Merge pull request #93 from dalurness/jesse/2025-08

Add janka102 day 8, 2025

Author: dalurness

src/content/communitySolutions/07/janka102.md

@@ -4,6 +4,8 @@ descriptions: ["ruby"]
 
 ## 2025
 
+One interesting thing I learned about Ruby is that the Hash class uses value equality for keys via `.hash` and `.eql?`. So for example two arrays with the same numbers in them can be used to check for the presense of the other (as apposed to only that _exact_ array). This was useful for me since I was using arrays for [x, y] coordinates and could then store those directly in the visits hash map.
+
 ```ruby
 require 'optparse'
 

src/content/communitySolutions/08/janka102.md

@@ -0,0 +1,92 @@
+---
+descriptions: ["ruby"]
+---
+
+## 2025
+
+I used a recursive approach filling in smaller and smaller sleigh sizes as presents are added. This gets very slow very quickly with a naive implementation. One trick is of course classic memoization! The trick to that trick is that you need to have a separate entry for each sleigh size _and previous present size_. Since present sizes can't repeat, then for example a remaining sleigh size of 5 can hold different amounts depending on what the previous present was (including no previous present).
+
+```
+1: 1
+...
+10: 123
+...
+100: 638760883391635670992582
+...
+1000: 9.849739E+240
+...
+10000: 7.486669E+2412
+________________________________________________________
+Executed in    1.01 secs
+```
+
+```ruby
+require 'optparse'
+
+class Dayd08
+  def initialize
+    @memo = {}
+  end
+
+  def solve(challenge)
+    if challenge
+      (1..).each do |size|
+        arrangements = fill_sleigh(size, nil)
+        # the numbers get very large, so format in 3.141592E+65 format
+        if size > 100
+          digits = Math.log10(arrangements)
+          sig_figs = 10**(digits % 1)
+          exponent = digits.floor
+          puts "#{size}: #{format('%.6fE+%d', sig_figs, exponent)}"
+        else
+          puts "#{size}: #{arrangements}"
+        end
+      end
+    else
+      size = 30
+      puts "#{size}: #{fill_sleigh(size, nil)}"
+    end
+  end
+
+  # Given a sleigh size, try to fit one of each size present
+  # then add up how many presents can fit in the new smaller sleigh
+  def fill_sleigh(size, prev)
+    return @memo[[size, prev]] if @memo.include?([size, prev])
+
+    total = 0
+
+    # order is important, check small to big
+    (1..9).each do |present|
+      # don't allow two presents of the same size next to each other
+      next if present == prev
+
+      remaining = size - present
+
+      # if there is the exact space required for this present then we found a solution
+      total += 1 if remaining.zero?
+
+      # if no more space then we can break early and skip checking even bigger presents
+      break if remaining <= 0
+
+      sub = fill_sleigh(remaining, present)
+      total += sub if sub.positive?
+    end
+
+    @memo[[size, prev]] = total
+
+    total
+  end
+end
+
+if __FILE__ == $PROGRAM_NAME
+  options = {}
+  OptionParser.new do |parser|
+    parser.banner = "Usage: #{$PROGRAM_NAME} [options]"
+
+    parser.on('-c', '--challenge', 'Solve for ever increasing sleigh sizes')
+  end.parse!(into: options)
+
+  challenge = Dayd08.new
+  challenge.solve(options[:challenge])
+end
+```