-- Mathlib/.lake/packages/lean4/src/lean/Std/Data/DHashMap/Internal/WF.lean
theorem wfImp_alterₘ [BEq α] [Hashable α] [LawfulBEq α] {m : Raw₀ α β} (h : Raw.WFImp m.1) {a : α}
    {f : Option (β a) → Option (β a)} : Raw.WFImp (m.alterₘ a f).1 where
  buckets_hash_self := isHashSelf_alterₘ m h a f
  distinct := DistinctKeys.perm (toListModel_alterₘ h) h.distinct.alterKey
  size_eq

Proof state:
α : Type u
β : α → Type v
inst✝² : BEq α
inst✝¹ : Hashable α
inst✝ : LawfulBEq α
m : Raw₀ α β
h : Raw.WFImp m.val
a : α
f : Option (β a) → Option (β a)
⊢ (if h : m.containsₘ a = true then
          let buckets' := updateBucket m.val.buckets ⋯ a fun l => AssocList.alter a f l;
          let size' := if containsₘ ⟨withComputedSize buckets', ⋯⟩ a = true then m.val.size else m.val.size - 1;
          ⟨{ size := size', buckets := buckets' }, ⋯⟩
        else
          match f none with
          | none => m
          | some b => (m.consₘ a b).expandIfNecessary).val.size =
    (alterKey a f (toListModel m.val.buckets)).length

Tactic:
split

Result:
case isTrue
α : Type u
β : α → Type v
inst✝² : BEq α
inst✝¹ : Hashable α
inst✝ : LawfulBEq α
m : Raw₀ α β
h : Raw.WFImp m.val
a : α
f : Option (β a) → Option (β a)
h✝ : m.containsₘ a = true
⊢ (let buckets' := updateBucket m.val.buckets ⋯ a fun l => AssocList.alter a f l;
        let size' := if containsₘ ⟨withComputedSize buckets', ⋯⟩ a = true then m.val.size else m.val.size - 1;
        ⟨{ size := size', buckets := buckets' }, ⋯⟩).val.size =
    (alterKey a f (toListModel m.val.buckets)).length

case isFalse
α : Type u
β : α → Type v
inst✝² : BEq α
inst✝¹ : Hashable α
inst✝ : LawfulBEq α
m : Raw₀ α β
h : Raw.WFImp m.val
a : α
f : Option (β a) → Option (β a)
h✝ : ¬m.containsₘ a = true
⊢ (match f none with
        | none => m
        | some b => (m.consₘ a b).expandIfNecessary).val.size =
    (alterKey a f (toListModel m.val.buckets)).length

Proof state:
case isTrue
α : Type u
β : α → Type v
inst✝² : BEq α
inst✝¹ : Hashable α
inst✝ : LawfulBEq α
m : Raw₀ α β
h : Raw.WFImp m.val
a : α
f : Option (β a) → Option (β a)
h✝ : m.containsₘ a = true
⊢ (let buckets' := updateBucket m.val.buckets ⋯ a fun l => AssocList.alter a f l;
        let size' := if containsₘ ⟨withComputedSize buckets', ⋯⟩ a = true then m.val.size else m.val.size - 1;
        ⟨{ size := size', buckets := buckets' }, ⋯⟩).val.size =
    (alterKey a f (toListModel m.val.buckets)).length

Tactic:
next h₁ =>
rw [containsₘ_eq_containsKey h] at h₁
simp only [length_alterKey, h.size_eq, dif_pos h₁]
rw [containsₘ_eq_containsKey (by apply wfImp_updateBucket_alter h)]
simp only [buckets_withComputedSize]
simp only [containsKey_of_perm <| toListModel_updateBucket_alter h]
rw [← getValueCast?_eq_some_getValueCast h₁]
conv => lhs; congr; rw [containsKey_alterKey_self h.distinct]

Result:
no goals

Proof state:
α : Type u
β : α → Type v
inst✝² : BEq α
inst✝¹ : Hashable α
inst✝ : LawfulBEq α
m : Raw₀ α β
h : Raw.WFImp m.val
a : α
f : Option (β a) → Option (β a)
h₁ : ¬containsKey a (toListModel m.val.buckets) = true
⊢ (match f none with
        | none => m
        | some b => (m.consₘ a b).expandIfNecessary).val.size =
    (alterKey a f (toListModel m.val.buckets)).length

Tactic:
rw [alterKey]

Result:
α : Type u
β : α → Type v
inst✝² : BEq α
inst✝¹ : Hashable α
inst✝ : LawfulBEq α
m : Raw₀ α β
h : Raw.WFImp m.val
a : α
f : Option (β a) → Option (β a)
h₁ : ¬containsKey a (toListModel m.val.buckets) = true
⊢ (match f none with
        | none => m
        | some b => (m.consₘ a b).expandIfNecessary).val.size =
    (match f (getValueCast? a (toListModel m.val.buckets)) with
      | none => eraseKey a (toListModel m.val.buckets)
      | some v => insertEntry a v (toListModel m.val.buckets)).length