Theory Bit

theory Bit
imports Main
```(*  Title:      HOL/Library/Bit.thy
Author:     Brian Huffman
*)

section ‹The Field of Integers mod 2›

theory Bit
imports Main
begin

subsection ‹Bits as a datatype›

typedef bit = "UNIV :: bool set"
morphisms set Bit ..

instantiation bit :: "{zero, one}"
begin

definition zero_bit_def: "0 = Bit False"

definition one_bit_def: "1 = Bit True"

instance ..

end

old_rep_datatype "0::bit" "1::bit"
proof -
fix P :: "bit ⇒ bool"
fix x :: bit
assume "P 0" and "P 1"
then have "∀b. P (Bit b)"
unfolding zero_bit_def one_bit_def
then show "P x"
by (induct x) simp
next
show "(0::bit) ≠ (1::bit)"
unfolding zero_bit_def one_bit_def
qed

lemma Bit_set_eq [simp]: "Bit (set b) = b"
by (fact set_inverse)

lemma set_Bit_eq [simp]: "set (Bit P) = P"
by (rule Bit_inverse) rule

lemma bit_eq_iff: "x = y ⟷ (set x ⟷ set y)"

lemma Bit_inject [simp]: "Bit P = Bit Q ⟷ (P ⟷ Q)"

lemma set [iff]:
"¬ set 0"
"set 1"
by (simp_all add: zero_bit_def one_bit_def Bit_inverse)

lemma [code]:
"set 0 ⟷ False"
"set 1 ⟷ True"
by simp_all

lemma set_iff: "set b ⟷ b = 1"
by (cases b) simp_all

lemma bit_eq_iff_set:
"b = 0 ⟷ ¬ set b"
"b = 1 ⟷ set b"

lemma Bit [simp, code]:
"Bit False = 0"
"Bit True = 1"

lemma bit_not_0_iff [iff]: "x ≠ 0 ⟷ x = 1" for x :: bit

lemma bit_not_1_iff [iff]: "x ≠ 1 ⟷ x = 0" for x :: bit

lemma [code]:
"HOL.equal 0 b ⟷ ¬ set b"
"HOL.equal 1 b ⟷ set b"

subsection ‹Type @{typ bit} forms a field›

instantiation bit :: field
begin

definition plus_bit_def: "x + y = case_bit y (case_bit 1 0 y) x"

definition times_bit_def: "x * y = case_bit 0 y x"

definition uminus_bit_def [simp]: "- x = x" for x :: bit

definition minus_bit_def [simp]: "x - y = x + y" for x y :: bit

definition inverse_bit_def [simp]: "inverse x = x" for x :: bit

definition divide_bit_def [simp]: "x div y = x * y" for x y :: bit

lemmas field_bit_defs =
plus_bit_def times_bit_def minus_bit_def uminus_bit_def
divide_bit_def inverse_bit_def

instance
by standard (auto simp: field_bit_defs split: bit.split)

end

lemma bit_add_self: "x + x = 0" for x :: bit
unfolding plus_bit_def by (simp split: bit.split)

lemma bit_mult_eq_1_iff [simp]: "x * y = 1 ⟷ x = 1 ∧ y = 1" for x y :: bit
unfolding times_bit_def by (simp split: bit.split)

text ‹Not sure whether the next two should be simp rules.›

lemma bit_add_eq_0_iff: "x + y = 0 ⟷ x = y" for x y :: bit
unfolding plus_bit_def by (simp split: bit.split)

lemma bit_add_eq_1_iff: "x + y = 1 ⟷ x ≠ y" for x y :: bit
unfolding plus_bit_def by (simp split: bit.split)

subsection ‹Numerals at type @{typ bit}›

text ‹All numerals reduce to either 0 or 1.›

lemma bit_minus1 [simp]: "- 1 = (1 :: bit)"
by (simp only: uminus_bit_def)

lemma bit_neg_numeral [simp]: "(- numeral w :: bit) = numeral w"
by (simp only: uminus_bit_def)

lemma bit_numeral_even [simp]: "numeral (Num.Bit0 w) = (0 :: bit)"

lemma bit_numeral_odd [simp]: "numeral (Num.Bit1 w) = (1 :: bit)"

subsection ‹Conversion from @{typ bit}›

context zero_neq_one
begin

definition of_bit :: "bit ⇒ 'a"
where "of_bit b = case_bit 0 1 b"

lemma of_bit_eq [simp, code]:
"of_bit 0 = 0"
"of_bit 1 = 1"

lemma of_bit_eq_iff: "of_bit x = of_bit y ⟷ x = y"
by (cases x) (cases y; simp)+

end

lemma (in semiring_1) of_nat_of_bit_eq: "of_nat (of_bit b) = of_bit b"
by (cases b) simp_all

lemma (in ring_1) of_int_of_bit_eq: "of_int (of_bit b) = of_bit b"
by (cases b) simp_all

hide_const (open) set

end
```