/-|
# Binary Search Trees

If the type of keys can be totally ordered -- that is, it supports a well-behaved `≤` comparison --
then maps can be implemented with binary search trees (BSTs). Insert and lookup operations on BSTs take time
proportional to the height of the tree. If the tree is balanced, the operations therefore take logarithmic time.

This example is based on a similar example found in the ["Sofware Foundations"](https://softwarefoundations.cis.upenn.edu/vfa-current/SearchTree.html)
book (volume 3).
-/

/-|
We use `Nat` as the key type in our implementation of BSTs,
since it has a convenient total order with lots of theorems and automation available.
We leave as an exercise to the reader the generalization to arbitrary types.
-/

inductive Tree (β : Type v) where
  | leaf
  | node (left : Tree β) (key : Nat) (value : β) (right : Tree β)
  deriving Repr

/-|
The function `contains` returns `true` iff the given gree contains the key `k`.
-/
def Tree.contains (t : Tree β) (k : Nat) : Bool :=
  match t with
  | leaf => false
  | node left key value right =>
    if k < key then
      left.contains k
    else if key < k then
      right.contains k
    else
      true

/-|
`t.find? k` returns `some v` if `v` is the value bound to key `k` in the tree `t`. It returns `none` otherwise.
-/
def Tree.find? (t : Tree β) (k : Nat) : Option β :=
  match t with
  | leaf => none
  | node left key value right =>
    if k < key then
      left.find? k
    else if key < k then
      right.find? k
    else
      some value

/-|
`t.insert k v` is the map containing all the bindings of `t` along with a binding of `k` to `v`.
-/
def Tree.insert (t : Tree β) (k : Nat) (v : β) : Tree β :=
  match t with
  | leaf => node leaf k v leaf
  | node left key value right =>
    if k < key then
      node (left.insert k v) key value right
    else if key < k then
      node left key value (right.insert k v)
    else
      node left k v right
/-|
Let's add a new operation to our tree: converting it to an association list that contains the key--value bindings from the tree stored as pairs.
If that list is sorted by the keys, then any two trees that represent the same map would be converted to the same list.
Here's a function that does so with an in-order traversal of the tree.
-/
def Tree.toList (t : Tree β) : List (Nat × β) :=
  match t with
  | leaf => []
  | node l k v r => l.toList ++ [(k, v)] ++ r.toList

#eval Tree.leaf.insert 2 "two"
      |>.insert 3 "three"
      |>.insert 1 "one"

#eval Tree.leaf.insert 2 "two"
      |>.insert 3 "three"
      |>.insert 1 "one"
      |>.toList

/-|
The implemention of `Tree.toList` is inefficient because of how it uses the `++` operator.
On a balanced tree its running time is linearithmic, because it does a linear number of
concatentations at each level of the tree. On an unbalanced tree it's quadratic time.
Here's a tail-recursive implementation than runs in linear time, regardless of whether the tree is balanced:
-/
def Tree.toListTR (t : Tree β) : List (Nat × β) :=
  go t []
where
  go (t : Tree β) (acc : List (Nat × β)) : List (Nat × β) :=
    match t with
    | leaf => acc
    | node l k v r => l.go ((k, v) :: r.go acc)

/-|
We now prove that `t.toList` and `t.toListTR` return the same list.
The proof is on induction, and as we used the auxiliary function `go`
to define `Tree.toListTR`, we use the auxiliary theorem `go` to prove the theorem.

The proof of the auxiliary theorem is by induction on `t`.
The `generalizing acc` modifier instructs Lean to revert `acc`, apply the
induction theorem for `Tree`s, and then reintroduce `acc` in each case.
By using `generalizing`, we obtain the more general induction hypotheses

- `left_ih : ∀ acc, toListTR.go left acc = toList left ++ acc`

- `right_ih : ∀ acc, toListTR.go right acc = toList right ++ acc`

Recall that the combinator `tac <;> tac'` runs `tac` on the main goal and `tac'` on each produced goal,
concatenating all goals produced by `tac'`. In this theorem, we use it to apply
`simp` and close each subgoal produced by the `induction` tactic.

The `simp` parameters `toListTR.go` and `toList` instruct the simplifier to try to reduce
and/or apply auto generated equation theorems for these two functions.
The parameter `*` intructs the simplifier to use any equation in a goal as rewriting rules.
In this particular case, `simp` uses the induction hypotheses as rewriting rules.
Finally, the parameter `List.append_assoc` intructs the simplifier to use the
`List.append_assoc` theorem as a rewriting rule.
-/
theorem Tree.toList_eq_toListTR (t : Tree β)
        : t.toList = t.toListTR := by
  simp [toListTR, go t []]
where
  go (t : Tree β) (acc : List (Nat × β))
     : toListTR.go t acc = t.toList ++ acc := by
    induction t generalizing acc <;>
      simp [toListTR.go, toList, *, List.append_assoc]

/-|
The `[csimp]` annotation instructs the Lean code generator to replace
any `Tree.toList` with `Tree.toListTR` when generating code.
-/
@[csimp] theorem Tree.toList_eq_toListTR_csimp
                 : @Tree.toList = @Tree.toListTR := by
  funext β t
  apply toList_eq_toListTR

/-|
The implementations of `Tree.find?` and `Tree.insert` assume that values of type tree obey the BST invariant:
for any non-empty node with key `k`, all the values of the `left` subtree are less than `k` and all the values
of the right subtree are greater than `k`. But that invariant is not part of the definition of tree.

So, let's formalize the BST invariant. Here's one way to do so. First, we define a helper `ForallTree`
to express that idea that a predicate holds at every node of a tree:
-/
inductive ForallTree (p : Nat → β → Prop) : Tree β → Prop
  | leaf : ForallTree p .leaf
  | node :
     ForallTree p left →
     p key value →
     ForallTree p right →
     ForallTree p (.node left key value right)

/-|
Second, we define the BST invariant:
An empty tree is a BST.
A non-empty tree is a BST if all its left nodes have a lesser key, its right nodes have a greater key, and the left and right subtrees are themselves BSTs.
-/
inductive BST : Tree β → Prop
  | leaf : BST .leaf
  | node :
     ForallTree (fun k v => k < key) left →
     ForallTree (fun k v => key < k) right →
     BST left → BST right →
     BST (.node left key value right)

/-|
We can use the `macro` command to create helper tactics for organizing our proofs.
The macro `have_eq x y` tries to prove `x = y` using linear arithmetic, and then
immediately uses the new equality to substitute `x` with `y` everywhere in the goal.

The modifier `local` specifies the scope of the macro.
-/
local macro "have_eq " lhs:term:max rhs:term:max : tactic =>
  `((have h : $lhs:term = $rhs:term :=
       -- TODO: replace with linarith
       by simp_arith at *; apply Nat.le_antisymm <;> assumption
     try subst $lhs:term))

/-|
The `by_cases' e` is just the regular `by_cases` followed by `simp` using all
hypotheses in the current goal as rewriting rules.
Recall that the `by_cases` tactic creates two goals. One where we have `h : e` and
another one containing `h : ¬ e`. The simplier uses the `h` to rewrite `e` to `True`
in the first subgoal, and `e` to `False` in the second. This is particularly
useful if `e` is the condition of an `if`-statement.
-/
local macro "by_cases' " e:term :  tactic =>
  `(by_cases $e:term <;> simp [*])


/-|
We can use the attribute `[simp]` to instruct the simplifier to reduce given definitions or
apply rewrite theorems. The `local` modifier limits the scope of this modification to this file.
-/
attribute [local simp] Tree.insert

/-|
We now prove that `Tree.insert` preserves the BST invariant using induction and case analysis.
Recall that the tactic `. tac` focuses on the main goal and tries to solve it using `tac`, or else fails.
It is used to structure proofs in Lean.
The notation `‹e›` is just syntax sugar for `(by assumption : e)`. That is, it tries to find a hypothesis `h : e`.
It is useful to access hypothesis that have auto generated names (aka "inaccessible") names.
-/

theorem Tree.forall_insert_of_forall
        (h₁ : ForallTree p t) (h₂ : p key value)
        : ForallTree p (t.insert key value) := by
  induction h₁ with
  | leaf => exact .node .leaf h₂ .leaf
  | node hl hp hr ihl ihr =>
    rename Nat => k
    by_cases' key < k
    . exact .node ihl hp hr
    . by_cases' k < key
      . exact .node hl hp ihr
      . have_eq key k
        exact .node hl h₂ hr

theorem Tree.bst_insert_of_bst
        {t : Tree β} (h : BST t) (key : Nat) (value : β)
        : BST (t.insert key value) := by
  induction h with
  | leaf => exact .node .leaf .leaf .leaf .leaf
  | node h₁ h₂ b₁ b₂ ih₁ ih₂ =>
    rename Nat => k
    simp
    by_cases' key < k
    . exact .node (forall_insert_of_forall h₁ ‹key < k›) h₂ ih₁ b₂
    . by_cases' k < key
      . exact .node h₁ (forall_insert_of_forall h₂ ‹k < key›) b₁ ih₂
      . have_eq key k
        exact .node h₁ h₂ b₁ b₂

/-|
Now, we define the type `BinTree` using a `Subtype` that states that only trees satisfying the BST invariant are `BinTree`s.
-/
def BinTree (β : Type u) := { t : Tree β // BST t }

def BinTree.mk : BinTree β :=
  ⟨.leaf, .leaf⟩

def BinTree.contains (b : BinTree β) (k : Nat) : Bool :=
  b.val.contains k

def BinTree.find? (b : BinTree β) (k : Nat) : Option β :=
  b.val.find? k

def BinTree.insert (b : BinTree β) (k : Nat) (v : β) : BinTree β :=
  ⟨b.val.insert k v, b.val.bst_insert_of_bst b.property k v⟩

/-|
Finally, we prove that `BinTree.find?` and `BinTree.insert` satisfy the map properties.
-/

attribute [local simp]
  BinTree.mk BinTree.contains BinTree.find?
  BinTree.insert Tree.find? Tree.contains Tree.insert

theorem BinTree.find_mk (k : Nat)
        : BinTree.mk.find? k = (none : Option β) := by
  simp

theorem BinTree.find_insert (b : BinTree β) (k : Nat) (v : β)
        : (b.insert k v).find? k = some v := by
  let ⟨t, h⟩ := b; simp
  induction t with simp
  | node left key value right ihl ihr =>
    by_cases' k < key
    . cases h; apply ihl; assumption
    . by_cases' key < k
      cases h; apply ihr; assumption

theorem BinTree.find_insert_of_ne (b : BinTree β) (h : k ≠ k') (v : β)
        : (b.insert k v).find? k' = b.find? k' := by
  let ⟨t, h⟩ := b; simp
  induction t with simp
  | leaf =>
    split <;> simp <;> split <;> simp
    have_eq k k'
    contradiction
  | node left key value right ihl ihr =>
    let .node hl hr bl br := h
    specialize ihl bl
    specialize ihr br
    by_cases' k < key; by_cases' key < k
    have_eq key k
    by_cases' k' < k; by_cases' k  < k'
    have_eq k k'
    contradiction
