Day 10 - Puzzle 1

Author: ariejan

lib/input/day_10.txt

@@ -0,0 +1,57 @@
+543213210349876210129890176034034565966563210587109092121
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+101210510123430192154345478776212656548989656743089876501

lib/solutions/day_10.rb

@@ -0,0 +1,61 @@
+class Day10
+  def part_one(input)
+    thf = TrailheadFinder.new(input)
+    thf.find_trailheads
+  end
+
+  def part_two(input)
+    0
+  end
+end
+
+class TrailheadFinder
+  attr_accessor :grid, :w, :h
+
+  def initialize(input)
+    # Parse grid
+    @grid = input.split("\n").map { |line| line.chars.map(&:to_i) }
+    @w = @grid[0].size
+    @h = @grid.size
+  end
+
+  def find_trailheads
+    total_nines = 0
+
+    @grid.each_with_index do |row, y|
+      row.each_with_index do |cell, x|
+        total_nines += count_reachable_nines(x, y) if cell == 0
+      end
+    end
+
+    total_nines
+  end
+
+  def count_reachable_nines(x, y)
+    visited = Array.new(@h) { Array.new(@w, false) }
+    queue = [[x, y, 0]]
+    reachable_nines = 0
+
+    until queue.empty?
+      cx, cy, current_value = queue.shift
+      next if visited[cy][cx]
+
+      visited[cy][cx] = true
+
+      if @grid[cy][cx] == 9
+        reachable_nines += 1
+        next
+      end
+
+      [[0, 1], [1, 0], [0, -1], [-1, 0]].each do |dx, dy|
+        nx = cx + dx
+        ny = cy + dy
+        if nx.between?(0, @w - 1) && ny.between?(0, @h - 1) && !visited[ny][nx] && @grid[ny][nx] == current_value + 1
+          queue << [nx, ny, current_value + 1]
+        end
+      end
+    end
+
+    reachable_nines
+  end
+end

spec/input/day_10_test.txt

@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732

spec/solutions/day_10_spec.rb

@@ -0,0 +1,28 @@
+require 'spec_helper'
+require 'solutions/day_10'
+
+RSpec.describe Day10 do
+  let(:input) { File.read(File.join(__dir__, '..', '..', 'spec', 'input', 'day_10_test.txt')).strip }
+
+  describe '#part_one' do
+    it 'small example' do
+      input = <<~INPUT
+        0123
+        1234
+        8765
+        9876
+      INPUT
+      expect(subject.part_one(input)).to eq(1)
+    end
+
+    it 'calculates the correct solutions for part one' do
+      expect(subject.part_one(input)).to eq(36)
+    end
+  end
+
+  describe '#part_two' do
+    it 'calculates the correct solutions for part two' do
+      expect(subject.part_two(input)).to eq(0)
+    end
+  end
+end