事前注文トラバーサルから始めます。空である場合は完了です。またはr0、ツリーのルートである最初の要素 があります。次に、 inorder トラバーサルで を検索しますr0。左のサブツリーはすべてそのポイントの前に来て、右のサブツリーはすべてそのポイントの後に来ます。したがって、その時点での順不同トラバーサルを、左サブツリーilの順トラバーサルと右サブツリーの順トラバーサルに分割できますir。
が空の場合il、残りの事前注文トラバーサルは右側のサブツリーに属し、帰納的に続行できます。が空の場合ir、反対側でも同じことが起こります。どちらも空でない場合はir、残りの先行順序探索で の最初の要素を見つけます。これにより、左側のサブツリーの事前順トラバーサルと右側のサブツリーの 1 つに分割されます。誘導は即時です。
正式な証明に興味がある人のために、私は (ついに) Idris で作成することができました。しかし、私はそれをひどく読みやすくするために時間をかけていなかったので、実際にはその多くを読むのはかなり難しい. トップレベルの型 (つまり、補題、定理、定義) を主に見て、証明 (用語) に行き詰まりすぎないようにすることをお勧めします。
最初にいくつかの準備事項:
module PreIn
import Data.List
%default total
さて、最初の本当のアイデアは二分木です。
data Tree : Type -> Type where
Tip : Tree a
Node : (l : Tree a) -> (v : a) -> (r : Tree a) -> Tree a
%name Tree t, u
2 番目の大きなアイデアは、特定のツリー内の特定の要素を見つける方法のアイデアです。これは、特定のリスト内の特定の要素を見つける方法を表すElemタイプ inに密接に基づいています。Data.List
data InTree : a -> Tree a -> Type where
AtRoot : x `InTree` (Node l x r)
OnLeft : x `InTree` l -> x `InTree` (Node l v r)
OnRight : x `InTree` r -> x `InTree` (Node l v r)
それから、恐ろしい補題がたくさんあります。そのうちのいくつかは、それについての私の質問への回答でEric Mertens ( glguy )によって提案されました。
恐ろしいレンマ
size : Tree a -> Nat
size Tip = Z
size (Node l v r) = size l + (S Z + size r)
onLeftInjective : OnLeft p = OnLeft q -> p = q
onLeftInjective Refl = Refl
onRightInjective : OnRight p = OnRight q -> p = q
onRightInjective Refl = Refl
inorder : Tree a -> List a
inorder Tip = []
inorder (Node l v r) = inorder l ++ [v] ++ inorder r
instance Uninhabited (Here = There y) where
uninhabited Refl impossible
instance Uninhabited (x `InTree` Tip) where
uninhabited AtRoot impossible
elemAppend : {x : a} -> (ys,xs : List a) -> x `Elem` xs -> x `Elem` (ys ++ xs)
elemAppend [] xs xInxs = xInxs
elemAppend (y :: ys) xs xInxs = There (elemAppend ys xs xInxs)
appendElem : {x : a} -> (xs,ys : List a) -> x `Elem` xs -> x `Elem` (xs ++ ys)
appendElem (x :: zs) ys Here = Here
appendElem (y :: zs) ys (There pr) = There (appendElem zs ys pr)
tThenInorder : {x : a} -> (t : Tree a) -> x `InTree` t -> x `Elem` inorder t
tThenInorder (Node l x r) AtRoot = elemAppend _ _ Here
tThenInorder (Node l v r) (OnLeft pr) = appendElem _ _ (tThenInorder _ pr)
tThenInorder (Node l v r) (OnRight pr) = elemAppend _ _ (There (tThenInorder _ pr))
listSplit_lem : (x,z : a) -> (xs,ys:List a) -> Either (x `Elem` xs) (x `Elem` ys)
-> Either (x `Elem` (z :: xs)) (x `Elem` ys)
listSplit_lem x z xs ys (Left prf) = Left (There prf)
listSplit_lem x z xs ys (Right prf) = Right prf
listSplit : {x : a} -> (xs,ys : List a) -> x `Elem` (xs ++ ys) -> Either (x `Elem` xs) (x `Elem` ys)
listSplit [] ys xelem = Right xelem
listSplit (z :: xs) ys Here = Left Here
listSplit {x} (z :: xs) ys (There pr) = listSplit_lem x z xs ys (listSplit xs ys pr)
mutual
inorderThenT : {x : a} -> (t : Tree a) -> x `Elem` inorder t -> InTree x t
inorderThenT Tip xInL = absurd xInL
inorderThenT {x} (Node l v r) xInL = inorderThenT_lem x l v r xInL (listSplit (inorder l) (v :: inorder r) xInL)
inorderThenT_lem : (x : a) ->
(l : Tree a) -> (v : a) -> (r : Tree a) ->
x `Elem` inorder (Node l v r) ->
Either (x `Elem` inorder l) (x `Elem` (v :: inorder r)) ->
InTree x (Node l v r)
inorderThenT_lem x l v r xInL (Left locl) = OnLeft (inorderThenT l locl)
inorderThenT_lem x l x r xInL (Right Here) = AtRoot
inorderThenT_lem x l v r xInL (Right (There locr)) = OnRight (inorderThenT r locr)
unsplitRight : {x : a} -> (e : x `Elem` ys) -> listSplit xs ys (elemAppend xs ys e) = Right e
unsplitRight {xs = []} e = Refl
unsplitRight {xs = (x :: xs)} e = rewrite unsplitRight {xs} e in Refl
unsplitLeft : {x : a} -> (e : x `Elem` xs) -> listSplit xs ys (appendElem xs ys e) = Left e
unsplitLeft {xs = []} Here impossible
unsplitLeft {xs = (x :: xs)} Here = Refl
unsplitLeft {xs = (x :: xs)} {ys} (There pr) =
rewrite unsplitLeft {xs} {ys} pr in Refl
splitLeft_lem1 : (Left (There w) = listSplit_lem x y xs ys (listSplit xs ys z)) ->
(Left w = listSplit xs ys z)
splitLeft_lem1 {w} {xs} {ys} {z} prf with (listSplit xs ys z)
splitLeft_lem1 {w} Refl | (Left w) = Refl
splitLeft_lem1 {w} Refl | (Right s) impossible
splitLeft_lem2 : Left Here = listSplit_lem x x xs ys (listSplit xs ys z) -> Void
splitLeft_lem2 {x} {xs} {ys} {z} prf with (listSplit xs ys z)
splitLeft_lem2 {x = x} {xs = xs} {ys = ys} {z = z} Refl | (Left y) impossible
splitLeft_lem2 {x = x} {xs = xs} {ys = ys} {z = z} Refl | (Right y) impossible
splitLeft : {x : a} -> (xs,ys : List a) ->
(loc : x `Elem` (xs ++ ys)) ->
Left e = listSplit {x} xs ys loc ->
appendElem {x} xs ys e = loc
splitLeft {e} [] ys loc prf = absurd e
splitLeft (x :: xs) ys Here prf = rewrite leftInjective prf in Refl
splitLeft {e = Here} (x :: xs) ys (There z) prf = absurd (splitLeft_lem2 prf)
splitLeft {e = (There w)} (y :: xs) ys (There z) prf =
cong $ splitLeft xs ys z (splitLeft_lem1 prf)
splitMiddle_lem3 : Right Here = listSplit_lem y x xs (y :: ys) (listSplit xs (y :: ys) z) ->
Right Here = listSplit xs (y :: ys) z
splitMiddle_lem3 {y} {x} {xs} {ys} {z} prf with (listSplit xs (y :: ys) z)
splitMiddle_lem3 {y = y} {x = x} {xs = xs} {ys = ys} {z = z} Refl | (Left w) impossible
splitMiddle_lem3 {y = y} {x = x} {xs = xs} {ys = ys} {z = z} prf | (Right w) =
cong $ rightInjective prf -- This funny dance strips the Rights off and then puts them
-- back on so as to change type.
splitMiddle_lem2 : Right Here = listSplit xs (y :: ys) pl ->
elemAppend xs (y :: ys) Here = pl
splitMiddle_lem2 {xs} {y} {ys} {pl} prf with (listSplit xs (y :: ys) pl) proof prpr
splitMiddle_lem2 {xs = xs} {y = y} {ys = ys} {pl = pl} Refl | (Left loc) impossible
splitMiddle_lem2 {xs = []} {y = y} {ys = ys} {pl = pl} Refl | (Right Here) = rightInjective prpr
splitMiddle_lem2 {xs = (x :: xs)} {y = x} {ys = ys} {pl = Here} prf | (Right Here) = (\Refl impossible) prpr
splitMiddle_lem2 {xs = (x :: xs)} {y = y} {ys = ys} {pl = (There z)} prf | (Right Here) =
cong $ splitMiddle_lem2 {xs} {y} {ys} {pl = z} (splitMiddle_lem3 prpr)
splitMiddle_lem1 : Right Here = listSplit_lem y x xs (y :: ys) (listSplit xs (y :: ys) pl) ->
elemAppend xs (y :: ys) Here = pl
splitMiddle_lem1 {y} {x} {xs} {ys} {pl} prf with (listSplit xs (y :: ys) pl) proof prpr
splitMiddle_lem1 {y = y} {x = x} {xs = xs} {ys = ys} {pl = pl} Refl | (Left z) impossible
splitMiddle_lem1 {y = y} {x = x} {xs = xs} {ys = ys} {pl = pl} Refl | (Right Here) = splitMiddle_lem2 prpr
splitMiddle : Right Here = listSplit xs (y::ys) loc ->
elemAppend xs (y::ys) Here = loc
splitMiddle {xs = []} prf = rightInjective prf
splitMiddle {xs = (x :: xs)} {loc = Here} Refl impossible
splitMiddle {xs = (x :: xs)} {loc = (There y)} prf = cong $ splitMiddle_lem1 prf
splitRight_lem1 : Right (There pl) = listSplit (q :: xs) (y :: ys) (There z) ->
Right (There pl) = listSplit xs (y :: ys) z
splitRight_lem1 {xs} {ys} {y} {z} prf with (listSplit xs (y :: ys) z)
splitRight_lem1 {xs = xs} {ys = ys} {y = y} {z = z} Refl | (Left x) impossible
splitRight_lem1 {xs = xs} {ys = ys} {y = y} {z = z} prf | (Right x) =
cong $ rightInjective prf -- Type dance: take the Right off and put it back on.
splitRight : Right (There pl) = listSplit xs (y :: ys) loc ->
elemAppend xs (y :: ys) (There pl) = loc
splitRight {pl = pl} {xs = []} {y = y} {ys = ys} {loc = loc} prf = rightInjective prf
splitRight {pl = pl} {xs = (x :: xs)} {y = y} {ys = ys} {loc = Here} Refl impossible
splitRight {pl = pl} {xs = (x :: xs)} {y = y} {ys = ys} {loc = (There z)} prf =
let rec = splitRight {pl} {xs} {y} {ys} {loc = z} in cong $ rec (splitRight_lem1 prf)
木とその順不同走査の対応
これらの恐ろしい補題は、順序通りのトラバーサルに関する次の定理につながります。これらの定理は、ツリー内の特定の要素を見つける方法と、その要素を順序通りのトラバーサルで見つける方法との間の 1 対 1 の対応を示しています。
---------------------------
-- tThenInorder is a bijection from ways to find a particular element in a tree
-- and ways to find that element in its inorder traversal. `inorderToFro`
-- and `inorderFroTo` together demonstrate this by showing that `inorderThenT` is
-- its inverse.
||| `tThenInorder t` is a retraction of `inorderThenT t`
inorderFroTo : {x : a} -> (t : Tree a) -> (loc : x `Elem` inorder t) -> tThenInorder t (inorderThenT t loc) = loc
inorderFroTo Tip loc = absurd loc
inorderFroTo (Node l v r) loc with (listSplit (inorder l) (v :: inorder r) loc) proof prf
inorderFroTo (Node l v r) loc | (Left here) =
rewrite inorderFroTo l here in splitLeft _ _ loc prf
inorderFroTo (Node l v r) loc | (Right Here) = splitMiddle prf
inorderFroTo (Node l v r) loc | (Right (There x)) =
rewrite inorderFroTo r x in splitRight prf
||| `inorderThenT t` is a retraction of `tThenInorder t`
inorderToFro : {x : a} -> (t : Tree a) -> (loc : x `InTree` t) -> inorderThenT t (tThenInorder t loc) = loc
inorderToFro (Node l v r) (OnLeft xInL) =
rewrite unsplitLeft {ys = v :: inorder r} (tThenInorder l xInL)
in cong $ inorderToFro _ xInL
inorderToFro (Node l x r) AtRoot =
rewrite unsplitRight {x} {xs = inorder l} {ys = x :: inorder r} (tThenInorder (Node Tip x r) AtRoot)
in Refl
inorderToFro {x} (Node l v r) (OnRight xInR) =
rewrite unsplitRight {x} {xs = inorder l} {ys = v :: inorder r} (tThenInorder (Node Tip v r) (OnRight xInR))
in cong $ inorderToFro _ xInR
ツリーとその事前順トラバーサルの対応
これらの同じ補題の多くを使用して、事前順序トラバーサルの対応する定理を証明できます。
preorder : Tree a -> List a
preorder Tip = []
preorder (Node l v r) = v :: (preorder l ++ preorder r)
tThenPreorder : (t : Tree a) -> x `InTree` t -> x `Elem` preorder t
tThenPreorder Tip AtRoot impossible
tThenPreorder (Node l x r) AtRoot = Here
tThenPreorder (Node l v r) (OnLeft loc) = appendElem _ _ (There (tThenPreorder _ loc))
tThenPreorder (Node l v r) (OnRight loc) = elemAppend (v :: preorder l) (preorder r) (tThenPreorder _ loc)
mutual
preorderThenT : (t : Tree a) -> x `Elem` preorder t -> x `InTree` t
preorderThenT {x = x} (Node l x r) Here = AtRoot
preorderThenT {x = x} (Node l v r) (There y) = preorderThenT_lem (listSplit _ _ y)
preorderThenT_lem : Either (x `Elem` preorder l) (x `Elem` preorder r) -> x `InTree` (Node l v r)
preorderThenT_lem {x = x} {l = l} {v = v} {r = r} (Left lloc) = OnLeft (preorderThenT l lloc)
preorderThenT_lem {x = x} {l = l} {v = v} {r = r} (Right rloc) = OnRight (preorderThenT r rloc)
splitty : Right pl = listSplit xs ys loc -> elemAppend xs ys pl = loc
splitty {pl = Here} {xs = xs} {ys = (x :: zs)} {loc = loc} prf = splitMiddle prf
splitty {pl = (There x)} {xs = xs} {ys = (y :: zs)} {loc = loc} prf = splitRight prf
preorderFroTo : {x : a} -> (t : Tree a) -> (loc : x `Elem` preorder t) ->
tThenPreorder t (preorderThenT t loc) = loc
preorderFroTo Tip Here impossible
preorderFroTo (Node l x r) Here = Refl
preorderFroTo (Node l v r) (There loc) with (listSplit (preorder l) (preorder r) loc) proof spl
preorderFroTo (Node l v r) (There loc) | (Left pl) =
rewrite sym (splitLeft {e=pl} (preorder l) (preorder r) loc spl)
in cong {f = There} $ cong {f = appendElem (preorder l) (preorder r)} (preorderFroTo _ _)
preorderFroTo (Node l v r) (There loc) | (Right pl) =
rewrite preorderFroTo r pl in cong {f = There} (splitty spl)
preorderToFro : {x : a} -> (t : Tree a) -> (loc : x `InTree` t) -> preorderThenT t (tThenPreorder t loc) = loc
preorderToFro (Node l x r) AtRoot = Refl
preorderToFro (Node l v r) (OnLeft loc) =
rewrite unsplitLeft {ys = preorder r} (tThenPreorder l loc)
in cong {f = OnLeft} (preorderToFro l loc)
preorderToFro (Node l v r) (OnRight loc) =
rewrite unsplitRight {xs = preorder l} (tThenPreorder r loc)
in cong {f = OnRight} (preorderToFro r loc)
ここまでいい？それを聞いてうれしい。あなたが求めている定理が近づいています！まず、ツリーが「単射」であるという概念が必要です。これは、このコンテキストでは「重複がない」という最も単純な概念だと思います。この概念が気に入らなくても心配しないでください。道を南下したところにもう 1 つあります。これは、とが で値を見つける方法でtある場合、木は単射であると言っています。loc1loc1xtloc1loc2
InjTree : Tree a -> Type
InjTree t = (x : a) -> (loc1, loc2 : x `InTree` t) -> loc1 = loc2
また、リストについても対応する概念が必要です。これは、木が単射である場合に限り、トラバーサルが単射であることを証明するためです。それらの証明はすぐ下にあり、前から続いています。
InjList : List a -> Type
InjList xs = (x : a) -> (loc1, loc2 : x `Elem` xs) -> loc1 = loc2
||| If a tree is injective, so is its preorder traversal
treePreInj : (t : Tree a) -> InjTree t -> InjList (preorder t)
treePreInj {a} t it x loc1 loc2 =
let foo = preorderThenT {a} {x} t loc1
bar = preorderThenT {a} {x} t loc2
baz = it x foo bar
in rewrite sym $ preorderFroTo t loc1
in rewrite sym $ preorderFroTo t loc2
in cong baz
||| If a tree is injective, so is its inorder traversal
treeInInj : (t : Tree a) -> InjTree t -> InjList (inorder t)
treeInInj {a} t it x loc1 loc2 =
let foo = inorderThenT {a} {x} t loc1
bar = inorderThenT {a} {x} t loc2
baz = it x foo bar
in rewrite sym $ inorderFroTo t loc1
in rewrite sym $ inorderFroTo t loc2
in cong baz
||| If a tree's preorder traversal is injective, so is the tree.
injPreTree : (t : Tree a) -> InjList (preorder t) -> InjTree t
injPreTree {a} t il x loc1 loc2 =
let
foo = tThenPreorder {a} {x} t loc1
bar = tThenPreorder {a} {x} t loc2
baz = il x foo bar
in rewrite sym $ preorderToFro t loc1
in rewrite sym $ preorderToFro t loc2
in cong baz
||| If a tree's inorder traversal is injective, so is the tree.
injInTree : (t : Tree a) -> InjList (inorder t) -> InjTree t
injInTree {a} t il x loc1 loc2 =
let
foo = tThenInorder {a} {x} t loc1
bar = tThenInorder {a} {x} t loc2
baz = il x foo bar
in rewrite sym $ inorderToFro t loc1
in rewrite sym $ inorderToFro t loc2
in cong baz
もっと恐ろしい補題
headsSame : {x:a} -> {xs : List a} -> {y : a} -> {ys : List a} -> (x :: xs) = (y :: ys) -> x = y
headsSame Refl = Refl
tailsSame : {x:a} -> {xs : List a} -> {y : a} -> {ys : List a} -> (x :: xs) = (y :: ys) -> xs = ys
tailsSame Refl = Refl
appendLeftCancel : {xs,ys,ys' : List a} -> xs ++ ys = xs ++ ys' -> ys = ys'
appendLeftCancel {xs = []} prf = prf
appendLeftCancel {xs = (x :: xs)} prf = appendLeftCancel {xs} (tailsSame prf)
lengthDrop : (xs,ys : List a) -> drop (length xs) (xs ++ ys) = ys
lengthDrop [] ys = Refl
lengthDrop (x :: xs) ys = lengthDrop xs ys
lengthTake : (xs,ys : List a) -> take (length xs) (xs ++ ys) = xs
lengthTake [] ys = Refl
lengthTake (x :: xs) ys = cong $ lengthTake xs ys
appendRightCancel_lem : (xs,xs',ys : List a) -> xs ++ ys = xs' ++ ys -> length xs = length xs'
appendRightCancel_lem xs xs' ys eq =
let foo = lengthAppend xs ys
bar = replace {P = \b => length b = length xs + length ys} eq foo
baz = trans (sym bar) $ lengthAppend xs' ys
in plusRightCancel (length xs) (length xs') (length ys) baz
appendRightCancel : {xs,xs',ys : List a} -> xs ++ ys = xs' ++ ys -> xs = xs'
appendRightCancel {xs} {xs'} {ys} eq with (appendRightCancel_lem xs xs' ys eq)
| lenEq = rewrite sym $ lengthTake xs ys
in let foo : (take (length xs') (xs ++ ys) = xs') = rewrite eq in lengthTake xs' ys
in rewrite lenEq in foo
listPartsEqLeft : {xs, xs', ys, ys' : List a} ->
length xs = length xs' ->
xs ++ ys = xs' ++ ys' ->
xs = xs'
listPartsEqLeft {xs} {xs'} {ys} {ys'} leneq appeq =
rewrite sym $ lengthTake xs ys
in rewrite leneq
in rewrite appeq
in lengthTake xs' ys'
listPartsEqRight : {xs, xs', ys, ys' : List a} ->
length xs = length xs' ->
xs ++ ys = xs' ++ ys' ->
ys = ys'
listPartsEqRight leneq appeq with (listPartsEqLeft leneq appeq)
listPartsEqRight leneq appeq | Refl = appendLeftCancel appeq
thereInjective : There loc1 = There loc2 -> loc1 = loc2
thereInjective Refl = Refl
injTail : InjList (x :: xs) -> InjList xs
injTail {x} {xs} xxsInj v vloc1 vloc2 = thereInjective $
xxsInj v (There vloc1) (There vloc2)
splitInorder_lem2 : ((loc1 : Elem v (v :: xs ++ v :: ysr)) ->
(loc2 : Elem v (v :: xs ++ v :: ysr)) -> loc1 = loc2) ->
Void
splitInorder_lem2 {v} {xs} {ysr} f =
let
loc2 = elemAppend {x=v} xs (v :: ysr) Here
in (\Refl impossible) $ f Here (There loc2)
-- preorderLength and inorderLength could be proven using the bijections
-- between trees and their traversals, but it's much easier to just prove
-- them directly.
preorderLength : (t : Tree a) -> length (preorder t) = size t
preorderLength Tip = Refl
preorderLength (Node l v r) =
rewrite sym (plusSuccRightSucc (size l) (size r))
in cong {f=S} $
rewrite sym $ preorderLength l
in rewrite sym $ preorderLength r
in lengthAppend _ _
inorderLength : (t : Tree a) -> length (inorder t) = size t
inorderLength Tip = Refl
inorderLength (Node l v r) =
rewrite lengthAppend (inorder l) (v :: inorder r)
in rewrite inorderLength l
in rewrite inorderLength r in Refl
preInLength : (t : Tree a) -> length (preorder t) = length (inorder t)
preInLength t = trans (preorderLength t) (sym $ inorderLength t)
splitInorder_lem1 : (v : a) ->
(xsl, xsr, ysl, ysr : List a) ->
(xsInj : InjList (xsl ++ v :: xsr)) ->
(ysInj : InjList (ysl ++ v :: ysr)) ->
xsl ++ v :: xsr = ysl ++ v :: ysr ->
v `Elem` (xsl ++ v :: xsr) ->
v `Elem` (ysl ++ v :: ysr) ->
xsl = ysl
splitInorder_lem1 v [] xsr [] ysr xsInj ysInj eq locxs locys = Refl
splitInorder_lem1 v [] xsr (v :: ysl) ysr xsInj ysInj eq Here Here with (ysInj v Here (elemAppend (v :: ysl) (v :: ysr) Here))
splitInorder_lem1 v [] xsr (v :: ysl) ysr xsInj ysInj eq Here Here | Refl impossible
splitInorder_lem1 v [] xsr (y :: ysl) ysr xsInj ysInj eq Here (There loc) with (headsSame eq)
splitInorder_lem1 v [] xsr (v :: ysl) ysr xsInj ysInj eq Here (There loc) | Refl = absurd $ splitInorder_lem2 (ysInj v)
splitInorder_lem1 v [] xsr (x :: xs) ysr xsInj ysInj eq (There loc) locys with (headsSame eq)
splitInorder_lem1 v [] xsr (v :: xs) ysr xsInj ysInj eq (There loc) locys | Refl = absurd $ splitInorder_lem2 (ysInj v)
splitInorder_lem1 v (v :: xs) xsr ysl ysr xsInj ysInj eq Here locys = absurd $ splitInorder_lem2 (xsInj v)
splitInorder_lem1 v (x :: xs) xsr [] ysr xsInj ysInj eq (There y) locys with (headsSame eq)
splitInorder_lem1 v (v :: xs) xsr [] ysr xsInj ysInj eq (There y) locys | Refl = absurd $ splitInorder_lem2 (xsInj v)
splitInorder_lem1 v (x :: xs) xsr (z :: ys) ysr xsInj ysInj eq (There y) locys with (headsSame eq)
splitInorder_lem1 v (v :: xs) xsr (_ :: ys) ysr xsInj ysInj eq (There y) Here | Refl = absurd $ splitInorder_lem2 (ysInj v)
splitInorder_lem1 v (x :: xs) xsr (x :: ys) ysr xsInj ysInj eq (There y) (There z) | Refl = cong {f = ((::) x)} $
splitInorder_lem1 v xs xsr ys ysr (injTail xsInj) (injTail ysInj) (tailsSame eq) y z
splitInorder_lem3 : (v : a) ->
(xsl, xsr, ysl, ysr : List a) ->
(xsInj : InjList (xsl ++ v :: xsr)) ->
(ysInj : InjList (ysl ++ v :: ysr)) ->
xsl ++ v :: xsr = ysl ++ v :: ysr ->
v `Elem` (xsl ++ v :: xsr) ->
v `Elem` (ysl ++ v :: ysr) ->
xsr = ysr
splitInorder_lem3 v xsl xsr ysl ysr xsInj ysInj prf locxs locys with (splitInorder_lem1 v xsl xsr ysl ysr xsInj ysInj prf locxs locys)
splitInorder_lem3 v xsl xsr xsl ysr xsInj ysInj prf locxs locys | Refl =
tailsSame $ appendLeftCancel prf
単純な事実: 木が単射である場合、その左右の部分木も単射です。
injLeft : {l : Tree a} -> {v : a} -> {r : Tree a} ->
InjTree (Node l v r) -> InjTree l
injLeft {l} {v} {r} injlvr x loc1 loc2 with (injlvr x (OnLeft loc1) (OnLeft loc2))
injLeft {l = l} {v = v} {r = r} injlvr x loc1 loc1 | Refl = Refl
injRight : {l : Tree a} -> {v : a} -> {r : Tree a} ->
InjTree (Node l v r) -> InjTree r
injRight {l} {v} {r} injlvr x loc1 loc2 with (injlvr x (OnRight loc1) (OnRight loc2))
injRight {l} {v} {r} injlvr x loc1 loc1 | Refl = Refl
主な目的！
tおよびuが二分木でtあり、単射であり、かつtおよびuが同じ前順序および順序内トラバーサルを持つ場合、およびtはu等しくなります。
travsDet : (t, u : Tree a) -> InjTree t -> preorder t = preorder u -> inorder t = inorder u -> t = u
-- The base case--both trees are empty
travsDet Tip Tip x prf prf1 = Refl
-- Impossible cases: only one tree is empty
travsDet Tip (Node l v r) x Refl prf1 impossible
travsDet (Node l v r) Tip x Refl prf1 impossible
-- The interesting case. `headsSame presame` proves
-- that the roots of the trees are equal.
travsDet (Node l v r) (Node t y u) lvrInj presame insame with (headsSame presame)
travsDet (Node l v r) (Node t v u) lvrInj presame insame | Refl =
let
foo = elemAppend (inorder l) (v :: inorder r) Here
bar = elemAppend (inorder t) (v :: inorder u) Here
inlvrInj = treeInInj _ lvrInj
intvuInj : (InjList (inorder (Node t v u))) = rewrite sym insame in inlvrInj
inorderRightSame = splitInorder_lem3 v (inorder l) (inorder r) (inorder t) (inorder u) inlvrInj intvuInj insame foo bar
preInL : (length (preorder l) = length (inorder l)) = preInLength l
inorderLeftSame = splitInorder_lem1 v (inorder l) (inorder r) (inorder t) (inorder u) inlvrInj intvuInj insame foo bar
inPreT : (length (inorder t) = length (preorder t)) = sym $ preInLength t
preLenlt : (length (preorder l) = length (preorder t))
= trans preInL (trans (cong inorderLeftSame) inPreT)
presame' = tailsSame presame
baz : (preorder l = preorder t) = listPartsEqLeft preLenlt presame'
quux : (preorder r = preorder u) = listPartsEqRight preLenlt presame'
-- Putting together the lemmas, we see that the
-- left and right subtrees are equal
recleft = travsDet l t (injLeft lvrInj) baz inorderLeftSame
recright = travsDet r u (injRight lvrInj) quux inorderRightSame
in rewrite recleft in rewrite recright in Refl
「重複なし」の代替概念
ツリー内の 2 つの場所が等しくない場合は常に、同じ要素を保持していないということになる場合、ツリーに「重複がない」と言いたい場合があります。NoDupsこれは、型を使用して表現できます。
NoDups : Tree a -> Type
NoDups {a} t = (x, y : a) ->
(loc1 : x `InTree` t) ->
(loc2 : y `InTree` t) ->
Not (loc1 = loc2) ->
Not (x = y)
これが必要なことを証明するのに十分強い理由は、ツリー内の 2 つのパスが等しいかどうかを判断する手順があるためです。
instance DecEq (x `InTree` t) where
decEq AtRoot AtRoot = Yes Refl
decEq AtRoot (OnLeft x) = No (\Refl impossible)
decEq AtRoot (OnRight x) = No (\Refl impossible)
decEq (OnLeft x) AtRoot = No (\Refl impossible)
decEq (OnLeft x) (OnLeft y) with (decEq x y)
decEq (OnLeft x) (OnLeft x) | (Yes Refl) = Yes Refl
decEq (OnLeft x) (OnLeft y) | (No contra) = No (contra . onLeftInjective)
decEq (OnLeft x) (OnRight y) = No (\Refl impossible)
decEq (OnRight x) AtRoot = No (\Refl impossible)
decEq (OnRight x) (OnLeft y) = No (\Refl impossible)
decEq (OnRight x) (OnRight y) with (decEq x y)
decEq (OnRight x) (OnRight x) | (Yes Refl) = Yes Refl
decEq (OnRight x) (OnRight y) | (No contra) = No (contra . onRightInjective)
Nodups tこれは、次のことを意味することを証明しInjTree tます。
noDupsInj : (t : Tree a) -> NoDups t -> InjTree t
noDupsInj t nd x loc1 loc2 with (decEq loc1 loc2)
noDupsInj t nd x loc1 loc2 | (Yes prf) = prf
noDupsInj t nd x loc1 loc2 | (No contra) = absurd $ nd x x loc1 loc2 contra Refl
最後に、すぐにNoDups t仕事が完了します。
travsDet2 : (t, u : Tree a) -> NoDups t -> preorder t = preorder u -> inorder t = inorder u -> t = u
travsDet2 t u ndt = travsDet t u (noDupsInj t ndt)