lean2/library/data/nat/pairing.lean

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/-
Copyright (c) 2015 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Module: data.nat.pairing
Authors: Leonardo de Moura
Elegant pairing function.
-/
import data.nat.sqrt data.nat.div
open prod decidable
namespace nat
definition mkpair (a b : nat) : nat :=
if a < b then b*b + a else a*a + a + b
definition unpair (n : nat) : nat × nat :=
let s := sqrt n in
if n - s*s < s then (n - s*s, s) else (s, n - s*s - s)
theorem mkpair_unpair (n : nat) : mkpair (pr1 (unpair n)) (pr2 (unpair n)) = n :=
let s := sqrt n in
by_cases
(λ h₁ : n - s*s < s,
begin
esimp [unpair],
rewrite [if_pos h₁],
esimp [mkpair],
rewrite [if_pos h₁, add_sub_of_le (sqrt_lower n)]
end)
(λ h₂ : ¬ n - s*s < s,
have g₁ : s ≤ n - s*s, from le_of_not_lt h₂,
assert g₂ : s + s*s ≤ n - s*s + s*s, from add_le_add_right g₁ (s*s),
assert g₃ : s*s + s ≤ n, by rewrite [sub_add_cancel (sqrt_lower n) at g₂, add.comm at g₂]; exact g₂,
have l₁ : n ≤ s*s + s + s, from sqrt_upper n,
have l₂ : n - s*s ≤ s + s, from calc
n - s*s ≤ (s*s + s + s) - s*s : sub_le_sub_right l₁ (s*s)
... = (s*s + (s+s)) - s*s : by rewrite add.assoc
... = s + s : by rewrite add_sub_cancel_left,
have l₃ : n - s*s - s ≤ s, from calc
n - s*s - s ≤ (s + s) - s : sub_le_sub_right l₂ s
... = s : by rewrite add_sub_cancel_left,
assert l₄ : ¬ s < n - s*s - s, from not_lt_of_le l₃,
begin
esimp [unpair],
rewrite [if_neg h₂], esimp,
esimp [mkpair],
rewrite [if_neg l₄, sub_sub, add_sub_of_le g₃],
end)
theorem unpair_mkpair (a b : nat) : unpair (mkpair a b) = (a, b) :=
by_cases
(λ h : a < b,
assert aux₁ : a ≤ b + b, from calc
a ≤ b : le_of_lt h
... ≤ b+b : !le_add_right,
begin
esimp [mkpair],
rewrite [if_pos h],
esimp [unpair],
rewrite [sqrt_offset_eq aux₁, add_sub_cancel_left, if_pos h]
end)
(λ h : ¬ a < b,
have h₁ : b ≤ a, from le_of_not_lt h,
assert aux₁ : a + b ≤ a + a, from add_le_add_left h₁ a,
have aux₂ : a + b ≥ a, from !le_add_right,
assert aux₃ : ¬ a + b < a, from not_lt_of_le aux₂,
begin
esimp [mkpair],
rewrite [if_neg h],
esimp [unpair],
rewrite [add.assoc (a * a) a b, sqrt_offset_eq aux₁, *add_sub_cancel_left, if_neg aux₃]
end)
end nat