feat(data/int/basic): bounded forall is decidable for integers

data/int/basic.lean

@@ -5,7 +5,7 @@ Authors: Jeremy Avigad
 
 The integers, with addition, multiplication, and subtraction.
 -/
-import data.nat.basic algebra.char_zero algebra.order_functions
+import data.nat.basic data.list.basic algebra.char_zero algebra.order_functions
 open nat
 
 namespace int
@@ -1126,4 +1126,35 @@ by simp [abs]
 
 end cast
 
+section decidable
+
+def range (m n : ℤ) : list ℤ :=
+(list.range (to_nat (n-m))).map $ λ r, m+r
+
+theorem mem_range_iff {m n r : ℤ} : r ∈ range m n ↔ m ≤ r ∧ r < n :=
+⟨λ H, let ⟨s, h1, h2⟩ := list.mem_map.1 H in h2 ▸
+  ⟨le_add_of_nonneg_right trivial,
+  add_lt_of_lt_sub_left $ match n-m, h1 with
+    | (k:ℕ), h1 := by rwa [list.mem_range, to_nat_coe_nat, ← coe_nat_lt] at h1
+    end⟩,
+λ ⟨h1, h2⟩, list.mem_map.2 ⟨to_nat (r-m),
+  list.mem_range.2 $ by rw [← coe_nat_lt, to_nat_of_nonneg (sub_nonneg_of_le h1),
+      to_nat_of_nonneg (sub_nonneg_of_le (le_of_lt (lt_of_le_of_lt h1 h2)))];
+    exact sub_lt_sub_right h2 _,
+  show m + _ = _, by rw [to_nat_of_nonneg (sub_nonneg_of_le h1), add_sub_cancel'_right]⟩⟩
+
+instance decidable_le_lt (P : int → Prop) [decidable_pred P] (m n : ℤ) : decidable (∀ r, m ≤ r → r < n → P r) :=
+decidable_of_iff (∀ r ∈ range m n, P r) $ by simp only [mem_range_iff, and_imp]
+
+instance decidable_le_le (P : int → Prop) [decidable_pred P] (m n : ℤ) : decidable (∀ r, m ≤ r → r ≤ n → P r) :=
+decidable_of_iff (∀ r ∈ range m (n+1), P r) $ by simp only [mem_range_iff, and_imp, lt_add_one_iff]
+
+instance decidable_lt_lt (P : int → Prop) [decidable_pred P] (m n : ℤ) : decidable (∀ r, m < r → r < n → P r) :=
+int.decidable_le_lt P _ _
+
+instance decidable_lt_le (P : int → Prop) [decidable_pred P] (m n : ℤ) : decidable (∀ r, m < r → r ≤ n → P r) :=
+int.decidable_le_le P _ _
+
+end decidable
+
 end int