diff --git a/main.lua b/main.lua
index 3733b7b..c9a0d5b 100644
--- a/main.lua
+++ b/main.lua
@@ -1,4 +1,4 @@
-require "sudoku.utils"
+sudoku = require "sudoku.sudoku"
 uiUtils = require "ui.utils"
 config = require "config"
 SudokuCanvas = require "ui.sudokucanvas"
@@ -11,10 +11,10 @@ function love.load()
     love.mouse.setGrabbed(false)
     love.keyboard.setKeyRepeat(true)
     math.randomseed(os.time())
-    local board = loadBoard(config.board.filePath)
+    board = sudoku.loadBoard(config.board.filePath)
     local width, height = love.graphics.getDimensions()
     sudokuCanvas = SudokuCanvas:new(200, 100, height/1.5, config.SudokuCanvas)
-    sudokuCanvas:setBoard(board)
+    sudokuCanvas:setBoard(board[1])
     myFont = love.graphics.newFont(24)
 end
 
diff --git a/sudoku/check.lua b/sudoku/check.lua
deleted file mode 100644
index 3c9b381..0000000
--- a/sudoku/check.lua
+++ /dev/null
@@ -1,132 +0,0 @@
-local function checkCT(ct)
-    local r = {}
-    for i = 1, 8 do
-        for j = i+1, 9 do
-            if ct[i] == ct[j] and ct[i] ~= 0 then
-                table.insert(r, {i, j})
-            end
-        end
-    end
-    if #r > 0 then return false, r end
-    return true
-end
-
-local function findThisTable(this, that)
-    for i = 1, #that do
-        if #that[i] == #this then
-            local flag = true
-            for j = 1, #this do
-                if this[j] ~= that[i][j] then flag = false; break end
-            end
-            if flag then return true, i end
-        end
-    end
-    return false
-end
-
-function check(board)
-    local rr = {}
-    for x = 1, 9 do
-        local ct = {}
-        for y = 1, 9 do
-            ct[y] = board[x][y]
-        end
-        local ok, r = checkCT(ct)
-        if not ok then
-            for _, p in ipairs(r) do
-                local t = {x, p[1], x, p[2], board[x][p[1]]}
-                if not findThisTable(t, rr) then
-                    table.insert(rr, t)
-                end
-            end
-        end
-    end
-    for y = 1, 9 do
-        local ct = {}
-        for x = 1, 9 do
-            ct[x] = board[x][y]
-        end
-        local ok, r = checkCT(ct)
-        if not ok then
-            for _, p in ipairs(r) do
-                local t = {p[1], y, p[2], y, board[p[1]][y]}
-                if not findThisTable(t, rr) then
-                    table.insert(rr, t)
-                end
-            end
-        end
-    end
-    for i = 0, 2 do
-        for j = 0, 2 do
-            local ct = {}
-            for x = 1, 3 do
-                for y = 1, 3 do
-                    ct[(x-1)*3+y] = board[x+i*3][y+j*3]
-                end
-            end
-            local ok, r = checkCT(ct)
-            if not ok then
-                for _, p in ipairs(r) do
-                    -- local x1 = ((p[1] - 1) // 3 + 1) + i * 3
-                    local x1 = math.floor((p[1] - 1) / 3 + 1) + i * 3
-                    local y1 = ((p[1] - 1) % 3 + 1) + j * 3
-                    -- local x2 = ((p[2] - 1) // 3 + 1) + i * 3
-                    local x2 = math.floor((p[2] - 1) / 3 + 1) + i * 3
-                    local y2 = ((p[2] - 1) % 3 + 1) + j * 3
-                    local t = {x1, y1, x2, y2, board[x1][y1]}
-                    if not findThisTable(t, rr) then
-                        table.insert(rr, t)
-                    end
-                end
-            end
-        end
-    end
-    if #rr > 0 then
-        return false, rr
-    end
-    return true
-end
-
-function checkSolved(board)
-    local r = {}
-    for i = 1, 9 do
-        for j = 1, 9 do
-            if board[i][j] < 1 or board[i][j] > 9 then
-                table.insert(r, {i, j, board[i][j]})
-            end
-        end
-    end
-    local ok, rr = check(board)
-    if #r > 0 or (rr and #rr > 0) then
-        return false, r, rr
-    end
-    return true
-end
-
-function pcheck(board)
-    local ok, r, rr = checkSolved(board)
-    if ok then
-        print("Board is solved!")
-        return 
-    end
-    for _, p in ipairs(r) do
-        print(S("(%d, %d) = %d", unpack(p)))
-    end
-    for _, p in ipairs(rr) do
-        print(S("(%d, %d) = (%d, %d) = %d", unpack(p)))
-    end
-end
-
-function scheck(board)
-    local msg = {}
-    local ok, r = check(board)
-    if ok then
-        return {"No Error!"}
-    end
-    for _, p in ipairs(r) do
-        local s = string.format("(%d, %d) = (%d, %d) = %d", unpack(p))
-        print(s)
-        table.insert(msg, s)
-    end
-    return msg
-end
diff --git a/sudoku/sudoku.lua b/sudoku/sudoku.lua
index ad40765..e2077a6 100644
--- a/sudoku/sudoku.lua
+++ b/sudoku/sudoku.lua
@@ -1,47 +1,85 @@
-require "sudoku.utils"
-require "sudoku.check"
+local utils = require "sudoku.utils"
 
-function calValidOptions(b, x, y)
-    if b[x][y] ~= 0 then return {} end
-    local elm
-    local u = {}
-    local v = {}
-    for i = 1, 9 do
-        elm = b[x][i]
-        if elm ~= 0 and find(u, elm) == 0 then
-            table.insert(u, elm)
+local sudoku = {}
+
+function sudoku.iterZones(board)
+    local row = 0
+    local column = 0
+    local block = 0
+    return function()
+        local a = {}
+        local p = {}
+        if row < 9 then
+            row = row + 1
+            for i = 1, 9 do
+                a[i] = board[row][i]
+                p[i] = {row, i}
+            end
+            return a, p
         end
-        elm = b[i][y]
-        if elm ~= 0 and find(u, elm) == 0 then
-            table.insert(u, elm)
+        if column < 9 then
+            column = column + 1
+            for i = 1, 9 do
+                a[i] = board[i][column]
+                p[i] = {i, column}
+            end
+            return a, p
+        end
+        if block < 9 then
+            block = block + 1
+            local dx = math.floor((block - 1) / 3)
+            local dy = (block - 1) % 3
+            local n = 0
+            for i = 1, 3 do
+                for j = 1, 3 do
+                    local x = i + 3 * dx
+                    local y = j + 3 * dy
+                    n = n + 1
+                    a[n] = board[x][y]
+                    p[n] = {x, y}
+                end
+            end
+            return a, p
         end
     end
-    -- local xc = ((x - 1) // 3) * 3
-    -- local yc = ((y - 1) // 3) * 3
-    local xc = math.floor((x - 1) / 3) * 3
-    local yc = math.floor((y - 1) / 3) * 3
-    for i = 1, 3 do
-        for j = 1, 3 do
-            elm = b[i+xc][j+yc]
-            if elm ~= 0 and find(u, elm) == 0 then
-                table.insert(u, elm)
+end
+
+local function checkAUX(zone, pos)
+    local isComplete = true
+    local ok = true
+    local conflictList = {}
+    local n = 0
+    for i = 1, 8 do
+        for j = i+1, 9 do
+            if zone[i] == 0 then
+                isComplete = false
+            elseif zone[i] == zone[j] then
+                ok = false
+                n = n + 1
+                conflictList[n] = {x1=pos[i][1], y1=pos[i][2], x2=pos[j][1], y2=pos[j][2]}
             end
         end
     end
-    for i = 1, 9 do
-        if find(u, i) == 0 then table.insert(v, i) end
-    end
-    return v
+    return ok, isComplete, conflictList
 end
 
-function buildSearchSpace(b)
-    local s = {}
-    for i = 1, 9 do
-        s[i] = {}
-        for j = 1, 9 do
-            s[i][j] = calValidOptions(b, i, j)
+function sudoku.checkBoard(board)
+    local ok = true
+    local isComplete = true
+    local conflictList = {}
+    for zone, pos in sudoku.iterZones(board) do
+        local tOk, tIsComplete, tConflictList = checkAUX(zone, pos)
+        if not tOk then ok = false end
+        if not tIsComplete then isComplete = false end
+        for _, conflict in ipairs(tConflictList) do
+            table.insert(conflictList, conflict)
         end
     end
-    return s
+    return ok, isComplete, conflictList
 end
 
+sudoku.cloneBoard = utils.cloneBoard
+sudoku.createEmptyBoard = utils.createEmptyBoard
+sudoku.loadBoard = utils.loadBoard
+return sudoku
+
diff --git a/sudoku/utils.lua b/sudoku/utils.lua
index c1516da..e9937b6 100644
--- a/sudoku/utils.lua
+++ b/sudoku/utils.lua
@@ -1,4 +1,21 @@
-function find(array, element)
+local function table2string(t)
+    local myself = table2string
+    local l = {}
+    local n = 0
+    for k, v in pairs(t) do
+        n = n + 1
+        if type(v) == "table" then
+            l[n] = k .. ': ' .. myself(v)
+        elseif type(v) == "string" then
+            l[n] = k .. ': ' .. '"' .. v .. '"'
+        else
+            l[n] = k .. ': ' .. v
+        end
+    end
+    return "{" .. table.concat(l, ", ") .. "}"
+end
+
+local function find(array, element)
     local index = 0
     for i = 1, #array do
         if array[i] == element then
@@ -9,13 +26,18 @@ function find(array, element)
     return index
 end
 
-function showBoard(board)
-    for row = 1, 9 do
-        print(table.concat(board[row], " "))
+local function createEmptyBoard()
+    local b = {}
+    for i = 1, 9 do
+        b[i] = {}
+        for j = 1, 9 do
+            b[i][j] = 0
+        end
     end
+    return b
 end
 
-function cloneBoard(b)
+local function cloneBoard(b)
     local c = {}
     for i = 1, 9 do
         c[i] = {}
@@ -26,14 +48,14 @@ function cloneBoard(b)
     return c
 end
 
-function loadBoard(fn)
-    local problem = {}
-    local solution = {}
-    local board = problem
+local function loadBoard(fn)
+    local boards = {}
     local r = 0
+    local board = {}
     for line in io.lines(fn) do
         if line:gsub("%s+", "") == "" then
-            board = solution
+            table.insert(boards, board)
+            board = {}
             r = 0
         else
             r = r + 1
@@ -45,5 +67,18 @@ function loadBoard(fn)
             end
         end
     end
-    return problem, solution
+    table.insert(boards, board)
+    return boards
 end
+
+local function showBoard(board)
+    for row = 1, 9 do
+        print(table.concat(board[row], " "))
+    end
+end
+
+return {
+    createEmptyBoard = createEmptyBoard,
+    cloneBoard = cloneBoard,
+    loadBoard = loadBoard,
+}
diff --git a/sudokuGUI b/sudokuGUI
new file mode 100755
index 0000000..1003ed0
--- /dev/null
+++ b/sudokuGUI
@@ -0,0 +1,3 @@
+#! /bin/sh
+
+love .
diff --git a/ui/sudokucanvas.lua b/ui/sudokucanvas.lua
index 073f319..26e5c33 100644
--- a/ui/sudokucanvas.lua
+++ b/ui/sudokucanvas.lua
@@ -1,20 +1,3 @@
-function table2string(t)
-    local myself = table2string
-    local l = {}
-    local n = 0
-    for k, v in pairs(t) do
-        n = n + 1
-        if type(v) == "table" then
-            l[n] = k .. ': ' .. myself(v)
-        elseif type(v) == "string" then
-            l[n] = k .. ': ' .. '"' .. v .. '"'
-        else
-            l[n] = k .. ': ' .. v
-        end
-    end
-    return "{" .. table.concat(l, ", ") .. "}"
-end
-
 SudokuCanvas = {}
 SudokuCanvas.__index = SudokuCanvas
 
@@ -187,9 +170,11 @@ end
 function SudokuCanvas:setBoard(board, board2)
     local board2 = board2 or board
     self.board = {}
-    self.board[1] = cloneBoard(board)
-    self.board[2] = cloneBoard(board2)
-    self.board[3] = cloneBoard(board2)
+    self.board[1] = sudoku.cloneBoard(board)
+    self.board[2] = sudoku.cloneBoard(board2)
+    self.board[3] = sudoku.cloneBoard(board2)
+    -- self.board[4] = createEmptyBoard()
+    self:checkBoard()
 end
 
 function SudokuCanvas:new(x, y, size, options)
@@ -225,6 +210,15 @@ end
 function SudokuCanvas:focus()
 end
 
+function SudokuCanvas:checkBoard()
+    local ok, isComplete, conflictList = sudoku.checkBoard(self.board[2])
+    self.board[4] = sudoku.createEmptyBoard()
+    for _, c in ipairs(conflictList) do
+        self.board[4][c.x1][c.y1] = 1
+        self.board[4][c.x2][c.y2] = 1
+    end
+end
+
 function SudokuCanvas:draw()
     local grid = self.grid
 
@@ -270,14 +264,22 @@ function SudokuCanvas:draw()
                 local px = self.bigNumbers[x][y][num].typed.x
                 local py = self.bigNumbers[x][y][num].typed.y
                 love.graphics.setFont(self.bigNumbers.typedFont)
-                uiUtils.setColor(self.colors.bigNumbersTyped)
+                if self.board[4][y][x] ~= 0 then
+                    uiUtils.setColor(self.colors.bigNumbersError)
+                else
+                    uiUtils.setColor(self.colors.bigNumbersTyped)
+                end
                 love.graphics.print(tostring(num), px, py)
             elseif self.board[2][y][x] ~= 0 then
                 local num = self.board[2][y][x]
                 local px = self.bigNumbers[x][y][num].handWritten.x
                 local py = self.bigNumbers[x][y][num].handWritten.y
                 love.graphics.setFont(self.bigNumbers.handWrittenFont)
-                uiUtils.setColor(self.colors.bigNumbersHandWritten)
+                if self.board[4][y][x] ~= 0 then
+                    uiUtils.setColor(self.colors.bigNumbersError)
+                else
+                    uiUtils.setColor(self.colors.bigNumbersHandWritten)
+                end
                 love.graphics.print(tostring(num), px, py)
             end
         end
@@ -315,17 +317,25 @@ function SudokuCanvas:keypressed(key)
         local num = key:byte() - 48
         if self.board[1][y][x] == 0 then
             if mode == "big" then
-                self.board[2][y][x] = num
-                self.board[3][y][x] = num
+                if self.board[2][y][x] == num then
+                    self.board[2][y][x] = 0
+                    self.board[3][y][x] = 0
+                else
+                    self.board[2][y][x] = num
+                    self.board[3][y][x] = num
+                end
+                self:checkBoard()
             else
                 self.smallNumbers[x][y][num].enabled = not self.smallNumbers[x][y][num].enabled
             end
+            self:checkBoard()
         end
     elseif key == "delete" or key == "0" then
         if self.board[1][y][x] == 0 then
             if mode == "big" then
                 self.board[2][y][x] = 0
                 self.board[3][y][x] = 0
+                self:checkBoard()
             else
                 for num = 1, 9 do
                     self.smallNumbers[x][y][num].enabled = false