2023 Day 9 Part 2 Completed

2023/09/run.rb

@@ -5,4 +5,5 @@
 input = File.read('./input.txt')
 
 ad = Advent::Sequences.new(input)
-puts "Part 1 Sum: #{ad.sum_next_number}"
+puts "Part 1 Sum: #{ad.sum_next_number}"
+puts "Part 2 Sum: #{ad.sum_prev_number}"

2023/09/sequences.rb

@@ -43,10 +43,34 @@ def next_number(sequence)
       working.last.last
     end
 
+    def prev_number(sequence)
+      working = [sequence.dup]
+      puts "Working A: #{working.inspect}" if @debug
+      until working.last.all?(&:zero?) do
+        working << diffs(working.last)
+        puts "Working B: #{working.inspect}" if @debug
+      end
+
+      while working.length > 1 do
+        puts "Working C: #{working.inspect}" if @debug
+        seq = working.pop
+        working.last.unshift(working.last.first - seq.first)
+      end
+
+      puts "Working D: #{working.inspect}" if @debug
+      working.last.first
+    end
+
     def sum_next_number
       @sequences.sum do |seq|
         next_number(seq)
       end
     end
+
+    def sum_prev_number
+      @sequences.sum do |seq|
+        prev_number(seq)
+      end
+    end
   end
 end

2023/09/spec/sequences_spec.rb

@@ -34,11 +34,23 @@
       end
     end
 
-    context "validation" do
+    describe "#prev_number" do
+      it "returns the next number in the sequence" do
+        # ad.debug!
+        expect(ad.prev_number([0, 3, 6, 9, 12, 15])).to eq(-3)
+        expect(ad.prev_number([1, 3, 6, 10, 15, 21])).to eq(0)
+        expect(ad.prev_number([10, 13, 16, 21, 30, 45])).to eq(5)
+      end
+    end
 
+    context "validation" do
       it "returns the sum of the next number in the sequence for part 1" do
         expect(ad.sum_next_number).to eq(114)
       end
+
+      it "returns the sum of the prev number in the sequence for part 2" do
+        expect(ad.sum_prev_number).to eq(2)
+      end
     end
   end
 end