src/HOL/ex/Word.thy
 author haftmann Tue, 04 Aug 2020 09:33:05 +0000 changeset 72082 41393ecb57ac parent 72079 8c355e2dd7db permissions -rw-r--r--
```
(*  Author:  Florian Haftmann, TUM
*)

section \<open>Proof of concept for algebraically founded bit word types\<close>

theory Word
imports
Main
"HOL-Library.Type_Length"
"HOL-Library.Bit_Operations"
begin

subsection \<open>Bit strings as quotient type\<close>

subsubsection \<open>Basic properties\<close>

quotient_type (overloaded) 'a word = int / "\<lambda>k l. take_bit LENGTH('a) k = take_bit LENGTH('a::len) l"
by (auto intro!: equivpI reflpI sympI transpI)

instantiation word :: (len) "{semiring_numeral, comm_semiring_0, comm_ring}"
begin

lift_definition zero_word :: "'a word"
is 0
.

lift_definition one_word :: "'a word"
is 1
.

lift_definition plus_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is plus

lift_definition uminus_word :: "'a word \<Rightarrow> 'a word"
is uminus
by (subst take_bit_minus [symmetric]) (simp add: take_bit_minus)

lift_definition minus_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is minus
by (subst take_bit_diff [symmetric]) (simp add: take_bit_diff)

lift_definition times_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is times
by (auto simp add: take_bit_eq_mod intro: mod_mult_cong)

instance
by standard (transfer; simp add: algebra_simps)+

end

instance word :: (len) comm_ring_1
by standard (transfer; simp)+

quickcheck_generator word
constructors:
"zero_class.zero :: ('a::len) word",
"numeral :: num \<Rightarrow> ('a::len) word",
"uminus :: ('a::len) word \<Rightarrow> ('a::len) word"

context
includes lifting_syntax
notes power_transfer [transfer_rule]
begin

lemma power_transfer_word [transfer_rule]:
\<open>(pcr_word ===> (=) ===> pcr_word) (^) (^)\<close>
by transfer_prover

end

subsubsection \<open>Conversions\<close>

context
includes lifting_syntax
notes
transfer_rule_of_bool [transfer_rule]
transfer_rule_numeral [transfer_rule]
transfer_rule_of_nat [transfer_rule]
transfer_rule_of_int [transfer_rule]
begin

lemma [transfer_rule]:
"((=) ===> (pcr_word :: int \<Rightarrow> 'a::len word \<Rightarrow> bool)) of_bool of_bool"
by transfer_prover

lemma [transfer_rule]:
"((=) ===> (pcr_word :: int \<Rightarrow> 'a::len word \<Rightarrow> bool)) numeral numeral"
by transfer_prover

lemma [transfer_rule]:
"((=) ===> pcr_word) int of_nat"
by transfer_prover

lemma [transfer_rule]:
"((=) ===> pcr_word) (\<lambda>k. k) of_int"
proof -
have "((=) ===> pcr_word) of_int of_int"
by transfer_prover
then show ?thesis by (simp add: id_def)
qed

end

lemma abs_word_eq:
"abs_word = of_int"
by (rule ext) (transfer, rule)

context semiring_1
begin

lift_definition unsigned :: "'b::len word \<Rightarrow> 'a"
is "of_nat \<circ> nat \<circ> take_bit LENGTH('b)"
by simp

lemma unsigned_0 [simp]:
"unsigned 0 = 0"
by transfer simp

end

context semiring_char_0
begin

lemma word_eq_iff_unsigned:
"a = b \<longleftrightarrow> unsigned a = unsigned b"
by safe (transfer; simp add: eq_nat_nat_iff)

end

instantiation word :: (len) equal
begin

definition equal_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> bool"
where "equal_word a b \<longleftrightarrow> (unsigned a :: int) = unsigned b"

instance proof
fix a b :: "'a word"
show "HOL.equal a b \<longleftrightarrow> a = b"
using word_eq_iff_unsigned [of a b] by (auto simp add: equal_word_def)
qed

end

context ring_1
begin

lift_definition signed :: "'b::len word \<Rightarrow> 'a"
is "of_int \<circ> signed_take_bit (LENGTH('b) - 1)"
by (cases \<open>LENGTH('b)\<close>)

lemma signed_0 [simp]:
"signed 0 = 0"
by transfer simp

end

lemma unsigned_of_nat [simp]:
"unsigned (of_nat n :: 'a word) = take_bit LENGTH('a::len) n"
by transfer (simp add: nat_eq_iff take_bit_eq_mod zmod_int)

lemma of_nat_unsigned [simp]:
"of_nat (unsigned a) = a"
by transfer simp

lemma of_int_unsigned [simp]:
"of_int (unsigned a) = a"
by transfer simp

lemma unsigned_nat_less:
\<open>unsigned a < (2 ^ LENGTH('a) :: nat)\<close> for a :: \<open>'a::len word\<close>

lemma unsigned_int_less:
\<open>unsigned a < (2 ^ LENGTH('a) :: int)\<close> for a :: \<open>'a::len word\<close>

context ring_char_0
begin

lemma word_eq_iff_signed:
"a = b \<longleftrightarrow> signed a = signed b"
by safe (transfer; auto simp add: signed_take_bit_eq_iff_take_bit_eq)

end

lemma signed_of_int [simp]:
"signed (of_int k :: 'a word) = signed_take_bit (LENGTH('a::len) - 1) k"
by transfer simp

lemma of_int_signed [simp]:
"of_int (signed a) = a"

subsubsection \<open>Properties\<close>

lemma exp_eq_zero_iff:
\<open>(2 :: 'a::len word) ^ n = 0 \<longleftrightarrow> LENGTH('a) \<le> n\<close>
by transfer simp

subsubsection \<open>Division\<close>

instantiation word :: (len) modulo
begin

lift_definition divide_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is "\<lambda>a b. take_bit LENGTH('a) a div take_bit LENGTH('a) b"
by simp

lift_definition modulo_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is "\<lambda>a b. take_bit LENGTH('a) a mod take_bit LENGTH('a) b"
by simp

instance ..

end

lemma zero_word_div_eq [simp]:
\<open>0 div a = 0\<close> for a :: \<open>'a::len word\<close>
by transfer simp

lemma div_zero_word_eq [simp]:
\<open>a div 0 = 0\<close> for a :: \<open>'a::len word\<close>
by transfer simp

context
includes lifting_syntax
begin

lemma [transfer_rule]:
"(pcr_word ===> (\<longleftrightarrow>)) even ((dvd) 2 :: 'a::len word \<Rightarrow> bool)"
proof -
have even_word_unfold: "even k \<longleftrightarrow> (\<exists>l. take_bit LENGTH('a) k = take_bit LENGTH('a) (2 * l))" (is "?P \<longleftrightarrow> ?Q")
for k :: int
proof
assume ?P
then show ?Q
by auto
next
assume ?Q
then obtain l where "take_bit LENGTH('a) k = take_bit LENGTH('a) (2 * l)" ..
then have "even (take_bit LENGTH('a) k)"
by simp
then show ?P
by simp
qed
show ?thesis by (simp only: even_word_unfold [abs_def] dvd_def [where ?'a = "'a word", abs_def])
transfer_prover
qed

end

instance word :: (len) semiring_modulo
proof
show "a div b * b + a mod b = a" for a b :: "'a word"
proof transfer
fix k l :: int
define r :: int where "r = 2 ^ LENGTH('a)"
then have r: "take_bit LENGTH('a) k = k mod r" for k
have "k mod r = ((k mod r) div (l mod r) * (l mod r)
+ (k mod r) mod (l mod r)) mod r"
also have "... = (((k mod r) div (l mod r) * (l mod r)) mod r
+ (k mod r) mod (l mod r)) mod r"
also have "... = (((k mod r) div (l mod r) * l) mod r
+ (k mod r) mod (l mod r)) mod r"
finally have "k mod r = ((k mod r) div (l mod r) * l
+ (k mod r) mod (l mod r)) mod r"
with r show "take_bit LENGTH('a) (take_bit LENGTH('a) k div take_bit LENGTH('a) l * l
+ take_bit LENGTH('a) k mod take_bit LENGTH('a) l) = take_bit LENGTH('a) k"
by simp
qed
qed

instance word :: (len) semiring_parity
proof
show "\<not> 2 dvd (1::'a word)"
by transfer simp
show even_iff_mod_2_eq_0: "2 dvd a \<longleftrightarrow> a mod 2 = 0"
for a :: "'a word"
by transfer (simp_all add: mod_2_eq_odd take_bit_Suc)
show "\<not> 2 dvd a \<longleftrightarrow> a mod 2 = 1"
for a :: "'a word"
by transfer (simp_all add: mod_2_eq_odd take_bit_Suc)
qed

subsubsection \<open>Orderings\<close>

instantiation word :: (len) linorder
begin

lift_definition less_eq_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> bool"
is "\<lambda>a b. take_bit LENGTH('a) a \<le> take_bit LENGTH('a) b"
by simp

lift_definition less_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> bool"
is "\<lambda>a b. take_bit LENGTH('a) a < take_bit LENGTH('a) b"
by simp

instance
by standard (transfer; auto)+

end

context linordered_semidom
begin

lemma word_less_eq_iff_unsigned:
"a \<le> b \<longleftrightarrow> unsigned a \<le> unsigned b"
by (transfer fixing: less_eq) (simp add: nat_le_eq_zle)

lemma word_less_iff_unsigned:
"a < b \<longleftrightarrow> unsigned a < unsigned b"
by (transfer fixing: less) (auto dest: preorder_class.le_less_trans [OF take_bit_nonnegative])

end

lemma word_greater_zero_iff:
\<open>a > 0 \<longleftrightarrow> a \<noteq> 0\<close> for a :: \<open>'a::len word\<close>

lemma of_nat_word_eq_iff:
\<open>of_nat m = (of_nat n :: 'a::len word) \<longleftrightarrow> take_bit LENGTH('a) m = take_bit LENGTH('a) n\<close>

lemma of_nat_word_less_eq_iff:
\<open>of_nat m \<le> (of_nat n :: 'a::len word) \<longleftrightarrow> take_bit LENGTH('a) m \<le> take_bit LENGTH('a) n\<close>

lemma of_nat_word_less_iff:
\<open>of_nat m < (of_nat n :: 'a::len word) \<longleftrightarrow> take_bit LENGTH('a) m < take_bit LENGTH('a) n\<close>

lemma of_nat_word_eq_0_iff:
\<open>of_nat n = (0 :: 'a::len word) \<longleftrightarrow> 2 ^ LENGTH('a) dvd n\<close>
using of_nat_word_eq_iff [where ?'a = 'a, of n 0] by (simp add: take_bit_eq_0_iff)

lemma of_int_word_eq_iff:
\<open>of_int k = (of_int l :: 'a::len word) \<longleftrightarrow> take_bit LENGTH('a) k = take_bit LENGTH('a) l\<close>
by transfer rule

lemma of_int_word_less_eq_iff:
\<open>of_int k \<le> (of_int l :: 'a::len word) \<longleftrightarrow> take_bit LENGTH('a) k \<le> take_bit LENGTH('a) l\<close>
by transfer rule

lemma of_int_word_less_iff:
\<open>of_int k < (of_int l :: 'a::len word) \<longleftrightarrow> take_bit LENGTH('a) k < take_bit LENGTH('a) l\<close>
by transfer rule

lemma of_int_word_eq_0_iff:
\<open>of_int k = (0 :: 'a::len word) \<longleftrightarrow> 2 ^ LENGTH('a) dvd k\<close>
using of_int_word_eq_iff [where ?'a = 'a, of k 0] by (simp add: take_bit_eq_0_iff)

subsection \<open>Bit structure on \<^typ>\<open>'a word\<close>\<close>

lemma word_bit_induct [case_names zero even odd]:
\<open>P a\<close> if word_zero: \<open>P 0\<close>
and word_even: \<open>\<And>a. P a \<Longrightarrow> 0 < a \<Longrightarrow> a < 2 ^ (LENGTH('a) - 1) \<Longrightarrow> P (2 * a)\<close>
and word_odd: \<open>\<And>a. P a \<Longrightarrow> a < 2 ^ (LENGTH('a) - 1) \<Longrightarrow> P (1 + 2 * a)\<close>
for P and a :: \<open>'a::len word\<close>
proof -
define m :: nat where \<open>m = LENGTH('a) - 1\<close>
then have l: \<open>LENGTH('a) = Suc m\<close>
by simp
define n :: nat where \<open>n = unsigned a\<close>
then have \<open>n < 2 ^ LENGTH('a)\<close>
then have \<open>n < 2 * 2 ^ m\<close>
then have \<open>P (of_nat n)\<close>
proof (induction n rule: nat_bit_induct)
case zero
show ?case
by simp (rule word_zero)
next
case (even n)
then have \<open>n < 2 ^ m\<close>
by simp
with even.IH have \<open>P (of_nat n)\<close>
by simp
moreover from \<open>n < 2 ^ m\<close> even.hyps have \<open>0 < (of_nat n :: 'a word)\<close>
by (auto simp add: word_greater_zero_iff of_nat_word_eq_0_iff l)
moreover from \<open>n < 2 ^ m\<close> have \<open>(of_nat n :: 'a word) < 2 ^ (LENGTH('a) - 1)\<close>
using of_nat_word_less_iff [where ?'a = 'a, of n \<open>2 ^ m\<close>]
by (cases \<open>m = 0\<close>) (simp_all add: not_less take_bit_eq_self ac_simps l)
ultimately have \<open>P (2 * of_nat n)\<close>
by (rule word_even)
then show ?case
by simp
next
case (odd n)
then have \<open>Suc n \<le> 2 ^ m\<close>
by simp
with odd.IH have \<open>P (of_nat n)\<close>
by simp
moreover from \<open>Suc n \<le> 2 ^ m\<close> have \<open>(of_nat n :: 'a word) < 2 ^ (LENGTH('a) - 1)\<close>
using of_nat_word_less_iff [where ?'a = 'a, of n \<open>2 ^ m\<close>]
by (cases \<open>m = 0\<close>) (simp_all add: not_less take_bit_eq_self ac_simps l)
ultimately have \<open>P (1 + 2 * of_nat n)\<close>
by (rule word_odd)
then show ?case
by simp
qed
then show ?thesis
qed

lemma bit_word_half_eq:
\<open>(of_bool b + a * 2) div 2 = a\<close>
if \<open>a < 2 ^ (LENGTH('a) - Suc 0)\<close>
for a :: \<open>'a::len word\<close>
proof (cases \<open>2 \<le> LENGTH('a::len)\<close>)
case False
have \<open>of_bool (odd k) < (1 :: int) \<longleftrightarrow> even k\<close> for k :: int
by auto
with False that show ?thesis
next
case True
obtain n where length: \<open>LENGTH('a) = Suc n\<close>
by (cases \<open>LENGTH('a)\<close>) simp_all
show ?thesis proof (cases b)
case False
moreover have \<open>a * 2 div 2 = a\<close>
using that proof transfer
fix k :: int
from length have \<open>k * 2 mod 2 ^ LENGTH('a) = (k mod 2 ^ n) * 2\<close>
by simp
moreover assume \<open>take_bit LENGTH('a) k < take_bit LENGTH('a) (2 ^ (LENGTH('a) - Suc 0))\<close>
with \<open>LENGTH('a) = Suc n\<close>
have \<open>k mod 2 ^ LENGTH('a) = k mod 2 ^ n\<close>
ultimately have \<open>take_bit LENGTH('a) (k * 2) = take_bit LENGTH('a) k * 2\<close>
with True show \<open>take_bit LENGTH('a) (take_bit LENGTH('a) (k * 2) div take_bit LENGTH('a) 2)
= take_bit LENGTH('a) k\<close>
by simp
qed
ultimately show ?thesis
by simp
next
case True
moreover have \<open>(1 + a * 2) div 2 = a\<close>
using that proof transfer
fix k :: int
from length have \<open>(1 + k * 2) mod 2 ^ LENGTH('a) = 1 + (k mod 2 ^ n) * 2\<close>
using pos_zmod_mult_2 [of \<open>2 ^ n\<close> k] by (simp add: ac_simps)
moreover assume \<open>take_bit LENGTH('a) k < take_bit LENGTH('a) (2 ^ (LENGTH('a) - Suc 0))\<close>
with \<open>LENGTH('a) = Suc n\<close>
have \<open>k mod 2 ^ LENGTH('a) = k mod 2 ^ n\<close>
ultimately have \<open>take_bit LENGTH('a) (1 + k * 2) = 1 + take_bit LENGTH('a) k * 2\<close>
with True show \<open>take_bit LENGTH('a) (take_bit LENGTH('a) (1 + k * 2) div take_bit LENGTH('a) 2)
= take_bit LENGTH('a) k\<close>
qed
ultimately show ?thesis
by simp
qed
qed

lemma even_mult_exp_div_word_iff:
\<open>even (a * 2 ^ m div 2 ^ n) \<longleftrightarrow> \<not> (
m \<le> n \<and>
n < LENGTH('a) \<and> odd (a div 2 ^ (n - m)))\<close> for a :: \<open>'a::len word\<close>
by transfer
(auto simp flip: drop_bit_eq_div simp add: even_drop_bit_iff_not_bit bit_take_bit_iff,

instantiation word :: (len) semiring_bits
begin

lift_definition bit_word :: \<open>'a word \<Rightarrow> nat \<Rightarrow> bool\<close>
is \<open>\<lambda>k n. n < LENGTH('a) \<and> bit k n\<close>
proof
fix k l :: int and n :: nat
assume *: \<open>take_bit LENGTH('a) k = take_bit LENGTH('a) l\<close>
show \<open>n < LENGTH('a) \<and> bit k n \<longleftrightarrow> n < LENGTH('a) \<and> bit l n\<close>
proof (cases \<open>n < LENGTH('a)\<close>)
case True
from * have \<open>bit (take_bit LENGTH('a) k) n \<longleftrightarrow> bit (take_bit LENGTH('a) l) n\<close>
by simp
then show ?thesis
next
case False
then show ?thesis
by simp
qed
qed

instance proof
show \<open>P a\<close> if stable: \<open>\<And>a. a div 2 = a \<Longrightarrow> P a\<close>
and rec: \<open>\<And>a b. P a \<Longrightarrow> (of_bool b + 2 * a) div 2 = a \<Longrightarrow> P (of_bool b + 2 * a)\<close>
for P and a :: \<open>'a word\<close>
proof (induction a rule: word_bit_induct)
case zero
from stable [of 0] show ?case
by simp
next
case (even a)
with rec [of a False] show ?case
using bit_word_half_eq [of a False] by (simp add: ac_simps)
next
case (odd a)
with rec [of a True] show ?case
using bit_word_half_eq [of a True] by (simp add: ac_simps)
qed
show \<open>bit a n \<longleftrightarrow> odd (a div 2 ^ n)\<close> for a :: \<open>'a word\<close> and n
by transfer (simp flip: drop_bit_eq_div add: drop_bit_take_bit bit_iff_odd_drop_bit)
show \<open>0 div a = 0\<close>
for a :: \<open>'a word\<close>
by transfer simp
show \<open>a div 1 = a\<close>
for a :: \<open>'a word\<close>
by transfer simp
show \<open>a mod b div b = 0\<close>
for a b :: \<open>'a word\<close>
apply transfer
apply (subst (3) mod_pos_pos_trivial [of _ \<open>2 ^ LENGTH('a)\<close>])
apply simp_all
apply (metis le_less mod_by_0 pos_mod_conj zero_less_numeral zero_less_power)
using pos_mod_bound [of \<open>2 ^ LENGTH('a)\<close>] apply simp
proof -
fix aa :: int and ba :: int
have f1: "\<And>i n. (i::int) mod 2 ^ n = 0 \<or> 0 < i mod 2 ^ n"
by (metis le_less take_bit_eq_mod take_bit_nonnegative)
have "(0::int) < 2 ^ len_of (TYPE('a)::'a itself) \<and> ba mod 2 ^ len_of (TYPE('a)::'a itself) \<noteq> 0 \<or> aa mod 2 ^ len_of (TYPE('a)::'a itself) mod (ba mod 2 ^ len_of (TYPE('a)::'a itself)) < 2 ^ len_of (TYPE('a)::'a itself)"
by (metis (no_types) mod_by_0 unique_euclidean_semiring_numeral_class.pos_mod_bound zero_less_numeral zero_less_power)
then show "aa mod 2 ^ len_of (TYPE('a)::'a itself) mod (ba mod 2 ^ len_of (TYPE('a)::'a itself)) < 2 ^ len_of (TYPE('a)::'a itself)"
using f1 by (meson le_less less_le_trans unique_euclidean_semiring_numeral_class.pos_mod_bound)
qed
show \<open>(1 + a) div 2 = a div 2\<close>
if \<open>even a\<close>
for a :: \<open>'a word\<close>
using that by transfer
(auto dest: le_Suc_ex simp add: mod_2_eq_odd take_bit_Suc elim!: evenE)
show \<open>(2 :: 'a word) ^ m div 2 ^ n = of_bool ((2 :: 'a word) ^ m \<noteq> 0 \<and> n \<le> m) * 2 ^ (m - n)\<close>
for m n :: nat
by transfer (simp, simp add: exp_div_exp_eq)
show "a div 2 ^ m div 2 ^ n = a div 2 ^ (m + n)"
for a :: "'a word" and m n :: nat
apply transfer
apply (auto simp add: not_less take_bit_drop_bit ac_simps simp flip: drop_bit_eq_div)
done
show "a mod 2 ^ m mod 2 ^ n = a mod 2 ^ min m n"
for a :: "'a word" and m n :: nat
by transfer (auto simp flip: take_bit_eq_mod simp add: ac_simps)
show \<open>a * 2 ^ m mod 2 ^ n = a mod 2 ^ (n - m) * 2 ^ m\<close>
if \<open>m \<le> n\<close> for a :: "'a word" and m n :: nat
using that apply transfer
apply (auto simp flip: take_bit_eq_mod)
apply (auto simp flip: push_bit_eq_mult simp add: push_bit_take_bit split: split_min_lin)
done
show \<open>a div 2 ^ n mod 2 ^ m = a mod (2 ^ (n + m)) div 2 ^ n\<close>
for a :: "'a word" and m n :: nat
by transfer (auto simp add: not_less take_bit_drop_bit ac_simps simp flip: take_bit_eq_mod drop_bit_eq_div split: split_min_lin)
show \<open>even ((2 ^ m - 1) div (2::'a word) ^ n) \<longleftrightarrow> 2 ^ n = (0::'a word) \<or> m \<le> n\<close>
for m n :: nat
show \<open>even (a * 2 ^ m div 2 ^ n) \<longleftrightarrow> n < m \<or> (2::'a word) ^ n = 0 \<or> m \<le> n \<and> even (a div 2 ^ (n - m))\<close>
for a :: \<open>'a word\<close> and m n :: nat
proof transfer
show \<open>even (take_bit LENGTH('a) (k * 2 ^ m) div take_bit LENGTH('a) (2 ^ n)) \<longleftrightarrow>
n < m
\<or> take_bit LENGTH('a) ((2::int) ^ n) = take_bit LENGTH('a) 0
\<or> (m \<le> n \<and> even (take_bit LENGTH('a) k div take_bit LENGTH('a) (2 ^ (n - m))))\<close>
for m n :: nat and k l :: int
by (auto simp flip: take_bit_eq_mod drop_bit_eq_div push_bit_eq_mult
simp add: div_push_bit_of_1_eq_drop_bit drop_bit_take_bit drop_bit_push_bit_int [of n m])
qed
qed

end

instantiation word :: (len) semiring_bit_shifts
begin

lift_definition push_bit_word :: \<open>nat \<Rightarrow> 'a word \<Rightarrow> 'a word\<close>
is push_bit
proof -
show \<open>take_bit LENGTH('a) (push_bit n k) = take_bit LENGTH('a) (push_bit n l)\<close>
if \<open>take_bit LENGTH('a) k = take_bit LENGTH('a) l\<close> for k l :: int and n :: nat
proof -
from that
have \<open>take_bit (LENGTH('a) - n) (take_bit LENGTH('a) k)
= take_bit (LENGTH('a) - n) (take_bit LENGTH('a) l)\<close>
by simp
moreover have \<open>min (LENGTH('a) - n) LENGTH('a) = LENGTH('a) - n\<close>
by simp
ultimately show ?thesis
qed
qed

lift_definition drop_bit_word :: \<open>nat \<Rightarrow> 'a word \<Rightarrow> 'a word\<close>
is \<open>\<lambda>n. drop_bit n \<circ> take_bit LENGTH('a)\<close>

lift_definition take_bit_word :: \<open>nat \<Rightarrow> 'a word \<Rightarrow> 'a word\<close>
is \<open>\<lambda>n. take_bit (min LENGTH('a) n)\<close>
by (simp add: ac_simps) (simp only: flip: take_bit_take_bit)

instance proof
show \<open>push_bit n a = a * 2 ^ n\<close> for n :: nat and a :: "'a word"
show \<open>drop_bit n a = a div 2 ^ n\<close> for n :: nat and a :: "'a word"
by transfer (simp flip: drop_bit_eq_div add: drop_bit_take_bit)
show \<open>take_bit n a = a mod 2 ^ n\<close> for n :: nat and a :: \<open>'a word\<close>
by transfer (auto simp flip: take_bit_eq_mod)
qed

end

instantiation word :: (len) ring_bit_operations
begin

lift_definition not_word :: "'a word \<Rightarrow> 'a word"
is not

lift_definition and_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is \<open>and\<close>
by simp

lift_definition or_word :: "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is or
by simp

lift_definition xor_word ::  "'a word \<Rightarrow> 'a word \<Rightarrow> 'a word"
is xor
by simp

lift_definition mask_word :: \<open>nat \<Rightarrow> 'a word\<close>

instance by (standard; transfer)
bit_not_iff bit_and_iff bit_or_iff bit_xor_iff)

end

definition even_word :: \<open>'a::len word \<Rightarrow> bool\<close>
where [code_abbrev]: \<open>even_word = even\<close>

lemma even_word_iff [code]:
\<open>even_word a \<longleftrightarrow> a AND 1 = 0\<close>
by (simp add: even_word_def and_one_eq even_iff_mod_2_eq_zero)

lemma bit_word_iff_drop_bit_and [code]:
\<open>bit a n \<longleftrightarrow> drop_bit n a AND 1 = 1\<close> for a :: \<open>'a::len word\<close>
by (simp add: bit_iff_odd_drop_bit odd_iff_mod_2_eq_one and_one_eq)

lifting_update word.lifting
lifting_forget word.lifting

end
```