email.cpp

// Copyright (C) 2020-2025 Jonathan Müller and lexy contributors
// SPDX-License-Identifier: BSL-1.0

#include <cstdio>
#include <optional>
#include <string>
#include <vector>

#include <lexy/action/parse.hpp> // lexy::parse
#include <lexy/callback.hpp>     // value callbacks
#include <lexy/dsl.hpp>          // lexy::dsl::*
#include <lexy/input/file.hpp>   // lexy::read_file

#include <lexy_ext/report_error.hpp> // lexy_ext::report_error

// Parses an email as defined by a simplified subset of RFC 5322
// (https://tools.ietf.org/html/rfc5322).
// It does not support groups or IPs as email addresses, nor most
// headers or the line width limitations.

namespace
{
namespace ast
{
    struct address
    {
        std::optional<std::string> display_name;
        std::string                local_part;
        std::string                domain;

        void print() const
        {
            if (display_name)
                std::printf("%s <%s@%s> ", display_name->c_str(), local_part.c_str(),
                            domain.c_str());
            else
                std::printf("%s@%s ", local_part.c_str(), domain.c_str());
        }
    };

    struct message
    {
        std::vector<address> from;
        std::vector<address> to;
        std::vector<address> cc;
        std::string          subject;
        std::string          body;

        void print() const
        {
            std::fputs("From: ", stdout);
            for (auto& addr : from)
                addr.print();
            std::putchar('\n');
            std::fputs("To: ", stdout);
            for (auto& addr : to)
                addr.print();
            std::putchar('\n');
            std::fputs("Cc: ", stdout);
            for (auto& addr : cc)
                addr.print();
            std::putchar('\n');
            std::printf("Subject:%s\n", subject.c_str());

            std::putchar('\n');

            std::printf("%s", body.c_str());
        }
    };
} // namespace ast

namespace grammar
{
    namespace dsl = lexy::dsl;

    // We're limiting whitespace to ASCII blank.
    // We don't distinguish here between folding and non-folding whitespace for simplicity.
    // We also don't support comments - they're really complicated in that they support nesting and
    // quotes and so on.
    constexpr auto ws = dsl::whitespace(dsl::ascii::blank);

    //=== https://tools.ietf.org/html/rfc5322#section-3.2.3 ===//
    constexpr auto atext
        = LEXY_CHAR_CLASS("atext",
                          dsl::ascii::alpha / dsl::ascii::digit / LEXY_LIT("!") / LEXY_LIT("#")
                              / LEXY_LIT("$") / LEXY_LIT("%") / LEXY_LIT("&") / LEXY_LIT("'")
                              / LEXY_LIT("*") / LEXY_LIT("+") / LEXY_LIT("-") / LEXY_LIT("/")
                              / LEXY_LIT("=") / LEXY_LIT("?") / LEXY_LIT("^") / LEXY_LIT("_")
                              / LEXY_LIT("`") / LEXY_LIT("{") / LEXY_LIT("|") / LEXY_LIT("}"));

    // Text of the specified characters.
    // In the grammar it is always surrounded by whitespace.
    // Here, we only add trailing whitespace; leading whitespace is taken care of earlier.
    constexpr auto atom = dsl::identifier(atext) + ws;

    struct dot_atom
    {
        // A list of atom separated by periods which are part of the content and thus captured
        // (unlike the whitespace).
        static constexpr auto rule  = dsl::list(atom, dsl::sep(dsl::capture(dsl::period) >> ws));
        static constexpr auto value = lexy::as_string<std::string>;
    };

    //=== https://tools.ietf.org/html/rfc5322#section-3.2.4 ===//
    struct quoted_string
    {
        static constexpr auto rule = [] {
            // \c, where c is a printable ASCII character, is replaced by c itself.
            auto escape = dsl::backslash_escape.capture(dsl::ascii::print);
            // Skip trailing whitespace.
            return dsl::quoted(dsl::ascii::print, escape) >> ws;
        }();
        static constexpr auto value = lexy::as_string<std::string>;
    };

    //=== https://tools.ietf.org/html/rfc5322#section-3.2.5 ===//
    // Word skips trailing whitespace.
    constexpr auto word = dsl::p<quoted_string> | dsl::else_ >> atom;

    struct phrase
    {
        // A phrase is a list of words which starts with another text character or quote.
        static constexpr auto rule  = dsl::list(dsl::peek(atext / LEXY_LIT("\"")) >> word);
        static constexpr auto value = lexy::as_string<std::string>;
    };

    //=== https://tools.ietf.org/html/rfc5322#section-3.4 ===//
    // Note: we're having no group so we merge mailbox and address.
    struct address
    {
        static constexpr auto rule = [] {
            auto local_part = dsl::p<quoted_string> | dsl::else_ >> dsl::p<dot_atom>;
            auto at         = dsl::at_sign + ws;
            auto domain     = dsl::p<dot_atom>; // No domain literal support.

            // Sets the local part and domain member.
            auto addr_spec = local_part + at + domain;

            // An address spec with an optional name.
            // Need to skip whitespace after the <, trailing whitespace skipped by addr_spec.
            auto angle_addr = dsl::angle_bracketed(ws + addr_spec);
            // Whitespace between phrase and < is skipped by phrase.
            auto name_addr = dsl::opt(dsl::p<phrase>) + angle_addr;

            // We're only having a named address if we can match the optional phrase followed by an
            // angle bracket.
            auto name_addr_condition = dsl::opt(dsl::p<phrase>) + LEXY_LIT("<");

            // An address spec without a name.
            auto unnamed_addr = dsl::nullopt + addr_spec;

            // Need to skip initial whitespace.
            return ws + (dsl::peek(name_addr_condition) >> name_addr | dsl::else_ >> unnamed_addr);
        }();
        static constexpr auto value = lexy::construct<ast::address>;
    };

    struct address_list
    {
        static constexpr auto rule  = dsl::list(dsl::p<address>, dsl::sep(dsl::comma));
        static constexpr auto value = lexy::as_list<std::vector<ast::address>>;
    };

    //=== https://tools.ietf.org/html/rfc5322#section-3.5/6 ===//
    struct unstructured
    {
        static constexpr auto rule  = dsl::opt(dsl::identifier(dsl::ascii::print));
        static constexpr auto value = lexy::as_string<std::string>;
    };

    struct body
    {
        // The body is literally anything until the end of the file.
        static constexpr auto rule  = dsl::capture(dsl::any);
        static constexpr auto value = lexy::as_string<std::string>;
    };

    struct fields
    {
        static constexpr auto rule = [] {
            // Some of the fields in the header of a message.
            auto from    = LEXY_LIT("From:") >> dsl::p<address_list> + dsl::newline;
            auto to      = LEXY_LIT("To:") >> dsl::p<address_list> + dsl::newline;
            auto cc      = LEXY_LIT("Cc:") >> dsl::p<address_list> + dsl::newline;
            auto subject = LEXY_LIT("Subject:") >> dsl::p<unstructured> + dsl::newline;

            // We allow a partial combination of the fields (i.e. each field at most once in an
            // arbitrary order). Some fields are mandatory, but this verification is better done
            // elsewhere.
            //
            // With LEXY_MEM() we can assign them to a member of the given name.
            return dsl::partial_combination(LEXY_MEM(from) = from, LEXY_MEM(to) = to,
                                            LEXY_MEM(cc) = cc, LEXY_MEM(subject) = subject);
        }();
        static constexpr auto value = lexy::as_aggregate<ast::message>;
    };

    struct message
    {
        // The fields followed by the body assigned to the body member.
        static constexpr auto rule
            = dsl::p<fields> + dsl::newline + (LEXY_MEM(body) = dsl::p<body>);
        static constexpr auto value = lexy::as_aggregate<ast::message>;
    };
} // namespace grammar
} // namespace

#ifndef LEXY_TEST
int main(int argc, char** argv)
{
    if (argc < 2)
    {
        std::fprintf(stderr, "usage: %s <filename>", argv[0]);
        return 1;
    }

    // Emails only support ASCII characters.
    auto file = lexy::read_file<lexy::ascii_encoding>(argv[1]);
    if (!file)
    {
        std::fprintf(stderr, "file '%s' not found", argv[1]);
        return 1;
    }

    auto message
        = lexy::parse<grammar::message>(file.buffer(), lexy_ext::report_error.path(argv[1]));
    if (!message)
        return 2;

    message.value().print();
}
#endif