Stacker

The Stacker stacks the shape received on its top input on top of the shape received on its bottom input and outputs the combined result. The machine always operates on east and west halves regardless of rotation; mirroring the machine swaps which physical input is considered the top vs bottom for stacking. If either input is null, the output will be null (the Simulated Stacker follows the same checks).

When performing a stack the top shape is first placed above the bottom shape with one empty layer between them. After that, the Shape Gravity Rules are applied so layers fall where possible. As a result, the top shape will fall down as far as possible onto the bottom shape. Two full layers stacked on each other will remain as separate layers, while complementary halves can combine into a single full layer during the gravity resolution. Any crystal pieces that fall as part of the stacking/gravity process will shatter.

The Stacker can be thought of as the counterpart to the Unstacker: it merges two shape signals into one stacked shape signal. The Simulated Stacker example shows that, by default, the left input is stacked on top of the right input and this is reversed if the machine is mirrored. The machine does not produce extra outputs beyond the single stacked result.

Practical notes and behavior summary:

• Inputs: two shape inputs (top and bottom); if either is null, output is null. Mirroring flips which input is considered top.
• Procedure: top shape is placed above bottom shape with one empty layer, then gravity rules are applied so layers fall and combine where possible.
• Result patterns: two full layers remain separate; opposite halves can merge into a single full layer; crystals that are vertically separated and then fall will shatter.
• Use cases: combine halves or whole shapes produced by Cutters or other machines to rebuild multi-layer shapes. Be aware that stacking can destroy crystals if they fall — use Swapper when you need to combine halves without causing crystals to drop and shatter, since the Swapper places halves beside each other rather than dropping them.
• Simulated machine: the Simulated Stacker follows the same crystal and floating-shape checks as the physical Stacker and outputs null if either input is null.
• Machine rotation does not change its logical east/west half behavior; plan belt routing accordingly so the intended halves are fed into the correct inputs.

The Stacker is essential for creating multi-layered outputs and recombining cut halves, but its gravity-driven finishing step and crystal fragility must be considered when designing production lines.