ETOOBUSY 🚀 minimal blogging for the impatient
AoC 2021/01 - Up and down
TL;DR
On with Advent of Code puzzle 01 from 2021: some tricky comparisons.
The 2021 edition of Advent of Code started with a trick. In hindsight, it was a sort of manifesto: there will be a lot of brute forcing to do, but you might find a clever solution every now and then.
And so… I was gullible enough to fall in the trap, and implemented it the full way:
#!/usr/bin/env raku
use v6;
sub MAIN ($filename = $?FILE.subst(/\.raku$/, '.sample')) {
my $inputs = get-inputs($filename);
my ($part1, $part2) = solve($inputs);
my $highlight = "\e[1;97;45m";
my $reset = "\e[0m";
put "part1 $highlight$part1$reset";
put "part2 $highlight$part2$reset";
}
sub get-inputs ($filename) {
$filename.IO.basename.IO.lines.Array;
} ## end sub get_inputs ($filename = undef)
sub solve ($inputs) {
return (part1($inputs), part2($inputs));
}
sub count-increases (@inputs) {
my $count = (1 .. @inputs.end)
.map({@inputs[$_] > @inputs[$_ - 1] ?? 1 !! 0 })
.sum;
}
sub part1 ($inputs) { return count-increases($inputs) }
sub part2 ($inputs) {
return count-increases(
(1 ..^ $inputs.end).map({$inputs[($_-1)..($_+1)].sum})
);
}
The trap is in part 2. I’ve been tricked into calculating the sliding window sum, but it was not needed. Consider four consecutive values, yielding two values to be compared:
...,xn,xn+1,xn+2,xn+3,...The two sums would be:
Sn=xn+xn+1+xn+2Sn+1=xn+1+xn+2+xn+3There’s a lot of similarity between the two… because they share two items out of three. So the comparison can be simplified like this:
Sn≷So… no need to do sums, it’s sufficient to skip two items for doing the comparison!
Stay safe folks!