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Function >>/2
Availability:Arithmetic function (see is/2)
[ISO]+IntExpr1 >> +IntExpr2
Bitwise shift IntExpr1 by IntExpr2 bits to the right. The ISO standard dictates shifting a negative value is implementation defined. SWI-Prolog defines shifting negative integers to be defined as -(-Int>>Shift). Shifting positive integers by more than their size results in 0 (zero). Shifting negative integers by more then their size results in -1. I.e., A is -3464 >> 100 binds A to -1. If IntExpr2 is negative, a right shift (see >>/2) is performed with the negated value of IntExpr2.
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  • bit_operation
  • bit_shift_operation
LogicalCaptain said (2020-11-23T23:31:01):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

Here is some functionality to transform integers into lists of 0,1 and the converse:

bitlist.pl

LogicalCaptain said (2020-11-23T21:47:45):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

The underlying bit pattern seems to be 2-complement with an infinite number of bits "to the left" (because we can have arbitrarily large numbers):

:- debug(bitshift_them).

bitshift_them(How,MaxBits) :-
   must_be(oneof([neg,pos]),How),
   must_be(nonneg,MaxBits),
   ((How==neg) -> Mult = -1 ; Mult = +1),
   bitshift_them(Mult,1,1,MaxBits).

bitshift_them(_,_,Bits,Bits) :- !.

bitshift_them(Mult,TwoExpN,Bits,MaxBits) :-
   Bits < MaxBits,!,
   P is Mult*TwoExpN,
   S is P>>1,
   debug(bitshift_them,"Shifted ~d (~d bits) 'rightwards' by ~d to get ~d",[P,Bits,1,S]),
   ((Mult > 0)
      -> assertion(((P == 1, S == 0); 2*S =:= P))
      ;  assertion(((P == -1, S == -1); (P == -2, S == -1); (2*S =:= P)))
   ),
   BitsNext is Bits+1,
   TwoExpNNext is TwoExpN*2,
   bitshift_them(Mult,TwoExpNNext,BitsNext,MaxBits).

Then:

?- bitshift_them(pos,100).
% Shifted 1 (1 bits) 'rightwards' by 1 to get 0
% Shifted 2 (2 bits) 'rightwards' by 1 to get 1
% Shifted 4 (3 bits) 'rightwards' by 1 to get 2
% Shifted 8 (4 bits) 'rightwards' by 1 to get 4
% Shifted 16 (5 bits) 'rightwards' by 1 to get 8
...
% Shifted 39614081257132168796771975168 (96 bits) 'rightwards' by 1 to get 19807040628566084398385987584
% Shifted 79228162514264337593543950336 (97 bits) 'rightwards' by 1 to get 39614081257132168796771975168
% Shifted 158456325028528675187087900672 (98 bits) 'rightwards' by 1 to get 79228162514264337593543950336
% Shifted 316912650057057350374175801344 (99 bits) 'rightwards' by 1 to get 158456325028528675187087900672
true.
?- bitshift_them(neg,100).
% Shifted -1 (1 bits) 'rightwards' by 1 to get -1
% Shifted -2 (2 bits) 'rightwards' by 1 to get -1
% Shifted -4 (3 bits) 'rightwards' by 1 to get -2
% Shifted -8 (4 bits) 'rightwards' by 1 to get -4
% Shifted -16 (5 bits) 'rightwards' by 1 to get -8
...
% Shifted -39614081257132168796771975168 (96 bits) 'rightwards' by 1 to get -19807040628566084398385987584
% Shifted -79228162514264337593543950336 (97 bits) 'rightwards' by 1 to get -39614081257132168796771975168
% Shifted -158456325028528675187087900672 (98 bits) 'rightwards' by 1 to get -79228162514264337593543950336
% Shifted -316912650057057350374175801344 (99 bits) 'rightwards' by 1 to get -158456325028528675187087900672
true.