day 5

Fun with sorting.

Author: robbkidd

2024/ruby/day05.rb

@@ -0,0 +1,65 @@
+require_relative 'day'
+
+class Day05 < Day # >
+
+  # @example
+  #   day.part1 #=> 143
+  def part1
+    updates
+      .select { correct_order? _1 }
+      .map { |update| update[ (update.length/2).ceil ] }
+      .reduce(&:+)
+  end
+
+  # @example
+  #   day.part2 #=> 123
+  def part2
+    updates
+      .reject { correct_order? _1 }
+      .map { |update| fix(update)[ (update.length/2).ceil ] }
+      .reduce(&:+)
+  end
+
+  # @example first
+  #   day.correct_order?([75,47,61,53,29]) #=> true
+  # @example fourth
+  #   day.correct_order?([75,97,47,61,53]) #=> false
+  def correct_order?(update)
+    update
+      .each_with_index { |page, idx|
+        return false if rules[page].intersect?(update[0..idx])
+      }
+    true
+  end
+
+  # @example fourth
+  #   day.fix([75,97,47,61,53]) #=> [97,75,47,61,53]
+  # @example fifth
+  #   day.fix([61,13,29]) #=> [61,29,13]
+  # @example last
+  #   day.fix([97,13,75,29,47]) #=> [97,75,47,29,13]
+  def fix(update)
+    update.sort { |x,y| rules[x].include?(y) ? -1 : 1 }
+  end
+
+  def rules
+    @rules ||=
+      input
+        .split("\n\n")[0]
+        .split("\n")
+        .each_with_object(Hash.new {|h,k| h[k]=[]}) { |line, rules_hash|
+          x, y = line.split("|").map(&:to_i)
+          rules_hash[x].append(y)
+        }
+  end
+
+  def updates
+    @updates ||=
+      input
+        .split("\n\n")[1]
+        .split("\n")
+        .map { |line| line.split(",").map(&:to_i) }
+  end
+
+  EXAMPLE_INPUT = File.read("../inputs/day05-example-input.txt")
+end