comm, Function.iterate_add_apply, first, second]

/-- The name is actually scanned, the key actually copied, and the resulting
payloads actually compared. Program premises are literal instruction equations. -/
theorem compareTrace (tape : Fin 5 → K) (distinct : Function.Injective tape)
    (labels : Label → Λ) (equalExit differentExit malformedExit : Option Λ)
    (program : Λ → TM2.Stmt (Alphabet (K := K)) Λ (State A))
    (atLabels : ∀ l, program (labels l) =
      instruction tape labels equalExit differentExit malformedExit l)
    (base : K → List Bool) (bits suffix key : List Bool)
    (canonical : BinaryNameMachine.canonical bits = true)
    (sourceWord : base (tape 0) = BinaryNameMachine.frame bits ++ suffix)
    (keyWord : base (tape 1) = key)
    (candidateEmpty : base (tape 2) = []) (copyEmpty : base (tape 3) = [])
    (scratchEmpty : base (tape 4) = []) (ambient : A) :
    (advance (TM2.step program))^[steps bits.length key.length]
      (some ⟨some (labels .scan), clean ambient, base⟩) =
      some ⟨if bits.reverse = key then equalExit else differentExit,
        clean ambient, Function.update base (tape 0) suffix⟩ := by
  have hd (i j : Fin 5) (hne : i ≠ j) : tape i ≠ tape j := fun h => hne (distinct h)
  let finalBase := Function.update base (tape 0) suffix
  let afterScan := Function.update finalBase (tape 2) bits.reverse
  let afterCopy := Function.update afterScan (tape 3) key
  have scanRun : (advance (TM2.step program))^[bits.length + 1]
      (some ⟨some (labels .scan), clean ambient, base⟩) =
      some ⟨some (labels .copyOut), clean ambient, afterScan⟩ := by
    let back := MachineStateEquiv.program (scanStateEquiv A).symm program
    have atScan : back (labels .scan) =
        BinaryNameMachine.scan (tape 0) (tape 2) (labels .scan)
          (some (labels .copyOut)) malformedExit := by
      change MachineStateEquiv.statement (scanStateEquiv A).symm
        (program (labels .scan)) = _
      rw [atLabels .scan]
      exact MachineStateEquiv.statement_symm_statement (scanStateEquiv A) _
    have backForward : MachineStateEquiv.program (scanStateEquiv A) back = program := by
      funext l
      change MachineStateEquiv.statement (scanStateEquiv A)
        (MachineStateEquiv.statement (scanStateEquiv A).symm (program l)) = _
      simpa only [Equiv.symm_symm] using
        MachineStateEquiv.statement_symm_statement (scanStateEquiv A).symm (program l)
    have initialTapes : UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt (tape 0) (tape 2)
        base (BinaryNameMachine.frame bits ++ suffix) [] = base := by
      rw [← sourceWord, ← candidateEmpty]
      exact UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt_self _ _ _
    have native := BinaryNameMachine.framedTrace (tape 0) (tape 2) (hd 0 2 (by decide))
      (labels .scan) (some (labels .copyOut)) malformedExit back atScan base (ambient, false)
      bits suffix [] none
    simp only [canonical, ↓reduceIte, initialTapes, List.append_nil] at native
    have transported := MachineStateEquiv.trace (scanStateEquiv A) back _ _ _ native
    rw [backForward] at transported
    simpa only [MachineStateEquiv.configuration, scanStateEquiv, Equiv.coe_fn_mk, BinaryNameMachine.clean,
      clean, afterScan, finalBase, UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt] using transported
  have afterScanKey : afterScan (tape 1) = key := by
    simpa [afterScan, finalBase, hd 1 2 (by decide), hd 1 0 (by decide)] using keyWord
  have afterScanCopy : afterScan (tape 3) = [] := by
    simpa [afterScan, finalBase, hd 3 2 (by decide), hd 3 0 (by decide)] using copyEmpty
  have afterScanScratch : afterScan (tape 4) = [] := by
    simpa [afterScan, finalBase, hd 4 2 (by decide), hd 4 0 (by decide)] using scratchEmpty
  have copyRun : (advance (TM2.step program))^[2 * (key.length + 1)]
      (some ⟨some (labels .copyOut), clean ambient, afterScan⟩) =
      some ⟨some (labels .compare), clean ambient, afterCopy⟩ := by
    have h := MachineCopy.copyTrace (tape 1) (tape 3) (tape 4)
      (hd 1 3 (by decide)) (hd 1 4 (by decide)) (hd 3 4 (by decide)) false
      (labels .copyOut) (labels .copyBack) (some (labels .compare)) program
      (atLabels .copyOut) (atLabels .copyBack) afterScan afterScanScratch
      (ambient, false, none) none
    simpa only [afterScanKey, afterScanCopy, List.append_nil, clean, afterCopy] using h
  have candidateWord : afterCopy (tape 2) = bits.reverse := by
    simp [afterCopy, afterScan, hd 2 3 (by decide)]
  have copiedWord : afterCopy (tape 3) = key := by simp [afterCopy]
  have restored : UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt (tape 2) (tape 3)
      afterCopy [] [] = finalBase := by
    funext k
    by_cases hc : k = tape 2
    · subst k
      simp [UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt, afterCopy, afterScan, finalBase,
        hd 2 3 (by decide), hd 2 0 (by decide), candidateEmpty]
    · by_cases hk : k = tape 3
      · subst k
        simp [UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt, afterCopy, afterScan, finalBase,
          hd 3 0 (by decide), copyEmpty]
      · simp [UniqueGamesTheorem.Reduction.MachineTransfer.tapesAt, afterCopy, afterScan, hc, hk]
  have comparisonRun : (advance (TM2.step program))^[max bits.length key.length + 1]
      (some ⟨some (labels .compare), clean ambient, afterCopy⟩) =
      some ⟨if bits.reverse = key then equalExit else differentExit,
        clean ambient, finalBase⟩ := by
    let back := MachineStateEquiv.program (compareStateEquiv A).symm program
    have atCompare : back (labels .compare) =
        MachineCompare.loop (tape 2) (tape 3) (labels .compare) equalExit differentExit := by
      change MachineStateEquiv.statement (compareStateEquiv A).symm
        (program (labels .compare)) = _
      rw [atLabels .compare]
      exact MachineStateEquiv.statement_symm_statement (compareStateEquiv A) _
    have backForward : MachineStateEquiv.program (compareStateEquiv A) back = program := by
      funext l
      change MachineStateEquiv.statement (compareStateEquiv A)
        (MachineStateEquiv.statement (compareStateEquiv A).symm (program l)) = _
      simpa only [Equiv.symm_symm] using
        MachineStateEquiv.statement_symm_statement (compareStateEquiv A).symm (program l)
    have native := MachineCompare.compareTrace_fromTapes (tape 2) (tape 3)
      (hd 2 3 (by decide)) (labels .compare) equalExit differentExit back atCompare afterCopy
      ambient none none
    rw [candidateWord, copiedWord, restored, List.length_reverse] at native
    have transported := MachineStateEquiv.trace (compareStateEquiv A) back _ _ _ native
    rw [backForward] at transported
    simpa only [MachineStateEquiv.configuration, compareStateEquiv, Equiv.coe_fn_mk, clean] using transported
  exact joinTrace (joinTrace scanRun copyRun) comparisonRun

/-- Linear runtime in the extracted candidate and stored key lengths. -/
def compareInTime (tape : Fin 5 → K) (distinct : Function.Injective tape)
    (labels : Label → Λ) (equalExit differentExit malformedExit : Option Λ)
    (program : Λ → TM2.Stmt (Alphabet (K := K)) Λ (State A))
    (atLabels : ∀ l, program (labels l) =
      instruction tape labels equalExit differentExit malformedExit l)
    (base : K → List Bool) (bits suffix key : List Bool)
    (canonical : BinaryNameMachine.canonical bits = true)
    (sourceWord : base (tape 0) = BinaryNameMachine.frame bits ++ suffix)
    (keyWord : base (tape 1) = key)
    (candidateEmpty : base (tape 2) = []) (copyEmpty : base (tape 3) = [])
    (scratchEmpty : base (tape 4) = []) (ambient : A) :
    StateTransition.EvalsToInTime (TM2.step program)
      ⟨some (labels .scan), clean ambient, base⟩
      (some ⟨if bits.reverse = key then equalExit else differentExit,
        clean ambient, Function.update base (tape 0) suffix⟩)
      (2 * bits.length + 3 * key.length + 4) where
  steps := steps bits.length key.length
  evals_in_steps := compareTrace tape distinct labels equalExit differentExit malformedExit
    program atLabels base bits suffix key canonical sourceWord keyWord
    candidateEmpty copyEmpty scratchEmpty ambient
  steps_le_m := steps_le bits.length key.length

/-- Concrete finite machine; exit labels 0, 1, 2 mean equal, different, malformed. -/
def machine : FinTM2 where
  K := Fin 5
  k₀ := 0
  k₁ := 1
  Γ _ := Bool
  Λ := Label ⊕ Fin 3
  main := .inl .scan
  σ := State Unit
  initialState := clean ()
  m label := match label with
    | .inl l => instruction id Sum.inl (some (.inr 0)) (some (.inr 1)) (some (.inr 2)) l
    | .inr _ => .halt

theorem machine_finiteAlphabet (k : machine.K) : Finite (machine.Γ k) := by
  change Finite Bool
  infer_instance

end UniqueGamesTheorem.BinaryNameCompare

end OAI