add day 07: Amplification Circuit

Author: petertseng

07_amplification_circuit.rb

@@ -0,0 +1,36 @@
+require_relative 'lib/intcode'
+
+VERBOSE = ARGV.delete('-v')
+
+def const_input(mem, phase, input)
+  ic = Intcode.new(mem, valid_ops: (1..8).to_a + [99]).continue(input: phase)
+  ic.continue(input: input) until ic.halted?
+  ic.output
+end
+
+# Assume each amplifier performs a linear mx+b transform.
+# Determine m and b by running the amplifiers once, instead of once per permutation.
+# Could take it even farther with dynamic programming:
+# https://www.reddit.com/r/adventofcode/comments/e7q8fp/2019_day_7_part_2_c_feed_it_forward/
+# But this is fast enough and I don't care.
+def chain(mem, phases)
+  amps = phases.to_h { |phase| [phase, {
+    b: b = const_input(mem, phase, 0),
+    m: const_input(mem, phase, 1).zip(b).map { |y, bb| y - bb },
+  }.freeze] }.freeze
+
+  puts amps if VERBOSE
+
+  sizes = amps.values.flat_map { |a| [a[:b].size, a[:m].size] }.uniq
+  raise "Incompatible sizes #{sizes}" if sizes.size != 1
+
+  phases.permutation.map { |perm|
+    ms = perm.map { |phase| amps[phase][:m] }.transpose.flatten
+    bs = perm.map { |phase| amps[phase][:b] }.transpose.flatten
+    ms.zip(bs).reduce(0) { |signal, (m, b)| m * signal + b }
+  }.max
+end
+
+input = (ARGV[0]&.include?(?,) ? ARGV[0] : ARGF.read).split(?,).map(&method(:Integer)).freeze
+
+[0...5, 5...10].each { |range| puts chain(input, range.to_a) }

README.md

@@ -45,6 +45,7 @@ Interesting approaches:
 * Day 02 (Intcode): Assume linearity, determine coefficients with a few runs, determine noun and verb.
 * Day 03 (Crossed Wires): Storing segment endpoints is faster than storing every point touched by the wires.
 * Day 04 (Password): Skip over entire swaths of non-increasing passwords. If considering 341234, skip straight to 344444.
+* Day 07 (Amplification Circuit): Assume all amplifiers perform a linear transform mx+b and determine m and b to reduce the number of times the amplifiers need to be run.
 
 # Takeaways
 

lib/intcode.rb

@@ -42,6 +42,10 @@ def initialize(
     @output = []
   end
 
+  def halted?
+    @halt
+  end
+
   def step(
     input: -> { raise 'no input' },
     mem_range: nil, mem_start: 0, mem_len: nil, mem_all: false,