Update solution for 2015 day 17

Remove redundant recursion step in solution for part 1
This speeds up the solution and makes the marshalling unnecessary

2015/day-17/.gitignore

@@ -1 +0,0 @@
-*.rbdump

2015/day-17/main.rb

@@ -6,39 +6,18 @@ require "set"
 
 Container = Struct.new(:id, :cap)
 
-# dumps result of previous solution to avoid recomputing
-def memo(file)
-  dump_path = "#{file.basename}.rbdump"
-
-  if File.exist?(dump_path)
-    File.open(dump_path, "rb") { |df| Marshal.load(df) }
-  else
-    File.open(dump_path, "wb") do |dumpfile|
-      result = yield
-      dumpfile.write(Marshal.dump(result))
-      result
-    end
-  end
-end
-
-# quite inefficient, probably needs a DP solution
 def combinations(amount, containers, stack = [], results = Set[])
-  stack_sum = stack.sum { |x| x.cap }
-
-  if stack_sum > amount
-    return results
-  elsif stack_sum == amount
-    results.add(stack.dup)
-    return results
-  end
+  next_sum = stack.sum(&:cap)
 
+  return results if next_sum > amount
+  return results.add(stack.dup) if next_sum == amount
   return results if containers.empty?
 
   containers.each_with_index do |container, idx|
     combinations(amount, containers[(idx+1)..], [*stack, container], results)
   end
 
-  combinations(amount, containers[1..], [], results)
+  results
 end
 
 def main(amount, file_paths)
@@ -49,13 +28,15 @@ def main(amount, file_paths)
 
     containers = file_path.each_line.with_index.map { |x, i| Container.new(i, x.to_i) }.freeze
 
-    result = memo(file_path) { combinations(amount, containers) }
+    result = combinations(amount, containers)
 
     solution1 = result.size
     puts "  Solution 1: #{solution1}"
 
-    solution2 = result.min_by { |x| x.size }.size
-      .then { |mincomb| result.select { |x| x.size == mincomb } }
+    solution2 = result
+      .group_by { |x| x.size }
+      .min { |(a, _), (b, _)| a <=> b }
+      .last
       .size
     puts "  Solution 2: #{solution2}"
   end