I am interacting with an api that accepts strings that are a maximum 5KB in size.
I want to take a string that may be more than 5KB and break it into chunks less than 5KB in size.
I then intend to pass each smaller-than-5kb-string to the api endpoint, and perform further actions when all requests have finished, probably using something like:
await Promise.all([get_thing_from_api(string_1), get_thing_from_api(string_2), get_thing_from_api(string_3)])
I have read that characters in a string can be between 1 - 4 bytes.
For this reason, to calculate string length in bytes we can use:
// in Node, string is UTF-8
Buffer.byteLength("here is some text");
// in Javascript
new Blob(["here is some text"]).size
Source:
https://stackoverflow.com/a/56026151
https://stackoverflow.com/a/52254083
My searches for "how to split strings into chunks of a certain size" return results that relate to splitting a string into strings of a particular character length, not byte length, eg:
var my_string = "1234 5 678905";
console.log(my_string.match(/.{1,2}/g));
// ["12", "34", " 5", " 6", "78", "90", "5"]Source:
https://stackoverflow.com/a/7033662
https://stackoverflow.com/a/6259543
https://gist.github.com/hendriklammers/5231994
Question
Is there a way to split a string into strings of a particular byte length?
I could either:
- assume that strings will only contain 1 byte per character
- allow for the 'worst case scenario' that each character is 4 bytes
but would prefer a more accurate solution.
I would be interested to know of both Node and plain JavaScript solutions, if they exist.
EDIT
This approach to calculating byteLength might be helpful - by iterating over characters in a string, getting their character code and incrementing byteLength accordingly:
function byteLength(str) {
// returns the byte length of an utf8 string
var s = str.length;
for (var i=str.length-1; i>=0; i--) {
var code = str.charCodeAt(i);
if (code > 0x7f && code <= 0x7ff) s++;
else if (code > 0x7ff && code <= 0xffff) s+=2;
if (code >= 0xDC00 && code <= 0xDFFF) i--; //trail surrogate
}
return s;
}
Source: https://stackoverflow.com/a/23329386
which led me to interesting experiments into the underlying data structures of Buffer:
var buf = Buffer.from('Hey! ф');
// <Buffer 48 65 79 21 20 d1 84>
buf.length // 7
buf.toString().charCodeAt(0) // 72
buf.toString().charCodeAt(5) // 1092
buf.toString().charCodeAt(6) // NaN
buf[0] // 72
for (let i = 0; i < buf.length; i++) {
console.log(buf[i]);
}
// 72 101 121 33 32 209 132 undefined
buf.slice(0,5).toString() // 'Hey! '
buf.slice(0,6).toString() // 'Hey! �'
buf.slice(0,7).toString() // 'Hey! ф'
but as @trincot pointed out in the comments, what is the correct way to handle multibyte characters? And how could I ensure chunks were split on spaces (so as not to 'break apart' a word?)
More info on Buffer: https://nodejs.org/api/buffer.html#buffer_buffer
EDIT
In case it helps anyone else understand the brilliant logic in the accepted answer, the snippet below is a heavily commented version I made so I could understand it better.
/**
* Takes a string and returns an array of substrings that are smaller than maxBytes.
*
* This is an overly commented version of the non-generator version of the accepted answer,
* in case it helps anyone understand its (brilliant) logic.
*
* Both plain js and node variations are shown below - simply un/comment out your preference
*
* @param {string} s - the string to be chunked
* @param {maxBytes} maxBytes - the maximum size of a chunk, in bytes
* @return {arrray} - an array of strings less than maxBytes (except in extreme edge cases)
*/
function chunk(s, maxBytes) {
// for plain js
const decoder = new TextDecoder("utf-8");
let buf = new TextEncoder("utf-8").encode(s);
// for node
// let buf = Buffer.from(s);
const result = [];
var counter = 0;
while (buf.length) {
console.log("=============== BEG LOOP " + counter + " ===============");
console.log("result is now:");
console.log(result);
console.log("buf is now:");
// for plain js
console.log(decoder.decode(buf));
// for node
// console.log(buf.toString());
/* get index of the last space character in the first chunk,
searching backwards from the maxBytes + 1 index */
let i = buf.lastIndexOf(32, maxBytes + 1);
console.log("i is: " + i);
/* if no space is found in the first chunk,
get index of the first space character in the whole string,
searching forwards from 0 - in edge cases where characters
between spaces exceeds maxBytes, eg chunk("123456789x 1", 9),
the chunk will exceed maxBytes */
if (i < 0) i = buf.indexOf(32, maxBytes);
console.log("at first condition, i is: " + i);
/* if there's no space at all, take the whole string,
again an edge case like chunk("123456789x", 9) will exceed maxBytes*/
if (i < 0) i = buf.length;
console.log("at second condition, i is: " + i);
// this is a safe cut-off point; never half-way a multi-byte
// because the index is always the index of a space
console.log("pushing buf.slice from 0 to " + i + " into result array");
// for plain js
result.push(decoder.decode(buf.slice(0, i)));
// for node
// result.push(buf.slice(0, i).toString());
console.log("buf.slicing with value: " + (i + 1));
// slice the string from the index + 1 forwards
// it won't erroneously slice out a value after i, because i is a space
buf = buf.slice(i + 1); // skip space (if any)
console.log("=============== END LOOP " + counter + " ===============");
counter++;
}
return result;
}
console.log(chunk("Hey there! € 100 to pay", 12));
