Bit Shifting

Question: Bit shifting is a low level operation that shifts all the bits of a number to either the left or right direction. Shifting a number's bits one place to the left will double the number. Shifting a number's bits one place to the right will halve the number. This bitwise operation performs much faster than the standard multiplication/division operations. Write the functions 'lshift' and 'rshift' in C that takes in two q objects (list or single long integer to represent number(s) to shift, single long integer to represent number of bits to shift) and shifts the number(s) that many bits to the left or right, respectively. This function should be exportable to q.

More Information:

https://en.wikipedia.org/wiki/Bitwise_operation#Bit_shifts

Example

                                
                                q).q.lshift:`shift 2: (`lshift;2) 
q).q.rshift:`shift 2: (`rshift;2) 
q)3 lshift 2 
12 
q)12 rshift 2 
3 
q)\t do[1000000;{x*2}/[20;1 2 3 4 5 6 7 8 9]] 
1958 
q)\t do[1000000;lshift[1 2 3 4 5 6 7 8 9;20]] 
86 
q)({x*2}/[20;1 2 3 4 5 6 7 8 9])~lshift[1 2 3 4 5 6 7 8 9;20] 
1b

Solution

                                
                                ######## shift.c ########

//gcc -shared -fPIC -DKXVER=3 shift.c -o shift.so -Wall
#include "k.h"
#include <stddef.h>

K lshift(K num, K n) {
    int t = num->t; 
    if(!((t == KJ) || (t == -KJ))) return krr("type");
    if(!(n->t == -KJ)) return krr("type");
    K result;
    if(t == KJ) {
        result = ktn(KJ, num->n);
        for(size_t i=0;i<num->n;++i){
            kJ(result)[i] = kJ(num)[i]<<n->j;
        }
    }
    else {
        result = kj((num->j)<<n->j);
    }
    return result;
}

K rshift(K num, K n) {
    int t = num->t; 
    if(!((t == KJ) || (t == -KJ))) return krr("type");
    if(!(n->t == -KJ)) return krr("type");
    K result;
    if(t == KJ) {
        result = ktn(KJ, num->n);
        for(size_t i=0;i<num->n;++i){
            kJ(result)[i] = kJ(num)[i]<<n->j;
        }
    }
    else {
        result = kj((num->j)>>n->j);
    }
    return result;
}

######## shift.q ########

.q.lshift:`shift 2: (`lshift;2);
.q.rshift:`shift 2: (`rshift;2);

Explanation: Create a function that takes in two K objects. Check their types, if either are not integers then return an error (ex. 'type'). If the first parameter is a list, iterate over the list. Perform a bitwise logical shift of the first parameter to N places (specified by second parameter) to the left or right with the '<<' or '>>' operator (respectively) and return the result as a K object.

Code:

https://bitbucket.org/alvikabir/dailyq/src/master/Intermediate/bit-shifting

Tags:

c

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daily q

Bit Shifting

Example

Solution

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