-- Clean/Utils/SourceSinkPath.lean

import Mathlib.Data.Finset.Basic

import Mathlib.Data.Fintype.Basic

import Mathlib.Data.Fintype.Prod

import Mathlib.Data.List.Basic

import Mathlib.Algebra.BigOperators.Group.Finset.Basic

import Mathlib.Algebra.Order.BigOperators.Group.Finset

import Mathlib.Algebra.BigOperators.Group.Finset.Piecewise

import Mathlib.Algebra.BigOperators.Ring.Finset

namespace Utils.StateTransition

variable {S : Type*} [DecidableEq S] [Fintype S]

def Transition (S : Type*) := S × S

def Run (S : Type*) := Transition S → ℕ

def countTransitionInPath [DecidableEq S] (t : Transition S) (path : List S) : ℕ :=
  (path.zip path.tail).count t

def Run.containsPath [DecidableEq S] (R : Run S) (path : List S) : Prop :=
  ∀ t : Transition S, countTransitionInPath t path ≤ R t

def Run.hasCycle [DecidableEq S] (R : Run S) : Prop :=
  ∃ (cycle : List S), cycle.length ≥ 2 ∧
    cycle.head? = cycle.getLast? ∧
    R.containsPath cycle

def Run.isAcyclic [DecidableEq S] (R : Run S) : Prop :=
  ¬R.hasCycle

-- Repository definitions used:
content: ...

-- Repository lemmas used:
content: theorem List.getElem_append_left{α : Type u_1} {l₁ l₂ : List α} {i : Nat} (hn : i < l₁.length) :\n(l₁ ++ l₂)[i] = l₁[i]

-- Local definitions used:
content: def Transition (S : Type*) := S × S

content: def Run (S : Type*) := Transition S → ℕ

content: def countTransitionInPath [DecidableEq S] (t : Transition S) (path : List S) : ℕ :=
  (path.zip path.tail).count t

content: def Run.containsPath [DecidableEq S] (R : Run S) (path : List S) : Prop :=
  ∀ t : Transition S, countTransitionInPath t path ≤ R t

content: def Run.hasCycle [DecidableEq S] (R : Run S) : Prop :=
  ∃ (cycle : List S), cycle.length ≥ 2 ∧
    cycle.head? = cycle.getLast? ∧
    R.containsPath cycle

content: def Run.isAcyclic [DecidableEq S] (R : Run S) : Prop :=
  ¬R.hasCycle

-- Local lemmas used:
content: lemma zip_drop_sublist (l : List S) (n : ℕ) :
    ((l.drop n).zip (l.drop (n + 1))).Sublist (l.zip l.tail)

content: lemma containsPath_drop (R : Run S) (path : List S) (n : ℕ)
    (h_contains : R.containsPath path) :
    R.containsPath (path.drop n)

content: lemma zip_take_sublist (l1 l2 : List S) (n m : ℕ) :
    ((l1.take n).zip (l2.take m)).Sublist (l1.zip l2)

content: lemma tail_take {α : Type*} (l : List α) (n : ℕ) :
    (l.take n).tail = (l.tail).take (n - 1)

content: lemma containsPath_take (R : Run S) (path : List S) (n : ℕ)
    (h_contains : R.containsPath path) :
    R.containsPath (path.take n)

content: lemma drop_take_length_ge_two {α : Type*} (path : List α) (n m : Fin path.length)
    (h_n_lt_m : n < m) :
    ((path.drop n.val).take (m.val - n.val + 1)).length ≥ 2

content: lemma getLast_drop_take {α : Type*} (path : List α) (n k : ℕ)
    (h_n_lt : n < path.length)
    (h_bound : n + k ≤ path.length)
    (h_k_pos : k > 0) :
    ((path.drop n).take k).getLast? = path[n + k - 1]?

content: lemma drop_take_cycle_same_endpoints (path : List S) (x : S) (n m : Fin path.length)
    (h_n_lt_m : n < m)
    (h_x_at_n : path[n] = x)
    (h_x_at_m : path[m] = x) :
    ((path.drop n.val).take (m.val - n.val + 1)).head? =
    ((path.drop n.val).take (m.val - n.val + 1)).getLast?

content: lemma containsPath_drop_take (R : Run S) (path : List S) (n m : ℕ)
    (h_contains : R.containsPath path) :
    R.containsPath ((path.drop n).take m)

content: lemma acyclic_containsPath_nodup (R : Run S) (path : List S)
    (h_acyclic : R.isAcyclic)
    (h_contains : R.containsPath path) :
    path.Nodup

content: lemma countTransitionInPath_append_singleton (path : List S) (x y : S)
    (h_nonempty : path ≠ [])
    (h_last : path.getLast? = some x)
    (h_not_in : (x, y) ∉ path.zip path.tail) :
    countTransitionInPath (x, y) (path ++ [y]) = 1

content: lemma countTransitionInPath_append_singleton_other (path : List S) (x y : S) (t : Transition S)
    (h_nonempty : path ≠ [])
    (h_last : path.getLast? = some x)
    (h_ne : t ≠ (x, y)) :
    countTransitionInPath t (path ++ [y]) = countTransitionInPath t path

content: lemma getLast_mem {α : Type*} (l : List α) (x : α) (h_last : l.getLast? = some x) :
    x ∈ l

content: lemma last_not_in_zip_tail {α : Type*} [DecidableEq α] (l : List α) (x : α)
    (h_nodup : l.Nodup)
    (h_last : l.getLast? = some x) :
    ∀ y : α, (x, y) ∉ l.zip l.tail

content: lemma drop_of_lt_length_nonempty {α : Type*} (path : List α) (i : ℕ)
    (h_i_lt : i < path.length) :
    path.drop i ≠ []

content: lemma cycle_from_suffix_contains (R : Run S) (suffix : List S) (current y : S)
    (h_suffix_nodup : suffix.Nodup)
    (h_contains_suffix : R.containsPath suffix)
    (h_suffix_nonempty : suffix ≠ [])
    (h_suffix_last : suffix.getLast? = some current)
    (h_edge : R (current, y) > 0) :
    ∀ t : Transition S, countTransitionInPath t (suffix ++ [y]) ≤ R t

-- Target theorem and proof:
lemma path_with_back_edge_creates_cycle (R : Run S) (path : List S) (current y : S)
    (h_acyclic : R.isAcyclic)
    (h_end : path.getLast? = some current)
    (h_contains : R.containsPath path)
    (h_y_in_path : y ∈ path)
    (h_edge : R (current, y) > 0) :
    R.hasCycle
:= by
  -- Find y's position in path
  rw [List.mem_iff_getElem] at h_y_in_path
  obtain ⟨i, h_i_lt, h_y_eq⟩ := h_y_in_path
  let suffix := path.drop i
  have h_suffix_nonempty : suffix ≠ [] := drop_of_lt_length_nonempty path i h_i_lt
  have h_suffix_head : suffix.head? = some y := by aesop
  have h_suffix_last : suffix.getLast? = some current := by grind
  let cycle := suffix ++ [y]
  have h_cycle_head : cycle.head? = some y := by grind
  have h_cycle_last : cycle.getLast? = some y := by grind
  have h_cycle_len : cycle.length ≥ 2 := by grind
  use cycle
  constructor
  · exact h_cycle_len
  constructor
  · rw [h_cycle_head, h_cycle_last]
  · unfold cycle
    have h_suffix_contains : R.containsPath suffix := by
      unfold suffix
      exact containsPath_drop R path i h_contains
    have h_path_nodup := acyclic_containsPath_nodup R path h_acyclic h_contains
    have h_suffix_nodup : suffix.Nodup := by grind
    exact cycle_from_suffix_contains R suffix current y h_suffix_nodup h_suffix_contains h_suffix_nonempty h_suffix_last h_edge