Year 2021 Day 02 solutions (#8)

- Solve both parts of day 2.

Author: sprak3000

2021/day-02.php

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+<?php
+
+require __DIR__ . '/../vendor/autoload.php';
+
+use adventofcode\Year2021\Navigation;
+
+/**
+ * --- Day 2: Dive! ---
+ * Now, you need to figure out how to pilot this thing.
+ *
+ * It seems like the submarine can take a series of commands like forward 1, down 2, or up 3:
+ *
+ * forward X increases the horizontal position by X units.
+ * down X increases the depth by X units.
+ * up X decreases the depth by X units.
+ *
+ * Note that since you're on a submarine, down and up affect your depth, and
+ * so they have the opposite result of what you might expect.
+ *
+ * The submarine seems to already have a planned course (your puzzle input).
+ * You should probably figure out where it's going. For example:
+ *
+ * forward 5
+ * down 5
+ * forward 8
+ * up 3
+ * down 8
+ * forward 2
+ *
+ * Your horizontal position and depth both start at 0. The steps above would then modify them as follows:
+ *
+ * forward 5 adds 5 to your horizontal position, a total of 5.
+ * down 5 adds 5 to your depth, resulting in a value of 5.
+ * forward 8 adds 8 to your horizontal position, a total of 13.
+ * up 3 decreases your depth by 3, resulting in a value of 2.
+ * down 8 adds 8 to your depth, resulting in a value of 10.
+ * forward 2 adds 2 to your horizontal position, a total of 15.
+ *
+ * After following these instructions, you would have a horizontal position of
+ * 15 and a depth of 10. (Multiplying these together produces 150.)
+ *
+ * Calculate the horizontal position and depth you would have after following
+ * the planned course. What do you get if you multiply your final horizontal
+ * position by your final depth?
+ */
+
+$navigation = new Navigation();
+
+$directions = file(__DIR__ . '/inputs/day-02.input', FILE_IGNORE_NEW_LINES);
+$position = $navigation->findPosition($directions);
+$product = $position['horizontal'] * $position['depth'];
+print(
+    'Your position is ' . $position['horizontal'] . ' horizontal and '
+    . $position['depth'] . ' depth for a product of ' . $product . ".\n"
+);
+
+/**
+ * --- Part Two ---
+ * Based on your calculations, the planned course doesn't seem to make any
+ * sense. You find the submarine manual and discover that the process is actually slightly more complicated.
+ *
+ * In addition to horizontal position and depth, you'll also need to track a
+ * third value, aim, which also starts at 0. The commands also mean something
+ * entirely different than you first thought:
+ *
+ * - down X increases your aim by X units.
+ * - up X decreases your aim by X units.
+ * - forward X does two things:
+ *      - It increases your horizontal position by X units.
+ *      - It increases your depth by your aim multiplied by X.
+ *
+ * Again note that since you're on a submarine, down and up do the opposite of
+ * what you might expect: "down" means aiming in the positive direction.
+ *
+ * Now, the above example does something different:
+ *
+ * - forward 5 adds 5 to your horizontal position, a total of 5. Because your aim is 0, your depth does not change.
+ * - down 5 adds 5 to your aim, resulting in a value of 5.
+ * - forward 8 adds 8 to your horizontal position, a total of 13. Because your aim is 5, your depth increases by 8*5=40.
+ * - up 3 decreases your aim by 3, resulting in a value of 2.
+ * - down 8 adds 8 to your aim, resulting in a value of 10.
+ * - forward 2 adds 2 to your horizontal position, a total of 15. Because
+ *   your aim is 10, your depth increases by 2*10=20 to a total of 60.
+ *
+ * After following these new instructions, you would have a horizontal
+ * position of 15 and a depth of 60. (Multiplying these produces 900.)
+ *
+ * Using this new interpretation of the commands, calculate the horizontal
+ * position and depth you would have after following the planned course. What
+ * do you get if you multiply your final horizontal position by your final depth?
+ */
+
+$position = $navigation->findPositionByAim($directions);
+$product = $position['horizontal'] * $position['depth'];
+print(
+    'Your position via aim approach is ' . $position['horizontal'] . ' horizontal and '
+    . $position['depth'] . ' depth for a product of ' . $product . ".\n"
+);

2021/inputs/day-02-sample.input

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+forward 5
+down 5
+forward 8
+up 3
+down 8
+forward 2