2019: Day 14 Part 2

2019/day14/part2.rb

@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+Chemical = Struct.new(:amount, :name) do
+  def pretty_print(pp)
+    pp.text to_s
+  end
+
+  def to_s
+    "#{amount} #{name}"
+  end
+end
+
+Formula = Struct.new(:product, :reactants) do
+  def produces?(name)
+    product.name == name
+  end
+
+  def pretty_print(pp)
+    pp.text to_s
+  end
+
+  def to_s
+    "#{product} <= #{reactants.join(', ')}"
+  end
+end
+
+def parse_chemical(text)
+  parts = text.split(' ')
+  amount = parts[0].to_i
+  name = parts[1].strip
+  Chemical.new(amount, name)
+end
+
+def parse(input)
+  input.lines.map do |line|
+    left, right = line.split('=>')
+
+    product = parse_chemical(right)
+    reactants = left.split(',').map { |text| parse_chemical(text) }
+    Formula.new(product, reactants)
+  end
+end
+
+def ore_needed(formulas, fuel)
+  have = Hash.new { |hash, key| hash[key] = 0 }
+  need = Hash.new { |hash, key| hash[key] = 0 }
+
+  have = Hash.new { |hash, key| hash[key] = 0 }
+  need = Hash.new { |hash, key| hash[key] = 0 }
+  need['FUEL'] = fuel
+  ore = 0
+
+  until need.empty?
+    product, amount = need.shift
+    next if amount.zero?
+
+    formula = formulas.find { |f| f.produces? product }
+
+    needed = (amount - have[product])
+
+    runs = (needed.fdiv formula.product.amount).ceil
+
+    formula.reactants.each do |r|
+      wanted = runs * r.amount
+      already_have = have[r.name]
+      can_use = [already_have, wanted].min
+      more = wanted - can_use
+
+      have[r.name] -= can_use
+      if r.name == 'ORE'
+        ore += more
+      else
+        need[r.name] += more
+      end
+    end
+
+    yielded = runs * formula.product.amount
+    extra = yielded - needed
+    have[product] += extra
+  end
+
+  ore
+end
+
+class OreCalculator
+  def initialize(formulas)
+    @formulas = formulas
+    @memo = {}
+  end
+
+  def calc(fuel)
+    return @memo[fuel] if @memo.key?(fuel)
+
+    ore = ore_needed(@formulas, fuel)
+    @memo[fuel] = ore
+    ore
+  end
+end
+
+def binary_search(f, lo, hi, target)
+  until (f.call(hi) - f.call(lo)).zero? || hi - lo < 2
+    mid = (hi + lo) / 2
+    case f.call(mid) <=> target
+    when -1
+      lo = mid
+    when 0
+      break
+    when +1
+      hi = mid
+    end
+  end
+
+  lo
+end
+
+input = File.read(File.join(__dir__, 'input'))
+formulas = parse(input)
+
+available_ore = 1_000_000_000_000
+
+upper_threshold = 0
+upper_fuel = 1
+until upper_threshold > available_ore
+  upper_threshold = ore_needed(formulas, upper_fuel)
+  upper_fuel *= 2
+end
+
+oc = OreCalculator.new(formulas)
+fuel_made = binary_search(oc.method(:calc), 0, upper_fuel, available_ore)
+puts fuel_made