如何反转UTF-8字符串？

Question

最近,有人询问了一种在C中反转字符串的算法.在处理非单字节字符串时,大多数提议的解决方案都存在问题.所以,我想知道什么是专门处理utf-8字符串的好算法.

我提出了一些代码,我将其作为答案发布,但我很高兴看到其他人的想法或建议.我更喜欢使用实际代码,因此我选择了C#,因为它似乎是本网站中最流行的语言之一,但我不介意你的代码是否是另一种语言,只要它可以合理任何熟悉命令式语言的人都能理解.而且,由于这是为了看看这样的算法如何在低级别实现(通过低级别我只是意味着处理字节),我们的想法是避免使用库来实现核心代码.

笔记:

我对算法本身,它的性能以及它如何进行优化感兴趣(我的意思是算法级优化,而不是用++ i替换i ++等等;我对实际的基准测试也不感兴趣).

我并不是要在生产代码中实际使用它或"重新发明轮子".这只是出于好奇和锻炼.

我正在使用C#字节数组,所以我假设您可以获得字符串的长度,而不会运行字符串直到找到NUL.也就是说,我没有考虑到找到字符串长度的复杂性.但是,如果你正在使用C语言,你可以在调用核心代码之前使用strlen()来解决这个问题.

编辑:

正如Mike F指出的那样,我的代码(以及此处发布的其他人的代码)并不处理复合字符.那些有些信息在这里.我不熟悉这个概念,但是如果这意味着存在"组合字符",即只与其他"基本"字符/代码点组合有效的字符/代码点,那么这样的查找表当反转时,字符可用于保持"全局"字符("基本"+"组合"字符)的顺序.

Answer 1

我将一次通过反转字节,然后第二次通过将任何多字节字符(在UTF8中很容易检测到)中的字节反转回正确的顺序.

你绝对可以在一次通过中处理这个问题,但除非例程成为瓶颈,否则我不会打扰.

Answer 2

此代码假定输入的UTF-8字符串有效且格式正确(即每个多字节字符最多4个字节):

#include "string.h"

void utf8rev(char *str)
{
    /* this assumes that str is valid UTF-8 */
    char    *scanl, *scanr, *scanr2, c;

    /* first reverse the string */
    for (scanl= str, scanr= str + strlen(str); scanl < scanr;)
        c= *scanl, *scanl++= *--scanr, *scanr= c;

    /* then scan all bytes and reverse each multibyte character */
    for (scanl= scanr= str; c= *scanr++;) {
        if ( (c & 0x80) == 0) // ASCII char
            scanl= scanr;
        else if ( (c & 0xc0) == 0xc0 ) { // start of multibyte
            scanr2= scanr;
            switch (scanr - scanl) {
                case 4: c= *scanl, *scanl++= *--scanr, *scanr= c; // fallthrough
                case 3: // fallthrough
                case 2: c= *scanl, *scanl++= *--scanr, *scanr= c;
            }
            scanr= scanl= scanr2;
        }
    }
}

// quick and dirty main for testing purposes
#include "stdio.h"

int main(int argc, char* argv[])
{
    char buffer[256];
    buffer[sizeof(buffer)-1]= '\0';

    while (--argc > 0) {
        strncpy(buffer, argv[argc], sizeof(buffer)-1); // don't overwrite final null
        printf("%s ? ", buffer);
        utf8rev(buffer);
        printf("%s\n", buffer);
    }
    return 0;
}

如果编译此程序(示例名称so199260.c:)并在UTF-8环境(在本例中为Linux安装)上运行它:

$ so199260 ???? ??? ???? français ???? a????b
a????b ? b????a
???? ? ????
français ? siaçnarf
???? ? ????
??? ? ???
???? ? ????

如果代码太含糊不清,我会很乐意澄清.

Answer 3

同意您的方法是在现场进行的唯一合理的方法.

就个人而言,我不喜欢在处理它的每个函数中重新验证UTF8,并且通常只做必要的事情来避免崩溃; 它增加了很多代码.Dunno很多C#所以这里是C:

(编辑以消除strlen)

void reverse( char *start, char *end )
{
    while( start < end )
    {
        char c = *start;
        *start++ = *end;
        *end-- = c;
    }
}

char *reverse_char( char *start )
{
    char *end = start;
    while( (end[1] & 0xC0) == 0x80 ) end++;
    reverse( start, end );
    return( end+1 );
}

void reverse_string( char *string )
{
    char *end = string;
    while( *end ) end = reverse_char( end );
    reverse( string, end-1 );
}

Answer 4

我最初的方法可以这样总结:

1)天真地反向字节

2)向后运行字符串并随时修复utf8序列.

在第二步中处理非法序列,在第一步中,我们检查字符串是否处于"同步"状态(即,如果它以合法的前导字节开始).

编辑:改进Reverse()中前导字节的验证

class UTF8Utils {


    public static void Reverse(byte[] str) {
        int len = str.Length;
        int i   = 0;
        int j   = len - 1;

        //  first, check if the string is "synced", i.e., it starts
        //  with a valid leading character. Will check for illegal 
        //  sequences thru the whole string later.
        byte leadChar = str[0];

        //  if it starts with 10xx xxx, it's a trailing char...
        //  if it starts with 1111 10xx or 1111 110x 
        //  it's out of the 4 bytes range.
    //  EDIT: added validation for 7 bytes seq and 0xff
        if( (leadChar & 0xc0) == 0x80 ||
            (leadChar & 0xfc) == 0xf8 ||
            (leadChar & 0xfe) == 0xfc ||
        (leadChar & 0xff) == 0xfe ||
        leadChar == 0xff) {

            throw new Exception("Illegal UTF-8 sequence");

        }

        //  reverse bytes in-place naïvely
        while(i < j) {
            byte tmp = str[i];
            str[i]  = str[j];
            str[j]  = tmp;
            i++;
            j--;
        }
        //  now, run the string again to fix the multibyte sequences
        UTF8Utils.ReverseMbSequences(str);

    }

    private static void ReverseMbSequences(byte[] str) {
        int i = str.Length - 1;
        byte leadChar = 0;
        int nBytes  = 0;

        //  loop backwards thru the reversed buffer
        while(i >= 0) {
            //  since the first byte in the unreversed buffer is assumed to be
            //  the leading char of that byte, it seems safe to assume that the  
            //  last byte is now the leading char. (Given that the string is
            //  not out of sync -- we checked that out already)
            leadChar = str[i];

            //  check how many bytes this sequence takes and validate against
            //  illegal sequences
            if(leadChar < 0x80) {
                nBytes = 1;
            } else if((leadChar & 0xe0) == 0xc0) {
                if((str[i-1] & 0xc0) != 0x80) {
                    throw new Exception("Illegal UTF-8 sequence");
                }
                nBytes = 2;
            } else if ((leadChar & 0xf0) == 0xe0) {
                if((str[i-1] & 0xc0) != 0x80 ||
                    (str[i-2] & 0xc0) != 0x80 ) {
                    throw new Exception("Illegal UTF-8 sequence");
                }
                nBytes = 3;
            } else if ((leadChar & 0xf8) == 0xf0) {
                if((str[i-1] & 0xc0) != 0x80 ||
                    (str[i-2] & 0xc0) != 0x80 ||
                    (str[i-3] & 0xc0) != 0x80  ) {
                    throw new Exception("Illegal UTF-8 sequence");
                }
                nBytes = 4;
            } else {
                throw new Exception("Illegal UTF-8 sequence");
            }

            //  now, reverse the current sequence and then continue
            //  whith the next one
            int back    = i;
            int front   = back - nBytes + 1;

            while(front < back) {
                byte tmp = str[front];
                str[front] = str[back];
                str[back] = tmp;
                front++;
                back--;
            }
            i -= nBytes;
        }
    }
}