Boat-bit

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A boat-bit is a binary digit represented by the presence or absence of a boat next to a snake (or other suitable object, such as an aircraft carrier). The bit can be toggled by a glider travelling along a certain path. A correctly timed glider on a crossing path can detect whether the transition was from 1 to 0 (in which case the crossing glider is deleted) or from 0 to 1 (in which case it passes unharmed). Three gliders therefore suffice for a non-destructive read.

The mechanisms involved in reading and writing a boat-bit are shown in the diagram below. The bit as shown in is the 0 state, with no boat present. It is about to be set to 1 by the incoming green glider, and then switched back to 0 again by the red glider. The first crossing glider will survive, but the second will be destroyed.

The glider-based read and write mechanisms of a boat-bit
Download RLE: click here

David Bell found a method of reading the bit while setting it to 0 in January 1997. It works by firing a middleweight spaceship at the boat-bit. If it is already 0 then the middleweight spaceship passes unharmed, but if it is 1 then the boat and the middleweight spaceship are destroyed and, with the help of an eater 1, converted into a glider that travels back along exactly the same path that is used by the gliders that toggle the boat-bit.

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