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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, which allows mimicking 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). 
Local Notation "1" := xH.

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.

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

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

Power

Definition pow (x:positive) := iter (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 (r:comparison) (x y:positive) {struct y} : comparison :=
  match x, y with
    | p~1, q~1 => compare_cont r p q
    | p~1, q~0 => compare_cont Gt p q
    | p~1, 1 => Gt
    | p~0, q~1 => compare_cont Lt p q
    | p~0, q~0 => compare_cont r p q
    | p~0, 1 => Gt
    | 1, q~1 => Lt
    | 1, q~0 => Lt
    | 1, 1 => r
  end.

Definition compare := compare_cont 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 proceed 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)) 1~0~0)
 end.

Fixpoint sqrtrem p : positive * mask :=
 match p with
  | 1 => (1,IsNul)
  | 1~0 => (1,IsPos 1)
  | 1~1 => (1,IsPos 1~0)
  | 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 
Set Printing Universes.
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) := nat_rect _ p (fun _ => xO).
Definition shiftr_nat (p:positive) := nat_rect _ p (fun _ => div2).

Definition shiftl (p:positive)(n:N) :=
  match n with
    | N0 => p
    | Npos n => iter xO p n
  end.

Definition shiftr (p:positive)(n:N) :=
  match n with
    | N0 => p
    | Npos n => iter div2 p n
  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).
Arguments to_nat x: simpl never.

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.

(* Another version that converts [n] into [n+1] *)

Fixpoint of_succ_nat (n:nat) : positive :=
  match n with
    | O => 1
    | S x => succ (of_succ_nat x)
  end.

Conversion with a decimal representation for printing/parsing

Notation ten := 1~0~1~0.

Fixpoint of_uint_acc (d:Decimal.uint)(acc:positive) :=
  match d with
  | Decimal.Nil => acc
  | Decimal.D0 l => of_uint_acc l (mul ten acc)
  | Decimal.D1 l => of_uint_acc l (add 1 (mul ten acc))
  | Decimal.D2 l => of_uint_acc l (add 1~0 (mul ten acc))
  | Decimal.D3 l => of_uint_acc l (add 1~1 (mul ten acc))
  | Decimal.D4 l => of_uint_acc l (add 1~0~0 (mul ten acc))
  | Decimal.D5 l => of_uint_acc l (add 1~0~1 (mul ten acc))
  | Decimal.D6 l => of_uint_acc l (add 1~1~0 (mul ten acc))
  | Decimal.D7 l => of_uint_acc l (add 1~1~1 (mul ten acc))
  | Decimal.D8 l => of_uint_acc l (add 1~0~0~0 (mul ten acc))
  | Decimal.D9 l => of_uint_acc l (add 1~0~0~1 (mul ten acc))
  end.

Fixpoint of_uint (d:Decimal.uint) : N :=
  match d with
  | Decimal.Nil => N0
  | Decimal.D0 l => of_uint l
  | Decimal.D1 l => Npos (of_uint_acc l 1)
  | Decimal.D2 l => Npos (of_uint_acc l 1~0)
  | Decimal.D3 l => Npos (of_uint_acc l 1~1)
  | Decimal.D4 l => Npos (of_uint_acc l 1~0~0)
  | Decimal.D5 l => Npos (of_uint_acc l 1~0~1)
  | Decimal.D6 l => Npos (of_uint_acc l 1~1~0)
  | Decimal.D7 l => Npos (of_uint_acc l 1~1~1)
  | Decimal.D8 l => Npos (of_uint_acc l 1~0~0~0)
  | Decimal.D9 l => Npos (of_uint_acc l 1~0~0~1)
  end.

Definition of_int (d:Decimal.int) : option positive :=
  match d with
  | Decimal.Pos d =>
    match of_uint d with
    | N0 => None
    | Npos p => Some p
    end
  | Decimal.Neg _ => None
  end.

Fixpoint to_little_uint p :=
  match p with
  | 1 => Decimal.D1 Decimal.Nil
  | p~1 => Decimal.Little.succ_double (to_little_uint p)
  | p~0 => Decimal.Little.double (to_little_uint p)
  end.

Definition to_uint p := Decimal.rev (to_little_uint p).

Definition to_int n := Decimal.Pos (to_uint n).

Numeral Notation positive of_int to_uint : positive_scope.

End Pos.
Re-export the notation for those who just Import BinPosDef 
Numeral Notation positive Pos.of_int Pos.to_uint : positive_scope.