AoC 2022 Day 12 - attempted rewrite with Grid

Looks like I did a lot, but it doesn't work. Save as a different file
for posterity.

Author: Robb Kidd

2022/ruby/day12_with_grid.rb

@@ -0,0 +1,100 @@
+require_relative "day"
+require_relative "grid"
+
+class Day12 < Day
+  def initialize(*)
+    super
+    @path_finder = HillClimbingAlgorithm.new(input)
+  end
+
+  # @example
+  #   day.part1 #=> 31
+  def part1
+    @path_finder
+      .shortest_hike_from_start
+      .tap { |path| @path_finder.print(path) }
+      .length - 1 # start doesn't count towards steps
+  end
+
+  # @example
+  #   day.part2 #=> 29
+  def part2
+    @path_finder
+      .shortest_hike_from_lowest_points
+      .tap { |path| @path_finder.print(path) }
+      .length - 1 # start doesn't count towards steps
+  end
+
+  EXAMPLE_INPUT = <<~INPUT
+    Sabqponm
+    abcryxxl
+    accszExk
+    acctuvwj
+    abdefghi
+  INPUT
+end
+
+class HillClimbingAlgorithm
+  def initialize(input = "")
+    @start_coords = nil
+    @goal_coords = nil
+
+    @hills = Grid.new(input)
+    @hills.parse do |coords, elevation|
+      @start_coords = coords if elevation == "S"
+      @goal_coords = coords if elevation == "E"
+    end
+    @hills.set_step_cost_calculator do |h, from, to|
+      if hikable?(h.at(from), h.at(to))
+        HIKABLE_HILL_COST
+      else
+        CLIMBING_GEAR_REQUIRED_COST
+      end
+    end
+  end
+
+  HIKABLE_HILL_COST = 1 # one step on the hike
+  CLIMBING_GEAR_REQUIRED_COST = Float::INFINITY # effectively ruling them out
+
+  # @example from puzzle
+  #   new.hikable? 'm', 'n' #=> true
+  #   new.hikable? 'm', 'a' #=> true
+  #   new.hikable? 'n', 'm' #=> true
+  #   new.hikable? 'm', 'o' #=> false
+  # @example treat start like a
+  #   new.hikable? 'S', 'a' #=> true
+  #   new.hikable? 'a', 'S' #=> true
+  #   new.hikable? 'S', 'c' #=> false
+  #   new.hikable? 'a', 'c' #=> false
+  # @example treat goal like z
+  #   new.hikable? 'y', 'z' #=> true
+  #   new.hikable? 'y', 'E' #=> true
+  #   new.hikable? 'x', 'E' #=> false
+  def hikable?(from_elevation, to_elevation)
+    [to_elevation, from_elevation].map do |el| # "S", "E", or [a-z],
+        el
+          .tr("S", "a") # treat S as lowest (a)
+          .tr("E", "z") # treat E as highest (z)
+          .ord # integer value for elevation character for comparison
+      end
+      .reduce(&:-) <= 1 # elevation diff is less than 1 higher
+  end
+
+  def shortest_hike_from_start
+    @hills.shortest_path_between(@start_coords, @goal_coords)
+  end
+
+  def shortest_hike_from_lowest_points
+    @hills
+      .select { |coords, elevation| %w[S a].include?(elevation) }
+      .map { |coords, _| @hills.shortest_path_between(coords, @goal_coords) }
+      .reject(
+        &:empty?
+      ) # reject if there was no hikable path from some lowest points
+      .min_by { |path| path.length }
+  end
+
+  def print(path)
+    puts "\n" + @hills.render_path(path)
+  end
+end