Number of character cells used by string

Question

I have a program that outputs a textual table using UTF-8 strings, and I need to measure the number of monospaced character cells used by a string so I can align it properly. If possible, I'd like to do this with standard functions.

Solution 1

From UTF-8 and Unicode FAQ for Unix/Linux:

The number of characters can be counted in C in a portable way using mbstowcs(NULL,s,0). This works for UTF-8 like for any other supported encoding, as long as the appropriate locale has been selected. A hard-wired technique to count the number of characters in a UTF-8 string is to count all bytes except those in the range 0x80 – 0xBF, because these are just continuation bytes and not characters of their own. However, the need to count characters arises surprisingly rarely in applications.

Solution 2

You may or may not have a UTF-8 compatible strlen(3) function available. However, there are some simple C functions readily available that do the job quickly.

The efficient C solutions examine the start of the character to skip continuation bytes. The simple code (referenced from the link above) is

int my_strlen_utf8_c(char *s) {
   int i = 0, j = 0;
   while (s[i]) {
     if ((s[i] & 0xc0) != 0x80) j++;
     i++;
   }
   return j;
}

The faster version uses the same technique, but prefetches data and does multi-byte compares, resulting is a substantial speedup. The code is longer and more complex, however.

Solution 3

I'm shocked that no one mentioned this, so here it goes for the record:

If you want to align text in a terminal, you need to use the POSIX functions wcwidth and wcswidth. Here's correct program to find the on-screen length of a string.

#define _XOPEN_SOURCE
#include <wchar.h>
#include <stdio.h>
#include <locale.h>
#include <stdlib.h>

int measure(char *string) {
    // allocate enough memory to hold the wide string
    size_t needed = mbstowcs(NULL, string, 0) + 1;
    wchar_t *wcstring = malloc(needed * sizeof *wcstring);
    if (!wcstring) return -1;

    // change encodings
    if (mbstowcs(wcstring, string, needed) == (size_t)-1) return -2;

    // measure width
    int width = wcswidth(wcstring, needed);

    free(wcstring);
    return width;
}

int main(int argc, char **argv) {
    setlocale(LC_ALL, "");

    for (int i = 1; i < argc; i++) {
        printf("%s: %d\n", argv[i], measure(argv[i]));
    }
}

Here's an example of it running:

$ ./measure hello 莊子 cＡb
hello: 5
莊子: 4
cＡb: 4

Note how the two characters "莊子" and the three characters "cＡb" (note the double-width Ａ) are both 4 columns wide.

As utf8everywhere.org puts it,

The size of the string as it appears on the screen is unrelated to the number of code points in the string. One has to communicate with the rendering engine for this. Code points do not occupy one column even in monospace fonts and terminals. POSIX takes this into account.

Windows does not have any built-in wcwidth function for console output; if you want to support multi-column characters in the Windows console you need to find a portable implementation of wcwidth give up because the Windows console doesn’t support Unicode without crazy hacks.

Solution 4

If you are able to use 3rd party libraries, have a look at the ICU library from IBM:

http://site.icu-project.org/

Solution 5

The following code takes ill-formed byte sequences into consideration. the example of string data comes from ""Table 3-8. Use of U+FFFD in UTF-8 Conversion"" in the Unicode Standard 6.3.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

#define is_trail(c) (c > 0x7F && c < 0xC0)
#define SUCCESS 1
#define FAILURE -1

int utf8_get_next_char(const unsigned char*, size_t, size_t*, int*, unsigned int*);
int utf8_length(unsigned char*, size_t);
void utf8_print_each_char(unsigned char*, size_t);

int main(void)
{
    unsigned char *str;
    str = (unsigned char *) "\x61\xF1\x80\x80\xE1\x80\xC2\x62\x80\x63\x80\xBF\x64";
    size_t str_size = strlen((const char*) str);

    puts(10 == utf8_length(str, str_size) ? "true" : "false");
    utf8_print_each_char(str, str_size);

    return EXIT_SUCCESS;
}

int utf8_length(unsigned char *str, size_t str_size)
{
    int length = 0;
    size_t pos = 0;
    size_t next_pos = 0;
    int is_valid = 0;
    unsigned int code_point = 0;

    while (
        utf8_get_next_char(str, str_size, &next_pos, &is_valid, &code_point) == SUCCESS
    ) {
        ++length;
    }

    return length;
}

void utf8_print_each_char(unsigned char *str, size_t str_size)
{
    int length = 0;
    size_t pos = 0;
    size_t next_pos = 0;
    int is_valid = 0;
    unsigned int code_point = 0;

    while (
        utf8_get_next_char(str, str_size, &next_pos, &is_valid, &code_point) == SUCCESS
    ) {
        if (is_valid == true) {
            printf("%.*s\n", (int) next_pos - (int) pos, str + pos);
        } else {
            puts("\xEF\xBF\xBD");
        }

        pos = next_pos;
    }
}

int utf8_get_next_char(const unsigned char *str, size_t str_size, size_t *cursor, int *is_valid, unsigned int *code_point)
{
    size_t pos = *cursor;
    size_t rest_size = str_size - pos;
    unsigned char c;
    unsigned char min;
    unsigned char max;

    *code_point = 0;
    *is_valid = SUCCESS;

    if (*cursor >= str_size) {
        return FAILURE;
    }

    c = str[pos];

    if (rest_size < 1) {
        *is_valid = false;
        pos += 1;
    } else if (c < 0x80) {
        *code_point = str[pos];
        *is_valid = true;
        pos += 1;
    } else if (c < 0xC2) {
        *is_valid = false;
        pos += 1;
    } else if (c < 0xE0) {

        if (rest_size < 2 || !is_trail(str[pos + 1])) {
            *is_valid = false;
            pos += 1;
        } else {
            *code_point = ((str[pos] & 0x1F) << 6) | (str[pos + 1] & 0x3F);
            *is_valid = true;
            pos += 2;
        }

    } else if (c < 0xF0) {

        min = (c == 0xE0) ? 0xA0 : 0x80;
        max = (c == 0xED) ? 0x9F : 0xBF;

        if (rest_size < 2 || str[pos + 1] < min || max < str[pos + 1]) {
            *is_valid = false;
            pos += 1;         
        } else if (rest_size < 3 || !is_trail(str[pos + 2])) {
            *is_valid = false;
            pos += 2;
        } else {
            *code_point = ((str[pos]     & 0x1F) << 12) 
                       | ((str[pos + 1] & 0x3F) <<  6) 
                       |  (str[pos + 2] & 0x3F);
            *is_valid = true;
            pos += 3;
        }

    } else if (c < 0xF5) {

        min = (c == 0xF0) ? 0x90 : 0x80;
        max = (c == 0xF4) ? 0x8F : 0xBF;

        if (rest_size < 2 || str[pos + 1] < min || max < str[pos + 1]) {
            *is_valid = false;
            pos += 1;
        } else if (rest_size < 3 || !is_trail(str[pos + 2])) {
            *is_valid = false;
            pos += 2;
        } else if (rest_size < 4 || !is_trail(str[pos + 3])) {
            *is_valid = false;
            pos += 3;
        } else {
            *code_point = ((str[pos]     &  0x7) << 18)
                       | ((str[pos + 1] & 0x3F) << 12)
                       | ((str[pos + 2] & 0x3F) << 6)
                       |  (str[pos + 3] & 0x3F);
            *is_valid = true;
            pos += 4;
        }

    } else {
        *is_valid = false;
        pos += 1;
    }

    *cursor = pos;

    return SUCCESS;
}

When I write code for UTF-8, I see "Table 3-7. Well-Formed UTF-8 Byte Sequences" in the Unicode Standard 6.3.

       Code Points    First Byte Second Byte Third Byte Fourth Byte
  U+0000 -   U+007F   00 - 7F
  U+0080 -   U+07FF   C2 - DF    80 - BF
  U+0800 -   U+0FFF   E0         A0 - BF     80 - BF
  U+1000 -   U+CFFF   E1 - EC    80 - BF     80 - BF
  U+D000 -   U+D7FF   ED         80 - 9F     80 - BF
  U+E000 -   U+FFFF   EE - EF    80 - BF     80 - BF
 U+10000 -  U+3FFFF   F0         90 - BF     80 - BF    80 - BF
 U+40000 -  U+FFFFF   F1 - F3    80 - BF     80 - BF    80 - BF
U+100000 - U+10FFFF   F4         80 - 8F     80 - BF    80 - BF

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