# ETOOBUSY ðŸš€ minimal blogging for the impatient

# PWC118 - Binary Palindrome

**TL;DR**

Here we are with TASK #1 from the Perl Weekly Challenge #118. Enjoy!

# The challenge

You are given a positive integer

`$N`

.Write a script to find out if the binary representation of the given integer is Palindrome. Print 1 if it is otherwise 0.

Example`Input: $N = 5 Output: 1 as binary representation of 5 is 101 which is Palindrome. Input: $N = 4 Output: 0 as binary representation of 4 is 100 which is NOT Palindrome.`

# The questions

One question is about the exact representation of the integer as a
decimal number. OK, *usually* any leading `0`

is ignored, but in base-2
this means ruling out all even numbers in a single stroke! Whatever, the
example seems explicit to this regard.

# The solution

Raku first, in the spirit of taking the challenge in the new language and try to learn something:

```
#!/usr/bin/env raku
use v6;
sub binary-palindrome (Int:D $N where * > 0 --> Bool) {
return False if $N %% 2;
my ($M, $n) = (0, $N);
($M, $n) = (($M +< 1) +| ($n +& 1), $n +> 1) while $n > 0;
return so $M == $N;
}
sub MAIN (*@args is copy) {
@args = 1 .. 31;
put $_, ' -> ', binary-palindrome($_) ?? 1 !! 0 for @args;
}
```

The approach chosen is to stick to bitwise operations instead of turning
the integers into a *string* representation and then check for
palindrome*ness*.

So what weâ€™re doing here is to *reverse* the input number `$N`

and check
whether the result (which is `$M`

, by the way) is the same as `$N`

.

One tricky part here was using the *correct* operators. Coming from
Perl, binary operations are `|`

and `&`

and the bitshifts are `<<`

and `>>`

.

The story in Raku is different though, so we have respectively `+|`

,
`+&`

, `+<`

, and `+>`

. The error messages were great to this regard.

Porting the solution to Perl is easy:

```
#!/usr/bin/env perl
use 5.024;
use warnings;
use experimental qw< postderef signatures >;
no warnings qw< experimental::postderef experimental::signatures >;
sub binary_palindrome ($N) {
die "invalid $N (positive integers are OK)\n"
if $N !~ m{\A [1-9]\d* \z}mxs;
return unless $N % 2;
my ($M, $n) = (0, $N);
($M, $n) = (($M << 1) | ($n & 1), $n >> 1) while $n > 0;
return $M == $N;
}
my @args = @ARGV ? @ARGV : 1 .. 31;
say $_, ' -> ', binary_palindrome($_) ? 1 : 0 for @args;
```

Well, thatâ€™s on the brink of *plagiarism*!

I decided to put a validation for the input (which I usually avoid) just to make the two solutions more aligned (Raku signatures allow setting validations so itâ€™s almost a sin to not put them).

Then, of course, thereâ€™s no *is-divisible-by* operator `%%`

in Perl,
so we have to resort to inverting the result of the *rest-by* operator
`%`

:

```
# Raku: is $N divisible by 2?
return False if $N %% 2;
# Perl: does integer division of $N by 2 have no rest?
return unless $N % 2;
```

No big deal, but itâ€™s good to be able and be precise in Raku.

Well.. enough for today, have fun and stay safe!

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