https://esolangs.org/wiki/Thue

thue.cpp

#include<iostream>
#include<fstream>
#include<string>
#include<vector>
#include<utility>
#include<algorithm>

int main(int argc,char** argv){
	std::streambuf *cinbuf=std::cin.rdbuf(); // save cin buffer in case it is redirected

	if(argc>1){
		// redirect file to stdin
		std::ifstream*f=new std::ifstream(argv[1]);
		if(!f->is_open()){
			std::cout<<"?file"<<std::endl;
			return 1;
		}
		std::cin.rdbuf(f->rdbuf());
	}

	std::vector<std::pair<std::string,std::string>> replacements;
	std::string buf;

	while(true){
		if(std::cin.eof()){
			std::cout<<"?eof"<<std::endl;
			return 2;
		}
		std::getline(std::cin,buf);

		// ignore empty lines
		if(buf.empty()) continue;

		size_t i=buf.find("::=");
		if(i!=std::string::npos&&i!=0){
			replacements.emplace_back(buf.substr(0,i),buf.substr(i+3,buf.size()));
		}else if(i==0){
			break;
		}else{
			// no ::= found
			std::cout<<"?syntax"<<std::endl;
			return 3;
		}
	}

	std::string state;

	while(!std::cin.eof()){
		std::getline(std::cin,buf);
		state+=buf;
	}

	// initial state loaded, can now return to normal cin
	std::cin.rdbuf(cinbuf); // also clears eof flag

	bool applied;
	do{
		std::random_shuffle(replacements.begin(),replacements.end());
		applied=false;

		for(std::pair<std::string,std::string> p:replacements){
			size_t i=state.find(p.first);
			
			// skip if rule didn't match
			if(i==std::string::npos) continue;
			
			// rule did match, apply it
			applied=true;

			if(p.second==":::"){
				// input
				std::cout<<"> ";
				std::getline(std::cin,buf);
				state.replace(i,p.first.size(),buf);
			}else if(p.second[0]=='~'){
				// output
				state.erase(i,p.first.size());
				std::cout<<(p.second.size()==1?"\n":p.second.substr(1));
			}else{
				state.replace(i,p.first.size(),p.second);
			}
		}
	}while(applied);

	std::cout<<"\n\t"<<state<<std::endl;

	return 0;
}