/* header->fragment generator in ISO C99
 * Uses nytpu's proposed algorithm: <//nytpu.com/gemlog/2021-12-11.gmi#my-proposed-algorithm>
 *
 * This IS NOT Unicode-aware!  It should be pretty trivial to modify it to
 * use libunistring and become Unicode-aware though.
 *
 * Written in 2021 by nytpu <alex [at] nytpu.com>
 * SPDX-License-Identifier: CC0-1.0
 */

#define _ISOC99_SOURCE
#include <ctype.h> // isalnum(), isspace(), tolower()
#include <stdbool.h> // bool, true, false
#include <stddef.h> // size_t
#include <stdlib.h> // calloc, free
#include <string.h> // strlen

char *
header_to_anchor(const char *header)
{
	if (header == NULL) return NULL;

	size_t header_len = strlen(header);
	// calloc to ensure that the string is always automatically NUL-terminated
	char *anchor = calloc(header_len + 1, sizeof(*anchor));
	if (anchor == NULL) return NULL;

	bool prev_is_dash = false;
	for (size_t hi = 0, ai = 0; hi < header_len; hi++) {
		if (isalnum(header[hi])) {
			anchor[ai++] = tolower(header[hi]);
			prev_is_dash = false;
		} else if (!prev_is_dash && (isspace(header[hi]) || header[hi] == '-')) {
			anchor[ai++] = '-';
			prev_is_dash = true;
		}
	}
	return anchor;
}