#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <limits.h>

static size_t sanitize(const char*format) {
	size_t slen = format ? strlen(format) : 0;
	if (format[0] != '%'
		|| (slen > 2 && format[1] != '.')
		|| strchr(&format[1], '%')
		|| (strspn(&format[slen-1], "aAeEfFgG") != 1)) return 0;
	else return slen;
}

int strfromd(char *restrict s, size_t n, const char *restrict format, double fp) {
	return sanitize(format) ? snprintf(s, n, format, fp) : -1;
}

int strfromf(char *restrict s, size_t n, const char *restrict format, float fp) {
	return sanitize(format) ? snprintf(s, n, format, fp) : -1;
}

int strfroml(char *restrict s, size_t n, const char *restrict format, long double fp) {
	enum { max_len = 1+sizeof "%.18446744073709551615Lg", };
	char ff[max_len];
	size_t slen = sanitize(format);
	if (!slen) return -1;
	if (slen < max_len-2) {
		memcpy(ff, format, slen-1);
		ff[slen-1] = 'L';
		ff[slen] = format[slen-1];
		ff[slen+1] = 0;
	} else {
		// If the precision is unreasonably long, fallback to
		// strtoull to parse it, and squeeze it into a
		// reasonable length, if possible.
		int eback = errno;
		unsigned long long prec = strtoull(format+2, NULL, 10);
		if (prec == ULLONG_MAX) errno = eback;
		snprintf(ff, max_len, "%%.%lldL%c", prec, format[slen-1]);
	}
	return snprintf(s, n, ff, fp);
}