Jolverine

Language version 1.0

The Jolverine language was devised as a conscious attempt to expand the genre of turning tarpit by adding the feature of modifying the instruction wheel during execution. It is not dissimilar to Wunnel, and was influenced slightly by Half-Broken Car in Heavy Traffic.

The name is a portmanteau of "jolly wolverine" (where "jolly" is a euphemism for "drunk",) which is an attempt to capture, in a noun phrase, the language's vicious, erratic nature.

Program Structure

A Jolverine program consists of a two-dimensional grid of symbols. An instruction pointer (IP) traverses the grid, starting in the upper-left corner (x=0, y=0) travelling right (dx=1, dy=0). dx and dy have only three possible values, -1, 0, and 1, like values on the tape (see below.)

Execution

On each tick, if the symbol under the IP is a *, the current instruction on the instruction wheel is executed. The instruction wheel is then advanced to the next instruction, regardless of what the symbols under the IP contains. (All symbols that are not * have the same meaning, that is to say, no meaning besides taking up space.) The IP is then advanced to the next position in the playfield, and the next tick begins. Execution halts when the IP travels beyond the extent of the playfield (i.e. when it can be proved that, in its direction of travel, it will never again hit a *.)

Storage

There is an unbounded tape, with a single head which can be moved left and right. Each cell on the tape may contain one of the three values -1, 0, or 1. Initially, all tape cells contain 0. Addition and subtraction of these values is always "mod 3 - 1": incrementing 1 yields -1 and decrementing -1 yields 1.

Instruction Wheel

The instruction wheel contains seven instructions. Initially, the wheel looks like this:

left <--
right
rot
adddx
adddy
input
output

The arrow indicates the current instruction on the wheel. Each time the wheel is advanced, the arrow moves down one row, wrapping around back to the top once it advances past the bottom of the wheel.

Each time an instruction is executed from the wheel, it is removed from the wheel and re-inserted at a different position. The first time this happens, it is re-inserted at the top of the wheel; the second time, it is re-inserted at the bottom; the third time, at the top again; the fourth time, at the bottom again; and so forth.

Instructions

• left: moves the tape head left one tape cell.
• right: moves the tape head right one tape cell.
• rot: increments the current tape cell.
• adddx: adds the contents of the current tape cell to the IP's dx.
• adddy: adds the contents of the current tape cell to the IP's dy.
• input: inputs a single bit (I/O is bitwise, and implementation-defined.)
• output: if the current tape cell is 0, output a 0; if the current tape cell is 1, output a 1; otherwise RESERVED for future expansion.

Notes

It is entirely possible for * to execute adddx or adddy with the result of both dx and dy being zero. In this case, execution does not halt, but the same * will be executed again, and again, until dx or dy changes.

Computational Class

Jolverine may or may not be Turing-complete. If it is not, it is not because it has insufficient storage, or no way to execute an instruction it has previously executed; it will be because there is no way to predictably execute the same series of instructions as was previously executed.

Discussion

I have yet to write a proper loop in Jolverine. I have only attempted to write an infinite loop, executing adddx and adddy alternately after roting a cell on the tape. After a few iterations, you can guarantee that adddx is at the top of the wheel, and adddy at the bottom, so you can choose to execute the next one, as appropriate, and it will not change the position of these instructions on the wheel (because they're already at the top and bottom.) However, I suspect even this technique really requires some number theory to do effectively -- probably finding the LCM of 7 (the number of instructions on the wheel) and a couple of other numbers (the amount you can change the x and y positions on each spin around the playfield.)

Happy wheel-mangling!
Chris Pressey
September 11, 2012
Montréal, Québec