Day 3 solution.

2015/days/day3.php

@@ -0,0 +1,96 @@
+<?php
+
+function part1() {
+  // Get elf instructions.
+  $directions = file_get_contents('./2015/inputs/day3.txt');
+  // Convert the string to an array for processing.
+  $directions = str_split($directions);
+  $position = [0,0];
+  $deliveries = [];
+  foreach ($directions as $direction) {
+    // First, deliver a present at current location.
+    $location_key = $position[0] . '_' . $position[1];
+    $location_key = crypt($location_key, '123');
+    if (isset($deliveries[$location_key])) {
+      $deliveries[$location_key]++;
+    }
+    else {
+      $deliveries[$location_key] = 1;
+    }
+    if ($direction == '^') {
+      $position[1]++;
+    }
+    elseif($direction == 'v') {
+      $position[1]--; 
+    }
+    elseif ($direction == '>') {
+      $position[0]++;
+    }
+    elseif ($direction == '<') {
+      $position[0]--; 
+    }
+  }
+
+  return $deliveries;
+}
+
+function part2() {
+  // Get elf instructions.
+  $directions = file_get_contents('./2015/inputs/day3.txt');
+  // Convert the string to an array for processing.
+  $directions = str_split($directions);
+  $santa_position = [0,0];
+  $robo_position = [0,0];
+  $deliveries = [];
+  $turn = 'santa';
+  foreach ($directions as $direction) {
+    if ($turn == 'santa') {
+      $deliveries = updateDeliveryList($deliveries, $santa_position);
+      $santa_position = changeCoordinates($santa_position, $direction);
+      $turn = 'robo';
+    }
+    elseif ($turn == 'robo') {
+      $deliveries = updateDeliveryList($deliveries, $robo_position);
+      $robo_position = changeCoordinates($robo_position, $direction);
+      $turn = 'santa';
+    }
+  }
+
+  return $deliveries;
+}
+
+function updateDeliveryList(array $list, $coords) {
+  $location_key = $coords[0] . '_' . $coords[1];
+  $location_key = crypt($location_key, '123');
+  if (!isset($list[$location_key])) {
+    $list[$location_key] = 1;
+  }
+  else {
+    $list[$location_key]++;
+  }
+
+  return $list;
+}
+
+function changeCoordinates($coords, $move) {
+  match ($move) {
+    '^' => $coords[1]++,
+    'v' => $coords[1]--,
+    '>' => $coords[0]++,
+    '<' => $coords[0]--,
+    default => null,
+  };
+
+  return $coords;
+}
+
+
+function day3($part = 1) {
+  if ($part == 1) {
+    return count(part1());
+  }
+  elseif ($part == 2) {
+    return count(part2());
+  }
+}
+

2015/solutions.php

@@ -10,6 +10,8 @@ $solutions['day_1']['part_1'] = day1solution1();
 $solutions['day_1']['part_2'] = day1solution1b();
 $solutions['day_2']['part_1'] = day2(1);
 $solutions['day_2']['part_2'] = day2(2);
+$solutions['day_3']['part_1'] = day3(1);
+$solutions['day_3']['part_2'] = day3(2);
 
 // OUTPUT THE SOLUTIONS WE HAVE.
 foreach ($solutions as $day => $parts) {