# Theory List-Index.List_Index

(* Author: Tobias Nipkow, with contributions from Dmitriy Traytel, Lukas Bulwahn, and Peter Lammich *)

section ‹Index-based manipulation of lists›

theory List_Index imports Main begin

text ‹\noindent
This theory collects functions for index-based manipulation of lists.
›

subsection ‹Finding an index›

text ‹
This subsection defines three functions for finding the index of items in a list:
\begin{description}
\item[‹find_index P xs›] finds the index of the first element in
‹xs› that satisfies ‹P›.
\item[‹index xs x›] finds the index of the first occurrence of
‹x› in ‹xs›.
\item[‹last_index xs x›] finds the index of the last occurrence of
‹x› in ‹xs›.
\end{description}
All functions return @{term "length xs"} if ‹xs› does not contain a
suitable element.

The argument order of ‹find_index› follows the function of the same
name in the Haskell standard library. For ‹index› (and ‹last_index›) the order is intentionally reversed: ‹index› maps
lists to a mapping from elements to their indices, almost the inverse of
function ‹nth›.›

primrec find_index :: "('a ⇒ bool) ⇒ 'a list ⇒ nat" where
"find_index _ [] = 0" |
"find_index P (x#xs) = (if P x then 0 else find_index P xs + 1)"

definition index :: "'a list ⇒ 'a ⇒ nat" where
"index xs = (λa. find_index (λx. x=a) xs)"

definition last_index :: "'a list ⇒ 'a ⇒ nat" where
"last_index xs x =
(let i = index (rev xs) x; n = size xs
in if i = n then i else n - (i+1))"

lemma find_index_append: "find_index P (xs @ ys) =
(if ∃x∈set xs. P x then find_index P xs else size xs + find_index P ys)"
by (induct xs) simp_all

lemma find_index_le_size: "find_index P xs <= size xs"
by(induct xs) simp_all

lemma index_le_size: "index xs x <= size xs"

lemma last_index_le_size: "last_index xs x <= size xs"

lemma index_Nil[simp]: "index [] a = 0"

lemma index_Cons[simp]: "index (x#xs) a = (if x=a then 0 else index xs a + 1)"

lemma index_append: "index (xs @ ys) x =
(if x : set xs then index xs x else size xs + index ys x)"
by (induct xs) simp_all

lemma index_conv_size_if_notin[simp]: "x ∉ set xs ⟹ index xs x = size xs"
by (induct xs) auto

lemma find_index_eq_size_conv:
"size xs = n ⟹ (find_index P xs = n) = (∀x ∈ set xs. ~ P x)"
by(induct xs arbitrary: n) auto

lemma size_eq_find_index_conv:
"size xs = n ⟹ (n = find_index P xs) = (∀x ∈ set xs. ~ P x)"
by(metis find_index_eq_size_conv)

lemma index_size_conv: "size xs = n ⟹ (index xs x = n) = (x ∉ set xs)"
by(auto simp: index_def find_index_eq_size_conv)

lemma size_index_conv: "size xs = n ⟹ (n = index xs x) = (x ∉ set xs)"
by (metis index_size_conv)

lemma last_index_size_conv:
"size xs = n ⟹ (last_index xs x = n) = (x ∉ set xs)"
apply(auto simp: last_index_def index_size_conv)
apply(drule length_pos_if_in_set)
apply arith
done

lemma size_last_index_conv:
"size xs = n ⟹ (n = last_index xs x) = (x ∉ set xs)"
by (metis last_index_size_conv)

lemma find_index_less_size_conv:
"(find_index P xs < size xs) = (∃x ∈ set xs. P x)"
by (induct xs) auto

lemma index_less_size_conv:
"(index xs x < size xs) = (x ∈ set xs)"
by(auto simp: index_def find_index_less_size_conv)

lemma last_index_less_size_conv:
"(last_index xs x < size xs) = (x : set xs)"
by(simp add: last_index_def Let_def index_size_conv length_pos_if_in_set
del:length_greater_0_conv)

lemma index_less[simp]:
"x : set xs ⟹ size xs <= n ⟹ index xs x < n"
apply(induct xs) apply auto
apply (metis index_less_size_conv less_eq_Suc_le less_trans_Suc)
done

lemma last_index_less[simp]:
"x : set xs ⟹ size xs <= n ⟹ last_index xs x < n"

lemma last_index_Cons: "last_index (x#xs) y =
(if x=y then
if x ∈ set xs then last_index xs y + 1 else 0
else last_index xs y + 1)"
using index_le_size[of "rev xs" y]
apply(auto simp add: last_index_def index_append Let_def)
done

lemma last_index_append: "last_index (xs @ ys) x =
(if x : set ys then size xs + last_index ys x
else if x : set xs then last_index xs x else size xs + size ys)"
by (induct xs) (simp_all add: last_index_Cons last_index_size_conv)

lemma last_index_Snoc[simp]:
"last_index (xs @ [x]) y =
(if x=y then size xs
else if y : set xs then last_index xs y else size xs + 1)"

lemma nth_find_index: "find_index P xs < size xs ⟹ P(xs ! find_index P xs)"
by (induct xs) auto

lemma nth_index[simp]: "x ∈ set xs ⟹ xs ! index xs x = x"
by (induct xs) auto

lemma nth_last_index[simp]: "x ∈ set xs ⟹ xs ! last_index xs x = x"

lemma index_rev: "⟦ distinct xs; x ∈ set xs ⟧ ⟹
index (rev xs) x = length xs - index xs x - 1"
by (induct xs) (auto simp: index_append)

lemma index_nth_id:
"⟦ distinct xs;  n < length xs ⟧ ⟹ index xs (xs ! n) = n"
by (metis in_set_conv_nth index_less_size_conv nth_eq_iff_index_eq nth_index)

lemma index_upt[simp]: "m ≤ i ⟹ i < n ⟹ index [m..<n] i = i-m"
by (induction n) (auto simp add: index_append)

lemma index_eq_index_conv[simp]: "x ∈ set xs ∨ y ∈ set xs ⟹
(index xs x = index xs y) = (x = y)"
by (induct xs) auto

lemma last_index_eq_index_conv[simp]: "x ∈ set xs ∨ y ∈ set xs ⟹
(last_index xs x = last_index xs y) = (x = y)"
by (induct xs) (auto simp:last_index_Cons)

lemma inj_on_index: "inj_on (index xs) (set xs)"

lemma inj_on_index2: "I ⊆ set xs ⟹ inj_on (index xs) I"
by (rule inj_onI) auto

lemma inj_on_last_index: "inj_on (last_index xs) (set xs)"

lemma find_index_conv_takeWhile:
"find_index P xs = size(takeWhile (Not o P) xs)"
by(induct xs) auto

lemma index_conv_takeWhile: "index xs x = size(takeWhile (λy. x≠y) xs)"
by(induct xs) auto

lemma find_index_first: "i < find_index P xs ⟹ ¬P (xs!i)"
unfolding find_index_conv_takeWhile
by (metis comp_apply nth_mem set_takeWhileD takeWhile_nth)

lemma index_first: "i<index xs x ⟹ x≠xs!i"
using find_index_first unfolding index_def by blast

lemma find_index_eqI:
assumes "i≤length xs"
assumes "∀j<i. ¬P (xs!j)"
assumes "i<length xs ⟹ P (xs!i)"
shows "find_index P xs = i"
by (metis (mono_tags, lifting) antisym_conv2 assms find_index_eq_size_conv
find_index_first find_index_less_size_conv linorder_neqE_nat nth_find_index)

lemma find_index_eq_iff:
"find_index P xs = i
⟷ (i≤length xs ∧ (∀j<i. ¬P (xs!j)) ∧ (i<length xs ⟶ P (xs!i)))"
by (auto intro: find_index_eqI
simp: nth_find_index find_index_le_size find_index_first)

lemma index_eqI:
assumes "i≤length xs"
assumes "∀j<i. xs!j ≠ x"
assumes "i<length xs ⟹ xs!i = x"
shows "index xs x = i"
unfolding index_def by (simp add: find_index_eqI assms)

lemma index_eq_iff:
"index xs x = i
⟷ (i≤length xs ∧ (∀j<i. xs!j ≠ x) ∧ (i<length xs ⟶ xs!i = x))"
by (auto intro: index_eqI
simp: index_le_size index_less_size_conv
dest: index_first)

lemma index_take: "index xs x >= i ⟹ x ∉ set(take i xs)"
apply(subst (asm) index_conv_takeWhile)
apply(subgoal_tac "set(take i xs) <= set(takeWhile ((≠) x) xs)")
apply(blast dest: set_takeWhileD)
apply(metis set_take_subset_set_take takeWhile_eq_take)
done

lemma last_index_drop:
"last_index xs x < i ⟹ x ∉ set(drop i xs)"
apply(subgoal_tac "set(drop i xs) = set(take (size xs - i) (rev xs))")
apply(simp add: last_index_def index_take Let_def split:if_split_asm)
apply (metis rev_drop set_rev)
done

lemma set_take_if_index: assumes "index xs x < i" and "i ≤ length xs"
shows "x ∈ set (take i xs)"
proof -
have "index (take i xs @ drop i xs) x < i"
using append_take_drop_id[of i xs] assms(1) by simp
thus ?thesis using assms(2)
qed

lemma index_take_if_index:
assumes "index xs x ≤ n" shows "index (take n xs) x = index xs x"
proof cases
assume "x : set(take n xs)" with assms show ?thesis
by (metis append_take_drop_id index_append)
next
assume "x ∉ set(take n xs)" with assms show ?thesis
by (metis order_le_less set_take_if_index le_cases length_take min_def size_index_conv take_all)
qed

lemma index_take_if_set:
"x : set(take n xs) ⟹ index (take n xs) x = index xs x"
by (metis index_take index_take_if_index linear)

lemma index_last[simp]:
"xs ≠ [] ⟹ distinct xs ⟹ index xs (last xs) = length xs - 1"
by (induction xs) auto

lemma index_update_if_diff2:
"n < length xs ⟹ x ≠ xs!n ⟹ x ≠ y ⟹ index (xs[n := y]) x = index xs x"
by(subst (2) id_take_nth_drop[of n xs])
(auto simp: upd_conv_take_nth_drop index_append min_def)

lemma set_drop_if_index: "distinct xs ⟹ index xs x < i ⟹ x ∉ set(drop i xs)"
by (metis in_set_dropD index_nth_id last_index_drop last_index_less_size_conv nth_last_index)

lemma index_swap_if_distinct: assumes "distinct xs" "i < size xs" "j < size xs"
shows "index (xs[i := xs!j, j := xs!i]) x =
(if x = xs!i then j else if x = xs!j then i else index xs x)"
proof-
have "distinct(xs[i := xs!j, j := xs!i])" using assms by simp
with assms show ?thesis
apply (auto simp: simp del: distinct_swap)
apply (metis index_nth_id list_update_same_conv)
apply (metis (erased, opaque_lifting) index_nth_id length_list_update list_update_swap nth_list_update_eq)
apply (metis index_nth_id length_list_update nth_list_update_eq)
by (metis index_update_if_diff2 length_list_update nth_list_update)
qed

lemma bij_betw_index:
"distinct xs ⟹ X = set xs ⟹ l = size xs ⟹ bij_betw (index xs) X {0..<l}"
apply simp
apply(rule bij_betw_imageI[OF inj_on_index])
by (auto simp: image_def) (metis index_nth_id nth_mem)

lemma index_image: "distinct xs ⟹ set xs = X ⟹ index xs  X = {0..<size xs}"

lemma index_map_inj_on:
"⟦ inj_on f S; y ∈ S; set xs ⊆ S ⟧ ⟹ index (map f xs) (f y) = index xs y"
by (induct xs) (auto simp: inj_on_eq_iff)

lemma index_map_inj: "inj f ⟹ index (map f xs) (f y) = index xs y"

subsection ‹Map with index›

primrec map_index' :: "nat ⇒ (nat ⇒ 'a ⇒ 'b) ⇒ 'a list ⇒ 'b list" where
"map_index' n f [] = []"
| "map_index' n f (x#xs) = f n x # map_index' (Suc n) f xs"

lemma length_map_index'[simp]: "length (map_index' n f xs) = length xs"
by (induct xs arbitrary: n) auto

lemma map_index'_map_zip: "map_index' n f xs = map (case_prod f) (zip [n ..< n + length xs] xs)"
proof (induct xs arbitrary: n)
case (Cons x xs)
hence "map_index' n f (x#xs) = f n x # map (case_prod f) (zip [Suc n ..< n + length (x # xs)] xs)" by simp
also have "… =  map (case_prod f) (zip (n # [Suc n ..< n + length (x # xs)]) (x # xs))" by simp
also have "(n # [Suc n ..< n + length (x # xs)]) = [n ..< n + length (x # xs)]" by (induct xs) auto
finally show ?case by simp
qed simp

abbreviation "map_index ≡ map_index' 0"

lemmas map_index = map_index'_map_zip[of 0, simplified]

lemma take_map_index: "take p (map_index f xs) = map_index f (take p xs)"
unfolding map_index by (auto simp: min_def take_map take_zip)

lemma drop_map_index: "drop p (map_index f xs) = map_index' p f (drop p xs)"
unfolding map_index'_map_zip by (cases "p < length xs") (auto simp: drop_map drop_zip)

lemma map_map_index[simp]: "map g (map_index f xs) = map_index (λn x. g (f n x)) xs"
unfolding map_index by auto

lemma map_index_map[simp]: "map_index f (map g xs) = map_index (λn x. f n (g x)) xs"
unfolding map_index by (auto simp: map_zip_map2)

lemma set_map_index[simp]: "x ∈ set (map_index f xs) = (∃i < length xs. f i (xs ! i) = x)"
unfolding map_index by (auto simp: set_zip intro!: image_eqI[of _ "case_prod f"])

lemma set_map_index'[simp]: "x∈set (map_index' n f xs)
⟷ (∃i<length xs. f (n+i) (xs!i) = x) "
unfolding map_index'_map_zip
by (auto simp: set_zip intro!: image_eqI[of _ "case_prod f"])

lemma nth_map_index[simp]: "p < length xs ⟹ map_index f xs ! p = f p (xs ! p)"
unfolding map_index by auto

lemma map_index_cong:
"∀p < length xs. f p (xs ! p) = g p (xs ! p) ⟹ map_index f xs = map_index g xs"
unfolding map_index by (auto simp: set_zip)

lemma map_index_id: "map_index (curry snd) xs = xs"
unfolding map_index by auto

lemma map_index_no_index[simp]: "map_index (λn x. f x) xs = map f xs"
unfolding map_index by (induct xs rule: rev_induct) auto

lemma map_index_congL:
"∀p < length xs. f p (xs ! p) = xs ! p ⟹ map_index f xs = xs"
by (rule trans[OF map_index_cong map_index_id]) auto

lemma map_index'_is_NilD: "map_index' n f xs = [] ⟹ xs = []"
by (induct xs) auto

declare map_index'_is_NilD[of 0, dest!]

lemma map_index'_is_ConsD:
"map_index' n f xs = y # ys ⟹ ∃z zs. xs = z # zs ∧ f n z = y ∧ map_index' (n + 1) f zs = ys"
by (induct xs arbitrary: n) auto

lemma map_index'_eq_imp_length_eq: "map_index' n f xs = map_index' n g ys ⟹ length xs = length ys"
proof (induct ys arbitrary: xs n)
case (Cons y ys) thus ?case by (cases xs) auto
qed (auto dest!: map_index'_is_NilD)

lemmas map_index_eq_imp_length_eq = map_index'_eq_imp_length_eq[of 0]

lemma map_index'_comp[simp]: "map_index' n f (map_index' n g xs) = map_index' n (λn. f n o g n) xs"
by (induct xs arbitrary: n) auto

lemma map_index'_append[simp]: "map_index' n f (a @ b)
= map_index' n f a @ map_index' (n + length a) f b"
by (induct a arbitrary: n) auto

lemma map_index_append[simp]: "map_index f (a @ b)
= map_index f a @ map_index' (length a) f b"
using map_index'_append[where n=0]
by (simp del: map_index'_append)

subsection ‹Insert at position›

primrec insert_nth :: "nat ⇒ 'a ⇒ 'a list ⇒ 'a list" where
"insert_nth 0 x xs = x # xs"
| "insert_nth (Suc n) x xs = (case xs of [] ⇒ [x] | y # ys ⇒ y # insert_nth n x ys)"

lemma insert_nth_take_drop[simp]: "insert_nth n x xs = take n xs @ [x] @ drop n xs"
proof (induct n arbitrary: xs)
case Suc thus ?case by (cases xs) auto
qed simp

lemma length_insert_nth: "length (insert_nth n x xs) = Suc (length xs)"
by (induct xs) auto

lemma set_insert_nth:
"set (insert_nth i x xs) = insert x (set xs)"

lemma distinct_insert_nth:
assumes "distinct xs"
assumes "x ∉ set xs"
shows "distinct (insert_nth i x xs)"
using assms proof (induct xs arbitrary: i)
case Nil
then show ?case by (cases i) auto
next
case (Cons a xs)
then show ?case
by (cases i) (auto simp add: set_insert_nth simp del: insert_nth_take_drop)
qed

lemma nth_insert_nth_front:
assumes "i < j" "j ≤ length xs"
shows "insert_nth j x xs ! i = xs ! i"
using assms by (simp add: nth_append)

lemma nth_insert_nth_index_eq:
assumes "i ≤ length xs"
shows "insert_nth i x xs ! i = x"
using assms by (simp add: nth_append)

lemma nth_insert_nth_back:
assumes "j < i" "i ≤ length xs"
shows "insert_nth j x xs ! i = xs ! (i - 1)"
using assms by (cases i) (auto simp add: nth_append min_def)

lemma nth_insert_nth:
assumes "i ≤ length xs" "j ≤ length xs"
shows "insert_nth j x xs ! i = (if i = j then x else if i < j then xs ! i else xs ! (i - 1))"
using assms by (simp add: nth_insert_nth_front nth_insert_nth_index_eq nth_insert_nth_back del: insert_nth_take_drop)

lemma insert_nth_inverse:
assumes "j ≤ length xs" "j' ≤ length xs'"
assumes "x ∉ set xs" "x ∉ set xs'"
assumes "insert_nth j x xs = insert_nth j' x xs'"
shows "j = j'"
proof -
from assms(1,3) have "∀i≤length xs. insert_nth j x xs ! i = x ⟷ i = j"
by (auto simp add: nth_insert_nth simp del: insert_nth_take_drop)
moreover from assms(2,4) have "∀i≤length xs'. insert_nth j' x xs' ! i = x ⟷ i = j'"
by (auto simp add: nth_insert_nth simp del: insert_nth_take_drop)
ultimately show "j = j'"
using assms(1,2,5) by (metis dual_order.trans nat_le_linear)
qed

text ‹Insert several elements at given (ascending) positions›

lemma length_fold_insert_nth:
"length (fold (λ(p, b). insert_nth p b) pxs xs) = length xs + length pxs"
by (induct pxs arbitrary: xs) auto

lemma invar_fold_insert_nth:
"⟦∀x∈set pxs. p < fst x; p < length xs; xs ! p = b⟧ ⟹
fold (λ(x, y). insert_nth x y) pxs xs ! p = b"
by (induct pxs arbitrary: xs) (auto simp: nth_append)

lemma nth_fold_insert_nth:
"⟦sorted (map fst pxs); distinct (map fst pxs); ∀(p, b) ∈ set pxs. p < length xs + length pxs;
i < length pxs; pxs ! i = (p, b)⟧ ⟹
fold (λ(p, b). insert_nth p b) pxs xs ! p = b"
proof (induct pxs arbitrary: xs i p b)
case (Cons pb pxs)
show ?case
proof (cases i)
case 0
with Cons.prems have "p < Suc (length xs)"
proof (induct pxs rule: rev_induct)
case (snoc pb' pxs)
then obtain p' b' where "pb' = (p', b')" by auto
with snoc.prems have "∀p ∈ fst  set pxs. p < p'" "p' ≤ Suc (length xs + length pxs)"
by (auto simp: image_iff sorted_wrt_append le_eq_less_or_eq)
with snoc.prems show ?case by (intro snoc(1)) (auto simp: sorted_append)
qed auto
with 0 Cons.prems show ?thesis unfolding fold.simps o_apply
by (intro invar_fold_insert_nth) (auto simp: image_iff le_eq_less_or_eq nth_append)
next
case (Suc n) with Cons.prems show ?thesis unfolding fold.simps
by (auto intro!: Cons(1))
qed
qed simp

subsection ‹Remove at position›

fun remove_nth :: "nat ⇒ 'a list ⇒ 'a list"
where
"remove_nth i [] = []"
| "remove_nth 0 (x # xs) = xs"
| "remove_nth (Suc i) (x # xs) = x # remove_nth i xs"

lemma remove_nth_take_drop:
"remove_nth i xs = take i xs @ drop (Suc i) xs"
proof (induct xs arbitrary: i)
case Nil
then show ?case by simp
next
case (Cons a xs)
then show ?case by (cases i) auto
qed

lemma remove_nth_insert_nth:
assumes "i ≤ length xs"
shows "remove_nth i (insert_nth i x xs) = xs"
using assms proof (induct xs arbitrary: i)
case Nil
then show ?case by simp
next
case (Cons a xs)
then show ?case by (cases i) auto
qed

lemma insert_nth_remove_nth:
assumes "i < length xs"
shows "insert_nth i (xs ! i) (remove_nth i xs) = xs"
using assms proof (induct xs arbitrary: i)
case Nil
then show ?case by simp
next
case (Cons a xs)
then show ?case by (cases i) auto
qed

lemma length_remove_nth:
assumes "i < length xs"
shows "length (remove_nth i xs) = length xs - 1"
using assms unfolding remove_nth_take_drop by simp

lemma set_remove_nth_subset:
"set (remove_nth j xs) ⊆ set xs"
proof (induct xs arbitrary: j)
case Nil
then show ?case by simp
next
case (Cons a xs)
then show ?case by (cases j) auto
qed

lemma set_remove_nth:
assumes "distinct xs" "j < length xs"
shows "set (remove_nth j xs) = set xs - {xs ! j}"
using assms proof (induct xs arbitrary: j)
case Nil
then show ?case by simp
next
case (Cons a xs)
then show ?case by (cases j) auto
qed

lemma distinct_remove_nth:
assumes "distinct xs"
shows "distinct (remove_nth i xs)"
using assms proof (induct xs arbitrary: i)
case Nil
then show ?case by simp
next
case (Cons a xs)
then show ?case
by (cases i) (auto simp add: set_remove_nth_subset rev_subsetD)
qed

end