02Credits Blog

Day42 - Finished Combos!

Built combo recognition


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Today I finally finished the combo recognition feature! It took me a couple of days to figure out the exact semantics, but I am fairly confident that I have gotten it correct. The combo feature in the original game was more complicated than I originally thought!

Problem Definition

Although easy to understand intuitively, the combo feature in Tetris attack is somewhat nuanced. The complication stems from the fact that combo extension can happen a ways after the original match occurred. Any block that falls as the result of a match may be used to extend the combo. To that end when implementing the combo recognition, every block effected by a match must be watched until we can prove the block has "settled" and will not fall anymore.

Another complication is that the current combo status must be reported at the start of a match while the tracked blocks must be collected after the match has been cleared. So to keep concerns properly separated I introduced events for match creation and completion and used them to track when to create and manage matches.


  MatchStarted.Subscribe(matchedBlocks => {
  for (let combo of combos) {
    if (matchedBlocks.some(matchedBlock => combo.trackedBlocks.has(matchedBlock))) {
      ComboExtended.Publish(matchedBlocks, combo.cascades);
MatchCompleted.Subscribe(clearedBlocks => {
  let foundExistingCombo = false;
  for (let combo of combos) {
    if (clearedBlocks.some(clearedBlock => combo.trackedBlocks.has(clearedBlock))) {
      foundExistingCombo = true;
  if (!foundExistingCombo) {
    combos.add(new Combo(clearedBlocks));

At match creation, any combos with tracked blocks participating in the match increment their counters and notify of their extension.

Upon completion, any combos with tracked blocks will search upward for new blocks to track and if no existing combos are participating a new combo is created for the newly matched blocks.

Combo Class

  export class Combo {
  constructor(matchedBlocks) {
    this.cascades = 0;
    this.trackedBlocks = new Set();

This new combo tracks the number of cascades it has participated in, a set of the blocks it is currently tracking, and searches upward from the initial block set for blocks to track.

  processMatchedBlocks(matchedBlocks) {
  for (let matchedBlock of matchedBlocks) {
    for (let y = matchedBlock.gridSlot.y - 1; y > 0; y--) {
      let fallingBlock = getBlock(matchedBlock.gridSlot.withY(y));
      if (!fallingBlock) break;
      if (fallingBlock.state === state.WAITING) {

Since the matched blocks are no longer needed, I search upward from each one until the top of the grid or the first empty block space. Any blocks currently in the Waiting state are added to the tracking list. Then every update loop the blocks are checked to ensure they are not settled. Any settled blocks are removed from the list and if the combo no is no longer tracking any blocks, then it is assumed that the combo is finished and it reports completion with the ComboFinished event as well as returning a true value to indicate that this combo should be removed from the combo list.

  update() {
  for (let trackedBlock of this.trackedBlocks) {
    if (trackedBlock.state !== state.FALLING &&
        trackedBlock.state !== state.MATCHED &&
        trackedBlock.state !== state.CLEARING &&
        !emptySlotBelow(trackedBlock)) {
  if (this.trackedBlocks.size == 0) {
    if (this.cascades > 0) {
    return true;
  return false;


A block is considered settled if it is not Falling, Matched, or Clearing, or if there is an empty block somewhere below it as defined by the emptySlotBelow function:

  function emptySlotBelow(block) {
  for (let y = block.gridSlot.y + 1; y <= previousFilledY; y++) {
    let possiblyEmptyBlock = getBlock(block.gridSlot.withY(y));
    if (!possiblyEmptyBlock || volatileStates.includes(possiblyEmptyBlock.state)) {
      return true;
  return false;

Which super cleanly searches downward from each block checking if any of them are blank or about to be blank.

And thats it! Not super complicated when put together, but it took me a long time to get to this point. And to top it all off, while writing this description I realized that my definition of settled will need some help since garbage blocks can let settled blocks rest above empty spaces. For now though this solution is good enough and I will cross that bridge when I get to it.

Not a super visual post today, but I plan on working on some better artwork soon so hopefully those posts will make for prettier reading.

Till tomorrow!