Add valid? method

spec/board_spec.cr

@@ -42,4 +42,15 @@ describe Sudoku::Board do
     board.extract_block(3, 1).should eq [[42, 43], [44, 45]]
   end
 
+  it "validates cells correctly" do
+    board = Sudoku::Board.new(9, 3)
+    board.put(1, 0, 0)
+    board.put(2, 0, 1)
+    board.put(3, 1, 0)
+    board.valid?(4, 0, 2).should be_true
+    board.valid?(4, 2, 0).should be_true
+    board.valid?(3, 0, 2).should be_false
+    board.valid?(3, 2, 0).should be_false
+  end
+
 end

src/board.cr

@@ -24,8 +24,14 @@ module Sudoku
       @grid[y][x]
     end
 
-    def check_valid(value : Value, x, y)
+    def valid?(value : Value, x, y)
-
+      if value == nil
+        true
+      else
+        extract_row(y).all? { |i| i != value } &&
+        extract_column(x).all? { |i| i != value } &&
+        extract_block(x, y).all? { |i| i.all? { |j| j != value } }
+      end
     end
 
     def extract_row(y)