Oncoming glider gun construction arm
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An oncoming glider gun construction arm is a highly efficient universal construction mechanism invented in 2026 by russellsprouts and inductivetype. In the technique, a single-channel stream of recipe gliders collides with the oncoming output of a glider gun. The most studied variant uses a p120 Simkin glider gun and a lane offset of 5. This variant can send slow salvos with an efficiency of around 3 fast gliders per slow glider.
The oncoming stream of gun gliders could be seen as a steady stream of moving elbows. An elbow "pull" simply requires delaying a recipe until the gliders advance to the desired location. An elbow "push" (aka swimming upstream) consumes one or more gun gliders to target ones further back.
It can also be seen as a special case of armless construction. Armless construction uses two recipe streams colliding in the same way as in the oncoming glider gun construction arm. Armless construction gives more freedom of movement — either recipe can delay to push or pull freely. It also has more degrees of freedom which may lead to more efficient recipes, since both sides of the recipe can be adjusted. However, with the oncoming glider gun technique, the circuitry for one of the recipes can be replaced with a much simpler glider gun pattern and there's no need to route or clone the second recipe. This leads to easier bootstrapping and compatibility with other single-channel technology.
Simkin gun-based arm
The Simkin glider gun has several advantages that make it useful for oncoming glider gun construction:
- Simple and easy to construct.
- Not too fast — recipe gliders may have a hard time swimming upstream against the similarly small Gosper's glider gun, especially if the recipe is limited by the 90 tick repeat time of a Scorbie Splitter.
- Not too slow — recipes can be spaced closer together and interact with multiple gun gliders without taking too much time.
A single-channel recipe has been found which can construct a (messy) oncoming Simkin glider gun from a standard pi-producing elbow block using only 67 gliders with a minimum repeat time of 90 ticks. The recipe produces a gun with 5-lane offset for the oncoming gliders. This pattern can be reduced back to a block with 19 gliders.
Theory
When two gliders collide head on at a specific lane offset, there are up to 5 unique patterns generated, and they repeat every 8 ticks of delay. See 2-glider collision for a table of all 180 degree collisions.
| The 8 reactions for a 5 lane offset. Note that patterns 5, 6, and 7 are rotated versions of 3, 2, and 1 due to rotational symmetry. Delays of 8, 9, ... 15 will repeat this sequence, but shifted by one full diagonal, and so on. (click above to open LifeViewer) |
Preliminary searches show that there are usable glider recipes in each offset from 1 to 6 (gliders with 0 offset will simply delete each other in at most 6 ticks), but the most studied is the 5 lane offset.
The 5 lane offset has the "180 degree kickback" reaction labeled 2 above. It sends a glider back towards the gun gliders with a 1 lane offset. With a p120 glider stream, the 1 lane offset happens to delete the next gun glider as well. This gives a convenient way to swim upstream -- one glider with 2 (mod 8) spacing will consume two gun gliders. Assuming a minimum follow distance of 90 ticks, this lets you use one glider to push up to 19fd.
Glider color and parity
With a p120 glider stream, recipes can only be used at a specific lane every 120 ticks, and can only be shifted in full diagonal amounts. This means that each 90 degree glider recipe has a fixed color and parity. To send arbitrary slow salvos, you need need a separate recipe for each color and phase in the direction you want to send gliders.
| 4 sample recipes for a 5 lane offset and p120 glider stream. These form a complete set of color and parity (assuming p2 targets) (click above to open LifeViewer) |
Gallery
(click above to open LifeViewer)