I recently found out about Advent of Code, and as part of my preparation for this year's event I'm solving the puzzles of the previous event (adventofcode.com/2015), in this short series I'll publish my solutions with a brief explanation.

A brief explanation about how Advent of Code works, there's one puzzle every day from December 1st to the 25th, each puzzle has two versions so you can earn upto two stars for every puzzle.

## --- Day 1: Not Quite Lisp ---

Problem description at day/1.

### Solution

For v1: Just use a variable for the current floor, increase by one when we find a (, descrease by 1 when we find a ).

        #!/usr/bin/env ruby
input = File.read('input.txt')
sequence = input.split(//)
floor = 0
sequence.each do |s|
if s == '('
floor += 1
else
floor -= 1
end
end
puts floor



For v2: Similar to v1, we just need to keep track of the positions.

        #!/usr/bin/env ruby
input = File.read('input.txt')
sequence = input.split(//)
floor = 0
sequence.each_with_index do |s, i|
if s == '('
floor += 1
else
floor -= 1
end
if floor == -1
puts i + 1
break
end
end



## --- Day 2: I Was Told There Would Be No Math ---

Problem description at day/2.

### Solution

For v1: Read all lines, parse the three integers and just apply the fomula from the problem description.

        paper = 0
File.readlines('input.txt').each do |line|
next if line.length == ''
dimensions = line.split('x').map(&:to_i).sort
l, w, h = dimensions
paper += (2 * l * w) + (2 * l * h) + (2 * w * h) + (l * w)
end
puts paper



For v2: Take the lowest 2 values (out of 3) and compute the perimeter, plus the product of all of them.

        ribbon = 0
File.readlines('input.txt').each do |line|
dimensions = line.split('x').map(&:to_i).sort
l, w, h = dimensions
ribbon += (2 * l + 2 * w) + (l * w * h)
end
puts ribbon



## --- Day 3: Perfectly Spherical Houses in a Vacuum ---

Problem description at day/3.

### Solution

For v1: A simulation problem, update your position in a 2-dimensional grid with every command, use a table to keep track of the unique positions you've been in.

        grid = {[0, 0] => true}
r = 0
c = 0
File.read('input.txt').split(//).each do |i|
r += 1 if i == 'v'
r -= 1 if i == '^'
c += 1 if i == '>'
c -= 1 if i == '<'
grid[[r, c]] = true
end
puts grid.length



For v2: Likewise, except that you have to alternate commands with Robo-Santa, the first command is for Santa, the second one isfor Robo-Santa, etc.

        grid = {[0, 0] => true}

def move(r, c, d)
r += 1 if d == 'v'
r -= 1 if d == '^'
c += 1 if d == '>'
c -= 1 if d == '<'
return r, c
end

sr = 0
sc = 0
rr = 0
rc = 0
File.read('input.txt').split(//).each_with_index do |d, i|
if i % 2 == 0
sr, sc = move(sr, sc, d)
grid[[sr, sc]] = true
else
rr, rc = move(rr, rc, d)
grid[[rr, rc]] = true
end
end
puts grid.length



## --- Day 4: The Ideal Stocking Stuffer ---

Problem description at day/4.

### Solution

For v1: Well, I just brute-forced this one, used Ruby's Digest::MD5 to compute the checksum.

        require 'digest'

secret = 'bgvyzdsv'
key = 1
loop do
md5 = Digest::MD5.new
md5.update(secret + key.to_s)
hex = md5.hexdigest
if hex.start_with?('00000')
puts key
break
end
key += 1
end



For v2: The same as v1, just add one more zero.

        require 'digest'

secret = 'bgvyzdsv'
key = 1
loop do
md5 = Digest::MD5.new
md5.update(secret + key.to_s)
hex = md5.hexdigest
if hex.start_with?('000000')
puts key
break
end
key += 1
end



## --- Day 5: Doesn't He Have Intern-Elves For This? ---

Problem description at day/5.

### Solution

For v1: Just follow the instructions carefully and you should be able to solve this problem.

        def nice?(word)
chars = word.split(//)
vowels = {
'a' => 0,
'e' => 0,
'i' => 0,
'o' => 0,
'u' => 0
}
forbidden = {
'ab' => true,
'cd' => true,
'pq' => true,
'xy' => true
}
consecutive = false
0.upto(chars.length - 2) do |i|
pair =  chars[i] + chars[i+1]
return false if forbidden[pair] == true
consecutive ||= (chars[i] == chars[i+1])
end

return false unless consecutive

chars.each do |c|
vowels[c] += 1 if vowels[c]
end
return vowels.values.reduce(&:+) >= 3
end

puts File.readlines('input.txt').select {|w| nice?(w.chomp) }.length



For v2: Just follow the instructions even more carefully, I was trapped by a moment by a pesky bug in this version.

        def nice?(word)
chars = word.split(//)
pairs = {}
contains_pair = false
prev = ''
(chars.length - 1).downto(1) do |i|
pair = chars[i-1] + chars[i]
contains_pair ||= (pairs[pair] == true)
pairs[prev] = true
prev = pair
end
return false unless contains_pair

0.upto(chars.length - 3) do |i|
return true if chars[i] == chars[i+2]
end
false
end

puts File.readlines('input.txt').select {|w| nice?(w.chomp) }.length
puts nice?('qjhvhtzxzqqjkmpb')
puts nice?('xxyxx')
puts nice?('uurcxstgmygtbstg')
puts nice?('ieodomkazucvgmuy')