euler11.rb

   1 grid_line = %w{
   2 08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
   3 49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
   4 81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
   5 52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91
   6 22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
   7 24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50
   8 32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
   9 67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
  10 24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
  11 21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
  12 78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
  13 16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
  14 86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
  15 19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
  16 04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
  17 88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
  18 04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
  19 20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
  20 20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
  21 01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48 }
  22
  23 $grid = []
  24 grid_line.map {|s| s.to_i}.each_slice(20) {|s| $grid << s}
  25 $line_length = 4
  26
  27 def find_largest_product
  28   largest = 0
  29   largest_row = 0
  30   largest_column = 0
  31   largest_direction = :east
  32   $grid.length.times do |row|
  33     $grid[0].length.times do |column|
  34       [:east, :south_east, :south, :south_west].each do |direction|
  35         this_product = find_product($grid, row, column, direction, $line_length)
  36         if this_product > largest
  37           largest = this_product
  38           largest_row = row
  39           largest_column = column
  40           largest_direction = direction
  41         end
  42       end
  43     end
  44   end
  45   return largest, largest_row, largest_column, largest_direction
  46 end
  47
  48 def offsets_of_direction(direction)
  49   case direction
  50   when :north
  51     return -1, 0
  52   when :south
  53     return 1, 0
  54   when :east
  55     return 0, 1
  56   when :west
  57     return 0, -1
  58   when :north_east
  59     return -1, 1
  60   when :south_east
  61     return 1, 1
  62   when :south_west
  63     return 1, -1
  64   when :north_west
  65     return -1, -1
  66   end
  67 end
  68
  69 def find_product(grid, row, column, direction, line_length)
  70   row_change, column_change = offsets_of_direction(direction)
  71   end_row = row + row_change * (line_length - 1)
  72   end_column = column + column_change * (line_length - 1)
  73   return 0 unless (0...grid.length).include? end_row and
  74     (0...grid[0].length).include? end_column
  75   if row_change == 0
  76     rows = [row] * line_length
  77   else
  78     rows = row.step(end_row, row_change).to_a
  79   end
  80   if column_change == 0
  81     cols = [column] * line_length
  82   else
  83     cols = column.step(end_column, column_change).to_a
  84   end
  85   rows.zip(cols).reduce(1) {|product, i| product *= grid[i[0]][i[1]]}
  86 end
  87
  88 if __FILE__==$0
  89   puts find_largest_product[0]
  90 end