Parse sudoku boards from strings

src/parser.cr

@@ -1,7 +1,41 @@
 module Sudoku
-  # Parse a sudoku board from a string. Return a board.
+
+  class ParseException < Exception
+  end
+
   class Parser
-    def self.parse(board_str)
+
+    # Parse a sudoku board from a string. Return a board.
+    # The string must contain valid board rows. Each row must be separated by a newline.
+    # Blanks are represented by dots.
+    # E.g.:
+    # .1.2
+    # 2..4
+    # .3..
+    # ....
+    def self.parse(board_str, block_size)
+      rows = board_str.split('\n')
+      board_size = rows.size
+      board = Sudoku::Board.new(board_size, block_size)
+
+      rows.each_with_index { |row, y|
+        if row.size != board_size
+          raise ParseException.new("Invalid row size: #{row.size} for row: #{row}")
+        end
+
+        row.each_char_with_index { |c, x|
+          case c
+          when '.'
+            board.put(nil, x, y)
+          when .number?
+            board.put(c.to_s.to_i, x, y)
+          else
+            raise ParseException.new("Invalid character: #{c}")
+          end
+        }
+      }
+
+      board
     end
   end
 end