Advent of Code - Day 9
The day I failed.
The input data
A very basic data file:
- each row contains a character and a number separated by space
- the characters represent directions (up, right, down, left):
U,R,D,L - the numbers represent the distance/step
Task: we pull a rope in the given direction for the given distance, and we need to figure out,
Part one
I will be honest: it was tough. I went down to a super primitive level, and wrote a low-performing code just to do what needs to be done and get the right result. Currently, I am not even sure, whether my code is actually working well or just accidentally gives the right result.
Should I share the code? Well, it’s definitely the worst practice.
Part two
I’m not proud of myself. This is the day I have to give up. I could pass part one, but this one requires skills that I don’t have yet. I failed. I tried, but couldn’t figure out the algorithm. I tried to reuse the functions from the part one, but it was a complete failure as well. I’m too tired, sad and disappointed now to try again. I give up…
This is how it should look like visualized.
When I was thinking about the solution, I thought if I treat every knot as a rope-head for the next knot, it will work. But I couldn’t even reproduce the example outputs of the task.
Maybe later I will re-try and update this article… But not today.
The next day
I was very upset that I couldn’t figure out the solution, so I spent my whole Saturday (10th of December) to find a solution. I analyzed the example in the puzzle description and watched that reddit video of the working solution a million times to understand the logic behind it. And then I just found it.
The solution I made for the part one was a total dead end, no wonder why lead me to nowhere. Yes, it accidentally gave the right result, but it was a disaster.
The new, working code covers both part one and two, you need only change the value of the ROPE_LENGTH constant. I also
added a print function to be able to visualize the result. For the puzzle example, it will look something like this:
The Code
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const ROPE_LENGTH = 10;
$knotPositions = array_fill(0, ROPE_LENGTH, [0,0]);
$moves = [];
$headX = 0;
$headY = 0;
$visitedByTail = [
'0,0' => 1,
];
$minY = 0;
$maxY = 0;
$row = 1;
if ($fn = fopen(__DIR__ . '/input.txt', 'r')) {
while (($line = fgets($fn, 1024)) !== false) {
$line = trim($line);
[$direction, $steps] = explode(' ', $line);
for ($i = 0; $i < $steps; $i++) {
switch ($direction) {
case 'U':
$headY++;
break;
case 'R':
$headX++;
break;
case 'D':
$headY--;
break;
case 'L':
$headX--;
}
$knotPositions[0] = [$headX, $headY];
$minY = min($minY, $headY);
$maxY = max($maxY, $headY);
for ($j = 1; $j < ROPE_LENGTH; $j++) {
moveKnot($j, $knotPositions);
}
$coordinate = $knotPositions[ROPE_LENGTH - 1][0].','.$knotPositions[ROPE_LENGTH - 1][1];
if (!isset($visitedByTail[$coordinate])) {
$visitedByTail[$coordinate] = 0;
}
$visitedByTail[$coordinate]++;
deleteLines($minY, $maxY);
printStep(
moveIndex: $row,
stepIndex: $i + 1,
direction: $direction,
knotPositions: $knotPositions,
visitedByTail: $visitedByTail,
minY: $minY,
maxY: $maxY
);
}
$row++;
}
fclose($fn);
}
function moveKnot(int $knotIndex, array &$knotPositions): void
{
[$currentTailX, $currentTailY] = $knotPositions[$knotIndex];
[$previousTailX, $previousTailY] = $knotPositions[$knotIndex - 1];
$diffX = abs($previousTailX - $currentTailX);
$diffY = abs($previousTailY - $currentTailY);
if ($diffX < 2 && $diffY < 2) {
return;
}
$currentTailX += ($diffX === 0 && $diffY > 1)
? 0
: ($previousTailX - $currentTailX > 0 ? 1 : -1);
$currentTailY += ($diffX > 1 && $diffY === 0)
? 0
: ($previousTailY - $currentTailY > 0 ? 1 : -1);
$knotPositions[$knotIndex] = [$currentTailX, $currentTailY];
}
function deleteLines(int $minY, int $maxY): void
{
for ($y = max($maxY +6, 20); $y >= min($minY -5, -20) -2; $y--) {
echo "\r\x1b[K"; // remove this line
echo "\033[1A\033[K";
}
}
function printStep(
int $moveIndex,
int $stepIndex,
string $direction,
array $knotPositions,
array $visitedByTail,
int $minY,
int $maxY
): void {
$matrix = [];
// Prepare matrix
for ($y = max($maxY +5, 20); $y >= min($minY -5, -20); $y--) {
for ($x = -180; $x <= 40; $x++) {
$matrix[$y][$x] = ' ';
if ($y === 0) {
$matrix[$y][$x] = '─';
}
if ($x === 0) {
$matrix[$y][$x] = '|';
}
}
}
foreach ($visitedByTail as $coordinate => $times) {
[$x, $y] = explode(',', $coordinate);
$matrix[$y][$x] = '.';
}
// Place markers
for ($i = ROPE_LENGTH - 1; $i >= 0; $i--) {
$sign = $i === 0 ? 'H' : $i;
$matrix[$knotPositions[$i][1]][$knotPositions[$i][0]] = $sign;
}
echo "Move #$moveIndex | Step #$stepIndex into $direction".PHP_EOL;
foreach ($matrix as $index => $row) {
echo str_pad($index, 4, ' ', STR_PAD_LEFT).'. '.implode($row).PHP_EOL;
}
echo ' ';
for ($x = -180; $x <= 40; $x++) {
echo ($x % 5 === 0) ? '|' : "'";
}
echo PHP_EOL;
usleep(100000);
}
echo count($visitedByTail).PHP_EOL;