day01.rb

# --- Day 1: Chronal Calibration ---
#
# "We've detected some temporal anomalies," one of Santa's Elves at the Temporal Anomaly Research
# and Detection Instrument Station tells you. She sounded pretty worried when she called you down
# here. "At 500-year intervals into the past, someone has been changing Santa's history!"
#
# "The good news is that the changes won't propagate to our time stream for another 25 days, and
# we have a device" - she attaches something to your wrist - "that will let you fix the changes
# with no such propagation delay. It's configured to send you 500 years further into the past
# every few days; that was the best we could do on such short notice."
#
# "The bad news is that we are detecting roughly fifty anomalies throughout time; the device will
# indicate fixed anomalies with stars. The other bad news is that we only have one device and
# you're the best person for the job! Good lu--" She taps a button on the device and you suddenly
# feel like you're falling. To save Christmas, you need to get all fifty stars by December 25th.
#
# Collect stars by solving 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!
#
# After feeling like you've been falling for a few minutes, you look at the device's tiny
# screen. "Error: Device must be calibrated before first use. Frequency drift detected. Cannot
# maintain destination lock." Below the message, the device shows a sequence of changes in
# frequency (your puzzle input). A value like +6 means the current frequency increases by 6; a
# value like -3 means the current frequency decreases by 3.
#
# For example, if the device displays frequency changes of +1, -2, +3, +1, then starting from a
# frequency of zero, the following changes would occur:
#
#
# Current frequency  0, change of +1; resulting frequency  1.
# Current frequency  1, change of -2; resulting frequency -1.
# Current frequency -1, change of +3; resulting frequency  2.
# Current frequency  2, change of +1; resulting frequency  3.
#
# In this example, the resulting frequency is 3.
#
# Here are other example situations:
#
#
# +1, +1, +1 results in  3
# +1, +1, -2 results in  0
# -1, -2, -3 results in -6
#
# Starting with a frequency of zero, what is the resulting frequency after all of the changes in
# frequency have been applied?
#

require_relative 'input'

day = __FILE__[/\d+/].to_i(10)
input = Input.for_day(day, 2018)

# puts "solving day #{day} from input:\n#{input}"

# --- Part Two ---
#
# You notice that the device repeats the same frequency change list over and over. To calibrate
# the device, you need to find the first frequency it reaches twice.
#
# For example, using the same list of changes above, the device would loop as follows:
#
# Current frequency  0, change of +1; resulting frequency  1.
# Current frequency  1, change of -2; resulting frequency -1.
# Current frequency -1, change of +3; resulting frequency  2.
# Current frequency  2, change of +1; resulting frequency  3.
# (At this point, the device continues from the start of the list.)
# Current frequency  3, change of +1; resulting frequency  4.
# Current frequency  4, change of -2; resulting frequency  2, which has already been seen.
#
# In this example, the first frequency reached twice is 2. Note that your device might need to
# repeat its list of frequency changes many times before a duplicate frequency is found, and that
# duplicates might be found while in the middle of processing the list.
#
# Here are other examples:
#
# +1, -1 first reaches 0 twice.
# +3, +3, +4, -2, -4 first reaches 10 twice.
# -6, +3, +8, +5, -6 first reaches 5 twice.
# +7, +7, -2, -7, -4 first reaches 14 twice.
#
# What is the first frequency your device reaches twice?

freq = input.each_line.reduce(0) do |acc,line|
  acc += line.to_i
end

puts "Part 1: Frequency: #{freq}"

require 'set'

freq=0
seen = Set.new
seen << freq
catch :duplicate do
  while true do
    input.each_line do |line|
      freq += line.to_i
      throw :duplicate if seen.include?(freq)
      seen << freq
    end
    print '.'
  end
end

puts "Part 2: first dup #{freq}"