import Mathlib.Tactic
import Mathlib.Data.Real.Basic
import Mathlib.Topology.Basic

/-!
Self-contained slice for Chapter11 Section 11.8 target theorem.
-/

namespace Chapter11

/-! From Section 11.1: basic bounded interval and partitions (minimal subset).-/


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

@[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

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

/-- Using ||ₗ subscript here to not override || -/
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

@[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

/-! From Section 11.8: Riemann–Stieltjes preliminaries (minimal subset).-/

open BoundedInterval

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

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

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

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

/-! Left and right limits (as in Section 11.8) -/
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)

/-- Definition 11.8.1 -/
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

/-- An improved version of BoundedInterval.joins that also controls α-length. -/
abbrev BoundedInterval.joins' (K I J: BoundedInterval) : Prop :=  K.joins I J ∧ ∀ α:ℝ → ℝ, α[K]ₗ = α[I]ₗ + α[J]ₗ

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.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

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 11.8.8 (h) (Laws of RS integration) / Exercise 11.8.8 -/
-- Target theorem
 theorem PiecewiseConstantOn.RS_integ_of_join {I J K: BoundedInterval} (hIJK: K.joins' I J)
  {f: ℝ → ℝ} (h: PiecewiseConstantOn f K) {α:ℝ → ℝ} (hα: Monotone α):
  RS_integ f K α = RS_integ f I α + RS_integ f J α := by
end Chapter11