lib/utils/utf8.c

/*
 * Copyright (c) 2021 Nordic Semiconductor ASA
 *
 * SPDX-License-Identifier: Apache-2.0
 */

#include <stdint.h>
#include <string.h>
#include <zephyr/sys/__assert.h>

#define ASCII_CHAR 0x7F
#define SEQUENCE_FIRST_MASK 0xC0
#define SEQUENCE_LEN_2_BYTE 0xC0
#define SEQUENCE_LEN_3_BYTE 0xE0
#define SEQUENCE_LEN_4_BYTE 0xF0

char *utf8_trunc(char *utf8_str)
{
	const size_t len = strlen(utf8_str);

	if (len == 0U) {
		/* no-op */
		return utf8_str;
	}

	char *last_byte_p = utf8_str + len - 1U;
	uint8_t bytes_truncated;
	char seq_start_byte;

	if ((*last_byte_p & ASCII_CHAR) == *last_byte_p) {
		/* Not part of an UTF8 sequence, return */
		return utf8_str;
	}

	/* Find the starting byte and NULL-terminate other bytes */
	bytes_truncated = 0;
	while ((*last_byte_p & SEQUENCE_FIRST_MASK) != SEQUENCE_FIRST_MASK &&
	       last_byte_p > utf8_str) {
		last_byte_p--;
		bytes_truncated++;
	}
	bytes_truncated++; /* include the starting byte */

	/* Verify if the last character actually need to be truncated
	 * Handles the case where the number of bytes in the last UTF8-char
	 * matches the number of bytes we searched for the starting byte
	 */
	seq_start_byte = *last_byte_p;
	if ((seq_start_byte & SEQUENCE_LEN_4_BYTE) == SEQUENCE_LEN_4_BYTE) {
		if (bytes_truncated == 4) {
			return utf8_str;
		}
	} else if ((seq_start_byte & SEQUENCE_LEN_3_BYTE) == SEQUENCE_LEN_3_BYTE) {
		if (bytes_truncated == 3) {
			return utf8_str;
		}
	} else if ((seq_start_byte & SEQUENCE_LEN_2_BYTE) == SEQUENCE_LEN_2_BYTE) {
		if (bytes_truncated == 2) {
			return utf8_str;
		}
	}

	/* NULL-terminate the unterminated starting byte */
	*last_byte_p = '\0';

	return utf8_str;
}

char *utf8_lcpy(char *dst, const char *src, size_t n)
{
	if (n > 0) {
		strncpy(dst, src, n - 1);
		dst[n - 1] = '\0';

		if (n != 1) {
			utf8_trunc(dst);
		}
	}

	return dst;
}