Generic C / Python Polyglot

A polyglot — coming from the Greek words πολύς (many) and γλώττα (tongue) — is a piece of source code which can run in multiple languages, often performing language-dependent tasks.
Even though such a source code’s feature may not seem particularily useful or possible with certain language combinations, trying to bend not one but multiple language’s syntactic rules and overall behavior is still interesting.

An example of a rather simple polyglot would be a Python 2 / Python 3 polyglot — if one counts those as two separate languages. Because of their enormous similarities, one can pick out differences and use those to determine which language runs the source code.

if 1/2>0:print("Python 3")
else:print("Python 2")

Utilizing Python’s version difference regarding integer division and real division is one of the most common ways to tell them apart, as it can also be used to only control a specific part of a program instead of having to write two nearly identical programs (increases the program’s style and cuts on bytes — an important consideration if one golfs a polyglot).

However, polyglots combining languages that are not as similar as Python 2 and Python 3 require more effort. The following is a general Python 2 / C polyglot, meaning that nearly all C and Python 2 programs can be mixed using this template (there are a few rules both languages need to abide which will come apparent later).

#define _\
"""
main(){printf("C");}
#define _"""
#define/*
print"Python 2"
#*/_f

In the above code, main(){printf("C");} can be nearly any C code and print"Python 2" can be nearly any Python 2 code.
Language determination is exclusively done via syntax. A C compiler sees a #define statement and a line continuation \, another two #define statements with a block comment in between and actual compilable C source code (view first emphasis).
Python, on the other hand, treats all octothorps, #, as comments, ignoring the line continuation, and triple-quoted strings, """...""", as strings rather than statements and thus only sees the Python code (view second emphasis).

My first ever polyglot used this exact syntactical language differentiation and solved a task titled Life and Death of Trees (link to my answer).

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BMP implementation in C

C is one cool and important language. CPython and Unix are based on it, the Mars Curiosity rover is run by it and even the GCC C compiler itself is written in C. However, as C is some years old by now, it lacks a lot of higher-level features most modern languages possess, being more down to the silicon, as the cool kids say. Concepts like pointer manipulation, bit fiddling and its string implementation — just to name a few — are at times cumbersome, insecure and error-prone; nevertheless is there a certain appeal to writing in C.

Being only one abstraction level away from Assembly — which itself is only one abstraction level above raw byte code — and having access to file manipulation down to the individual bit, I set out to write a Microsoft Bitmap (.bmp) implementation in pure C. As Microsoft’s standard for this image file format is quite feature-rich, I decided to focus on the bar minimum — a bitmap with 24-bit color depth (three colors, one byte per), one color plane, no compression, no palette and 300 DPI.
My Bitmap implementation supports both reading and writing .bmp files, as well as generating some test images — including a Mandelbrot Set fractal renderer, of course. Implementation source code can be downloaded (bmp.c) or seen below.

Mandelbrot Set fractal
A Mandelbrot Set fractal rendering.

Implementing a file format requires knowing its specification. Although it is not the best article I have ever seen, this Wikipedia article gave me some insights. The missing pieces were reverse engineered using Adobe Photoshop CC and the HxD hex editor.
The following is a snippet of the implementation’s savebmp function (full source code listed below). It illustrates the Bitmap file’s byte layout only showing the file header, omitting a lengthy data part concatenated to the header. S, K, W, H and B are all byte arrays of length four (little-endian format) which contain the file’s total size, the bitmap data offset (which is constant, since the header is always exactly 54 bytes large), the image’s dimensions (horizontal and vertical) and the bitmap data’s section’s size, respectively.

/*  bitmap file header  */
0x42, 0x4D,             // BM
S[0], S[1], S[2], S[3], // file size
0x00, 0x00, 0x00, 0x00, // unused
K[0], K[1], K[2], K[3], // bitmap data offset
/*      DIB header      */
0x28, 0x00, 0x00, 0x00, // DIB size
W[0], W[1], W[2], W[3], // pixel width
H[0], H[1], H[2], H[3], // pixel height
0x01, 0x00,             // one color plane
0x18, 0x00,             // 24 bit color depth
0x00, 0x00, 0x00, 0x00, // no compression
B[0], B[1], B[2], B[3], // bitmap data size
0x23, 0x2E, 0x00, 0x00, // 300 DPI (horizontal)
0x23, 0x2E, 0x00, 0x00, // 300 DPI (vertical)
0x00, 0x00, 0x00, 0x00, // no palette
0x00, 0x00, 0x00, 0x00  // color importance
/*  data bytes follow   */

Key bytes to note are the first two identifying the file type (the ASCII-encoded letters BM) and the DPI bytes, 0x23, 0x2E, which indicate 0x00002E23 = 11811 pixels per meter in both the horizontal and vertical direction. Converting from pixels per meter to dots per inch results in 11811 / (1 meter / 1 inch) = 11811 * 127 / 5000 = 300 DPI (roughly).
Most values are represented using four bytes in little-endian format. Translating an 32-bit integer into four little-endian formatted bytes can be achieved as follows.

/* unsigned 32-bit integer */
unsigned int n = 0b10100100010000100000100000010000;
/*                 < m sig><sm sig><sl sig>< l sig> */

/* byte (unsigned char) array of size four */
unsigned char N[4] = {          
	(n & 0xff000000) >>  0, // most significant byte
	(n & 0x00ff0000) >>  8, // second most significant byte
	(n & 0x0000ff00) >> 16, // second least significant byte
	(n & 0x000000ff) >> 24  // least significant byte
};

Other than rendering a fractal, I also implemented three nested loops which output an image containing every possible color exactly once ((2**8)**3 = 16777216 pixels in total).

All sixteen million colors
All sixteen million colors in one image.

An image’s data type is implemented as a struct image which contains three variables — width and height, two integers specifying the image’s dimensions, and *px, a pointer to an one-dimensional integer array of size width*height which holds the entire image data.
Defined functions are listed ahead.

  • image * readbmp(char []);
    • Reads an image specified by a file name. If reading fails, a NULL pointer is returned.
  • void savebmp(image *, char []);
    • Saves given image to a file with specified name.
  • image * newimage (int, int);
    • Returns a pointer to an image struct with specified dimensions (image will be filled with nothing but black pixels).
  • void freeimage (image *);
    • Frees an image struct’s memory.
  • int getpx (image *, int, int);
    • Returns the pixel color at specified coordinates.
  • void setpx (image *, int, int, int);
    • Sets the pixel color at specified coordinates.
  • void fill (image *, int);
    • Fills a given image with a given color (all pixels are set to specified color).
  • int color(byte, byte, byte);
    • Returns a 32-bit integer representing a color specified by three bytes (byte is defined through typedef unsigned char byte;).
  • int hsl (double, double, double);
    • Returns a 32-bit integer representing a color specified by three doubles in the HSL color format.

Images shown in this post were converted to .png files as WordPress does not allow .bmp file uploads; the raw pixel data should, however, be identical.


/* ================================================== *
 *                GENERAL INFORMATION                 *
 * ================================================== *
 * Bitmap file format implementation in C.            *
 * Supported functionality: 24-bit color depth image  *
 *  struct, reading and writing .bmp files and simple *
 *  image manipulation.                               *
 * Supported color formats: RGB and HSL.              *
 * Additional functionality: Mandelbrot Set fractal   *
 *  rendering.                                        *
 *                                                    *
 * Author: Jonathan Frech                             *
 *                                                    *
 * Edit history: 23rd, 24th, 27th, 28th, 29th, 30th   *
 *  of June, 1st, 2nd, 3rd, 10th, 11th, 13th, 14th,   *
 *  15th, 16th, 17th, 18th, 19th, 20th, 25th, 26th,   *
 *  27th, 29th of July, 11th, 16th of August, 17th,   *
 *  18th, 19th of October 2017                        */
 
/* ================================================== *
 *                    COMPILATION                     *
 * ================================================== *
 * $ rm bmp; gcc bmp.c -lm -o bmp; ./bmp              */

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TImg

Texas Instrument’s TI-84 Plus is a graphing calculator with a variety of features. It has built-in support for both fractions and complex numbers, can differentiate and integrate given functions and supports programming capabilities. The latter allows to directly manipulate the calculator’s monochrome display’s 5985 pixels (the screen has dimensions 95x63). TImg is a Python program (source code is listed below and can also be downloaded) which takes in an image and outputs TI-BASIC source code which, when run on the graphing calculator, will produce the given image — in potentially lower quality.

TImg
TI-84 Plus’ screen dimensions (bitmap).

PIL — the Python Imaging Library — is used to read in the image and further for processing. The supplied image may be rotated and resized to better fit the TI-84’s screen and any color or even grayscale information is reduced to an actual bitmap — every pixel only has two distinct values.
Direct pixel manipulation on the TI-84 is done via the Graph screen. To get remove any pixels the system draws on its own, the first three commands are ClrDraw, GridOff and AxesOff which should result in a completely blank screen — assuming that no functions are currently being drawn. All subsequent commands are in charge of drawing the previously computed bitmap. To turn certain pixels on, Pxl-On(Y,X is used where Y and X are the pixel’s coordinates.

fractal
A fractal (bitmap).

Since the TI-84 Plus only has 24 kilobytes of available RAM, the source code for a program which would turn on every single pixel individually does not fit. Luckily, though, a program which only individually turns on half of the screen’s pixels fits. To ensure that TImg’s output fits on the hardware it is designed to be used with, an image’s bitmap is inverted when the required code would otherwise exceed 3500 lines — a value slightly above the required code to draw half of the pixels.

jblog
A J-Blog screenshot (bitmap).

By default, the resulting code draws pixels starting at the screen’s top-left corner and ending at its bottom-right. A command line flag --shuffle can be set which changes this behavior to let pixels pseudo-randomly appear on the screen (pseudo-randomness is calculated in the Python script; the TI-BASIC source code is completely deterministic).
And — of course — one can feed the program an image of the calculator the BASIC code runs on; self-referential TIception.

tiception
TIception (input image).

# Python 2.7 code; Jonathan Frech; 5th, 6th of October 2017

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