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// Copyright (c) 2019 Egor Tensin <Egor.Tensin@gmail.com>
// This file is part of the "math-server" project.
// For details, see https://github.com/egor-tensin/math-server.
// Distributed under the MIT License.
#pragma once
#include "../lexer/lexer.hpp"
#include "error.hpp"
#include "operator.hpp"
#include <optional>
#include <string_view>
namespace math::server {
class Parser {
public:
using Type = lexer::Token::Type;
// 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() {
const auto result = exec_dmas();
if (m_lexer.has_token()) {
throw ParserError{"expected a binary operator"};
}
return result;
}
private:
// DMAS as in Division, Multiplication, Addition and Subtraction
double exec_dmas() { return exec_binary_op(exec_factor(), parser::BinaryOp::min_precedence()); }
// Exponentiation operator
double exec_exp() {
return exec_binary_op(exec_atom(), parser::BinaryOp::get_precedence(Type::CARET));
}
double exec_binary_op(double lhs, unsigned min_prec) {
for (auto op = peek_operator(min_prec); op.has_value();) {
const auto prev = *op;
const auto prev_prec = prev.get_precedence();
m_lexer.drop_token();
auto rhs = exec_factor();
for (op = peek_operator(min_prec); op.has_value(); op = peek_operator(min_prec)) {
const auto next = *op;
const auto next_prec = next.get_precedence();
{
const auto acc_left_assoc = next.is_left_associative() && next_prec > prev_prec;
const auto acc_right_assoc =
next.is_right_associative() && next_prec == prev_prec;
const auto acc = acc_left_assoc || acc_right_assoc;
if (!acc) {
break;
}
}
rhs = exec_binary_op(rhs, next_prec);
}
lhs = prev.exec(lhs, rhs);
}
return lhs;
}
std::optional<parser::BinaryOp> peek_operator(unsigned min_prec) {
const auto token = m_lexer.peek_token();
if (!token.has_value() || !parser::BinaryOp::is(*token)) {
return {};
}
const auto op = parser::BinaryOp::from_token(*token);
if (op.get_precedence() < min_prec) {
return {};
}
return op;
}
double exec_factor() {
if (!m_lexer.has_token()) {
throw ParserError{"expected '-', '+', '(' or a number"};
}
if (m_lexer.drop_token_of_type(Type::MINUS).has_value()) {
return -exec_factor();
}
if (m_lexer.drop_token_of_type(Type::PLUS).has_value()) {
return exec_factor();
}
return exec_exp();
}
double exec_atom() {
if (!m_lexer.has_token()) {
throw ParserError{"expected '-', '+', '(' or a number"};
}
if (m_lexer.drop_token_of_type(Type::LEFT_PAREN).has_value()) {
const auto inner = exec_dmas();
if (!m_lexer.has_token() ||
!m_lexer.drop_token_of_type(Type::RIGHT_PAREN).has_value()) {
throw ParserError{"missing closing ')'"};
}
return inner;
}
if (const auto token = m_lexer.drop_token_of_type(Type::NUMBER); token.has_value()) {
return token.value().as_number();
}
throw ParserError{"expected '-', '+', '(' or a number"};
}
Lexer m_lexer;
};
} // namespace math::server
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