day 9, part 2

More naive map-reduce! Redo the work! It's fast enough!

Author: Robb Kidd

2023/ruby/day09.rb

@@ -2,28 +2,41 @@
 
 class Day09 < Day # >
 
+  # @example
+  #  day.oasis_report #=> [[0, 3, 6, 9, 12, 15], [1, 3, 6, 10, 15, 21], [10, 13, 16, 21, 30, 45]]
+  def oasis_report
+    @oasis_report ||=
+      input
+        .each_line
+        .map { |line| line.split(" ").map(&:to_i) }
+  end
+
   # @example
   #   day.part1 #=> 114
   def part1
-    input
-      .each_line
-      .map { |line| line.split(" ").map(&:to_i) }
+    oasis_report
       .map { |history|
         sequences = [history]
         while !sequences.last.all?(0) do
           sequences << sequences.last.each_cons(2).map { |a,b| b - a }
-        end
-        sequences
-          .reverse
-          .map(&:last)
-          .reduce(&:+)
+        end                                 #                  ^^^^^
+        sequences.map(&:last).reduce(&:+)   # <--- last and next-minus-prev
       }
       .reduce(&:+)
   end
 
   # @example
-  #   day.part2 #=> 'how are you'
+  #   day.part2 #=> 2
   def part2
+    oasis_report
+      .map { |history|
+        sequences = [history]
+        while !sequences.last.all?(0) do
+          sequences << sequences.last.each_cons(2).map { |a,b| a - b }
+        end                                 #                  ^^^^^
+        sequences.map(&:first).reduce(&:+)  # <--- first and prev-minus-next
+      }
+      .reduce(&:+)
   end
 
   EXAMPLE_INPUT = <<~INPUT