day 9, part 1

A naive map-reduce implementation pretty much as described by the day's prose. But it works.

Author: Robb Kidd

2023/ruby/day09.rb

@@ -0,0 +1,34 @@
+require_relative 'day'
+
+class Day09 < Day # >
+
+  # @example
+  #   day.part1 #=> 114
+  def part1
+    input
+      .each_line
+      .map { |line| line.split(" ").map(&:to_i) }
+      .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(&:+)
+      }
+      .reduce(&:+)
+  end
+
+  # @example
+  #   day.part2 #=> 'how are you'
+  def part2
+  end
+
+  EXAMPLE_INPUT = <<~INPUT
+    0 3 6 9 12 15
+    1 3 6 10 15 21
+    10 13 16 21 30 45
+  INPUT
+end