implemented day 3 part 2.

src/advent/day3.rb

@@ -16,6 +16,26 @@ def group_neighbour(lst, &block)
   ret
 end
 
+class Gear
+  def initialize(line_index, column_index)
+    @line_index = line_index
+    @column_index = column_index
+  end
+
+  def get_neighbourhood
+    ret = []
+    (-1..1).each do |line_delta|
+      (-1..1).each do |col_delta|
+        ret.append({ line_index: @line_index + line_delta, column_index: @column_index + col_delta})
+      end
+    end
+    ret
+  end
+
+  attr_accessor :line_index
+  attr_accessor :column_index
+end
+
 class EngineNumber
   def initialize(line_index, start, end_, value)
     @line_index = line_index
@@ -24,6 +44,10 @@ class EngineNumber
     @value = value
   end
 
+  def is_located_on(line_index, column_index)
+    line_index == @line_index and ( @start <= column_index and column_index < @end_)
+  end
+
 
   attr_accessor :line_index
   attr_accessor :start
@@ -77,6 +101,16 @@ class EngineLine
     group_digit_indexes(digit_indexes).map &method(:create_engine_number)
   end
 
+  def extract_gears
+    ret = []
+    @line.split("").each_with_index do |letter, index|
+      if letter == "*"
+        ret.append Gear.new @idx, index
+      end
+    end
+    ret
+  end
+
 end
 
 class Engine
@@ -123,6 +157,29 @@ class Engine
     extract_engine_numbers.filter {|engine_number| touches_symbol? engine_number}
   end
 
+  def is_adjacent(engine_number, gear)
+    gear.get_neighbourhood
+        .map {|point| engine_number.is_located_on(point[:line_index], point[:column_index])}
+        .any?
+  end
+
+  def get_adjacent_number_parts(gear)
+    extract_engine_numbers
+      .filter {|eg| is_adjacent(eg, gear)}
+  end
+
+  def extract_gears
+    @engine_lines
+      .map {|line| line.extract_gears}
+      .flatten
+  end
+
+  def get_gear_ratios
+    extract_gears
+      .map {|gear| get_adjacent_number_parts(gear)}
+      .filter {|adjacent_numbers| adjacent_numbers.length == 2}
+      .map {|adjacent_numbers| adjacent_numbers.map {|a| a.value} .reduce(1, :*)}
+  end
   attr_accessor :engine_lines
 end
 
@@ -134,4 +191,5 @@ input_content = File.open(input_file_name).read
 engine = Engine.deserialize input_content
 
 puts engine.extract_engine_numbers_that_touches_symbols.map {|e| e.value} .sum
+puts engine.get_gear_ratios.sum
 

src/advent/day3_test.rb

@@ -31,6 +31,8 @@ class Day3Test < Test::Unit::TestCase
 
     assert_equal "hello"[4..10], "o"
 
+    assert_true 0 <= 10 and 10 < 100
+
 
 
   end
@@ -102,4 +104,19 @@ class EngineLineTest < Test::Unit::TestCase
     assert_equal expt, engine.extract_engine_numbers_that_touches_symbols.map {|e| e.value} .sum
   end
 
+  def test_extract_gears
+    engine = Engine.deserialize @@easy_engine
+    assert_equal 3, engine.extract_gears.length
+  end
+
+  def test_get_gear_ratios
+    engine = Engine.deserialize @@easy_engine
+    assert_equal [16345, 451490], engine.get_gear_ratios
+  end
+
+  def test_day_3_part_2_easy
+    engine = Engine.deserialize @@easy_engine
+    assert_equal 467835, engine.get_gear_ratios.sum
+  end
+
 end