Extract substring by utf-8 byte positions

I have a string and start and length with which to extract a substring. Both positions (start and length) are based on the byte offsets in the original UTF8 string.

However, there is a problem:

The start and length are in bytes, so I cannot use “substring”. The UTF8 string contains several multi-byte characters. Is there a hyper-efficient way of doing this? (I don’t need to decode the bytes…)

Example： var orig = ‘你好吗？’

The s,e might be 3,3 to extract the second character (好). I’m looking for

var result = orig.substringBytes(3,3);

Help!

Update #1 In C/C++ I would just cast it to a byte array, but not sure if there is an equivalent in javascript. BTW, yes we could parse it into a byte array and parse it back to a string, but it seems that there should be a quick way to cut it at the right place. Imagine that ‘orig’ is 1000000 characters, and s = 6 bytes and l = 3 bytes.

Update #2 Thanks to zerkms helpful re-direction, I ended up with the following, which does NOT work right – works right for multibyte but messed up for single byte.

function substrBytes(str, start, length)
{
    var ch, startIx = 0, endIx = 0, re = '';
    for (var i = 0; 0 < str.length; i++)
    {
        startIx = endIx++;

        ch = str.charCodeAt(i);
        do {
            ch = ch >> 8;   // a better way may exist to measure ch len
            endIx++;
        }
        while (ch);

        if (endIx > start + length)
        {
            return re;
        }
        else if (startIx >= start)
        {
            re += str[i];
        }
    }
}

Update #3 I don’t think shifting the char code really works. I’m reading two bytes when the correct answer is three… somehow I always forget this. The codepoint is the same for UTF8 and UTF16, but the number of bytes taken up on encoding depends on the encoding!!! So this is not the right way to do this.

Answer

I had a fun time fiddling with this. Hope this helps.

Because Javascript does not allow direct byte access on a string, the only way to find the start position is a forward scan.

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Update #3 I don’t think shifting the char code really works. I’m reading two bytes when the correct answer is three… somehow I always forget this. The codepoint is the same for UTF8 and UTF16, but the number of bytes taken up on encoding depends on the encoding!!! So this is not the right way to do this.

This is not correct – Actually there is no UTF-8 string in javascript. According to the ECMAScript 262 specification all strings – regardless of the input encoding – must be internally stored as UTF-16 (“[sequence of] 16-bit unsigned integers”).

Considering this, the 8 bit shift is correct (but unnecessary).

Wrong is the assumption that your character is stored as a 3-byte sequence…
In fact, all characters in a JS (ECMA-262) string are 16 bit (2 byte) long.

This can be worked around by converting the multibyte characters to utf-8 manually, as shown in the code below.

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UPDATE This solution doesn’t handle codepoints >= U+10000 including emoji. See APerson’s Answer for a more complete solution.

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See the details explained in my example code:

function encode_utf8( s )
{
  return unescape( encodeURIComponent( s ) );
}

function substr_utf8_bytes(str, startInBytes, lengthInBytes) {

   /* this function scans a multibyte string and returns a substring. 
    * arguments are start position and length, both defined in bytes.
    * 
    * this is tricky, because javascript only allows character level 
    * and not byte level access on strings. Also, all strings are stored
    * in utf-16 internally - so we need to convert characters to utf-8
    * to detect their length in utf-8 encoding.
    *
    * the startInBytes and lengthInBytes parameters are based on byte 
    * positions in a utf-8 encoded string.
    * in utf-8, for example: 
    *       "a" is 1 byte, 
            "ü" is 2 byte, 
       and  "你" is 3 byte.
    *
    * NOTE:
    * according to ECMAScript 262 all strings are stored as a sequence
    * of 16-bit characters. so we need a encode_utf8() function to safely
    * detect the length our character would have in a utf8 representation.
    * 
    * http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf
    * see "4.3.16 String Value":
    * > Although each value usually represents a single 16-bit unit of 
    * > UTF-16 text, the language does not place any restrictions or 
    * > requirements on the values except that they be 16-bit unsigned 
    * > integers.
    */

    var resultStr = '';
    var startInChars = 0;

    // scan string forward to find index of first character
    // (convert start position in byte to start position in characters)

    for (bytePos = 0; bytePos < startInBytes; startInChars++) {

        // get numeric code of character (is >128 for multibyte character)
        // and increase "bytePos" for each byte of the character sequence

        ch = str.charCodeAt(startInChars);
        bytePos += (ch < 128) ? 1 : encode_utf8(str[startInChars]).length;
    }

    // now that we have the position of the starting character,
    // we can built the resulting substring

    // as we don't know the end position in chars yet, we start with a mix of
    // chars and bytes. we decrease "end" by the byte count of each selected 
    // character to end up in the right position
    end = startInChars + lengthInBytes - 1;

    for (n = startInChars; startInChars <= end; n++) {
        // get numeric code of character (is >128 for multibyte character)
        // and decrease "end" for each byte of the character sequence
        ch = str.charCodeAt(n);
        end -= (ch < 128) ? 1 : encode_utf8(str[n]).length;

        resultStr += str[n];
    }

    return resultStr;
}

var orig = 'abc你好吗？';

alert('res: ' + substr_utf8_bytes(orig, 0, 2)); // alerts: "ab"
alert('res: ' + substr_utf8_bytes(orig, 2, 1)); // alerts: "c"
alert('res: ' + substr_utf8_bytes(orig, 3, 3)); // alerts: "你"
alert('res: ' + substr_utf8_bytes(orig, 6, 6)); // alerts: "好吗"