libcruft-util/utf8.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2017 Danny Robson <danny@nerdcruft.net>
 */

#include "./utf8.hpp"


///////////////////////////////////////////////////////////////////////////////
template <typename T>
struct test {
    constexpr
    test (T _mask, T _bits) noexcept:
        mask (_mask),
        bits (_bits)
    { ; }

    constexpr bool
    valid (T t) const noexcept
    {
        return (t & mask) == bits;
    }

    constexpr T
    value (T t) const noexcept
    {
        return t & ~mask;
    }

    T mask;
    T bits;
};


//-----------------------------------------------------------------------------
static constexpr test<uint32_t>
operator"" _test (const char *str, size_t len)
{
    uint32_t mask = 0;
    uint32_t bits = 0;

    if (str[0] != '0' || str[1] != 'b')
        throw std::invalid_argument ("invalid bit test prefix");

    for (size_t i = 2; i < len; ++i) {
        auto c = str[i];

        mask <<= 1;
        bits <<= 1;

        switch (c) {
        case '0': mask |= 0x1; bits |= 0x0; break;
        case '1': mask |= 0x1; bits |= 0x1; break;
        case 'x': mask |= 0x0; bits |= 0x0; break;
        default:
            throw std::invalid_argument ("invalid bit test character");
        }

    }

    return { mask, bits };
}


///////////////////////////////////////////////////////////////////////////////
template <
    typename InputT,
    typename OutputT>
static OutputT
decode (cruft::view<InputT> src, OutputT dst)
{
    using namespace cruft::utf8;

    static constexpr
    test<codepoint_t> PREFIX[] = {
        "0b0xxxxxxx"_test,
        "0b110xxxxx"_test,
        "0b1110xxxx"_test,
        "0b11110xxx"_test
    };

    for (auto cursor = src.cbegin (); cursor != src.cend (); ) {
        codepoint_t c = std::to_integer<codepoint_t> (*cursor++);

        int len = PREFIX[0].valid (c) ? 0 :
                  PREFIX[1].valid (c) ? 1 :
                  PREFIX[2].valid (c) ? 2 :
                  PREFIX[3].valid (c) ? 3 :
                  throw malformed_error {};

        // get the simple ANSI case out of the way
        if (!len) {
            *dst++ = c;
            continue;
        }

        codepoint_t accum { PREFIX[len].value (c) };

        // prepend each of the remaining bytes data to an accumulator
        for (int i = 1; i <= len; ++i) {
            static constexpr auto CONTINUATION = "0b10xxxxxx"_test;
            if (cursor == src.cend ())
                throw malformed_error {};

            codepoint_t now = std::to_integer<codepoint_t> (*cursor++);
            if (!CONTINUATION.valid (now))
                throw malformed_error {};

            accum <<= 6;
            accum  |= CONTINUATION.value (now);
        }

        // check that the codepoint is the right size by seeing if the unique
        // bits present in the decoded size codepoint are actually used.
        //
        // these could theoretically be provided to the user, but they may be
        // misused so we will throw an error instead.
        static constexpr
        codepoint_t LEVEL_MASK[] {
            0b00000000'00000000'01111111,
            0b00000000'00000111'10000000,
            0b00000000'11111000'00000000,
            0b00011111'00000000'00000000
        };

        if (!(accum & LEVEL_MASK[len]))
            throw overlong_error {};

        // utf16 surrogates should not be present in utf8
        if (accum >= 0xD800 && accum <= 0xDFFF)
            throw illegal_codepoint {};

        // reject the BOM
        if (accum == 0xfffe || accum == 0xffff)
            throw illegal_codepoint {};

        *dst++ = accum;
    }


    return dst;
}


///////////////////////////////////////////////////////////////////////////////
std::vector<cruft::utf8::codepoint_t>
cruft::utf8::decode (view<const std::byte*> src)
{
    std::vector<codepoint_t> dst;
    dst.reserve (src.size ());

    ::decode (src, std::back_inserter (dst));
    return dst;
}