solution for day 11 part 2

Author: Adrian Reimann

day_11/11-1.php

@@ -3,7 +3,6 @@
 const MAX_ENERGY = 9; //if charge is higher than this, the octopus flashes
 const MAX_STEPS = 100; //these are the steps we wanna run
 
-
 //read input and format it into multidimensional of octopuses (10 by 10)
 $octopus_group = [];
 $input = fopen("input.txt", "r");

day_11/11-2.php

@@ -0,0 +1,147 @@
+<?php
+
+//SAME CODE AS PART 1 solution - just instead of looping a fixed x times, we just loop until the flash count of a single loop is exactly 100 (the count of octopuses)
+
+const MAX_ENERGY = 9; //if charge is higher than this, the octopus flashes
+
+//read input and format it into multidimensional of octopuses (10 by 10)
+$octopus_group = [];
+$octopus_count = 0; //with the given 10x10 grids this will always be 100 - but checking it during input makes the code dynamic for different size grids
+$input = fopen("input.txt", "r");
+while (($input_line = fgets($input)) !== false) {
+    $split_line = str_split(trim($input_line)); //split numbers into array
+    $octopus_group[] = $split_line; //add array to octopus_group array
+    $octopus_count += count($split_line);
+}
+fclose($input);
+
+$steps = 0;
+$flash_count = 0;
+//loop until the flash count of the last loop was exactly same as octopus count
+while ($flash_count != $octopus_count) {
+    $steps++; //increase steps
+    $flash_list = [];
+
+    //first, increase all octopuse values by 1
+    increaseOctopusEnergy($octopus_group);
+
+    //now recursively check for flashes
+    findFlashes($octopus_group);
+
+    //lastly, set the flashed octopuses to zero (this is a good time to count how many octopuses flashed, so we pass the flash list to see WHICH flashed)
+    $flash_count = resetFlashedOctopuses($octopus_group, $flash_list);
+}
+
+//result
+echo "At " . $steps . " steps, all octopuses synchronise." . PHP_EOL;
+
+
+/**
+ * Loop through the multidimensional array and increase the energy level of every octopus by 1
+ *
+ * @author Adrian
+ * @date_created 2021-12-12
+ *
+ * @param array $group the multidimensional array of octopuses
+ *
+ * @return array
+ */
+function increaseOctopusEnergy(&$group) {
+    foreach ($group as $k_row => $row) {
+        foreach ($row as $k_octo => $octopus) {
+            $group[$k_row][$k_octo]++;
+        }
+    }
+
+    return $group;
+}
+
+/**
+ * Loop through the multidimensional array and trigger a flash if energy level reached MAX.
+ * This calls a recursive function that checks followup-flashes from surrounding octopuses.
+ *
+ * @author Adrian
+ * @date_created 2021-12-12
+ *
+ * @param array $group the multidimensional array of octopuses
+ *
+ * @return array
+ */
+function findFlashes(&$group) {
+    foreach ($group as $k_row => $row) {
+        foreach ($row as $k_octo => $octopus) {
+            //access the octopus from array instead of $octopus, as the referenced array may have been modified in the recursive function - this does not reflect on $row in this loop yet
+            if($group[$k_row][$k_octo] > MAX_ENERGY) {
+                triggerFlash($group, $k_octo, $k_row);
+            }
+        }
+    }
+
+    return $group;
+}
+
+/**
+ * Loop through the multidimensional array and reset every "null" value back to 0 so it is a natural number again
+ *
+ * @author Adrian
+ * @date_created 2021-12-12
+ *
+ * @param array $group the multidimensional array of octopuses
+ *
+ * @return int how many octopuses where reset (read: how many octopuses flashed)
+ */
+function resetFlashedOctopuses(&$group, &$flash_list) {
+    $reset_count = 0;
+    foreach ($group as $k_row => $row) {
+        foreach ($row as $k_octo => $octopus) {
+            if($octopus == null) {
+                //reset the energy to 0 and increase the reset (flash) counter
+                $group[$k_row][$k_octo] = 0;
+                $reset_count++;
+            }
+        }
+    }
+
+    return $reset_count;
+}
+
+/**
+ * Loop through the multidimensional array and trigger a flash if energy level reached MAX.
+ * This calls a recursive function that checks followup-flashes from surrounding octopuses.
+ *
+ * @author Adrian
+ * @date_created 2021-12-12
+ *
+ * @param array $group the multidimensional array of octopuses
+ * @param int $x_loc array location of this octopus on the "inner" (second) array
+ * @param int $y_loc array location of this octopus on the "outer" (first) array
+ *
+ */
+function triggerFlash(&$group, $x_loc, $y_loc) {
+
+    //set this octopus to "null" instead of a number, as an octopus flash cannot be triggered twice in the same step, so we do NOT want to increase more after a flash
+    $group[$y_loc][$x_loc] = null;
+
+    //array of all locations the flash would affect/increase (array of array as some $x keys are the same and would overwrite)
+    $locations = [
+        [$x_loc-1, $y_loc-1], //top left
+        [$x_loc, $y_loc-1], //top middle
+        [$x_loc+1, $y_loc-1], //top right
+        [$x_loc-1, $y_loc], //left
+        [$x_loc+1, $y_loc], //right
+        [$x_loc-1, $y_loc+1], //bottom left
+        [$x_loc, $y_loc+1], //bottom
+        [$x_loc+1, $y_loc+1], //bottom right
+    ];
+
+    //only increase surrounding values if isset - this not only skips outer bounds, but also the octopuses that already flashed this step, as they are now "null"
+    foreach ($locations as list($x, $y)) {
+        if(isset($group[$y][$x])) {
+            $group[$y][$x]++;
+            //if the increase surpasses max energy, trigger a new flash from this octopus
+            if($group[$y][$x] > MAX_ENERGY) {
+                triggerFlash($group, $x, $y);
+            }
+        }
+    }
+}