NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t read any further: just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.

Q(uine)R code generator

What is difficult?

As you may know, QR code (specification: ISO/IEC 18004) is quite complex. It requires a bit stream, Reed-Solomon codes, BCH codes, data relocation, big (and irregular!) numerical tables, human-oriented pattern arrangement (difficult to implement by a small program), etc.

On the other hand, a quine has a big restriction: it needs to have two copies of the program!

Do you think that it is possible to write QR code encoder in C within 1kB?

How did I achieve that?

It is known that we can mitigate the quine restriction by eval. For example, this is a quine written in Ruby:

    eval s="print 'eval s='; p s"

Note that print appears just once in the program.

To emulate this approach in C, I adopted the following structure.

    char s[]="<main code>int main() { <VM code> }";
    int main() { <VM code>; }

• <VM code> : An own VM implementation (written in C) that executes the main code.
• <main code> : An object code of quine and QR code encoder (written in the assembly language of the VM).

<main code> (about 1100 bytes) appears once and <VM code> (about 450 bytes) appears twice. So the whole code is about 1100 + 450 * 2 = 2000 bytes. Hooray!

What sort of VM is that?

The VM is a kind of stack machine which makes the object code compact. The instruction set is carefully selected to minimize the sizes of both <main code> and <VM code>.

• 21 integer registers access
  • read, write, inc-and-read, dec-and-read
• Binary operations
  • add, sub, mul, div, mod, xor, less than, equal, right shift
• Stack operation
  • push (an immediate value)
• Heap access
  • load, store
• If-Then-Else
• While-Loop
• Misc.
  • putchar, exit

The instructions have variable lengths, but many of them are represented as one printable character. (See machine.rb and assemble.rb in detail.) The mapping between instructions and characters is also carefully designed to minimize <VM code>, which makes the resulted program non-human-readable.

How did I create the main code?

It is too tedious to write the machine language directly. So I wrote the source code in Ruby syntax and created a Ruby-to-VM compiler that involves alpha renaming and automatic register allocation only for this purpose.

• src.rb: the source code of QR code encoder written in Ruby syntax.
• vm.c: the source code of the VM.

The compiler tool chain is the following.

    src.rb              interp.c
      |                   |
      | [parse.rb]        |
      V                   |
    ast.txt               |
      |                   |
      | [serialize.rb]    |
      V                   |
    seq.txt               |
      |                   |
      | [alpha.rb]        |
      V                   |
    alpha.txt             |
      |                   |
      | [register.rb]     |
      V                   |
    reg.txt               |
      |                   |
      | [assemble.rb]     |
      V                   |
    raw.txt               |
      |                   |
      +---------+---------+
                |
                | [link.rb]
                V
              main.c
                |
                | [gen-prog.c]
                V
              prog.c

• parse.rb converts Ruby to AST.
• serialize.rb converts AST to intermediate instruction sequence.
• alpha.rb applies “alpha-renaming” to the instruction sequence.
• register.rb applies “register allocation”.
• assemble.rb converts the sequence to the final stack-machine instructions.
• vm.c is an implementation of the VM.
• link.rb generates main.c by linking the instructions and interpreter source.
• gen-prog.rb generates prog.c based on main.c.

You can run the process by rake command. ruby-minisat gem is needed.

    $ gem install ruby-minisat
    $ rake

Tools

I checked that the following QR code readers can read the output of my program.

• Linux: ZBar bar code reader.

A note from the judges in 2023:

The author also tested an Android app called Barcode Scanner which used to be found at https://play.google.com/store/apps/details?id=com.google.zxing.client.android but it appears to no longer exist.

You can use tool.rb to convert the output to a png file.

    ./main Hello | ruby tool.rb > hello.png

    $ zbarimg -q hello.png
    QR-Code:Hello

    $ ruby tool.rb prog.c
    $ ruby tool.rb prog.c | ruby tool.rb

Limitations

• QR code error correction level is fixed to L (Low).
• It supports only byte mode (not numeric, alphanumeric, kanji mode…).
• The mask pattern is fixed to 4.
• The mask is not applied to the remained bits.

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