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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 "error.hpp"
#include "operator.hpp"
#include "../lexer/lexer.hpp"
#include <optional>
#include <string_view>
namespace math::server {
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() {
const auto result = exec_expr();
if (m_lexer.has_token()) {
throw ParserError{"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 ParserError{"expected '-', '(' or a number"};
}
using Type = lexer::Token::Type;
if (m_lexer.drop_token_of_type(Type::MINUS).has_value()) {
return -exec_primary();
}
if (m_lexer.drop_token_of_type(Type::LEFT_PAREN).has_value()) {
const auto inner = exec_expr();
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;
};
}
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