(* Author: Andreas Lochbihler, ETH Zürich
Author: Florian Haftmann, TU Muenchen *)
header \<open>Less common functions on lists\<close>
theory More_List
imports Main
begin
definition strip_while :: "('a \<Rightarrow> bool) \<Rightarrow> 'a list \<Rightarrow> 'a list"
where
"strip_while P = rev \<circ> dropWhile P \<circ> rev"
lemma strip_while_rev [simp]:
"strip_while P (rev xs) = rev (dropWhile P xs)"
by (simp add: strip_while_def)
lemma strip_while_Nil [simp]:
"strip_while P [] = []"
by (simp add: strip_while_def)
lemma strip_while_append [simp]:
"\<not> P x \<Longrightarrow> strip_while P (xs @ [x]) = xs @ [x]"
by (simp add: strip_while_def)
lemma strip_while_append_rec [simp]:
"P x \<Longrightarrow> strip_while P (xs @ [x]) = strip_while P xs"
by (simp add: strip_while_def)
lemma strip_while_Cons [simp]:
"\<not> P x \<Longrightarrow> strip_while P (x # xs) = x # strip_while P xs"
by (induct xs rule: rev_induct) (simp_all add: strip_while_def)
lemma strip_while_eq_Nil [simp]:
"strip_while P xs = [] \<longleftrightarrow> (\<forall>x\<in>set xs. P x)"
by (simp add: strip_while_def)
lemma strip_while_eq_Cons_rec:
"strip_while P (x # xs) = x # strip_while P xs \<longleftrightarrow> \<not> (P x \<and> (\<forall>x\<in>set xs. P x))"
by (induct xs rule: rev_induct) (simp_all add: strip_while_def)
lemma strip_while_not_last [simp]:
"\<not> P (last xs) \<Longrightarrow> strip_while P xs = xs"
by (cases xs rule: rev_cases) simp_all
lemma split_strip_while_append:
fixes xs :: "'a list"
obtains ys zs :: "'a list"
where "strip_while P xs = ys" and "\<forall>x\<in>set zs. P x" and "xs = ys @ zs"
proof (rule that)
show "strip_while P xs = strip_while P xs" ..
show "\<forall>x\<in>set (rev (takeWhile P (rev xs))). P x" by (simp add: takeWhile_eq_all_conv [symmetric])
have "rev xs = rev (strip_while P xs @ rev (takeWhile P (rev xs)))"
by (simp add: strip_while_def)
then show "xs = strip_while P xs @ rev (takeWhile P (rev xs))"
by (simp only: rev_is_rev_conv)
qed
lemma strip_while_snoc [simp]:
"strip_while P (xs @ [x]) = (if P x then strip_while P xs else xs @ [x])"
by (simp add: strip_while_def)
lemma strip_while_map:
"strip_while P (map f xs) = map f (strip_while (P \<circ> f) xs)"
by (simp add: strip_while_def rev_map dropWhile_map)
definition no_leading :: "('a \<Rightarrow> bool) \<Rightarrow> 'a list \<Rightarrow> bool"
where
"no_leading P xs \<longleftrightarrow> (xs \<noteq> [] \<longrightarrow> \<not> P (hd xs))"
lemma no_leading_Nil [simp, intro!]:
"no_leading P []"
by (simp add: no_leading_def)
lemma no_leading_Cons [simp, intro!]:
"no_leading P (x # xs) \<longleftrightarrow> \<not> P x"
by (simp add: no_leading_def)
lemma no_leading_append [simp]:
"no_leading P (xs @ ys) \<longleftrightarrow> no_leading P xs \<and> (xs = [] \<longrightarrow> no_leading P ys)"
by (induct xs) simp_all
lemma no_leading_dropWhile [simp]:
"no_leading P (dropWhile P xs)"
by (induct xs) simp_all
lemma dropWhile_eq_obtain_leading:
assumes "dropWhile P xs = ys"
obtains zs where "xs = zs @ ys" and "\<And>z. z \<in> set zs \<Longrightarrow> P z" and "no_leading P ys"
proof -
from assms have "\<exists>zs. xs = zs @ ys \<and> (\<forall>z \<in> set zs. P z) \<and> no_leading P ys"
proof (induct xs arbitrary: ys)
case Nil then show ?case by simp
next
case (Cons x xs ys)
show ?case proof (cases "P x")
case True with Cons.hyps [of ys] Cons.prems
have "\<exists>zs. xs = zs @ ys \<and> (\<forall>a\<in>set zs. P a) \<and> no_leading P ys"
by simp
then obtain zs where "xs = zs @ ys" and "\<And>z. z \<in> set zs \<Longrightarrow> P z"
and *: "no_leading P ys"
by blast
with True have "x # xs = (x # zs) @ ys" and "\<And>z. z \<in> set (x # zs) \<Longrightarrow> P z"
by auto
with * show ?thesis
by blast next
case False
with Cons show ?thesis by (cases ys) simp_all
qed
qed
with that show thesis
by blast
qed
lemma dropWhile_idem_iff:
"dropWhile P xs = xs \<longleftrightarrow> no_leading P xs"
by (cases xs) (auto elim: dropWhile_eq_obtain_leading)
abbreviation no_trailing :: "('a \<Rightarrow> bool) \<Rightarrow> 'a list \<Rightarrow> bool"
where
"no_trailing P xs \<equiv> no_leading P (rev xs)"
lemma no_trailing_unfold:
"no_trailing P xs \<longleftrightarrow> (xs \<noteq> [] \<longrightarrow> \<not> P (last xs))"
by (induct xs) simp_all
lemma no_trailing_Nil [simp, intro!]:
"no_trailing P []"
by simp
lemma no_trailing_Cons [simp]:
"no_trailing P (x # xs) \<longleftrightarrow> no_trailing P xs \<and> (xs = [] \<longrightarrow> \<not> P x)"
by simp
lemma no_trailing_append_Cons [simp]:
"no_trailing P (xs @ y # ys) \<longleftrightarrow> no_trailing P (y # ys)"
by simp
lemma no_trailing_strip_while [simp]:
"no_trailing P (strip_while P xs)"
by (induct xs rule: rev_induct) simp_all
lemma strip_while_eq_obtain_trailing:
assumes "strip_while P xs = ys"
obtains zs where "xs = ys @ zs" and "\<And>z. z \<in> set zs \<Longrightarrow> P z" and "no_trailing P ys"
proof -
from assms have "rev (rev (dropWhile P (rev xs))) = rev ys"
by (simp add: strip_while_def)
then have "dropWhile P (rev xs) = rev ys"
by simp
then obtain zs where A: "rev xs = zs @ rev ys" and B: "\<And>z. z \<in> set zs \<Longrightarrow> P z"
and C: "no_trailing P ys"
using dropWhile_eq_obtain_leading by blast
from A have "rev (rev xs) = rev (zs @ rev ys)"
by simp
then have "xs = ys @ rev zs"
by simp
moreover from B have "\<And>z. z \<in> set (rev zs) \<Longrightarrow> P z"
by simp
ultimately show thesis using that C by blast
qed
lemma strip_while_idem_iff:
"strip_while P xs = xs \<longleftrightarrow> no_trailing P xs"
proof -
def ys \<equiv> "rev xs"
moreover have "strip_while P (rev ys) = rev ys \<longleftrightarrow> no_trailing P (rev ys)"
by (simp add: dropWhile_idem_iff)
ultimately show ?thesis by simp
qed
lemma no_trailing_map:
"no_trailing P (map f xs) = no_trailing (P \<circ> f) xs"
by (simp add: last_map no_trailing_unfold)
lemma no_trailing_upt [simp]:
"no_trailing P [n..<m] \<longleftrightarrow> (n < m \<longrightarrow> \<not> P (m - 1))"
by (auto simp add: no_trailing_unfold)
definition nth_default :: "'a \<Rightarrow> 'a list \<Rightarrow> nat \<Rightarrow> 'a"
where
"nth_default x xs n = (if n < length xs then xs ! n else x)"
lemma nth_default_Nil [simp]:
"nth_default y [] n = y"
by (simp add: nth_default_def)
lemma nth_default_Cons_0 [simp]:
"nth_default y (x # xs) 0 = x"
by (simp add: nth_default_def)
lemma nth_default_Cons_Suc [simp]:
"nth_default y (x # xs) (Suc n) = nth_default y xs n"
by (simp add: nth_default_def)
lemma nth_default_map_eq:
"f y = x \<Longrightarrow> nth_default x (map f xs) n = f (nth_default y xs n)"
by (simp add: nth_default_def)
lemma nth_default_strip_while_eq [simp]:
"nth_default x (strip_while (HOL.eq x) xs) n = nth_default x xs n"
proof -
from split_strip_while_append obtain ys zs
where "strip_while (HOL.eq x) xs = ys" and "\<forall>z\<in>set zs. x = z" and "xs = ys @ zs" by blast
then show ?thesis by (simp add: nth_default_def not_less nth_append)
qed
lemma nth_default_Cons:
"nth_default y (x # xs) n = (case n of 0 \<Rightarrow> x | Suc n' \<Rightarrow> nth_default y xs n')"
by (simp split: nat.split)
lemma nth_default_nth:
"n < length xs \<Longrightarrow> nth_default y xs n = xs ! n"
by (simp add: nth_default_def)
lemma nth_default_beyond:
"length xs \<le> n \<Longrightarrow> nth_default y xs n = y"
by (simp add: nth_default_def)
lemma range_nth_default [simp]:
"range (nth_default dflt xs) = insert dflt (set xs)"
by (auto simp add: nth_default_def[abs_def] in_set_conv_nth)
lemma nth_strip_while:
assumes "n < length (strip_while P xs)"
shows "strip_while P xs ! n = xs ! n"
proof -
have "length (dropWhile P (rev xs)) + length (takeWhile P (rev xs)) = length xs"
by (subst add.commute)
(simp add: arg_cong [where f=length, OF takeWhile_dropWhile_id, unfolded length_append])
then show ?thesis using assms
by (simp add: strip_while_def rev_nth dropWhile_nth)
qed
lemma length_strip_while_le:
"length (strip_while P xs) \<le> length xs"
unfolding strip_while_def o_def length_rev
by (subst (2) length_rev[symmetric])
(simp add: strip_while_def length_dropWhile_le del: length_rev)
lemma finite_nth_default_neq_default [simp]:
"finite {k. nth_default dflt xs k \<noteq> dflt}"
by (simp add: nth_default_def)
lemma sorted_list_of_set_nth_default:
"sorted_list_of_set {k. nth_default dflt xs k \<noteq> dflt} = map fst (filter (\<lambda>(_, x). x \<noteq> dflt) (zip [0..<length xs] xs))"
by (rule sorted_distinct_set_unique) (auto simp add: nth_default_def in_set_conv_nth sorted_filter distinct_map_filter intro: rev_image_eqI)
lemma nth_default_snoc_default [simp]:
"nth_default dflt (xs @ [dflt]) = nth_default dflt xs"
by (auto simp add: nth_default_def fun_eq_iff nth_append)
lemma nth_default_strip_while_dflt [simp]:
"nth_default dflt (strip_while (op = dflt) xs) = nth_default dflt xs"
by (induct xs rule: rev_induct) auto
lemma nth_default_eq_dflt_iff:
"nth_default dflt xs k = dflt \<longleftrightarrow> (k < length xs \<longrightarrow> xs ! k = dflt)"
by (simp add: nth_default_def)
end