9,0,0, 0,5,0, 0,6,0,

  0,5,3, 7,0,0, 0,8,0,

  4,0,0, 0,0,0, 0,0,3,

  0,9,0, 0,0,0, 0,0,0,

  0,8,5, 0,6,0, 0,0,1,

  0,0,0, 4,0,0, 6,0,0,

  0,6,1, 0,4,0, 0,0,8,

  0,0,0, 0,0,2, 0,7,0,

  3,0,0, 0,0,0, 0,0,0

];

​

let state = [];

​

const gridSize = 3;

const numCells = gridSize * gridSize;

let cellSize;

​

let drawPartialState = true;

​

function setup() {

  createCanvas(600, 600);

  cellSize = min(width, height)/numCells;

  createState();

  textAlign(CENTER, CENTER);

}

​

function draw() {

  background(220);

  collapseNext();

  drawGrid();

}

​

function drawGrid() {

  strokeWeight(1);

  for(let j = 0; j < numCells; j ++) {

    for(let i = 0; i < numCells; i ++) {

      square(i * cellSize, j * cellSize, cellSize);

      const possible = Array.from(getCell(i, j).values());

      const cx = (i + 0.5) * cellSize;

      const cy = (j + 0.5) * cellSize;

      if(possible.length === 1) {

        textSize(cellSize/2);

        text(possible[0], cx, cy);

      } else if(drawPartialState) {

        textSize(cellSize/numCells);

        for(let k = 0; k < possible.length; k ++) {

          let x = k % gridSize;

          x = cx + x * cellSize/gridSize - cellSize/gridSize;

          let y = (k / gridSize) | 0;

          y = cy + y * cellSize/gridSize - cellSize/gridSize;

          text(possible[k], x, y);

        }

      }

    }

  }

  // thick borders

  strokeWeight(3);

  const interval = cellSize * gridSize;

  for(let i = 0; i <= gridSize; i ++) {

    line(i * interval, 0, i * interval, height);

    line(0, i * interval, width, i * interval);

  }

}

​

function createState() {

  state = [];

  for(let i = 0; i < original.length; i ++) {

    const v = original[i];

    const possible = new Set();

    if(v) {

      possible.add(v);

    } else {

      for(let n = 1; n <= gridSize * gridSize; n ++) {

        possible.add(n);

      }

    }

    state.push(possible);

  }

  propagateToAll();

}

​

function collapseNext() {

  let fewestOptsIdx = -1;

  let fewestOptsNum = Infinity;

  for(let i = 0; i < state.length; i ++) {

    const num = state[i].size;

    if(num === 0) {

      // should never have 0 options

      // restart

      console.log("fuck!");

      createState();

    } else if(num > 1 && num < fewestOptsNum) {

      fewestOptsIdx = i;

      fewestOptsNum = num;

    }

  }

  if(fewestOptsIdx != -1) {

    const opts = Array.from(state[fewestOptsIdx].values());

    const collapsed = new Set();

    collapsed.add(random(opts));

    state[fewestOptsIdx] = collapsed;

  }

  propagateToAll();

}

​

function propagateToAll() {

 for(let j = 0; j < numCells; j ++) {

    for(let i = 0; i < numCells; i ++) {

      propagate(i, j);

    }

  }

}

​

function propagate(x, y) {

  let impossible = superCell(x, y);

  impossible = union(impossible, column(x));

  impossible = union(impossible, row(y));

  const cell = getCell(x, y);

  if(cell.size === 1) {

    return;

  }

  impossible.forEach(v => cell.delete(v));

}

​

function superCell(x, y) {

  const set = new Set();

  x = ((x / gridSize) | 0) * gridSize;

  y = ((y / gridSize) | 0) * gridSize;

  for(let j = 0; j < gridSize; j ++) {

    for(let i = 0; i < gridSize; i ++) {

      const cell = getCell(x + i, y + j);

      if(cell.size > 1) {

        continue;

      }

      cell.forEach(c => set.add(c));

    }

  }

  return set;

}

​

function column(i) {

  const set = new Set();

  for(let j = 0; j < numCells; j ++) {

    const cell = getCell(i, j);

    if(cell.size > 1) {

      continue;

    }

    cell.forEach(c => set.add(c));

  }

  return set;

}

​

function row(j) {

  const set = new Set();

  for(let i = 0; i < numCells; i ++) {

    const cell = getCell(i, j);

    if(cell.size > 1) {

      continue;

    }

    cell.forEach(c => set.add(c));

  }

  return set;

}

​

function getCell(x, y) {

  return state[x + y * numCells];

}

​

​

// Set functions

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

​

function union(setA, setB) {

  const _union = new Set(setA);

  for (const elem of setB) {

    _union.add(elem);

  }

  return _union;

}