chore: work on day24b

lib/day24.rb

@@ -1,4 +1,20 @@
 # frozen_string_literal: true
+
+require 'matrix'
+
+class Point3D
+  attr_reader :x, :y, :z, :cx, :cy, :cz
+
+  def initialize(x, y, z, cx, cy, cz)
+    @x, @y, @z = x.to_f, y.to_f, z.to_f
+    @cx, @cy, @cz = cx.to_f, cy.to_f, cz.to_f
+  end
+
+  def to_s
+    "Position: (#{x}, #{y}, #{z}), Velocity: (#{cx}, #{cy}, #{cz})"
+  end
+end
+
 class Line
   attr_reader :x, :y, :cx, :cy
 
@@ -36,6 +52,32 @@ def intersection_point(other_line)
   end
 end
 
+def find_intersection_point(points)
+  return nil if points.size < 3
+
+  a_rows = []
+  b_elements = []
+
+  3.times do |i|
+    p1, p2 = points[i], points[(i+1) % 3]
+
+    d = Vector[p2.cx - p1.cx, p2.cy - p1.cy, p2.cz - p1.cz]
+    n = d.cross_product(Vector[p1.cx, p1.cy, p1.cz])
+
+    a_rows << n.to_a
+    b_elements << n.dot(Vector[p1.x, p1.y, p1.z])
+  end
+
+  a = Matrix.rows(a_rows)
+  b = Vector.elements(b_elements)
+
+  begin
+    a.inverse * b
+  rescue ExceptionForMatrix::ErrNotRegular
+    return nil
+  end
+end
+
 # Day24
 class Day24
   def load(file, min, max)
@@ -51,6 +93,17 @@ def load(file, min, max)
     end
   end
 
+  def parse(file)
+    content = File.read(file)
+    @parts = []
+    content.split("\n").each do |line|
+      f, e = line.split("@")
+      fs = f.split(",").map(&:to_i)
+      es = e.split(",").map(&:to_i)
+      @parts << Point3D.new(fs[0], fs[1], fs[2], es[0], es[1], es[2])
+    end
+  end
+
   def part1
     result = []
     @parts.each_with_index do |part, i|
@@ -68,5 +121,15 @@ def part1
     end
     result.length
   end
+
+  def part2
+    solution = find_intersection_point(@parts) 
+    if solution.nil?
+      return -1
+    else
+      return soluion[0] + solution[1] + solution[2]
+    end
+    0
+  end
 end
 

test/unit/day24_test.rb

@@ -8,6 +8,12 @@ def test_day24a
     assert_equal(2, sut.part1)
   end
 
+  def test_day24b
+    sut = Day24.new
+    sut.parse('data/day24a.txt')
+    assert_equal(47, sut.part2)
+  end
+
   def test_day24
     sut = Day24.new
     sut.load('data/day24.txt', 200000000000000, 400000000000000)