# RabbitFarm

### 2021-07-11

#### Swapping Bits / Time Angle

*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

posted at: 17:41 by: Adam Russell | path: /perl | permanent link to this entry

#### The Weekly Challenge 120 (Prolog Solutions)

*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

posted at: 14:37 by: Adam Russell | path: /prolog | permanent link to this entry