// Simple sudoku solver, written by anvaka (https://twitter.com/anvaka)

​

// JSBin - don't break the loops!

// noprotect

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let grid = [

  8, 0, 0,  0, 0, 0,  0, 0, 0,

  0, 0, 3,  6, 0, 0,  0, 0, 0,

  0, 7, 0,  0, 9, 0,  2, 0, 0,

  0, 5, 0,  0, 0, 7,  0, 0, 0,

  0, 0, 0,  0, 4, 5,  7, 0, 0,

  0, 0, 0,  1, 0, 0,  0, 3, 0,

  0, 0, 1,  0, 0, 0,  0, 6, 8,

  0, 0, 8,  5, 0, 0,  0, 1, 0,

  0, 9, 0,  0, 0, 0,  4, 0, 0  

]

solve(grid); 

renderGrid(grid)

​

function solve(grid) {

  let emptyCellIndex = grid.findIndex(v => v === 0);

  if (emptyCellIndex < 0) return true;

  let x = emptyCellIndex % 9, y = Math.floor(emptyCellIndex / 9);

​

  for (let candidate = 1; candidate < 10; ++candidate) {

    if (isTaken(grid, candidate, x ,y)) continue;

    grid[x + y * 9] = candidate;

    if (solve(grid)) return true;

    grid[x + y * 9] = 0;

  }

}

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function isTaken(grid, value, x, y) {

  let cellTaken = other => other === value;

  return verticalIsTaken(grid, x, cellTaken) ||

    horizontalIsTaken(grid, y, cellTaken) ||

    blockIsTaken(grid, x, y, cellTaken)

}

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function verticalIsTaken(grid, x, cellTaken) {

  for (let y = 0; y < 9; ++y) if (cellTaken(grid[y * 9 + x])) return true;

}

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function horizontalIsTaken(grid, y, cellTaken) {

  for (let x = 0; x < 9; ++x) if (cellTaken(grid[y * 9 + x])) return true;

}

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function blockIsTaken(grid, x, y, cellTaken) {

  let bx = Math.floor(x/3) * 3, by = Math.floor(y / 3) * 3;

  for (let i = 0; i < 9; ++i) 

    if (cellTaken(grid[(by + Math.floor(i / 3)) * 9 + (bx + i % 3)])) return true;   

}

​

// That's it! The code below is just the UI

​

function renderGrid(grid) {

  let el = document.querySelector('.container');

  el.innerHTML = '';

  let w = 24; 

  let h = 24;

  el.style.width = (w * 9) + 'px';

  el.style.height = (h * 9) + 'px';

  grid.forEach((value, idx) => {

    let x = idx % 9, y = Math.floor(idx / 9);

    let bx = Math.floor(x/3) * 3, by = Math.floor(y / 3) * 3;

    let div = document.createElement('div');

    div.classList.add('cell');

    div.classList.add('x-' + x);

    div.classList.add('y-' + y);

    if ((bx ^ by) & 1) div.classList.add('alt-background');

    div.style.left = x * w + 'px';

    div.style.top = y * h + 'px';

    div.style.width = w + 'px';

    div.style.height = h + 'px';

    div.innerText = value === 0 ? '' : value;

    el.appendChild(div);

  });

}

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