CASTLE

CASTLE, which stands for "Cellular Automaton Specification by Transforming Lists of Examples", is a 2014 experimental language by User:Quintopia. Inspired by example-driven languages like Clue (oklopol) and CA specification languages, particularly the .rule file format for Golly. The goal was to create a method for specifying CAs in which nearly arbitrary transformations can be specified, even those with non-traditional neighborhood shapes and sizes and those with neighborhoods that change over time, in a human-readable format.

File Format
Files are organized thusly (comments and placeholders in italics are not a part of the format):

def list,of,state,symbols def id(list of included states) or def id[list of disincluded states] partition (tesselation symmetry) This section is optional, used only for block CAs symmetries of blocks ''Description of partition time-dependence. See relevant section.'' (blank line) The following section is not optional, because it is the actual specification of the CA rules [prob:]state1|id1->state2|id2 describe which state/class of states we are transitioning from and to (with optional probability) state2|id2->state3|id3 it's possible for multiple states to be simultaneously transformed in a single transition description (etc.) [symmetries to apply to this example (space for none) .r.e.c.t .a.n.g.u .l.a*r.p .a.t.t.e .r.n.b.l .o.c.k.? === symmetries for this example .a.n.o.t .h.e.r.p .a.t*e.r .n.f.o.r .t.h.i.s .c.a.s.e ]

It's also possible to specify hexagonal grid by starting alternating lines with spaces: .h.e.x.a .g.o.n.a.l .g.r.i.d .s.l*o.o.k  .l.i.k.e .t.h.i.s.? So a hexagonal moore neighborhood: .6.x .m*o.o .r.e No file may contain both hexagonal and rectangular grids.

States and state classes
Any string of characters not given a special meaning by this document can symbolize a state (all of =*,.:?!@;[]-&gt; are disincluded, for example), and an automaton may have any number of states. For instance, CGoL has two, Wireworld has three, and REDGREEN has 23. These are defined using a "def" statement followed by the comma-separated list of state names. For convenience, it is recommended to make state names as short as possible and all the same length. It is also possible to assign a name to an entire class of states. "def id" and "def id[]" serve this purpose. The brackets should contain a comma-separated list of state or class names which were previously defined. In the case of square brackets, all states except for those listed will be assigned to the class. There is one special predefined class called ?, which automatically contains all defined states.

Defining block automata
Using the "partition" keyword, it is possible to define automata which use irregularly shaped blocks in the place of cells, as long as the blocks can be tessellated by a rectangular or hexagonal symmetry, such that the position of each block varies with each time step. This is done by describing the shape of a neighborhood of blocks visually, and providing a description of the symmetry by which it is tesselated as described below. The shape of the block description must be either a rectangle or hexagon, in which regions belonging to separate blocks are assigned different numbers. For each cell is given a list of numbers enclosed by curly braces, providing block numbers corresponding to different partitions at each time tick (mod the length of a list). Each list must be the same length, and all blocks must all be congruent in each time tick described. The compiler will check that the description can be tessellated according to the symmetry provided. See the example for the Billiard ball machine to see how a time-dependent moving square partition can be described.

When defining blocks, arbitrary ways of tessellating the blocks are permitted. By default, they will be tessellated by straight translation in a rectangular or hexagonal grid as seems most appropriate, but any permutation of edges may be defined using permutation cycle notation. An edge permutation is given using i for no reflection and r for reflection with edges numbered clockwise from the top. So (1r3r)(2i4i) is a Klein bottle and (1r2r3r4r) is another type of Klein bottle. (1r2i3r4i) is a type of projective plane. (If no such symmetry is specified, it's assumed to be (1i3i)(2i4i) for rectangular and (1i4i)(2i5i)(3i6i) for hexagonal.)

Describing State Transitions by Example
Central to CASTLE is the ability to give examples to describe the ways in which states are updated on each tick. Because CASTLE truly allows arbitrary (local) conditions to play a role in determining state transitions, it must be possible to easily describe large classes of conditions with relatively few examples. In CASTLE, this is done by providing a list of examples, each one consisting of a list of transitions that should take place in the described conditions; a list of descriptions of representative neighborhoods, each consisting of a set of symmetries that should apply to the example (and only that example!); and a visual description of a representative neighborhood of the cell/block in which those state transitions should be performed. The representative neighborhoods are separated from each other by a line consisting only of "===". The compiler will generate from these examples a rule such that the overall board state is matched against each example in the order they are provided, and the transitions are performed in the first one that matches. By default, if no example is matched for a given cell/block, that cell or block will remain in the same state, which one could take to mean that there is an implied example at the end of the list that matches all conditions and maps every state to itself. See example programs at the bottom to make this clearer.

Note that the representative neighborhoods and their corresponding symmetries are optional. If absent, the provided transitions will always occur, unless an earlier transition is matched. Also note that not all states/classes on the left sides of transitions must be marked in a matched representative neighborhood. Only those transitions whose left sides match a marked state/class will be performed. It is possible for multiple cells to be marked, each corresponding to different states/classes, and therefore to different transitions. It is illegal to mark a state/class for which there is no associated transition with that state/class on the left-hand side. Since symmetries are applied only with respect to the "first marked cell", it is possible for the other marked cells to be permuted by symmetries, matching different cells depending on the permutation chosen. When multiple ways to match a neighborhood are possible, one should be chosen uniformly at random (where randomness is available) or else the first available match should be chosen (where no randomness is available).

Class names or state names may appear on either side of the '-&gt;' in a transition description. In the event that a class name appears on the right side of a transition, it may followed by a number in square brackets. In each example provided for that transition, a cell labeled with that class may also include the same number in square brackets to indicate which specific state in that class is to be matched and transitioned to. If no numbers are given in square brackets, the first cell labeled with that class in the examples in row major order will be matched. If no cell is labeled with that class, the first cell in the example region which happens to contain a member of that class (matched perhaps by ? or some other more inclusive class) will be matched. If it is possible that the example will match without containing any member of that class, compilation must fail.

Each state transition provided for a given set of examples can also be given an optional probability. This shall be a decimal number between 0 and 1. If present, before performing the transition, the automaton simulator will pick a random number between 0 and 1 and perform the transition if and only if the resulting number is less than or equal to the provided probability. This ability is only supported when the compilation target is one which is capable of probabilistic transitions in cellular automata, and the probability will be ignored otherwise. One may also specify conditional transitions using the "also" and "else" keywords, which would be placed where a probability would be placed. "also" evaluates to 1 if the transition on the previous line probabilistically succeeded and 0 otherwise. "else" evaluates to 1 if the previous line probabilistically failed and 0 otherwise.

The visual neighborhood description is a multi-line arrangement of cell descriptions, each of which consists of a special cell marker and a state/class id. In a block automaton, the shape of the example must correspond exactly to the shape of the partition in the orientation assigned the number 1 according to the partition description or compilation must fail.

Special Syntax for Examples
Individual cells in an a neighborhood description can be marked to modify the way they are parsed and permuted when matching state transitions.

Symmetry
All cellular automata require a grid of cells of some sort, each of which can be in one of a number of states. Depending on the nature of the grid and the nature of the automaton, the neighborhoods of each cell can be different shapes and sizes. In CASTLE, one describes a state transition by simply "drawing" the state of a cell's neighborhood in which the transition would take place, but for, for example, a totalistic automaton like Conway's Game of Life, it would be tedious to draw all 56 neighborhood arrangements that would result in an off cell turning on. Therefore, it is necessary to be able to define symmetries that can be applied to an example, so that 55 of those neighborhood arrangements can be inferred from the example given by simply permuting the neighborhood in all ways allowed by the symmetry. Symmetries are applied relative to the "first marked cell", which is the first cell in row-major order whose current state/class corresponds to the left-hand-side of the first transition for this example set. CASTLE supports the following symmetry types:

Examples
CGoL: def 0,1 0->1 + .1.1.1 .0*0.0 .0.0.0 1->0 + .?.0.0 .0*1.0 .0.0.0 === + .1.1.1 .1*1.? .?.?.?

Schrödinger's Game of Life: def 0,1,C def A(1,C) def D(0,C) D->1 + .1.1.1 .0*D.0 .0.0.0 A->0 + .?.0.0 .0*A.0 .0.0.0 === + .1.1.1 .1*A.? .?.?.? 1->1 + .1.1.? .0*1.0 .0.0.0 0->0 + .?.?.0 .0*0.0 .0.0.0 === + .1.1.1 .1*0.? .?.?.? 0->C 1->C

Rule 110 (notice the blank lines containing only space characters indicating no symmetries will be applied, as all variation is done with the use of wildcards): def 0,1 0->1 .?*0.1 1->0 .1*1.1

Billiard ball machine: def o,# partition {1,4}{1,3}{2,3}{2,4} {1,2}{1,1}{2,1}{2,2} {3,2}{3,1}{4,1}{4,2} {3,4}{3,3}{4,3}{4,4} o-># #->o 4 *#.# .#*o === 4 *#*o *o*#

REDGREEN: def a,~,%,#,&,s,k,o,f,p,t,^,-,=,D,O,1,z,Z,¿,«,»,T def F(p,:,O,1) def P(a,~,%,s,k,o,z,Z) def M(D,O,1) def S(s,k,o) def B(%,&,^,z,Z) def _(#,&,:,^,-,=,D,1,¿) def |[_,D] def C[T,D,1] def d[~,f] def n[%,&] F->a + .a.a:D :D*F:D :D!|:D F->~ + .~.~.~ .~*F:D :D!|:D F->F |[(«,»)] :P*F .?.« P->F .F *P === |[(«,»)] *P.F .?.« M->% + .B.?.? .?*M.? .?.?.? S->~ + .~.?.? .?*S.? S->a a->~ + .~.?.? !?*a!? === | .~*a ._.? === | .~*a .~.~ a->s + !?*a!? .s.?.? a->k + !?*a!? .k.?.? a->z + .^.?.? .?*a.? .?.?.? ~->s + .%.?.? .?*~.? .?.?.? === + .&.?.? .?*~.? .?.?.? ~->o *~ .S ~->f + .f.f.f .~*~.~ .~.~.~ %->k + .~.?.? .?*%.? .?.?.? === :a:a:a :a*%:a :a:a:a === .C.C.C .C*%.C .C.C.C #->& + .%.?.? .?*#.? .?.?.? &-># + :%.n.n .n*&.n .n.n.n f->a + .d:~:~ :~*f:~ :~:~:~ f->~ + .f.f.f .f*f.? .?.?.? === + .~.~.~ .~*f.~ .~.~.? p->t .p *p ._ =->^ + .^.?:^ :^*=:^ :^:^:^ D->O + .?:D:D :D*D:D :D:D:D === + :_:_:_ :_*D:_ :_:_:_ 0.5:«->» 0.5:»->« |[(«,»)] *« .¿