Extract a UTF-8 Substring by Codepoint Indices

Task

• Given a UTF-8 encoded null-terminated char[] string, and two integers start (inclusive) and end (exclusive) representing codepoint indices
• Print the substring between start and end to standard output (stdout), follow by a newline
• Substring should be null-terminated in a local buffer that you declare inside your function

Function Signature

// Given a UTF-8 encoded string, extract the substring from codepoint index start (inclusive) to end (exclusive)
// and print it to standard output (stdout), followed by a newline. The substring should be null-terminated in a local buffer.
void utf8_substring(const char str[], int start, int end);

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Examples

$ ./utf8_substring
café 1 3
af
你好世界 1 3
好世
Привет 2 5
иве

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Code

#include <string.h>
#include <stdio.h>
 
// Extracts the substring from codepoint index start (inclusive) to end (exclusive)
void utf8_substring(const char str[], int start, int end) {
        char result[strlen(str)];
        int result_index = 0;
        int codepoint = 0;
        for(int byte = 0; str[byte] != 0 && codepoint < end; byte++){
                char c = str[byte];
                if ((c & 0x80) == 0) {
                        if(codepoint >= start) {
                                result[result_index] = str[byte];
                                result_index++;
                        }
                        codepoint++;
                } else if ((c & 0xE0) == 0xC0) {
                        if (codepoint >= start){
                                for(int i = 0; i < 2; i++, result_index++, byte++) {
                                        result[result_index] = str[byte];
                                }
                                byte--;
                        }
                        codepoint++;
                } else if ((c & 0xF0) == 0xE0) {
                        if(codepoint >= start){
                                for (int i = 0; i < 3; i++, result_index++, byte++) {
                                        result[result_index] = str[byte];
                                }
                                byte--;
                        }
                        codepoint++;
                } else if ((c & 0xF8) == 0xF0) {
                        if(codepoint >= start) {
                                for(int i = 0; i < 4; i++, result_index++, byte++) {
                                        result[result_index] = str[byte];
                                }
                                byte--;
                        }
                        codepoint++;
                }
        }
        result[result_index] = 0;
        printf("%s \n", result);
}