Module Type NBitsProp
(Import A : NAxiomsSig')
(Import B : NSubProp A)
(Import C : NParityProp A B)
(Import D : NPowProp A B C)
(Import E : NDivProp A B)
(Import F : NLog2Prop A B C D).
Include BoolEqualityFacts A.
Ltac order_nz := try apply pow_nonzero; order'.
Hint Rewrite div_0_l mod_0_l div_1_r mod_1_r : nz.
(** Some properties of power and division *)
(** An injection from bits [true] and [false] to numbers 1 and 0. We declare it as a (local) coercion for shorter statements. *)
Definition b2n (b:bool) := if b then 1 else 0.
Local Coercion b2n : bool >-> t.
Instance b2n_proper : Proper (Logic.eq ==> eq) b2n.
(** We can compact [testbit_odd_0] [testbit_even_0] [testbit_even_succ] [testbit_odd_succ] in only two lemmas. *)
(** Alternative characterisations of [testbit] *)
(** This concise equation could have been taken as specification for testbit in the interface, but it would have been hard to implement with little initial knowledge about div and mod *)
(** This characterisation that uses only basic operations and power was initially taken as specification for testbit. We describe [a] as having a low part and a high part, with the corresponding bit in the middle. This characterisation is moderatly complex to implement, but also moderately usable... *)
(** Results about the injection [b2n] *)
(** The specification of testbit by low and high parts is complete *)
(** All bits of number 0 are 0 *)
(** Various ways to refer to the lowest bit of a number *)
(** Hence testing a bit is equivalent to shifting and testing parity *)
(** [log2] gives the highest nonzero bit *)
(** Hence the number of bits of [a] is [1+log2 a] (see [Pos.size_nat] and [Pos.size]). *)
(** Testing bits after division or multiplication by a power of two *)
(** Selecting the low part of a number can be done by a modulo *)

Lemma mod_pow2_bits_low : forall a n m, m<n ->
(a mod 2^n).[m] = a.[m].