day 15, part 2

Author: Robb Kidd

2023/ruby/day15.rb

@@ -6,12 +6,42 @@ class Day15 < Day # >
   #   day.part1 #=> 1320
   def part1
     initialization_sequence
-      .reduce(0) { |sum, step| sum += HASH.hash(step) }
+      .reduce(0) { |sum, step| sum += hash(step) }
   end
 
-  # example
-  #   day.part2 #=> 'how are you'
+  # @example
+  #   day.part2 #=> 145
   def part2
+    # "... a series of 256 boxes numbered 0 through 255 ..."
+    boxes = Array.new(256) { Hash.new }
+
+    # "... perform each step in the initialization sequence ..."
+    initialization_sequence
+      .each do |step|
+        label, op, focal_length = /\A(.*)([-,\=])(.*)?\z/.match(step)[1..3]
+        case op
+        when "-" ; boxes[hash(label)].delete(label)
+        when "=" ; boxes[hash(label)].store(label, focal_length.to_i)
+        else     ; raise("lolwut")
+        end
+      end
+
+    # "To confirm that all of the lenses are installed correctly,
+    #  add up the focusing power of all of the lenses."
+    boxes
+      .map.with_index { |box, box_number| # box numbers are 0-255
+        box
+          .values # only need the focal lengths not the labels
+          .map.with_index(1) { |focal_length, slot_number| # box slot numbers start at 1
+            # "The focusing power of a single lens is the result of multiplying together:
+            # " - One plus the box number of the lens in question.
+            # " - The slot number of the lens within the box: 1 for the first lens, ...
+            # " - The focal length of the lens."
+            (box_number + 1) * slot_number * focal_length
+          }
+      }
+      .flatten
+      .reduce(&:+)
   end
 
   # @example parses_input
@@ -25,33 +55,30 @@ def initialization_sequence
   EXAMPLE_INPUT = <<~INPUT
     rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7
   INPUT
-end
 
-#class Holiday_ASCII_String_Helper
-module HASH
-  # @example
-  #   HASH.hash("") #=> 0
+  # @example default_start
+  #   new.hash("") #=> 0
   # @example HASH
-  #   HASH.hash("HASH") #=> 52
+  #   new.hash("HASH") #=> 52
   # @example from_example_input
-  #   HASH.hash("rn=1") #=> 30
-  #   HASH.hash("cm-") #=> 253
-  #   HASH.hash("qp=3") #=> 97
-  #   HASH.hash("cm=2") #=> 47
-  #   HASH.hash("qp-") #=> 14
-  #   HASH.hash("pc=4") #=> 180
-  #   HASH.hash("ot=9") #=> 9
-  #   HASH.hash("ab=5") #=> 197
-  #   HASH.hash("pc-") #=> 48
-  #   HASH.hash("pc=6") #=> 214
-  #   HASH.hash("ot=7") #=> 231
-  def self.hash(str="")
-    str
-      .each_byte             # Determine the ASCII code for the current character of the string.
-      .inject(0) do |current_value, ord| # start with a current value of 0
-        current_value += ord # Increase the current value by the ASCII code you just determined.
-        current_value *= 17  # Set the current value to itself multiplied by 17.
-        current_value %= 256 # Set the current value to the remainder of dividing itself by 256.
+  #   new.hash("rn=1") #=> 30
+  #   new.hash("cm-") #=> 253
+  #   new.hash("qp=3") #=> 97
+  #   new.hash("cm=2") #=> 47
+  #   new.hash("qp-") #=> 14
+  #   new.hash("pc=4") #=> 180
+  #   new.hash("ot=9") #=> 9
+  #   new.hash("ab=5") #=> 197
+  #   new.hash("pc-") #=> 48
+  #   new.hash("pc=6") #=> 214
+  #   new.hash("ot=7") #=> 231
+  def hash(str="")
+    str                      # "... for each character in the string starting from the beginning:"
+      .each_byte             # "Determine the ASCII code for the current character of the string."
+      .inject(0) do |current_value, ord| # "... start with a current value of 0."
+        current_value += ord # "Increase the current value by the ASCII code you just determined."
+        current_value *= 17  # "Set the current value to itself multiplied by 17."
+        current_value %= 256 # "Set the current value to the remainder of dividing itself by 256."
       end
   end
 end