First commit, Complete 2015 day 1

2015/day1/README.md

@@ -0,0 +1,36 @@
+# Day 1: Not Quite Lisp
+
+## Part 1
+
+Santa was hoping for a white Christmas, but his weather machine's "snow" function is powered by stars, and he's fresh out! To save Christmas, he needs you to collect fifty stars by December 25th.
+
+Collect stars by helping Santa solve puzzles. Two puzzles will be made available on each day in the Advent calendar; the second puzzle is unlocked when you complete the first. Each puzzle grants one star. Good luck!
+
+Here's an easy puzzle to warm you up.
+
+Santa is trying to deliver presents in a large apartment building, but he can't find the right floor - the directions he got are a little confusing. He starts on the ground floor (floor 0) and then follows the instructions one character at a time.
+
+An opening parenthesis, (, means he should go up one floor, and a closing parenthesis, ), means he should go down one floor.
+
+The apartment building is very tall, and the basement is very deep; he will never find the top or bottom floors.
+
+For example:
+
+- `(())` and `()()` both result in floor 0.
+- `(((` and `(()(()(` both result in floor 3.
+- `))(((((` also results in floor 3.
+- `())` and `))(` both result in floor -1 (the first basement level).
+- `)))` and `)())())` both result in floor -3.
+
+**To what floor do the instructions take Santa?**
+
+## Part 2
+
+Now, given the same instructions, find the **position** of the first character that causes him to enter the basement (floor -1). The first character in the instructions has position 1, the second character has position 2, and so on.
+
+For example:
+
+* `)` causes him to enter the basement at character position 1.
+* `()())` causes him to enter the basement at character position 5.
+
+What is the position of the character that causes Santa to first enter the basement?

2015/day1/run.php

@@ -0,0 +1,28 @@
+#!/usr/bin/env php
+<?php declare(strict_types=1);
+
+$inputStr = file_get_contents('input.txt');
+$input = str_split($inputStr);
+$actions = array_map(fn (string $item): int => $item === '(' ? 1 : -1, $input);
+
+echo "Part 1: final floor:";
+echo array_reduce($actions, fn (int $a, int $b): int => $a + $b, 0) . "\n";
+
+function find_sum_index(array $actions, int $target = -1): int
+{
+    $current = 0;
+
+    foreach ($actions as $idx => $val)
+    {
+        $current += $val;
+
+        if ($current === $target)
+        {
+            return $idx + 1;
+        }
+
+    }
+}
+
+echo "Part 2: first step in basement:";
+echo  find_sum_index($actions, -1) . "\n";

README.md

@@ -0,0 +1 @@
+# Advent of Code