encoding.hpp

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// Copyright (C) 2020-2023 Jonathan Müller and lexy contributors
// SPDX-License-Identifier: BSL-1.0
 
#ifndef LEXY_ENCODING_HPP_INCLUDED
#define LEXY_ENCODING_HPP_INCLUDED
 
#include <cstdint>
#include <lexy/_detail/assert.hpp>
#include <lexy/_detail/config.hpp>
 
//=== encoding classes ===//
namespace lexy
{
enum class encoding_endianness
{
    little,
    big,
    bom,
};
 
struct default_encoding
{
    using char_type = char;
    using int_type  = int;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return false;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        return -1;
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        if constexpr (std::is_unsigned_v<char_type>)
            // We can just convert it to int directly.
            return static_cast<int_type>(c);
        else
        {
            // We first need to prevent negative values, by making it unsigned.
            auto value = static_cast<unsigned char>(c);
            return static_cast<int_type>(value);
        }
    }
};
 
// An encoding where the input is assumed to be valid ASCII.
struct ascii_encoding
{
    using char_type = char;
    using int_type  = char;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return false;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        if constexpr (std::is_signed_v<char_type>)
            return int_type(-1);
        else
            return int_type(0xFFu);
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        return int_type(c);
    }
};
 
struct utf8_encoding
{
    using char_type = LEXY_CHAR8_T;
    using int_type  = LEXY_CHAR8_T;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return std::is_same_v<OtherCharType, char>;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        // 0xFF is not part of valid UTF-8.
        return int_type(0xFF);
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        return int_type(c);
    }
};
 
struct utf8_char_encoding
{
    using char_type = char;
    using int_type  = char;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return std::is_same_v<OtherCharType, LEXY_CHAR8_T>;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        // 0xFF is not part of valid UTF-8.
        return int_type(0xFF);
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        return int_type(c);
    }
};
 
struct utf16_encoding
{
    using char_type = char16_t;
    using int_type  = std::int_least32_t;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return sizeof(wchar_t) == sizeof(char16_t) && std::is_same_v<OtherCharType, wchar_t>;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        // Every value of char16_t is valid UTF16.
        return int_type(-1);
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        return int_type(c);
    }
};
 
struct utf32_encoding
{
    using char_type = char32_t;
    using int_type  = char32_t;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return sizeof(wchar_t) == sizeof(char32_t) && std::is_same_v<OtherCharType, wchar_t>;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        // The highest unicode code point is U+10'FFFF, so this is never a valid code point.
        return int_type(0xFFFF'FFFF);
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        return c;
    }
};
 
struct byte_encoding
{
    using char_type = unsigned char;
    using int_type  = int;
 
    template <typename OtherCharType>
    static constexpr bool is_secondary_char_type()
    {
        return std::is_same_v<OtherCharType, char> || std::is_same_v<OtherCharType, std::byte>;
    }
 
    static LEXY_CONSTEVAL int_type eof()
    {
        return -1;
    }
 
    static constexpr int_type to_int_type(char_type c)
    {
        return int_type(c);
    }
};
} // namespace lexy
 
//=== deduce_encoding ===//
namespace lexy
{
template <typename CharT>
struct _deduce_encoding;
template <typename CharT>
using deduce_encoding = typename _deduce_encoding<CharT>::type;
 
template <>
struct _deduce_encoding<char>
{
#if defined(LEXY_ENCODING_OF_CHAR)
    using type = LEXY_ENCODING_OF_CHAR;
    static_assert(std::is_same_v<type, default_encoding>      //
                      || std::is_same_v<type, ascii_encoding> //
                      || std::is_same_v<type, utf8_encoding>  //
                      || std::is_same_v<type, utf8_char_encoding>,
                  "invalid value for LEXY_ENCODING_OF_CHAR");
#else
    using type = default_encoding; // Don't know the exact encoding.
#endif
};
 
#if LEXY_HAS_CHAR8_T
template <>
struct _deduce_encoding<LEXY_CHAR8_T>
{
    using type = utf8_encoding;
};
#endif
template <>
struct _deduce_encoding<char16_t>
{
    using type = utf16_encoding;
};
template <>
struct _deduce_encoding<char32_t>
{
    using type = utf32_encoding;
};
 
template <>
struct _deduce_encoding<unsigned char>
{
    using type = byte_encoding;
};
template <>
struct _deduce_encoding<std::byte>
{
    using type = byte_encoding;
};
} // namespace lexy
 
//=== impls ===//
namespace lexy::_detail
{
template <typename Encoding, typename CharT>
constexpr bool is_compatible_char_type
    = std::is_same_v<typename Encoding::char_type,
                     CharT> || Encoding::template is_secondary_char_type<CharT>();
 
template <typename Encoding, typename CharT>
using require_secondary_char_type
    = std::enable_if_t<Encoding::template is_secondary_char_type<CharT>()>;
 
template <typename CharT>
constexpr bool is_ascii(CharT c)
{
    if constexpr (std::is_signed_v<CharT>)
        return 0 <= c && c <= 0x7F;
    else
        return c <= 0x7F;
}
 
template <typename TargetCharT, typename CharT>
LEXY_CONSTEVAL TargetCharT transcode_char(CharT c)
{
    if constexpr (std::is_same_v<CharT, TargetCharT>)
    {
        return c;
    }
#if !LEXY_HAS_CHAR8_T
    else if constexpr (std::is_same_v<CharT, char> && std::is_same_v<TargetCharT, LEXY_CHAR8_T>)
    {
        // If we don't have char8_t, `LEXY_LIT(u8"ä")` would have the type char, not LEXY_CHAR8_T
        // (which is unsigned char). So we disable checking in that case, to allow such usage. Note
        // that this prevents catching `LEXY_LIT("ä")`, but there is nothing we can do.
        return static_cast<LEXY_CHAR8_T>(c);
    }
#endif
    else
    {
        LEXY_ASSERT(is_ascii(c), "character type of string literal didn't match, "
                                 "so only ASCII characters are supported");
        // Note that we don't need to worry about signed/unsigned conversion, all ASCII values are
        // positive.
        return static_cast<TargetCharT>(c);
    }
}
 
template <typename Encoding, typename CharT>
LEXY_CONSTEVAL auto transcode_int(CharT c) -> typename Encoding::int_type
{
    return Encoding::to_int_type(transcode_char<typename Encoding::char_type>(c));
}
} // namespace lexy::_detail
 
#endif // LEXY_ENCODING_HPP_INCLUDED