Resrap

Just a parser… in reverse.

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What is Resrap?

Resrap is a seedable, grammar-based code snippet generator. Instead of parsing code, it generates code from formal grammars — producing endless, realistic-looking (or hilariously nonsensical) snippets.

It works with any language that can be described with a grammar (even English if you like!) and is perfect for:

• Typing practice with realistic-looking snippets
• Stress-testing parsers, syntax highlighters, or linters
• Fun exploration of procedural code generation

Resrap now also supports probabilistic and infinitely repeatable grammars via the ABNF (Awesome BNF) format — see docs/ABNF.md for full reference.

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How?

Resrap reads a grammar and builds a graph of expansions. It then randomly traverses the graph (or deterministically with a seed) to produce snippets that:

• Follow the grammar’s syntax rules
• Look structurally like real code
• Include probabilities for weighted choices (<0.2>)
• Support infinite loops via the ^ operator

Example grammar snippet (simplified C):

program : (header+<0.4>) function^;
header:'#include<'identifier'.h>\n';
function:functionheader'{''\n'functioncontent'}';
functionheader:datatype ' ' identifier '(' ')' ;
...
...

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Generated code example

#include<success.h>
#include<email.h>
double class(){
while(variable < query && password < variable){
int result = variable + (user / hello);
}
}double user(){
if(class > user && result < class){
float password = 1024.13 - (13.7);
}
}double hello(

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Installation

go get github.com/ItsArnavSh/[email protected]

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Usage

	//Resrap with Single threaded
	rs := resrap.NewResrap()
	err := rs.ParseGrammarFile("C", "example/C.g4")
	if err != nil {
		fmt.Println(err)
		return
	}
	code := rs.GenerateRandom("C", "program", 10)
	fmt.Println(code)

	//Lets get a multithreaded API set up quick
	r := resrap.NewResrapMT(20, 1000) //20 worker pool and 1000 wait queue max size
	err = r.ParseGrammarFile("C", "example/C.g4")
	if err != nil {
		fmt.Println(err)
		return
	}
	//Receive from this
	r.StartResrap()
	defer r.ShutDownResrap()
	codeChan := r.GetCodeChannel()
	id := "12321"
	r.GenerateRandom(id, "C", "program", 10)
	res := <-codeChan
	fmt.Println(res.Code)

Notes

• IDs: You must create unique IDs for each job; results are returned with the ID.
• CodeChannel: A blocking, unbounded channel — handle results yourself.
• Why multithreaded? Efficiently handles many concurrent jobs, fully utilizing CPU cores while keeping grammar graphs immutable and lock-free.

For benchmarks and performance comparisons, see benchmark-results/Multithreading.md.

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Roadmap

• Maintain generation sessions (e.g., generate snippets in chunks)
• Dynamic worker scaling and idle worker shutdown

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Motivation

Resrap was created to:

• Generate unlimited, realistic code snippets
• Avoid copyright issues from using real code
• Provide a fun, deterministic, and probabilistic code generator
• Give programmers a playground for syntax, speed, and randomness

“Just a parser… in reverse.”

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ABNF (Awesome BNF)

• ^ → Infinite generation (loops nodes without halting)
• <prob> → Weighted probabilities for branching
• Compatible with standard EBNF operators: +, *, ?, ()

See docs/ABNF.md for full syntax and examples.