c++ - 在 C++ 中从 UTF-8 转换为 ISO8859-15

Question

我想在 C/C++ 中进行从 UTF-8 到 ISO 8859-15 的转换，而不包括额外的库。

我怎样才能做到这一点？

我找到了以下适用于 ISO 8859-1 的代码，但我不确定如何处理 ISO 8859-15 和 ISO 8859-1 (https://en.wikipedia.org/wiki/ISO/IEC_8859-15) 之间的差异:

std::string UTF8toISO8859_1(const char * in) {
    std::string out;
    if (in == NULL)
        return out;

    unsigned int codepoint;
    while (*in != 0) {
        unsigned char ch = static_cast<unsigned char>(*in);
        if (ch <= 0x7f)
            codepoint = ch;
        else if (ch <= 0xbf)
            codepoint = (codepoint << 6) | (ch & 0x3f);
        else if (ch <= 0xdf)
            codepoint = ch & 0x1f;
        else if (ch <= 0xef)
            codepoint = ch & 0x0f;
        else
            codepoint = ch & 0x07;
        ++in;
        if (((*in & 0xc0) != 0x80) && (codepoint <= 0x10ffff)) {
            if (codepoint <= 255) {
                out.append(1, static_cast<char>(codepoint));
            }
            else {
                out.append("?");
            }
        }
    }
    return out;
}

Answer 1

我喜欢这段代码。它出奇地短。大多数代码只是处理将多字节序列解码为代码点。一旦代码点被解码，向 ISO-8859-1 的转换就非常简单:

• 如果它小于或等于 255，它也是一个有效的 ISO-8859-1 字符:out.append(1, static_cast<char>(codepoint));
• 如果不是，则不能在 ISO-8859-1 中表示，并用问号代替:out.append("?");

因此，要使其适用于 ISO-8859-15，需要更多代码来处理引入 ISO-8859-15 时已被替换的字符(参见 Comparing ISO-8859-1 and ISO-8859-15)。不幸的是，它大大增加了代码大小。

下面的代码应该很容易理解。如果这是主要问题，可以对其进行优化以获得更好的性能。

std::string UTF8toISO8859_1(const char * in) {
    std::string out;
    if (in == NULL)
        return out;

    unsigned int codepoint;
    while (*in != 0) {
        unsigned char ch = static_cast<unsigned char>(*in);
        if (ch <= 0x7f)
            codepoint = ch;
        else if (ch <= 0xbf)
            codepoint = (codepoint << 6) | (ch & 0x3f);
        else if (ch <= 0xdf)
            codepoint = ch & 0x1f;
        else if (ch <= 0xef)
            codepoint = ch & 0x0f;
        else
            codepoint = ch & 0x07;
        ++in;

        if (((*in & 0xc0) != 0x80) && (codepoint <= 0x10ffff)) {
            // a valid codepoint has been decoded; convert it to ISO-8859-15               
            char outc;
            if (codepoint <= 255) {
                // codepoints up to 255 can be directly converted wit a few exceptions
                if (codepoint != 0xa4 && codepoint != 0xa6 && codepoint != 0xa8
                        && codepoint != 0xb4 && codepoint != 0xb8 && codepoint != 0xbc
                        && codepoint != 0xbd && codepoint != 0xbe) {
                    outc = static_cast<char>(codepoint);
                }
                else {
                    outc = '?';
                }
            }
            else {
                // With a few exceptions, codepoints above 255 cannot be converted
                if (codepoint == 0x20AC) {
                    outc = 0xa4;
                }
                else if (codepoint == 0x0160) {
                    outc = 0xa6;
                }
                else if (codepoint == 0x0161) {
                    outc = 0xa8;
                }
                else if (codepoint == 0x017d) {
                    outc = 0xb4;
                }
                else if (codepoint == 0x017e) {
                    outc = 0xb8;
                }
                else if (codepoint == 0x0152) {
                    outc = 0xbc;
                }
                else if (codepoint == 0x0153) {
                    outc = 0xbd;
                }
                else if (codepoint == 0x0178) {
                    outc = 0xbe;
                }
                else {
                    outc = '?';
                }
            }
            out.append(1, outc);
        }
    }
    return out;
}