From 23a90beb38550820733e952016ad6d95bbe06816 Mon Sep 17 00:00:00 2001
From: Clement Desmidt <clement@desmidt.fr>
Date: Fri, 10 Dec 2021 16:10:34 +0100
Subject: [PATCH] :sparkles: Finit le jour 9

---
 day_9/part_2.php | 99 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 day_9/part_2.php

diff --git a/day_9/part_2.php b/day_9/part_2.php
new file mode 100644
index 0000000..02412bb
--- /dev/null
+++ b/day_9/part_2.php
@@ -0,0 +1,99 @@
+<?php
+
+$grid = array_map(static function ($row) {
+    return str_split($row);
+}, explode("\n", file_get_contents('./input.txt')));
+
+$points_in_bassin = [];
+$low_points = [];
+
+foreach ($grid as $i => $row) {
+    foreach ($row as $j => $cell) {
+        $count = 0;
+        if (!array_key_exists($i - 1, $grid) || $grid[$i - 1][$j] > $cell) {
+            $count++;
+        }
+        if (!array_key_exists($i + 1, $grid) || $grid[$i + 1][$j] > $cell) {
+            $count++;
+        }
+        if (!array_key_exists($j - 1, $row) || $row[$j - 1] > $cell) {
+            $count++;
+        }
+        if (!array_key_exists($j + 1, $row) || $row[$j + 1] > $cell) {
+            $count++;
+        }
+
+        if ($count === 4) {
+            $low_points[] = [$i, $j];
+        }
+    }
+}
+
+foreach ($low_points as $i => $low_point) {
+    $points_in_bassin[$i] = build_bassin($grid, [$low_point],0, [$low_point]);
+}
+
+rsort($points_in_bassin);
+
+echo array_product(array_slice($points_in_bassin, 0, 3));
+
+function build_bassin($grid, $neighbours, $count_neighbours = 0, $current_bassin = [])
+{
+    $step = 0;
+    $new_neighbours = [];
+    foreach ($neighbours as $value) {
+        [$i, $j] = $value;
+
+        if (
+            isset($grid[$i][$j - 1]) &&
+            $grid[$i][$j - 1] != 9 &&
+            !in_array([$i, $j - 1], $current_bassin)
+        ) {
+            $step++;
+            $count_neighbours++;
+            $current_bassin[] = [$i, $j - 1];
+            $new_neighbours[] = [$i, $j - 1];
+        }
+
+        if (
+            isset($grid[$i][$j + 1]) &&
+            $grid[$i][$j + 1] != 9 &&
+            !in_array([$i, $j + 1], $current_bassin)
+        ) {
+            $step++;
+            $count_neighbours++;
+            $current_bassin[] = [$i, $j + 1];
+            $new_neighbours[] = [$i, $j + 1];
+        }
+
+        if (
+            isset($grid[$i - 1][$j]) &&
+            $grid[$i - 1][$j] != 9 &&
+            !in_array([$i - 1, $j], $current_bassin)
+        ) {
+            $step++;
+            $count_neighbours++;
+            $current_bassin[] = [$i - 1, $j];
+            $new_neighbours[] = [$i - 1, $j];
+        }
+
+        if (
+            isset($grid[$i + 1][$j]) &&
+            $grid[$i + 1][$j] != 9 &&
+            !in_array([$i + 1, $j], $current_bassin)
+        ) {
+            $step++;
+            $count_neighbours++;
+            $current_bassin[] = [$i + 1, $j];
+            $new_neighbours[] = [$i + 1, $j];
+        }
+    }
+
+    if ($step !== 0) {
+        return build_bassin($grid, $new_neighbours, $count_neighbours, $current_bassin, 1);
+    }
+
+    return $count_neighbours + 1;
+}
+
+