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A BitChanger-like language where the tape head is a turtle [Public domain]

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README.markdown

The Etcha Programming Language

Introduction

Etcha is an esoteric programming language based on Jeffry Johnston's BitChanger. Like BitChanger, Etcha has four instructions, two of which are used to form Brainfuck-like while-loops. Unlike BitChanger, Etcha has a 2-dimensional storage model based on turtle graphics, which permits it to be immediately used for an alternative purpose: graphical composition. Unlike a classical turtle in a language such as LOGO however, the turtle in Etcha is an integral part of the computation, playing a role similar to the tape head of a Turing machine.

Instructions

  • + -- equivalent to FD 1
  • > -- equivalent to RT 90; toggles PU/PD every 4 executions
  • [ -- equivalent to While
  • ] -- equivalent to Wend

In Etcha, instructions control a turtle. The turtle exists vis-a-vis an unbounded Cartesian grid called the playfield. The turtle has a position in that it occupies exactly one of the points on the playfield (which are referred to as pixels). Each pixel has a state, which is either black or white; all pixels are initially black.

The turtle also has an orientation which describes the direction it would move in should it travel forward. Unlike a conventional turtle, because of its Cartesian context, there are only four possible orientations which the Etcha turtle can possess: north, east, south, and west, corresponding to headings of 0, 90, 180, and 270 degrees. When an Etcha program starts, the turtle is initially oriented north. Because position is relative, it doesn't matter where the turtle is initially located, but solely for psychological satisfaction we can say that it is initially situated in the very center of this unbounded Cartesian grid.

The turtle is also equipped with a pen, which has a pen mode and a pen position. The pen mode is always XOR, meaning that when moving forward by execution of +, the state of the pixel that was previously occupied by the turtle gets inverted (from black to white, or vice versa.) The pen position may be up or down. It is initially down. Every fourth time an > instruction is executed, the pen's position is toggled from up to down or vice versa. These executions need not be consecutive; there may be any number of intervening instructions executed.

Examples

-> Tests for functionality "Interpret Etcha Program"

-> Functionality "Interpret Etcha Program" is implemented by shell
-> command "java -cp bin tc.catseye.etcha.Main %(test-file)"

| >+++>+++>+++>+++>[+]>>>>+
= ----
= #####
=    # 
=    # 
=  ### 
= ----

Computational Class

Etcha is Turing-complete if BitChanger is, because we can easily translate any BitChanger program into an equivalent Etcha program. Let the Etcha program begin with >>>>, to initially lift the pen. The BitChanger instruction < is translated to the Etcha instructions >>>+>>>>>, and the BitChanger instruction } is translated to >>>>>+>>>. The instructions [ and ] remain the same. The relation between the BitChanger tape state and the Etcha playfield is quite literal; the y dimension of the grid is simply ignored.

Implementation

In a particular implementation of Etcha on a microcomputer with a finite storage, the playfield cannot be truly unbounded, and there will come a point in a long enough program execution, entirely dependent on the capabilities of the hardware and the implementation, where a pixel change cannot be correctly stored. The behaviour after this point is undefined. Such (m)icro(c)omputer implementations of Etcha may be marketed under the name "MC Etcha".

Cat's Eye Technologies provides an implementation of Etcha written in the Java[TM] programming language. This implementation attempts to demonstrate that the Model-View-Controller design pattern can be applied not only to user interfaces, but also to programming language interpreters. The Model is the state of the program (which is also the state of the Turtle graphics engine.) The View is the interpreter's interpretation of that state, and the Controller is the interpreter's behaviour with respect to the View. For example, the Model exposes the pen up/down semantics, but it is the Controller that implements the rule that >>>> toggles the pen position.

Cat's Eye Technologies' ridiculously over-engineered implementation of Etcha is in the public domain.

Happy Etchin'!
Chris Pressey
October 4th, 2009
Mold City, USA

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