RabbitFarm

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

Part 1

You are given a positive integer $N less than or equal to 255. Write a script to swap the odd positioned bits with the even positioned bits and print the decimal equivalent of the new binary representation.

Solution

use strict;
use warnings;
sub swap_bits{
    my($n) = @_;
    my $bits = substr(unpack("B32", pack("N", shift)), 24, 8);
    my @bits = split(//, $bits);
    for(my $i = 0; $i < @bits; $i += 2){
        @bits[$i, $i + 1] = @bits[$i + 1, $i]; 
    }  
    my $swapped_decimal = unpack("N", pack("B32", substr("0" x 32 . join("", @bits), -32)));
    return $swapped_decimal; 
}

MAIN:{
    my $N;
    $N = 101; 
    print swap_bits($N) . "\n";
    $N = 18; 
    print swap_bits($N) . "\n";
}   

Sample Run

$ perl perl/ch-1.pl
154
33

Notes

This code re-uses much of the code from last week's challenge solution. The only difference here is the for loop which swaps the even/odd bits.

Part 2

You are given time $T in the format hh:mm. Write a script to find the smaller angle formed by the hands of an analog clock at a given time.

Solution

use strict;
use warnings;
sub clock_angle{
    my($h, $m) = split(/:/, $_[0]);
    my $angle = abs(0.5 * (60 * $h - 11 * $m)); 
    $angle = 360 - $angle if $angle > 180; 
    return $angle;
}

MAIN:{
    my $T;
    $T = "03:10";  
    print clock_angle($T) . "\n";  
    $T = "04:00";  
    print clock_angle($T) . "\n";  
}

Sample Run

$ perl perl/ch-1.pl
35
120

Notes

Perhaps not a whole lot going on here: the time is broken into hour and minute parts and then the angle is computed directly from those values.

References

Challenge 120

The examples used here are from the weekly challenge problem statement and demonstrate the working solution.

Part 1

You are given a positive integer $N less than or equal to 255. Write a script to swap the odd positioned bits with the even positioned bits and print the decimal equivalent of the new binary representation.

Solution

:-initialization(main).

pad(Bits, Padded):-
    length(Bits, L),
    PadLength is 8 - L, 
    length(Padding, PadLength),  
    maplist(=(0), Padding),
    append(Padding, Bits, Padded).  

bits(N, Bits):-
    bits(N, [], Bits).
bits(0, Bits, Bits).    
bits(N, Bit_Accum, Bits):-    
    B is N /\ 1,
    N0 is N >> 1,
    bits(N0, [B|Bit_Accum], Bits).

swap([], []). 
swap([H0, H1|T], [H1, H0|ST]):-
    swap(T, ST).

decimal(Bits, Decimal):-
    decimal(Bits, 0, Decimal).
decimal([], Decimal, Decimal).
decimal([H|T], DecimalAccum, Decimal):-
    length([H|T], B), 
    D is (H * 2 ** (B - 1)) + DecimalAccum,
    decimal(T, D, Decimal).  

main:-
    bits(18, B),
    pad(B, Padded),  
    swap(Padded, Swapped), 
    decimal(Swapped, Decimal), 
    write(Decimal), nl,
    halt.

Sample Run

$ gprolog --consult-file prolog/ch-1.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
Copyright (C) 1999-2020 Daniel Diaz
compiling /home/adamcrussell/Projects/perlweeklychallenge-club/challenge-120/adam-russell/prolog/ch-1.p for byte code...
/home/adamcrussell/Projects/perlweeklychallenge-club/challenge-120/adam-russell/prolog/ch-1.p compiled, 36 lines read - 3978 bytes written, 98 ms
33.0

Notes

Working with bitwise operators in Prolog is something I have done a bit before. This code most notably borrows from a solution to Challenge 079. Here that set_bit/2 predicate is repurposed as bits/2 which returns a list of bits for any given number.

The list of bits must be padded (pad/2), the bits swapped (swap/2), and then the decimal value of the swapped bits calculated (decimal/2).

Part 2

You are given time $T in the format hh:mm. Write a script to find the smaller angle formed by the hands of an analog clock at a given time.

Solution

:-initialization(main).

clock_angle(Time, Angle):-
    append(H, [58|M], Time),  
    number_codes(Hour, H),
    number_codes(Minutes, M),
    A is abs(0.5 * (60 * Hour - 11 * Minutes)), 
    ((A > 180, Angle is 360 - A); Angle = A). 

main:-
    clock_angle("03:10", Angle),
    write(Angle), nl,  
    halt. 

Sample Run

$ gprolog --consult-file prolog/ch-2.p
GNU Prolog 1.4.5 (32 bits)
Compiled Dec  3 2020, 00:37:14 with gcc
By Daniel Diaz
Copyright (C) 1999-2020 Daniel Diaz
compiling /home/adamcrussell/Projects/perlweeklychallenge-club/challenge-120/adam-russell/prolog/ch-2.p for byte code...
/home/adamcrussell/Projects/perlweeklychallenge-club/challenge-120/adam-russell/prolog/ch-2.p compiled, 13 lines read - 2384 bytes written, 40 ms
35.0

Notes

clock_angle/2 starts out by splitting the hh:mm formatted time into the hour and minute parts, by way of append. 58 is the character code for the ':'. Once this is done, and that is perhaps the most prological part of the code, the formula for the angle is applied.

References

Challenge 120