Parsing a string to create a geometry - c++

Question

What is the algorithm for developing a string parser to create a geometry? The geometry is generated in 2 steps: at the first step, we create primitives; at the second, we combine primitives into objects.
The syntax is presented in the string below.
string str="[GEOMETRY]
PRIMITIVE1=SPHERE(RADIUS=5.5);
PRIMITIVE2=BOX(A=-5.2, B=7.3);
//...
OBJECT1=PRIMITIVE2*(-PRIMITIVE1);
//..."
class PRIMITIVE{
int number;
public:
Primitive& operator+ (Primitive& primitive) {}; //overloading arithmetic operations
Primitive& operator* (Primitive& primitive) {};
Primitive& operator- (Primitive& primitive) {};
virtual bool check_in_point_inside_primitive = 0;
};
class SPHERE:public PRIMITIVE{
double m_radius;
public:
SPHERE(double radius): m_radius(radius) {}; //In which part of the parser to create objects?
bool check_in_point_inside_sphere(Point& point){};
};
class BOX:public PRIMITIVE{
double m_A;
double m_B;
public:
BOX(double A, double B): m_A(A), m_B(B) {};
bool check_in_point_inside_box(Point& point){};
};
class OBJECT{
int number;
PRIMITIVE& primitive;
public:
OBJECT(){};
bool check_in_point_inside_object(Primitive& PRIMITIVE1, Primitive& PRIMITIVE2, Point& point){
//>How to construct a function from an expression 'PRIMITIVE2*(-PRIMITIVE1)' when parsing?
}
};
How to analyze the string PRIMITIVE1=SPHERE(RADIUS=5.5) and pass a parameter to the constructor of SPHERE()? How to identify this object with the name PRIMITIVE 1 to call to it in OBJECT? Is it possible to create a pair<PRIMITIVE1,SPHERE(5.5)> and store all primitives in map?
How to parse the string of the OBJECT1 and to construct a function from an expression PRIMITIVE2*(-PRIMITIVE1) inside an OBJECT1? This expression will be required multiple times when determining the position of each point relative to the object.
How to use boost::spirit for this task? Tokenize a string using boost::spirit::lex, and then develop rules using boost::spirit::qi?

Answer 1

As a finger exercise, and despite the serious problems I see with the chosen virtual type hierarchy, let's try to make a value-oriented container of Primitives that can be indexed by their id (ById):
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#include <boost/intrusive/set.hpp>
#include <boost/poly_collection/base_collection.hpp>
#include <iostream>
namespace bi = boost::intrusive;
struct Point {
};
using IndexHook = bi::set_member_hook<bi::link_mode<bi::auto_unlink>>;
class Primitive {
int _id;
public:
struct ById {
bool operator()(auto const&... oper) const { return std::less<>{}(access(oper)...); }
private:
static int access(int id) { return id; }
static int access(Primitive const& p) { return p._id; }
};
IndexHook _index;
Primitive(int id) : _id(id) {}
virtual ~Primitive() = default;
int id() const { return _id; }
Primitive& operator+= (Primitive const& primitive) { return *this; } //overloading arithmetic operations
Primitive& operator*= (Primitive const& primitive) { return *this; }
Primitive& operator-= (Primitive const& primitive) { return *this; }
virtual bool check_in_point_inside(Point const&) const = 0;
};
using Index =
bi::set<Primitive, bi::constant_time_size<false>,
bi::compare<Primitive::ById>,
bi::member_hook<Primitive, IndexHook, &Primitive::_index>>;
class Sphere : public Primitive {
double _radius;
public:
Sphere(int id, double radius)
: Primitive(id)
, _radius(radius) {} // In which part of the parser to create objects?
bool check_in_point_inside(Point const& point) const override { return false; }
};
class Box : public Primitive {
double _A;
double _B;
public:
Box(int id, double A, double B) : Primitive(id), _A(A), _B(B) {}
bool check_in_point_inside(Point const& point) const override { return false; }
};
class Object{
int _id;
Primitive& _primitive;
public:
Object(int id, Primitive& p) : _id(id), _primitive(p) {}
bool check_in_point_inside_object(Primitive const& p1, Primitive const& p2,
Point const& point) const
{
//>How to construct a function from an expression
//'PRIMITIVE2*(-PRIMITIVE1)' when parsing?
return false;
}
};
using Primitives = boost::poly_collection::base_collection<Primitive>;
int main() {
Primitives test;
test.insert(Sphere{2, 4.0});
test.insert(Sphere{4, 4.0});
test.insert(Box{2, 5, 6});
test.insert(Sphere{1, 4.0});
test.insert(Box{3, 5, 6});
Index idx;
for (auto& p : test)
if (not idx.insert(p).second)
std::cout << "Duplicate id " << p.id() << " not indexed\n";
for (auto& p : idx)
std::cout << typeid(p).name() << " " << p.id() << "\n";
std::cout << "---\n";
for (auto& p : test)
std::cout << typeid(p).name() << " " << p.id() << "\n";
}
Prints
Duplicate id 2 not indexed
6Sphere 1
3Box 2
3Box 3
6Sphere 4
---
3Box 2
3Box 3
6Sphere 2
6Sphere 4
6Sphere 1
So far so good. This is an important building block to prevent all manner of pain when dealing with virtual types in Spirit grammars¹
PS: I've since dropped the idea of intrusive_set. It doesn't work because the base_container moves items around on reallocation, and that unlinks the items from their intrusive set.
Instead, see below for an approach that doesn't try to resolve ids during the parse.
Parsing primitives
We get the ID from the PRIMITIVE1. We could store it somewhere before naturally parsing the primitives themselves, then set the id on it on commit.
Let's start with defining a State object for the parser:
struct State {
Ast::Id next_id;
Ast::Primitives primitives;
Ast::Objects objects;
template <typename... T> void commit(boost::variant<T...>& val) {
boost::apply_visitor([this](auto& obj) { commit(obj); }, val);
}
template <typename T> void commit(T& primitiveOrExpr) {
auto id = std::exchange(next_id, 0);
if constexpr (std::is_base_of_v<Ast::Primitive, T>) {
primitiveOrExpr.id = id;
primitives.insert(std::move(primitiveOrExpr));
} else {
objects.push_back(Ast::Object{id, std::move(primitiveOrExpr)});
}
}
};
As you can see, we just have a place to store the primitives, objects. And then there is the temporary storage for our next_id while we're still parsing the next entity.
The commit function helps sorting the products of the parser rules. As it happens, they can be variant, which is why we have the apply_visitor dispatch for commit on a variant.
Again, as the footnote¹ explains, Spirit's natural attribute synthesis favors static polymorphism.
The semantic actions we need are now:
static inline auto& state(auto& ctx) { return get<State>(ctx); }
auto draft = [](auto& ctx) { state(ctx).next_id = _attr(ctx); };
auto commit = [](auto& ctx) { state(ctx).commit(_attr(ctx)); };
Now let's jump ahead to the primitives:
auto sphere = as<Ast::Sphere>(eps >> "sphere" >>'(' >> param("radius") >> ')');
auto box = as<Ast::Box>(eps >> "box" >> '(' >> param('a') >> ',' >> param('b') >> ')');
auto primitive =
("primitive" >> uint_[draft] >> '=' >> (sphere | box)[commit]) > ';';
That's still cheating a little, as I've used the param helper to reduce typing:
auto number = as<Ast::Number>(double_, "number");
auto param(auto name, auto p) { return eps >> omit[name] >> '=' >> p; }
auto param(auto name) { return param(name, number); }
As you can see I've already assumed most parameters will have numerical nature.
What Are Objects Really?
Looking at it for a while, I concluded that really an Object is defined as an id number (OBJECT1, OBJECT2...) which is tied to an expression. The expression can reference primitives and have some unary and binary operators.
Let's sketch an AST for that:
using Number = double;
struct RefPrimitive { Id id; };
struct Binary;
struct Unary;
using Expr = boost::variant< //
Number, //
RefPrimitive, //
boost::recursive_wrapper<Unary>, //
boost::recursive_wrapper<Binary> //
>;
struct Unary { char op; Expr oper; };
struct Binary { Expr lhs; char op; Expr rhs; };
struct Object { Id id; Expr expr; };
Now To Parse Into That Expression AST
It's really 1:1 rules for each Ast node type. E.g.:
auto ref_prim = as<Ast::RefPrimitive>(lexeme["primitive" >> uint_]);
Now many of the expression rules can recurse, so we need declared rules with definitions via BOOST_SPIRIT_DEFINE:
// object expression grammar
rule<struct simple_tag, Ast::Expr> simple{"simple"};
rule<struct unary_tag, Ast::Unary> unary{"unary"};
rule<struct expr_tag, Ast::Expr> expr{"expr"};
rule<struct term_tag, Ast::Expr> term{"term"};
rule<struct factor_tag, Ast::Expr> factor{"factor"};
As you can tell, some of these are not 1:1 with the Ast nodes, mainly because of the recursion and the difference in operator precedence (term vs factor vs. simple). It's easier to see with the rule definition:
auto unary_def = char_("-+") >> simple;
auto simple_def = ref_prim | unary | '(' >> expr >> ")";
auto factor_def = simple;
auto term_def = factor[assign] >> *(char_("*/") >> term)[make_binary];
auto expr_def = term[assign] >> *(char_("-+") >> expr)[make_binary];
Because none of the rules actually expose a Binary, automatic attribute propagation is not convenient there². Instead, we use assign and make_binary semantic actions:
auto assign = [](auto& ctx) { _val(ctx) = _attr(ctx); };
auto make_binary = [](auto& ctx) {
using boost::fusion::at_c;
auto& attr = _attr(ctx);
auto op = at_c<0>(attr);
auto& rhs = at_c<1>(attr);
_val(ctx) = Ast::Binary { _val(ctx), op, rhs };
};
Finally, let's tie the defintions to the declared rules (using their tag types):
BOOST_SPIRIT_DEFINE(simple, unary, expr, term, factor)
All we need is a similar line to primitive:
auto object =
("object" >> uint_[draft] >> '=' >> (expr)[commit]) > ';';
And we can finish up by defining each line as a primitive|object:
auto line = primitive | object;
auto file = no_case[skip(ws_comment)[*eol >> "[geometry]" >> (-line % eol) >> eoi]];
At the top level we expect the [GEOMETRY] header, specify that we want to be case insensitive and ... that ws_comment is to be skipped³:
auto ws_comment = +(blank | lexeme["//" >> *(char_ - eol) >> eol]);
This allows us to ignore the // comments as well.
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//#define BOOST_SPIRIT_X3_DEBUG
#include <boost/fusion/adapted.hpp>
#include <boost/poly_collection/base_collection.hpp>
#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <list>
#include <map>
namespace x3 = boost::spirit::x3;
namespace Ast {
using Id = uint32_t;
struct Point { }; // ?? where does this belong?
struct Primitive {
Id id;
virtual ~Primitive() = default;
};
struct Sphere : Primitive { double radius; };
struct Box : Primitive { double a, b; };
using Number = double;
struct RefPrimitive { Id id; };
struct Binary;
struct Unary;
using Expr = boost::variant< //
Number, //
RefPrimitive, //
boost::recursive_wrapper<Unary>, //
boost::recursive_wrapper<Binary> //
>;
struct Unary { char op; Expr oper; };
struct Binary { Expr lhs; char op; Expr rhs; };
struct Object { Id id; Expr expr; };
using Primitives = boost::poly_collection::base_collection<Primitive>;
using Objects = std::list<Object>;
using Index = std::map<Id, std::reference_wrapper<Primitive const>>;
std::ostream& operator<<(std::ostream& os, Primitive const& p) {
return os << boost::core::demangle(typeid(p).name()) << " "
<< "(id: " << p.id << ")";
}
std::ostream& operator<<(std::ostream& os, Object const& o) {
return os << "object(id:" << o.id << ", expr:" << o.expr << ")";
}
std::ostream& operator<<(std::ostream& os, RefPrimitive ref) {
return os << "reference(prim:" << ref.id << ")";
}
std::ostream& operator<<(std::ostream& os, Binary const& b) {
return os << '(' << b.lhs << b.op << b.rhs << ')';
}
std::ostream& operator<<(std::ostream& os, Unary const& u) {
return os << '(' << u.op << u.oper << ')';
}
} // namespace Ast
BOOST_FUSION_ADAPT_STRUCT(Ast::Primitive, id)
BOOST_FUSION_ADAPT_STRUCT(Ast::Sphere, radius)
BOOST_FUSION_ADAPT_STRUCT(Ast::Box, a, b)
BOOST_FUSION_ADAPT_STRUCT(Ast::Object, id)
BOOST_FUSION_ADAPT_STRUCT(Ast::RefPrimitive, id)
BOOST_FUSION_ADAPT_STRUCT(Ast::Unary, op, oper)
namespace Parser {
using namespace x3;
struct State {
Ast::Id next_id;
Ast::Primitives primitives;
Ast::Objects objects;
template <typename... T> void commit(boost::variant<T...>& val) {
boost::apply_visitor([this](auto& obj) { commit(obj); }, val);
}
template <typename T> void commit(T& val) {
auto id = std::exchange(next_id, 0);
if constexpr (std::is_base_of_v<Ast::Primitive, T>) {
val.id = id;
primitives.insert(std::move(val));
} else {
objects.push_back(Ast::Object{id, std::move(val)});
}
}
};
static inline auto& state(auto& ctx) { return get<State>(ctx); }
auto draft = [](auto& ctx) { state(ctx).next_id = _attr(ctx); };
auto commit = [](auto& ctx) { state(ctx).commit(_attr(ctx)); };
template <typename T>
auto as = [](auto p, char const* name = "as") {
return rule<struct _, T>{name} = p;
};
auto ws_comment = +(blank | lexeme["//" >> *(char_ - eol) >> (eol | eoi)]);
auto number = as<Ast::Number>(double_, "number");
auto param(auto name, auto p) { return eps >> omit[name] >> '=' >> p; }
auto param(auto name) { return param(name, number); }
auto sphere = as<Ast::Sphere>(eps >> "sphere" >>'(' >> param("radius") >> ')');
auto box = as<Ast::Box>(eps >> "box" >> '(' >> param('a') >> ',' >> param('b') >> ')');
auto primitive =
("primitive" >> uint_[draft] >> '=' >> (sphere | box)[commit]) > ';';
auto ref_prim = as<Ast::RefPrimitive>(lexeme["primitive" >> uint_], "ref_prim");
// object expression grammar
rule<struct simple_tag, Ast::Expr> simple{"simple"};
rule<struct unary_tag, Ast::Unary> unary{"unary"};
rule<struct expr_tag, Ast::Expr> expr{"expr"};
rule<struct term_tag, Ast::Expr> term{"term"};
rule<struct factor_tag, Ast::Expr> factor{"factor"};
auto assign = [](auto& ctx) { _val(ctx) = _attr(ctx); };
auto make_binary = [](auto& ctx) {
using boost::fusion::at_c;
auto& attr = _attr(ctx);
auto op = at_c<0>(attr);
auto& rhs = at_c<1>(attr);
_val(ctx) = Ast::Binary { _val(ctx), op, rhs };
};
auto unary_def = char_("-+") >> simple;
auto simple_def = ref_prim | unary | '(' >> expr >> ")";
auto factor_def = simple;
auto term_def = factor[assign] >> *(char_("*/") >> term)[make_binary];
auto expr_def = term[assign] >> *(char_("-+") >> expr)[make_binary];
BOOST_SPIRIT_DEFINE(simple, unary, expr, term, factor)
auto object =
("object" >> uint_[draft] >> '=' >> (expr)[commit]) > ';';
auto line = primitive | object;
auto file = no_case[skip(ws_comment)[*eol >> "[geometry]" >> (-line % eol) >> eoi]];
} // namespace Parser
int main() {
for (std::string const input :
{
R"(
[geometry]
primitive1=sphere(radius=5.5);
primitive2=box(a=-5.2, b=7.3);
//...
object1=primitive2*(-primitive1);
//...)",
R"(
[GEOMETRY]
PRIMITIVE1=SPHERE(RADIUS=5.5);
PRIMITIVE2=BOX(A=-5.2, B=7.3);
//...
OBJECT1=PRIMITIVE2*(-PRIMITIVE1);
//...)",
}) //
{
Parser::State state;
bool ok = parse(begin(input), end(input),
x3::with<Parser::State>(state)[Parser::file]);
std::cout << "Parse success? " << std::boolalpha << ok << "\n";
Ast::Index index;
for (auto& p : state.primitives)
if (auto[it,ok] = index.emplace(p.id, p); not ok) {
std::cout << "Duplicate id " << p
<< " (conflicts with existing " << it->second.get()
<< ")\n";
}
std::cout << "Primitives by ID:\n";
for (auto& [id, prim] : index)
std::cout << " - " << prim << "\n";
std::cout << "Objects in definition order:\n";
for (auto& obj: state.objects)
std::cout << " - " << obj << "\n";
}
}
Prints
Parse success? true
Primitives by ID:
- Ast::Sphere (id: 1)
- Ast::Box (id: 2)
Objects in definition order:
- object(id:1, expr:(reference(prim:2)*(-reference(prim:1))))
Parse success? true
Primitives by ID:
- Ast::Sphere (id: 1)
- Ast::Box (id: 2)
Objects in definition order:
- object(id:1, expr:(reference(prim:2)*(-reference(prim:1))))
¹ How can I use polymorphic attributes with boost::spirit::qi parsers?
² and insisting on that leads to classical in-efficiency with rules that cause a lot of backtracking
³ outside of lexemes