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// Copyright Oliver Kowalke 2014.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
#include <cstdlib>
#include <functional>
#include <iostream>
#include <sstream>
#include <boost/context/all.hpp>
/*
* grammar:
* P ---> E '\0'
* E ---> T {('+'|'-') T}
* T ---> S {('*'|'/') S}
* S ---> digit | '(' E ')'
*/
class Parser{
char next;
std::istream& is;
std::function<void(char)> cb;
char pull(){
return std::char_traits<char>::to_char_type(is.get());
}
void scan(){
do{
next=pull();
}
while(isspace(next));
}
public:
Parser(std::istream& is_,std::function<void(char)> cb_) :
next(), is(is_), cb(cb_)
{}
void run() {
scan();
E();
}
private:
void E(){
T();
while (next=='+'||next=='-'){
cb(next);
scan();
T();
}
}
void T(){
S();
while (next=='*'||next=='/'){
cb(next);
scan();
S();
}
}
void S(){
if (std::isdigit(next)){
cb(next);
scan();
}
else if(next=='('){
cb(next);
scan();
E();
if (next==')'){
cb(next);
scan();
}else{
exit(2);
}
}
else{
exit(3);
}
}
};
int main() {
std::istringstream is("1+1");
bool done=false;
char c;
// create handle to main execution context
boost::context::execution_context main_ctx(
boost::context::execution_context::current() );
// executes parser in new execution context
boost::context::execution_context parser_ctx(
boost::context::fixedsize_stack(),
[&main_ctx,&is,&c,&done](){
// create parser with callback function
Parser p( is,
[&main_ctx,&c](char ch){
c=ch;
// resume main execution context
main_ctx.resume();
});
// start recursive parsing
p.run();
done=true;
main_ctx.resume();
});
// user-code pulls parsed data from parser
// invert control flow
parser_ctx.resume();
do {
printf("Parsed: %c\n",c);
parser_ctx.resume();
} while( ! done);
std::cout << "main: done" << std::endl;
return EXIT_SUCCESS;
}
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