lexer.cpp

#include <iostream>
#include <string>
#include <vector>
#include <sstream>

#include "lexer.h"
#include "tokens.h"

namespace Math_Interpreter {

Lexer::Lexer(const std::string& text)
    : text(text), pos(0), current(0)
{
    advance();
}

void Lexer::advance() {
    if (pos < text.length()) {
        current = text.at(pos++);
    } else {
        current = 0; // 0 means nothing, therefore in the while loop, it will break if current == 0.
    }
}

std::vector<Token> Lexer::generate_tokens() {
    std::vector<Token> tokens;

    while (current) {
        if (WHITESPACE.find_first_of(current) != std::string::npos) {
            advance();
        } else if (current == '.' || DIGITS.find_first_of(current) != std::string::npos) {
            tokens.push_back(generate_number());
        } else if (current == '+') {
            advance();
            tokens.push_back(Token(TokenType::PLUS));
        } else if (current == '-') {
            advance();
            tokens.push_back(Token(TokenType::MINUS));
        } else if (current == '*') {
            advance();
            tokens.push_back(Token(TokenType::MULTIPLY));
        } else if (current == '/') {
            advance();
            tokens.push_back(Token(TokenType::DIVIDE));
        } else if (current == '(') {
            advance();
            tokens.push_back(Token(TokenType::LPAREN));
        } else if (current == ')') {
            advance();
            tokens.push_back(Token(TokenType::RPAREN));
        } else {
            std::ostringstream oss;
            oss << "Illegal character '" << current << "'";
            throw oss.str();
        }
    }

    return tokens;
}

Token Lexer::generate_number() {
    std::size_t decimal_point_count = 0;
    std::string number_str(1, current);
    advance();

    while (current && (current == '.' || DIGITS.find_first_of(current) != std::string::npos)) {
        if (current == '.') {
            if (++decimal_point_count > 1) {
                break;
            }
        }

        number_str += current;
        advance();
    }

    if (number_str.at(0) == '.') {
        number_str = '0' + number_str;
    }

    if (number_str.at(number_str.length() - 1) == '.') {
        number_str += '0';
    }

    return Token(TokenType::NUMBER, std::stod(number_str));
}

} // namespace Math_Interpreter