Advent Calendar 2021
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The gift is presented by Arne Sommer. Today he is talking about his solution to “The Weekly Challenge - 097”. This is re-produced for Advent Calendar 2021 from the original post by Arne Sommer.
Task #1: Caesar Cipher
You are given string $S containing alphabets A..Z only and a number $N.
Write a script to encrypt the given string $S using Caesar Cipher with left shift of size $N.
The expression alphabets A..Z only is wrong, as the example has several spaces as well. So they should be allowed.
File: caesar-cipher
#! /usr/bin/env raku
subset AZ-space of Str where /^ <[ A .. Z \s ]>+ $/; # [1]
subset PosInt of Int where -25 <= $_ <= 25; # [2]
unit sub MAIN (AZ-space $S = 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG',
PosInt $N = 3); # [3]
say $S.comb.map({ caesar($_, $N) }).join; # [4]
sub caesar ($char, $shift)
{
return $char if $char eq " "; # [5]
my $code = $char.ord; # [6]
$code -= $shift; # [7]
$code += 26 if $code < 65; # 'A' # [8]
$code -= 26 if $code > 90; # 'Z' # [8a]
return $code.chr; # [9]
}
[1] The allowed characters (or «domain specific alphabet»).
[2] The challenge says that the left shift value is a number. It does not make sense to allow anything other than integers, so I restrict the value to that type. Negative values should be ok, and they mean a right shift value (instead of left).
[3] The arguments, with default values as given in the challenge.
[4] Split the string into single charactes (with comb, apply the «caesar» function on each one (with map), join the characters together as a string again (with join), and print it.
[5] Do not shift spaces.
[6] Get the character codepoint.
[7] Subtract the shift value (as we shift to the left, or lower in the alphabet).
[8] Wrap around if we shift out of the A-Z range, here lower - or higher in [8b]
[9] Get the character with the specified codepoint.
See docs.raku.org/routine/ord for more information about ord.
See docs.raku.org/routine/chr for more information about chr.
Running it:
$ ./caesar-cipher 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' 3
QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
$ ./caesar-cipher 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' -3
WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ
$ ./caesar-cipher 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' 13
GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT
$ ./caesar-cipher 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' -13
GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT
Raku has a ords variant that takes a whole string, and not a single character as ord. And chrs which takes an array of codepoints and turns them into a string, and not a single codepoint to a character as chr. Let us use them to write a shorter program:
File: caesar-cipher-map
#! /usr/bin/env raku
subset AZ-space of Str where /^ <[ A .. Z \s ]>+ $/;
subset PosInt of Int where -25 <= $_ <= 25;
unit sub MAIN (AZ-space $S = 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG',
PosInt $N = 3);
say caesar($S, $N);
sub caesar ($string, $shift)
{
return $string.ords.map({$_ == 32 ?? 32 !! (($_ - $shift - 65) % 26 ) + 65}).chrs;
# #################### # 1a ############# ############ # 1b # 1c ## 1d
}
[1] We use map to change the individual codepoints. We let the spaces with codepoint 32 alone [1a]. Every other value we reduce to a number between 0 and 25 (by subtracting the codepoint of the first letter (A: 65) and the shift value [1b]. The modulo operator (%) takes care of negative values for us, doing the right thing. E.g. -2 % 26 -> 24 [1c]. Then we add adjust the values up to where they should be (A to Z) [1d] before we turn the whole array of codepints into a string.
See docs.raku.org/routine/ords for more information about ords.
See docs.raku.org/routine/chrs for more information about chrs.
Running it gives the same result as before:
$ ./caesar-cipher-map 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' 3
QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
$ ./caesar-cipher-map 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' -3
WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ
$ ./caesar-cipher-map 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' 13
GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT
$ ./caesar-cipher-map 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' -13
GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT
A Perl Version
This is straight forward translation of the first Raku version.
File: caesar-cipher-perl
#! /usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use feature 'signatures';
no warnings "experimental::signatures";
my $S = shift(@ARGV) // 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG';
die "Illegal characters" unless $S =~ /^[A-Z\s]+$/;
my $N = shift(@ARGV) // 3;
die "Illegal shift $N" if $N !~ /^\-?\d+$/ || $N < -25 || $N > 25;
say join("", map { caesar($_, $N) } split(//, $S));
sub caesar ($char, $shift)
{
return $char if $char eq " ";
my $code = ord($char);
$code -= $shift;
$code += 26 if $code < 65; # 'A'
$code -= 26 if $code > 90; # 'Z'
return chr($code);
}
Running it gives the same result as the Raku version:
$ ./caesar-cipher-perl 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' 3
QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
$ ./caesar-cipher-perl 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' -3
WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ
$ ./caesar-cipher-perl 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' 13
GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT
$ ./caesar-cipher-perl 'THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG' -13
GUR DHVPX OEBJA SBK WHZCF BIRE GUR YNML QBT
If you have any suggestion then please do share with us perlweeklychallenge@yahoo.com.