# Library Coq.PArith.BinPosDef

# Binary positive numbers, operations

Initial development by Pierre Crégut, CNET, Lannion, France
The type positive and its constructors xI and xO and xH are now defined in BinNums.v
Require Export BinNums.
Postfix notation for positive numbers, allowing to mimic the position of bits in a big-endian representation. For instance, we can write 1~1~0 instead of (xO (xI xH)) for the number 6 (which is 110 in binary notation).
Notation "p ~ 1" := (xI p)
(at level 7, left associativity, format "p '~' '1'") : positive_scope.
Notation "p ~ 0" := (xO p)
(at level 7, left associativity, format "p '~' '0'") : positive_scope.
Local Open Scope positive_scope.
Local Local
Module Pos.
Definition t := positive.

# Operations over positive numbers

## Successor

Fixpoint succ x :=
match x with
| p~1 => (succ p)~0
| p~0 => p~1
| 1 => 1~0
end.

## Addition

Fixpoint add x y :=
match x, y with
| p~1, q~1 => (add_carry p q)~0
| p~1, q~0 => (add p q)~1
| p~1, 1 => (succ p)~0
| p~0, q~1 => (add p q)~1
| p~0, q~0 => (add p q)~0
| p~0, 1 => p~1
| 1, q~1 => (succ q)~0
| 1, q~0 => q~1
| 1, 1 => 1~0
end
with add_carry x y :=
match x, y with
| p~1, q~1 => (add_carry p q)~1
| p~1, q~0 => (add_carry p q)~0
| p~1, 1 => (succ p)~1
| p~0, q~1 => (add_carry p q)~0
| p~0, q~0 => (add p q)~1
| p~0, 1 => (succ p)~0
| 1, q~1 => (succ q)~1
| 1, q~0 => (succ q)~0
| 1, 1 => 1~1
end.
Infix "+" := add : positive_scope.

## Operation x -> 2*x-1

Fixpoint pred_double x :=
match x with
| p~1 => p~0~1
| p~0 => (pred_double p)~1
| 1 => 1
end.

## Predecessor

Definition pred x :=
match x with
| p~1 => p~0
| p~0 => pred_double p
| 1 => 1
end.

## The predecessor of a positive number can be seen as a N

Definition pred_N x :=
match x with
| p~1 => Npos (p~0)
| p~0 => Npos (pred_double p)
| 1 => N0
end.

## An auxiliary type for subtraction

Inductive mask : Set :=
| IsNul : mask
| IsPos : positive -> mask
| IsNeg : mask.

## Operation x -> 2*x+1

Definition succ_double_mask (x:mask) : mask :=
match x with
| IsNul => IsPos 1
| IsNeg => IsNeg
| IsPos p => IsPos p~1
end.

## Operation x -> 2*x

Definition double_mask (x:mask) : mask :=
match x with
| IsNul => IsNul
| IsNeg => IsNeg
| IsPos p => IsPos p~0
end.

## Operation x -> 2*x-2

Definition double_pred_mask x : mask :=
match x with
| p~1 => IsPos p~0~0
| p~0 => IsPos (pred_double p)~0
| 1 => IsNul
end.

## Predecessor with mask

Definition pred_mask (p : mask) : mask :=
match p with
| IsPos 1 => IsNul
| IsPos q => IsPos (pred q)
| IsNul => IsNeg
| IsNeg => IsNeg
end.

## Subtraction, result as a mask

Fixpoint sub_mask (x y:positive) {struct y} : mask :=
match x, y with
| p~1, q~1 => double_mask (sub_mask p q)
| p~1, q~0 => succ_double_mask (sub_mask p q)
| p~1, 1 => IsPos p~0
| p~0, q~1 => succ_double_mask (sub_mask_carry p q)
| p~0, q~0 => double_mask (sub_mask p q)
| p~0, 1 => IsPos (pred_double p)
| 1, 1 => IsNul
| 1, _ => IsNeg
end
with sub_mask_carry (x y:positive) {struct y} : mask :=
match x, y with
| p~1, q~1 => succ_double_mask (sub_mask_carry p q)
| p~1, q~0 => double_mask (sub_mask p q)
| p~1, 1 => IsPos (pred_double p)
| p~0, q~1 => double_mask (sub_mask_carry p q)
| p~0, q~0 => succ_double_mask (sub_mask_carry p q)
| p~0, 1 => double_pred_mask p
| 1, _ => IsNeg
end.

## Subtraction, result as a positive, returning 1 if x<=y

Definition sub x y :=
match sub_mask x y with
| IsPos z => z
| _ => 1
end.
Infix "-" := sub : positive_scope.

## Multiplication

Fixpoint mul x y :=
match x with
| p~1 => y + (mul p y)~0
| p~0 => (mul p y)~0
| 1 => y
end.
Infix "*" := mul : positive_scope.

## Iteration over a positive number

Fixpoint iter (n:positive) {A} (f:A -> A) (x:A) : A :=
match n with
| xH => f x
| xO n' => iter n' f (iter n' f x)
| xI n' => f (iter n' f (iter n' f x))
end.

## Power

Definition pow (x y:positive) := iter y (mul x) 1.
Infix "^" := pow : positive_scope.

## Square

Fixpoint square p :=
match p with
| p~1 => (square p + p)~0~1
| p~0 => (square p)~0~0
| 1 => 1
end.

## Division by 2 rounded below but for 1

Definition div2 p :=
match p with
| 1 => 1
| p~0 => p
| p~1 => p
end.
Division by 2 rounded up
Definition div2_up p :=
match p with
| 1 => 1
| p~0 => p
| p~1 => succ p
end.

## Number of digits in a positive number

Fixpoint size_nat p : nat :=
match p with
| 1 => S O
| p~1 => S (size_nat p)
| p~0 => S (size_nat p)
end.
Same, with positive output
Fixpoint size p :=
match p with
| 1 => 1
| p~1 => succ (size p)
| p~0 => succ (size p)
end.

## Comparison on binary positive numbers

Fixpoint compare_cont (x y:positive) (r:comparison) {struct y} : comparison :=
match x, y with
| p~1, q~1 => compare_cont p q r
| p~1, q~0 => compare_cont p q Gt
| p~1, 1 => Gt
| p~0, q~1 => compare_cont p q Lt
| p~0, q~0 => compare_cont p q r
| p~0, 1 => Gt
| 1, q~1 => Lt
| 1, q~0 => Lt
| 1, 1 => r
end.
Definition compare x y := compare_cont x y Eq.
Infix "?=" := compare (at level 70, no associativity) : positive_scope.
Definition min p p' :=
match p ?= p' with
| Lt | Eq => p
| Gt => p'
end.
Definition max p p' :=
match p ?= p' with
| Lt | Eq => p'
| Gt => p
end.

## Boolean equality and comparisons

Fixpoint eqb p q {struct q} :=
match p, q with
| p~1, q~1 => eqb p q
| p~0, q~0 => eqb p q
| 1, 1 => true
| _, _ => false
end.
Definition leb x y :=
match x ?= y with Gt => false | _ => true end.
Definition ltb x y :=
match x ?= y with Lt => true | _ => false end.
Infix "=?" := eqb (at level 70, no associativity) : positive_scope.
Infix "<=?" := leb (at level 70, no associativity) : positive_scope.
Infix "<?" := ltb (at level 70, no associativity) : positive_scope.

## A Square Root function for positive numbers

We procede by blocks of two digits : if p is written qbb' then sqrt(p) will be sqrt(q)~0 or sqrt(q)~1. For deciding easily in which case we are, we store the remainder (as a mask, since it can be null). Instead of copy-pasting the following code four times, we factorize as an auxiliary function, with f and g being either xO or xI depending of the initial digits. NB: (sub_mask (g (f 1)) 4) is a hack, morally it's g (f 0).
Definition sqrtrem_step (f g:positive->positive) p :=
match p with
| (s, IsPos r) =>
let s' := s~0~1 in
let r' := g (f r) in
if s' <=? r' then (s~1, sub_mask r' s')
else (s~0, IsPos r')
| (s,_) => (s~0, sub_mask (g (f 1)) 4)
end.
Fixpoint sqrtrem p : positive * mask :=
match p with
| 1 => (1,IsNul)
| 2 => (1,IsPos 1)
| 3 => (1,IsPos 2)
| p~0~0 => sqrtrem_step xO xO (sqrtrem p)
| p~0~1 => sqrtrem_step xO xI (sqrtrem p)
| p~1~0 => sqrtrem_step xI xO (sqrtrem p)
| p~1~1 => sqrtrem_step xI xI (sqrtrem p)
end.
Definition sqrt p := fst (sqrtrem p).

## Greatest Common Divisor

Definition divide p q := exists r, q = r*p.
Notation "( p | q )" := (divide p q) (at level 0) : positive_scope.
Instead of the Euclid algorithm, we use here the Stein binary algorithm, which is faster for this representation. This algorithm is almost structural, but in the last cases we do some recursive calls on subtraction, hence the need for a counter.
Fixpoint gcdn (n : nat) (a b : positive) : positive :=
match n with
| O => 1
| S n =>
match a,b with
| 1, _ => 1
| _, 1 => 1
| a~0, b~0 => (gcdn n a b)~0
| _ , b~0 => gcdn n a b
| a~0, _ => gcdn n a b
| a'~1, b'~1 =>
match a' ?= b' with
| Eq => a
| Lt => gcdn n (b'-a') a
| Gt => gcdn n (a'-b') b
end
end
end.
We'll show later that we need at most (log2(a.b)) loops
Definition gcd (a b : positive) := gcdn (size_nat a + size_nat b)%nat a b.
Generalized Gcd, also computing the division of a and b by the gcd
Fixpoint ggcdn (n : nat) (a b : positive) : (positive*(positive*positive)) :=
match n with
| O => (1,(a,b))
| S n =>
match a,b with
| 1, _ => (1,(1,b))
| _, 1 => (1,(a,1))
| a~0, b~0 =>
let (g,p) := ggcdn n a b in
(g~0,p)
| _, b~0 =>
let '(g,(aa,bb)) := ggcdn n a b in
(g,(aa, bb~0))
| a~0, _ =>
let '(g,(aa,bb)) := ggcdn n a b in
(g,(aa~0, bb))
| a'~1, b'~1 =>
match a' ?= b' with
| Eq => (a,(1,1))
| Lt =>
let '(g,(ba,aa)) := ggcdn n (b'-a') a in
(g,(aa, aa + ba~0))
| Gt =>
let '(g,(ab,bb)) := ggcdn n (a'-b') b in
(g,(bb + ab~0, bb))
end
end
end.
Definition ggcd (a b: positive) := ggcdn (size_nat a + size_nat b)%nat a b.
Local copies of the not-yet-available N.double and N.succ_double
Definition Nsucc_double x :=
match x with
| N0 => Npos 1
| Npos p => Npos p~1
end.
Definition Ndouble n :=
match n with
| N0 => N0
| Npos p => Npos p~0
end.
Operation over bits.
Logical or
Fixpoint lor (p q : positive) : positive :=
match p, q with
| 1, q~0 => q~1
| 1, _ => q
| p~0, 1 => p~1
| _, 1 => p
| p~0, q~0 => (lor p q)~0
| p~0, q~1 => (lor p q)~1
| p~1, q~0 => (lor p q)~1
| p~1, q~1 => (lor p q)~1
end.
Logical and
Fixpoint land (p q : positive) : N :=
match p, q with
| 1, q~0 => N0
| 1, _ => Npos 1
| p~0, 1 => N0
| _, 1 => Npos 1
| p~0, q~0 => Ndouble (land p q)
| p~0, q~1 => Ndouble (land p q)
| p~1, q~0 => Ndouble (land p q)
| p~1, q~1 => Nsucc_double (land p q)
end.
Logical diff
Fixpoint ldiff (p q:positive) : N :=
match p, q with
| 1, q~0 => Npos 1
| 1, _ => N0
| _~0, 1 => Npos p
| p~1, 1 => Npos (p~0)
| p~0, q~0 => Ndouble (ldiff p q)
| p~0, q~1 => Ndouble (ldiff p q)
| p~1, q~1 => Ndouble (ldiff p q)
| p~1, q~0 => Nsucc_double (ldiff p q)
end.
xor
Fixpoint lxor (p q:positive) : N :=
match p, q with
| 1, 1 => N0
| 1, q~0 => Npos (q~1)
| 1, q~1 => Npos (q~0)
| p~0, 1 => Npos (p~1)
| p~0, q~0 => Ndouble (lxor p q)
| p~0, q~1 => Nsucc_double (lxor p q)
| p~1, 1 => Npos (p~0)
| p~1, q~0 => Nsucc_double (lxor p q)
| p~1, q~1 => Ndouble (lxor p q)
end.
Shifts. NB: right shift of 1 stays at 1.
Definition shiftl_nat (p:positive)(n:nat) := nat_iter n xO p.
Definition shiftr_nat (p:positive)(n:nat) := nat_iter n div2 p.
Definition shiftl (p:positive)(n:N) :=
match n with
| N0 => p
| Npos n => iter n xO p
end.
Definition shiftr (p:positive)(n:N) :=
match n with
| N0 => p
| Npos n => iter n div2 p
end.
Checking whether a particular bit is set or not
Fixpoint testbit_nat (p:positive) : nat -> bool :=
match p with
| 1 => fun n => match n with
| O => true
| S _ => false
end
| p~0 => fun n => match n with
| O => false
| S n' => testbit_nat p n'
end
| p~1 => fun n => match n with
| O => true
| S n' => testbit_nat p n'
end
end.
Same, but with index in N
Fixpoint testbit (p:positive)(n:N) :=
match p, n with
| p~0, N0 => false
| _, N0 => true
| 1, _ => false
| p~0, Npos n => testbit p (pred_N n)
| p~1, Npos n => testbit p (pred_N n)
end.

## From binary positive numbers to Peano natural numbers

Definition iter_op {A}(op:A->A->A) :=
fix iter (p:positive)(a:A) : A :=
match p with
| 1 => a
| p~0 => iter p (op a a)
| p~1 => op a (iter p (op a a))
end.
Definition to_nat (x:positive) : nat := iter_op plus x (S O).

## From Peano natural numbers to binary positive numbers

A version preserving positive numbers, and sending 0 to 1.
Fixpoint of_nat (n:nat) : positive :=
match n with
| O => 1
| S O => 1
| S x => succ (of_nat x)
end.
Fixpoint of_succ_nat (n:nat) : positive :=
match n with
| O => 1
| S x => succ (of_succ_nat x)
end.
End Pos.