pl/include/parser/lexer.hpp

#pragma once
#include <cstring>
#include <cctype>

#include "prelude/string.hpp"

enum class TokenType
{
    Eof = 256,
    Id,
    IntLiteral,

    Extern,
    Fn,
    Local,

    Unknown,
};

template <typename... Ts>
inline bool is_one_of(TokenType t, Ts... ts) {
    return ((t == ts) || ...);
}

// operator== TokenType == char
inline bool operator==(TokenType t, char c)
{
    return static_cast<char>(t) == c;
}

// operator== char == TokenType
inline bool operator==(char c, TokenType t)
{
    return t == c;   // reuse the function above
}

// operator!=
inline bool operator!=(TokenType t, char c)
{
    return !(t == c);
}

inline bool operator!=(char c, TokenType t)
{
    return !(t == c);
}

struct Token
{
    TokenType token = TokenType::Unknown;
    long int_number = 0;
    // null-terminated
    StringView string;
    long line_number = 0;
    long offset_start = 0;
    long offset_end = 0;
public:
    Token(TokenType t) : token(t) {}
    Token(TokenType t, long lnumber, long soffset, long eoffset)
        : token(t), line_number(lnumber), offset_start(soffset), offset_end(eoffset) {}
    Token() : token(TokenType::Unknown) {}
};

class Lexer
{
public:
    const Token& token() const { return m_token; }
    const Token* seek_token()
    {
        auto s = m_token;
        auto p = m_pos;
        auto l = m_line;
        auto lnl = m_last_newline;
        if (!NextToken())
        {
            return new Token(TokenType::Eof, m_line, m_pos - m_last_newline, m_pos - m_last_newline);
        }
        auto seeked = m_token;
        m_token = s;
        m_pos = p;
        m_line = l;
        m_last_newline = lnl;
        return new Token(seeked);
    }

    const char* filename() const { return m_filename; }
public:
    Lexer(char* filename, StringView code)
        : m_filename(filename), m_code(code) {}

    Lexer(const Lexer&) = delete;
public:
    bool NextToken()
    {
        auto len = m_code.len();

        auto peek = [&]() -> char {
            return (m_pos < len) ? m_code.data[m_pos] : '\0';
        };

        auto advance = [&]() -> char {
            return (m_pos < len) ? m_code.data[m_pos++] : '\0';
        };

        // IMPORTANT: >= not >
        if (m_pos >= len)
        {
            m_token = Token(TokenType::Eof);
            return false;
        }

        char c = advance();

        // skip whitespace safely
        while (c != '\0' && std::isspace((unsigned char)c))
        {
            if (c == '\n')
            {
                m_line++;
                m_last_newline = m_pos;
            }

            if (m_pos >= len) // reached real EOF while skipping whitespace
            {
                m_token = Token(TokenType::Eof);
                return false;
            }

            c = advance();
        }

        if (c == '\0' || m_pos > len) // paranoia guard
        {
            m_token = Token(TokenType::Eof);
            return false;
        }

        // identifier
        if (std::isalpha((unsigned char)c) || c == '_')
        {
            StringBuilder s;
            long offset_start = m_pos - m_last_newline - 1; // -1 because we already consumed c
            s.Push(c);

            // NOTE: usually identifiers allow digits after first char; add isdigit if you want
            while (true)
            {
                char p = peek();
                if (!(std::isalpha((unsigned char)p) || p == '_'))
                    break;
                s.Push(advance());
            }

            s.Push('\0');
            m_token = Token(TokenType::Id, m_line, offset_start, m_pos - m_last_newline);
            m_token.string = s.view();

            if (strcmp("extern", m_token.string.c_str()) == 0) m_token.token = TokenType::Extern;
            else if (strcmp("fn", m_token.string.c_str()) == 0) m_token.token = TokenType::Fn;
            else if (strcmp("local", m_token.string.c_str()) == 0) m_token.token = TokenType::Local;

            return true;
        }

        // integer (hex supported)
        if (std::isdigit((unsigned char)c))
        {
            StringBuilder s;
            long offset_start = m_pos - m_last_newline - 1;

            bool hex = (c == '0' && peek() == 'x');
            s.Push(c);

            if (hex) s.Push(advance()); // consume 'x'

            while (true)
            {
                char p = peek();
                if (std::isdigit((unsigned char)p) ||
                    (hex && std::isxdigit((unsigned char)p)))
                {
                    s.Push(advance());
                }
                else break;
            }

            s.Push('\0');
            m_token = Token(TokenType::IntLiteral, m_line, offset_start, m_pos - m_last_newline);
            m_token.int_number = std::strtol(s.data, nullptr, hex ? 16 : 10);
            m_token.string = s.data;
            return true;
        }

        // single-char token fallback
        m_token = Token((TokenType)c, m_line, m_pos - m_last_newline - 1, m_pos - m_last_newline);
        return true;
    }

    void NextExpect(TokenType expected)
    {
        if (!NextToken())
        {
            fprintf(stderr, "%s:%d:%d: ERROR: expected %ld, but got EOF\n", m_filename, 0, 0, expected);
            Exit(1);
        }
        if (token().token != expected)
        {
            fprintf(stderr, "%s:%d:%d: ERROR: expected %ld, but got %ld\n", m_filename, token().line_number, token().offset_start, expected, token().token);
            Exit(1);
        }
    }

    void NextExpect(char expected)
    {
        NextExpect((TokenType)expected);
    }
private:
    void Exit(int status)
    {
        std::exit(status);
    }
private:
    StringView m_code;
    char* m_filename;

    Token m_token;

    size_t m_pos = 0;
    size_t m_line = 1;
    size_t m_last_newline = 0;
};