import Mathlib
import Analysis.Section_11_6
import Analysis.Tags.Section_11_6

namespace Chapter11

-- <CONTEXT>
namespace Chapter9
abbrev Convergesto (X : Set ℝ) (f : ℝ → ℝ) (L x₀ : ℝ) : Prop :=
  (nhdsWithin x₀ X).Tendsto f (nhds L)
end Chapter9

inductive BoundedInterval where
  | Ioo (a b:ℝ) : BoundedInterval
  | Icc (a b:ℝ) : BoundedInterval
  | Ioc (a b:ℝ) : BoundedInterval
  | Ico (a b:ℝ) : BoundedInterval

open BoundedInterval Chapter9

@[coe]
def BoundedInterval.toSet (I: BoundedInterval) : Set ℝ := match I with
  | Ioo a b => .Ioo a b
  | Icc a b => .Icc a b
  | Ioc a b => .Ioc a b
  | Ico a b => .Ico a b

instance BoundedInterval.inst_coeSet : Coe BoundedInterval (Set ℝ) where
  coe := toSet

open Classical in
noncomputable instance BoundedInterval.decidableEq : DecidableEq BoundedInterval := Classical.decEq BoundedInterval

instance BoundedInterval.instMembership : Membership ℝ BoundedInterval where
  mem I x := x ∈ (I:Set ℝ)

instance BoundedInterval.instSubset : HasSubset BoundedInterval where
  Subset I J := ∀ x, x ∈ I → x ∈ J

abbrev BoundedInterval.a (I: BoundedInterval) : ℝ := match I with
  | Ioo a _ => a
  | Icc a _ => a
  | Ioc a _ => a
  | Ico a _ => a

abbrev BoundedInterval.b (I: BoundedInterval) : ℝ := match I with
  | Ioo _ b => b
  | Icc _ b => b
  | Ioc _ b => b
  | Ico _ b => b

abbrev BoundedInterval.length (I: BoundedInterval) : ℝ := max (I.b - I.a) 0

macro:max atomic("|" noWs) a:term noWs "|ₗ" : term => `(BoundedInterval.length $a)

abbrev BoundedInterval.joins (K I J: BoundedInterval) : Prop := (I:Set ℝ) ∩ (J:Set ℝ) = ∅
  ∧ (K:Set ℝ) = (I:Set ℝ) ∪ (J:Set ℝ) ∧ |K|ₗ = |I|ₗ + |J|ₗ

theorem BoundedInterval.join_Ico_Icc {a b c:ℝ} (hab: a ≤ b) (hbc: b ≤ c) : (Icc a c).joins (Ico a b) (Icc b c) := by
  sorry

@[ext]
structure Partition (I: BoundedInterval) where
  intervals : Finset BoundedInterval
  exists_unique (x:ℝ) (hx : x ∈ I) : ∃! J, J ∈ intervals ∧ x ∈ J
  contains (J : BoundedInterval) (hJ : J ∈ intervals) : J ⊆ I

instance Partition.instMembership (I: BoundedInterval) : Membership BoundedInterval (Partition I) where
  mem P J := J ∈ P.intervals

instance Partition.instBot (I: BoundedInterval) : Bot (Partition I) where
  bot := {
    intervals := {I}
    exists_unique := by
      sorry
    contains := by
      sorry
    }

noncomputable abbrev Partition.join {I J K:BoundedInterval} (P: Partition I) (Q: Partition J) (h: K.joins I J) : Partition K :=
{
  intervals := P.intervals ∪ Q.intervals
  exists_unique := by
    sorry
  contains := by
    sorry
}

abbrev Constant {X Y:Type} (f: X → Y) : Prop := ∃ c, ∀ x, f x = c

open Classical in
noncomputable abbrev constant_value {X Y:Type} [hY: Nonempty Y] (f:X → Y) : Y :=
  if h: Constant f then h.choose else hY.some

abbrev ConstantOn (f: ℝ → ℝ) (X: Set ℝ) : Prop := Constant (fun x : X ↦ f ↑x)

noncomputable abbrev constant_value_on (f:ℝ → ℝ) (X: Set ℝ) : ℝ := constant_value (fun x : X ↦ f ↑x)

abbrev PiecewiseConstantWith (f:ℝ → ℝ) {I: BoundedInterval} (P: Partition I) : Prop := ∀ J ∈ P, ConstantOn f (J:Set ℝ)

abbrev PiecewiseConstantOn (f:ℝ → ℝ) (I: BoundedInterval) : Prop := ∃ P : Partition I, PiecewiseConstantWith f P

noncomputable abbrev PiecewiseConstantWith.integ (f:ℝ → ℝ) {I: BoundedInterval} (P: Partition I)  :
  ℝ := ∑ J ∈ P.intervals, constant_value_on f (J:Set ℝ) * |J|ₗ

noncomputable abbrev right_lim (f: ℝ → ℝ) (x₀:ℝ) : ℝ := lim ((nhdsWithin x₀ (.Ioi x₀)).map f)

noncomputable abbrev left_lim (f: ℝ → ℝ) (x₀:ℝ) : ℝ := lim ((nhdsWithin x₀ (.Iio x₀)).map f)

theorem right_lim_def {f: ℝ → ℝ} {x₀ L:ℝ} (h: Convergesto (.Ioi x₀) f L x₀) :
  right_lim f x₀ = L := by
  sorry

theorem left_lim_def {f: ℝ → ℝ} {x₀ L:ℝ} (h: Convergesto (.Iio x₀) f L x₀) :
  left_lim f x₀ = L := by
  sorry

noncomputable abbrev jump (f: ℝ → ℝ) (x₀:ℝ) : ℝ :=
  right_lim f x₀ - left_lim f x₀

theorem right_lim_of_continuous {X:Set ℝ} {f: ℝ → ℝ} {x₀:ℝ}
  (h : ∃ ε>0, .Ico x₀ (x₀+ε) ⊆ X) (hf: ContinuousWithinAt f X x₀) :
  right_lim f x₀ = f x₀ := by
  sorry

theorem left_lim_of_continuous {X:Set ℝ} {f: ℝ → ℝ} {x₀:ℝ}
  (h : ∃ ε>0, .Ioc (x₀-ε) x₀ ⊆ X) (hf: ContinuousWithinAt f X x₀) :
  left_lim f x₀ = f x₀ := by
  sorry

theorem jump_of_continuous {X:Set ℝ} {f: ℝ → ℝ} {x₀:ℝ}
  (h : X ∈ nhds x₀) (hf: ContinuousWithinAt f X x₀) :
  jump f x₀ = 0 := by
  sorry

theorem right_lim_of_monotone {f: ℝ → ℝ} (x₀:ℝ) (hf: Monotone f) :
  Convergesto (.Ioi x₀) f (sInf (f '' .Ioi x₀)) x₀ := by
  sorry

theorem right_lim_of_monotone' {f: ℝ → ℝ} (x₀:ℝ) (hf: Monotone f) :
  right_lim f x₀ = sInf (f '' .Ioi x₀) := by
  sorry

theorem left_lim_of_monotone {f: ℝ → ℝ} (x₀:ℝ) (hf: Monotone f) :
  Convergesto (.Iio x₀) f (sSup (f '' .Iio x₀)) x₀ := by
  sorry

theorem left_lim_of_monotone' {f: ℝ → ℝ} (x₀:ℝ) (hf: Monotone f) :
  left_lim f x₀ = sSup (f '' .Iio x₀) := by
  sorry

theorem jump_of_monotone {f: ℝ → ℝ} (x₀:ℝ) (hf: Monotone f) :
  0 ≤ jump f x₀  := by
  sorry

theorem right_lim_le_left_lim_of_monotone {f:ℝ → ℝ} {a b:ℝ} (hab: a < b)
  (hf: Monotone f) :
  right_lim f a ≤ left_lim f b := by
  sorry

noncomputable abbrev α_length (α: ℝ → ℝ) (I: BoundedInterval) : ℝ := match I with
| Icc a b => if a ≤ b then (right_lim α b) - (left_lim α a) else 0
| Ico a b => if a ≤ b then (left_lim α b) - (left_lim α a) else 0
| Ioc a b => if a ≤ b then (right_lim α b) - (right_lim α a) else 0
| Ioo a b => if a < b then (left_lim α b) - (right_lim α a) else 0

notation3:max α"["I"]ₗ" => α_length α I

theorem α_length_of_empty (α: ℝ → ℝ) {I: BoundedInterval} (hI: (I:Set ℝ) = ∅) : α[I]ₗ = 0 := by
  sorry

@[simp]
theorem α_length_of_pt {α: ℝ → ℝ} (a:ℝ) : α[Icc a a]ₗ = jump α a := by
  sorry

theorem α_length_of_cts {α:ℝ → ℝ} {I: BoundedInterval} {a b: ℝ}
  (haa: a < I.a) (hab: I.a ≤ I.b) (hbb: I.b < b)
  (hI : I ⊆ Ioo a b) (hα: ContinuousOn α (Ioo a b)) :
  α[I]ₗ = α I.b - α I.a := by
  sorry

@[simp]
theorem α_len_of_id (I: BoundedInterval) : (fun x ↦ x)[I]ₗ = |I|ₗ := by
  sorry

abbrev BoundedInterval.joins' (K I J: BoundedInterval) : Prop :=  K.joins I J ∧ ∀ α:ℝ → ℝ, α[K]ₗ = α[I]ₗ + α[J]ₗ

theorem BoundedInterval.join_Icc_Ioc' {a b c:ℝ} (hab: a ≤ b) (hbc: b ≤ c) : (Icc a c).joins' (Icc a b) (Ioc b c) := by
  sorry

theorem BoundedInterval.join_Icc_Ioo' {a b c:ℝ} (hab: a ≤ b) (hbc: b < c) : (Ico a c).joins' (Icc a b) (Ioo b c) := by
  sorry

theorem BoundedInterval.join_Ioc_Ioc' {a b c:ℝ} (hab: a ≤ b) (hbc: b ≤ c) : (Ioc a c).joins' (Ioc a b) (Ioc b c) := by
  sorry

theorem BoundedInterval.join_Ioc_Ioo' {a b c:ℝ} (hab: a ≤ b) (hbc: b < c) : (Ioo a c).joins' (Ioc a b) (Ioo b c) := by
  sorry

theorem BoundedInterval.join_Ico_Icc' {a b c:ℝ} (hab: a ≤ b) (hbc: b ≤ c) : (Icc a c).joins' (Ico a b) (Icc b c) := by
  sorry

theorem BoundedInterval.join_Ico_Ico' {a b c:ℝ} (hab: a ≤ b) (hbc: b ≤ c) : (Ico a c).joins' (Ico a b) (Ico b c) := by
  sorry

theorem BoundedInterval.join_Ioo_Icc' {a b c:ℝ} (hab: a < b) (hbc: b ≤ c) : (Ioc a c).joins' (Ioo a b) (Icc b c) := by
  sorry

theorem BoundedInterval.join_Ioo_Ico' {a b c:ℝ} (hab: a < b) (hbc: b ≤ c) : (Ioo a c).joins' (Ioo a b) (Ico b c) := by
  sorry

noncomputable abbrev PiecewiseConstantWith.RS_integ (f:ℝ → ℝ) {I: BoundedInterval} (P: Partition I) (α: ℝ → ℝ)   :
  ℝ := ∑ J ∈ P.intervals, constant_value_on f (J:Set ℝ) * α[J]ₗ

noncomputable abbrev f_11_8_6 (x:ℝ) : ℝ := if x < 2 then 4 else 2

noncomputable abbrev P_11_8_6 : Partition (Icc 1 3) :=
  (⊥: Partition (Ico 1 2)).join (⊥ : Partition (Icc 2 3))
  (join_Ico_Icc (by norm_num) (by norm_num) )

theorem f_11_8_6_RS_integ : PiecewiseConstantWith.RS_integ f_11_8_6 P_11_8_6 (fun x ↦ x) = 22 := by
  sorry

theorem PiecewiseConstantWith.RS_integ_eq_integ {f:ℝ → ℝ} {I: BoundedInterval} (P: Partition I) :RS_integ f P (fun x ↦ x) = integ f P := by
  sorry

theorem PiecewiseConstantWith.RS_integ_eq {f:ℝ → ℝ} {I: BoundedInterval} {P P': Partition I}
  (hP: PiecewiseConstantWith f P) (hP': PiecewiseConstantWith f P') (α:ℝ → ℝ): RS_integ f P α = RS_integ f P' α := by
  sorry

open Classical in
noncomputable abbrev PiecewiseConstantOn.RS_integ (f:ℝ → ℝ) (I: BoundedInterval) (α:ℝ → ℝ):
  ℝ := if h: PiecewiseConstantOn f I then PiecewiseConstantWith.RS_integ f h.choose α else 0

theorem PiecewiseConstantOn.RS_integ_def {f:ℝ → ℝ} {I: BoundedInterval} {P: Partition I}
  (h: PiecewiseConstantWith f P) (α:ℝ → ℝ) : RS_integ f I α = PiecewiseConstantWith.RS_integ f P α := by
  sorry

theorem α_length_nonneg_of_monotone {α:ℝ → ℝ}  (hα: Monotone α) (I: BoundedInterval):
  0 ≤ α[I]ₗ := by
  sorry
-- </CONTEXT>
-- <TARGET_THEOREM>
open Classical in
theorem PiecewiseConstantOn.RS_of_extend {I J: BoundedInterval} (hIJ: I ⊆ J)
  {f: ℝ → ℝ} (h: PiecewiseConstantOn f I) {α:ℝ → ℝ} (hα: Monotone α):
  PiecewiseConstantOn (fun x ↦ if x ∈ I then f x else 0) J := by
-- </TARGET_THEOREM>

end Chapter11