Lane Merge

Four lanes, four blocks, one gate on the right-hand edge. Every block slides along its lane one square at a time, and a two-square car can also merge into the lane above or below when the whole span there is clear — but a three-square rig is locked to the lane it starts in. Get the marked car ★ to the gate ◎ inside a slide budget that is always achievable, because it is the shortest route the solver found plus four.

It runs entirely on your device: 0 network requests, works offline. No ads, no account, nothing to buy, and no betting, money or prizes of any kind.

1. Focus the board — click it, or press Tab until it is focused.
2. Hold a block. Press Enter to hold the next one, Shift for the previous, or just press any square the block covers. The block you are holding has a thick ring around it.
3. Slide it with ← and →, or A and D. One square per press, into an empty square.
4. Merge it with ↑ and ↓, or W and S. A merge needs the block's whole span clear in the next lane, and only two-square cars can do it — a rig says locked on its left end, a car says merges.
5. Or press an empty square and the block you are holding takes one step toward it: across lanes first, then along the lane. There is also a pad under the board for touch.
6. Get ★ flush against the gate ◎. The marked car has to end up in the gate lane with its nose on the right-hand edge.
7. Read the two numbers. Best possible is the shortest route this board allows from the start; from here is the shortest still available from where the traffic now stands, recalculated after every move. An optimal slide drops it by exactly one. A slide that leaves it alone cost you one of your four spare moves.

A press that cannot move anything costs nothing, so nudging a block against a rig to see what happens is free. When the slides left can no longer cover the route left, the round ends there and says so — shuffling on from a position you cannot win from teaches nothing.

How do you solve a sliding block puzzle where one block has to escape?

Work backwards from the gate, and treat every blocker as its own small puzzle with a price. Ask what has to be true for the last slide: the marked car has to be in the gate lane with a clear square in front of it. Now ask what is standing in that square, and where that block could go — for a car, whether there is a whole clear span in the lane above or below; for a rig, how far along its own lane it can be shoved and whether anything is behind it. Each answer is a sub-goal with a cost in slides, and adding those costs up is usually faster than trying moves. Two habits make the difference. Move the rig first when it is in the way, because it is the block with the fewest options and every other block can route around it afterwards. And do not fill the lane you are about to merge into — the commonest wasted pair of slides is parking a car exactly where the marked one needed to go, then spending another slide moving it out again.

Is every board actually solvable?

Yes, and by construction rather than by luck. The board is built starting from a solved position — the marked car already on the gate — and then every position reachable from it is enumerated. The board you are dealt is drawn from that set, so a position with no way back to the gate is never rejected because it is never proposed. A breadth-first search run backwards from the solved positions labels each one with the exact minimum number of slides it needs, and the deepest label inside the difficulty band is the board you get.

Why is the slide budget the number it is?

It is the solver's own answer plus four. Because one slide is one edge of an unweighted graph, the breadth-first search returns the true shortest route rather than an estimate, so the target is always reachable and always tight. The four spare slides are the margin for going the long way round: an optimal slide keeps the margin intact, a wasted one spends a unit of it, and when the margin runs out the round ends rather than letting you shuffle in a position you can no longer win from.

Is there any money, betting or in-app purchase in this?

None, and there never will be. No currency, nothing to buy, no lives or hints for sale, no prizes, and no gambling of any kind anywhere in Demystify Play.

Do I need to see colour to play this?

No. Every square carries a shape as well as a tint: a star is the marked car, a disc is a two-square car, a bar is a three-square rig and a dashed ring is the gate. Each block also writes its own rule — merges or locked — on its left-hand end, and the block you are holding has a thick outline. Turn the screen greyscale and nothing is lost. A screen reader hears each square named in words, plus what pressing it would do.

Can I play it with a keyboard or a screen reader?

Both. Arrow keys or W A S D move the block you are holding, Enter and Shift change which block that is, and every square is a real button you can reach with Tab. The live region announces which block you are holding, where the marked car and the gate are, how many slides remain and the shortest route still available.

Does it work offline?

Yes. Once the page has loaded once, the whole game is on your device — boards are generated and solved in your browser from a seed, so there is no level server to be offline from.

Four lanes by six columns, four blocks, and a shortest route between eight and twelve slides — deals outside that band are redrawn rather than dealt. Blocks move one square per slide, never two, so the counter and the route are the same unit. There is no undo: an undo would remove the only thing that makes a slide a decision, which is that it costs you one. The solver runs on the whole board after every move, which is a few thousand positions and imperceptible on a phone.

Tilt Maze is the other spatial puzzle here where the route matters more than the direction, and Slide Fifteen shares the discipline of moving the thing that is in the way before the thing you want.

Bookmark this page (Ctrl+D, or ⌘D on Mac) or install the app — it works offline the next time you need it.