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#pragma once
#include "error.hpp"
#include "lexer.hpp"
#include <optional>
#include <string>
#include <string_view>
namespace math::server {
namespace parser {
class Error : public server::Error {
public:
explicit Error(const std::string& what)
: server::Error{"parser error: " + what}
{ }
};
class BinaryOp {
public:
static bool is(const lexer::Token& token) {
using Type = lexer::Token::Type;
switch (token.get_type()) {
case Type::PLUS:
case Type::MINUS:
case Type::ASTERISK:
case Type::SLASH:
return true;
default:
return false;
}
}
static BinaryOp from_token(const lexer::Token& token) {
if (!is(token)) {
throw Error{"internal: token is not a binary operator"};
}
return BinaryOp{token};
}
static constexpr unsigned min_precedence() { return 0; }
unsigned get_precedence() const {
using Type = lexer::Token::Type;
switch (m_type) {
case Type::PLUS:
case Type::MINUS:
return min_precedence();
case Type::ASTERISK:
case Type::SLASH:
return min_precedence() + 1;
default:
throw Error{"internal: undefined operator precedence"};
}
}
double exec(double lhs, double rhs) const {
using Type = lexer::Token::Type;
switch (m_type) {
case Type::PLUS:
return lhs + rhs;
case Type::MINUS:
return lhs - rhs;
case Type::ASTERISK:
return lhs * rhs;
case Type::SLASH:
// Trapping the CPU would be better?
if (rhs == 0.) {
throw Error{"division by zero"};
}
return lhs / rhs;
default:
throw Error{"internal: unsupported operator"};
}
}
private:
explicit BinaryOp(const lexer::Token& token)
: m_type{token.get_type()}
{ }
lexer::Token::Type m_type;
};
}
class Parser {
public:
// I did simple recursive descent parsing a long time ago (see
// https://github.com/egor-tensin/simple-interpreter), this appears to be
// a finer algorithm for parsing arithmetic expressions.
// Reference: https://en.wikipedia.org/wiki/Operator-precedence_parser
explicit Parser(const std::string_view& input)
: m_lexer{input}
{ }
double exec() {
m_lexer.parse_token();
const auto result = exec_expr();
if (m_lexer.has_token()) {
throw parser::Error{"expected a binary operator"};
}
return result;
}
private:
double exec_expr() {
return exec_expr(exec_primary(), parser::BinaryOp::min_precedence());
}
double exec_expr(double lhs, unsigned min_prec) {
for (auto op = peek_operator(); op.has_value() && op->get_precedence() >= min_prec;) {
const auto lhs_op = *op;
m_lexer.drop_token();
auto rhs = exec_primary();
for (op = peek_operator(); op.has_value() && op->get_precedence() > lhs_op.get_precedence(); op = peek_operator()) {
rhs = exec_expr(rhs, op->get_precedence());
}
lhs = lhs_op.exec(lhs, rhs);
}
return lhs;
}
std::optional<parser::BinaryOp> peek_operator() {
const auto token = m_lexer.peek_token();
if (!token.has_value() || !parser::BinaryOp::is(*token)) {
return {};
}
return parser::BinaryOp::from_token(*token);
}
double exec_primary() {
if (!m_lexer.has_token()) {
throw parser::Error{"expected '-', '(' or a number"};
}
using Type = lexer::Token::Type;
if (m_lexer.drop_token_if(Type::MINUS).has_value()) {
return -exec_primary();
}
if (m_lexer.drop_token_if(Type::LEFT_PAREN).has_value()) {
const auto inner = exec_expr();
if (!m_lexer.has_token() || !m_lexer.drop_token_if(Type::RIGHT_PAREN).has_value()) {
throw parser::Error{"missing closing ')'"};
}
return inner;
}
if (const auto token = m_lexer.drop_token_if(Type::NUMBER); token.has_value()) {
return token.value().get_number_value();
}
throw parser::Error{"expected '-', '(' or a number"};
}
Lexer m_lexer;
};
}
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