Here are the important file formats used by Golly:

  Extended RLE format (.rle)
  Macrocell format (.mc)
  Rule colors (.colors)
  Rule icons (.icons)
  Rule tables (.table)
  Rule trees (.tree)
  Zip files (.zip)

Extended RLE format

Golly prefers to store patterns and pattern fragments in a simple concise textual format we call "Extended RLE" (it's a modified version of the RLE format created by Dave Buckingham). The data is run-length encoded which works well for sparse patterns while still being easy to interpret (either by a machine or by a person). The format permits retention of the most critical data:

- The cell configuration; ie. which cells have what values.
- The transition rule to be applied.
- Any comments or description.
- The generation count.
- The absolute position on the screen.

Golly uses this format for internal cuts and pastes, which makes it very convenient to move cell configurations to and from text files. For instance, the r-pentomino is represented as

  `x = 3, y = 3, rule = B3/S23`
  `b2o$2o$bo!`

I just drew this pattern in Golly, selected the whole thing, copied it to the clipboard, and then in my editor I did a paste to get the textual version. Similarly, data in this format can be cut from a browser or email window and pasted directly into Golly.

RLE data is indicated by a file whose first non-comment line starts with "x". A comment line is either a blank line or a line beginning with "#". The line starting with "x" gives the dimensions of the pattern and usually the rule, and has the following format:

  `x = width, y = height, rule = rule`

where width and height are the dimensions of the pattern and rule is the rule to be applied. Whitespace can be inserted at any point in this line except at the beginning or where it would split a token. The dimension data is ignored when Golly loads a pattern, so it need not be accurate, but it is not ignored when Golly pastes a pattern; it is used as the boundary of what to paste, so it may be larger or smaller than the smallest rectangle enclosing all live cells.

Any line that is not blank, or does not start with a "#" or "x " or "x=" is treated as run-length encoded pattern data. The data is ordered a row at a time from top to bottom, and each row is ordered left to right. A "$" represents the end of each row and an optional "!" represents the end of the pattern.

For two-state rules, a "b" represents an off cell, and a "o" represents an on cell. For rules with more than two states, a "." represents a zero state; states 1..24 are represented by "A".."X", states 25..48 by "pA".."pX", states 49..72 by "qA".."qZ", and on up to states 241..255 represented by "yA".."yO". The pattern reader is flexible and will permit "b" and "." interchangeably and "o" and "A" interchangeably.

Any data value or row terminator can be immediately preceded with an integer indicating a repetition count. Thus, "3o" and "ooo" both represent a sequence of three on cells, and "5$" means finish the current row and insert four blank rows, and start at the left side of the row after that.

The pattern writer attempts to keep lines about 70 characters long for convenience when sharing patterns or storing them in text files, but the reader will accept much longer lines.

If the File menu's "Save Extended RLE" option is ticked then comment lines with a specific format will be added at the start of the file to convey extra information. These comment lines start with "#CXRLE" and contain keyword/value pairs. The keywords currently supported are "Pos", which denotes the absolute position of the upper left cell (which may be on or off), and "Gen", which denotes the generation count. For instance,

  `#CXRLE Pos=0,-1377 Gen=3480106827776`

indicates that the upper left corner of the enclosing rectange is at an X coordinate of 0 and a Y coordinate of -1377, and that the pattern stored is at generation 3,480,106,827,776.

All comment lines that are not CXRLE lines, and occur at the top or bottom of the file, are treated as information lines and are displayed when the user clicks the "information" button in Golly's tool bar. Any comment lines interspersed with the pattern data will not be displayed.

Macrocell format

The size of an Extended RLE file is frequently proportional to the number of cells it contains, yet Golly supports universes that can contain trillions of cells or more, using hashlife-based algorithms. The storage of these huge universes, for which Extended RLE is not feasible, is done by essentially dumping the in-memory compressed representation of the universe in "Macrocell format". Since little translation need be done between external and internal representation, this format is also used to store backups of universes at certain points in Golly's operation when using one of the hashlife-based algorithms.

The macrocell format has two parts: the header, and the tree. The first portion of the file is the header; this contains the format identifier, the rule, the generation count (if non-zero), and any comments associated with this file. The format identifier is a line that starts with "[M2]" and contains the name and version of the program used to write it:

  `[M2] (golly 2.0)`

Following this is any comment lines, which are lines that begin with '#'. If the first two characters of a line are '#R', then the remainder of the line (after intervening whitespace) is the rule for the pattern. If the first two characters are '#G', then the remainder of the line is the current generation count. Any other line starting with a '#' is treated as an ordinary comment line.

Following the header is is a child-first textual representation of a canonicalized quadtree. Each line is either a leaf node, or a non-leaf node. For two-state algorithms, the leaf nodes contain an 8x8 pixel array in a simplified raster format, left-to-right, then top-down, with "." representing an empty cell, "*" representing a live cell, and "$" representing the end of line; empty cells at the end of each row are suppressed. No whitespace is permitted; lines representing leaf nodes for two-state algorithms are recognized because they start with ".", "*", or "$". A sample leaf node containing a glider is:

  `$$..*$...*$.***$$$$`

For algorithms with more than two states, leaf nodes represent a two-by-two square of the universe. They contain five integers separated by single spaces; the first integer is 1, and the next four are the state values for the northwest, northeast, southwest, and southeast values of the square.

Nodes with children are represented by lines with five integers. The first integer is the logarithm base 2 of the size of the square this node representes; for two-state patterns, this must be 4 or larger; for multi-state patterns, this must be 2 or larger. The next four values are the node numbers of the northwest, northeast, southwest, and southeast child nodes. Each of these child nodes must be the appropriate size; that is, a square one fourth the area of the current node.

All nodes, both leaf and non-leaf, are numbered in the order they occur in the file, starting with 1. No node number can point to a node that has yet been defined. The special node number "0" is used to represent all squares that have no live cells.

The total universe represented by a macrocell file is that of the last node in the file (the root node), which also must be the single node with the largest size. By convention, the upper left cell of the southeast child of the root node is at coordinate position (x=0,y=1).

Macrocell files saved from two-state algorithms and from multi-state algorithms are not compatible.

Rule colors

Whenever Golly switches to a new rule it searches for a matching rule.colors file and, if found, loads the color information stored in the file. Golly looks for the file in your rules folder first, then it looks in the supplied Rules folder. If rule.colors is not found but rule contains one or more hyphens then Golly searches for prefix.colors, where prefix consists of all the characters before the final hyphen. This allows related rules to share a single .colors file.

Note that a rule string can contain characters not allowed in file names ("/", "\" and ":"), so Golly will substitute those characters with underscores. For example, if you wanted to change the colors associated with the rule 12/34/6 then you'd need to create a file called 12_34_6.colors.

A .colors file is an ordinary text file. Any blank lines or lines starting with "#" are ignored. You can specify the RGB colors for one or more states using lines like these:

  `color = 0 48 48 48 (dark gray)`
  `color = 1 0 128 255 (light blue)`
  `color = 2 255 255 255 (white)`
  `color = 3 255 128 0 (orange)`

Golly silently ignores any states that are invalid for the matching rule. Any text after the final number on each line is ignored. To specify a color gradient for all live states (all states except 0) you can use a line like this:

  `gradient = 0 0 255 255 0 0 (blue to red)`

Note that a .colors file is loaded after switching to the current algorithm's default color scheme, so you have the choice of completely changing all the default colors, or only changing some of them. Use Preferences > Color to change the default colors for each algorithm.

Rule icons

Whenever Golly switches to a new rule it searches for a matching rule.icons file and, if found, loads the icon bitmaps stored in the file. Golly looks for the file in your rules folder first, then it looks in the supplied Rules folder. If rule.icons is not found but rule contains one or more hyphens then Golly will look for prefix.icons, where prefix consists of all the characters before the final hyphen. This allows related rules to share a single .icons file.

Note that a rule string can contain characters not allowed in file names ("/", "\" and ":"), so Golly will substitute those characters with underscores. For example, if you wanted to change the icons associated with the rule 12/34/6 then you'd need to create a file called 12_34_6.icons.

Golly can load the bitmaps from BMP, GIF, PNG or TIFF files. The following picture shows how the bitmaps must be arranged:

The top row contains one or more icons of size 15x15, so the total width must be a multiple of 15. The bottom row contains icons of size 7x7, where each icon is under the bottom left corner of its corresponding 15x15 icon. The 7x7 icons are optional; if not supplied (i.e., the total height is 15 rather than 22) then Golly will create them by scaling down the 15x15 icons. Scaled icons can look rather ugly, so this isn't recommended. Pure black (0,0,0) must be used for the areas you want to be transparent. The non-black areas will be replaced with the current cell's state color. If the number of icons supplied is fewer than the number of live states then the last icon is automatically duplicated. If there are more icons than required then the extra icons are simply ignored.

If Golly can't find any rule-specific icons it will use the default icons associated with each algorithm. You can change these icons by using the "Load Icons" button in Preferences > Color to load files with the same format described above. The only difference is that there is no need for a .icons extension.

Rule tables

Golly allows you to add new rules by creating transition tables stored in external files. Given the rule string "foo", the RuleTable algorithm will search for a file called foo.table. It looks for the file in your rules folder first, then it looks in the supplied Rules folder. Here's a simple example file:

  `# Signals (2/3) pass alongside a wire (1):`
  `n_states:4`
  `neighborhood:vonNeumann`
  `symmetries:rotate4`
  `var a={2,3}`
  `var b={2,3}`
  `var c={2,3}`
  `a,0,b,1,c,b`

See the `.table` files in the Rules folder for more examples. Empty lines and those beginning with the hash symbol "#" are ignored.

Required descriptors (must appear before other content):

- `n_states:` specifies the number of states in the CA (from 0 to n_states-1 inclusive).
- `neighborhood:` specifies the cell neighborhood for the CA update step. Must be one of: `vonNeumann`, `Moore`.
- `symmetries:` can be one of the following: `none`, `rotate4`, `rotate8`, `reflect`, `rotate4reflect`, `rotate8reflect`. Hopefully these are self-explanatory.

The rest of the file contains variables and transitions. Variable names should only use the letters a-z,A-Z (case-sensitive). Each variable line should follow the form above to list the states. Variables should appear before the first transition that uses them.

Transition lines should have states or variables separated by commas. If there are no variables and `n_states`<11 then no comma should be used. Only one transition (or variable) should appear on each line. Inputs are listed in the order C,N,E,S,W,C' for the von Neumann neighborhood, and C,N,NE,E,SE,S,SW,W,NW,C' for the Moore neighborhood.

Where the same variable appears more than once in a transition, it stands for the same state each time. A transition can have a variable as its output (C') but only where that variable appears more than once, so that it has a definite value. The transition in the example above expands to the following: `20212->2`, `20213->2`, `20312->3`, `20313->3`, `30212->2`, `30213->2`, `30312->3`, `30313->3`, and all 90-degree rotations of those (because of the `rotate4` symmetry).

Rule tables usually don't specify every possible set of inputs. For those not listed, the central cell remains unchanged.

Rules are checked in the order given in the file - the first rule that matches is applied. If you want, you can write rules in the form of general cases and exceptions, as long as the exceptions appear first in the file.

(This form of CA rule table representation comes from Gianluca Tempesti's PhD thesis: http://lslwww.epfl.ch/pages/embryonics/thesis/AppendixA.html.)

If you have a C/C++ implementation of a transition function, there is a way to automatically produce a rule table file. See `Rules/TableGenerators/make-ruletable.cpp` for instructions.

To share your rule tables with others, you can archive them at the public Rule Table Repository.

Rule trees

Golly permits the loading of rules from external files, where those files are in RuleTree format. If a rule named Foo is requested, Golly will search for Foo.tree and, if found, load the rule from that file. It looks for the file in your rules folder first, then it looks in the supplied Rules folder.

Golly includes programs that permit you to transform a given transition function in C++, Perl, or Java into a RuleTree file (see Rules/TreeGenerators) if the number of states is sufficiently small (approximately 10 states for eight-neighbor rules, and 32 states for four-neighbor rules). Essentially, the RuleTree format allows you to add your own rules to Golly without needing to know how to recompile Golly and without dealing with the intricacies of external libraries; it generates relatively compact files, and the data structure is defined for very fast execution.

A rule tree is nothing more than a complete transition table for a rule, expressed in a compressed, canonicalized tree format. For an n state rule, each tree node has n children; each child is either another tree node or a next state value. To look up a function of m variables, each of which is a state value, you start at the root node and select the child node corresponding to the value of the first variable. From that node, you select the child node corresponding to the value of the second variable, and so on. When you finally look up the value of the final variable in the last node, the result value is the actual next state value, rather than another node.

The RuleTree format has fixed the order of variables used for these lookups. For a four-neighbor rule, the order is always north, west, east, south, center; for an eight-neighbor rule, the order is always northwest, northeast, southwest, southeast, north, west, east, south, center.

Without compression, for an n-state rule, there would be a total of 1+n+n^2+n^3+n^4 nodes for a four-neighbor rule, and 1+n+...+n^8 for an eight-neighbor rule; this could quickly get unmanageable. Almost all rules show significant redundancy, with identical rows in the transition table, and identical nodes in the rule tree. To compress this tree, all we do is merge identical nodes, from the bottom up. This can be done explicitly as we construct the tree from a transition function (see Rules/TreeGenerators/RuleTreeGen.java) or symbolically as we evaluate a more expressive format.

The RuleTree format itself is simple, and has similarities to the macrocell format. It is not intended for human authorship or consumption. The RuleTree format has two parts: a header, and the rule tree itself. The header consists of comments (lines starting with a "#") that are ignored, and three required parameter values that must be defined before the first tree node. These values are defined, one per line, starting with the parameter name, then an equals sign, and finally an integer value. The three parameters are num_states, which must be in the range 2..256 inclusive, num_neighbors, which must be 4 or 8, and num_nodes, which must match the number of node lines in the file.

The tree is represented by a sequence of node lines. Each node line consists of exactly num_states integers separated by single spaces. The first integer of each node line is the depth of that node, which must range from 1..num_neighbors+1. The remaining integers for nodes of depth one are state values. The remaining integers for nodes of depth greater than one are node numbers. Nodes are numbered in the order they appear in the file, starting with zero; each node must be defined before its node number is used. The root node, which must be the single node at depth num_neighbors+1, must be the last node defined in the file.

For comparison purposes, the program "RuleTableToTree" is supplied with Golly; this executable will convert a rule table file into a rule tree file. It operates symbolically (it does not enumerate all the possibilities) so it should be fast on any rule table. To use it, change to the main golly directory (the directory that contains the Rules directory); this is probably where the executable is installed. Type

  `RuleTableToTree Codd >Rules/Codd.tree`

to convert the Codd rule table file (as an example) to a rule tree file. Now you can load a Codd universe into either the RuleTable or the RuleTree algorithm.

Zip files

Golly can open a standard zip file and process its contents in the following way:

If the zip file is "complex" then Golly builds a temporary HTML file with special unzip links to each included file and shows the result in the help window. A complex zip file is one that contains:

- Any folders.
- Any rule-related files (.colors or .icons or .table or .tree extension). All such files are automatically extracted and installed into your rules folder.
- Any HTML files (.htm or .html extension).
- Any text files (.txt or .doc extension, or a name containing "readme").
- Multiple scripts (.pl or .py extension).
- Multiple patterns.

If the zip file contains at most one pattern and at most one script, Golly will load the pattern (if present) and then run the script (if present). For security reasons, if the zip file was downloaded via a get link then Golly will ask if you really want to run the included script. Both the pattern and the script must be at the root level of the zip file; ie. not inside a folder. For an example of such a zip file, open Langtons-ant.zip from the Patterns/Other-Rules/WireWorld/ folder. This zip file contains a pattern file and a simple Python script that sets up a nicer initial view and step size.

A number of pattern collections in the form of zip files can be downloaded from the online archives.