#include <stdlib.h>
#include <limits.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <string.h>
#include "syscall.h"

void __procfdname(char *, unsigned);

int mergepaths(char *restrict a, const char *restrict r)
{
	size_t i, j;
	i = j = 0;

	/* if r is absolute, skip a content */
	if (r[0] == '/') {
		a[0] = '/';
		j = 1;
	} else {
		/* slash terminate a if needed */
		i = strlen(a)-1;
		if (r[0] && (a[i-1] != '/')) a[i] = '/';
		else --i;
	}

	/* resolve . and .. */
	for (; r[j]; ++j) {
		if (r[j] == '.') {
			if (r[j+1] == '/' || !r[j+1]) continue;
			if (r[j+1] == '.' && (r[j+2] == '/' || !r[j+2])) {
				while (i > 0 && a[--i] != '/');
				continue;
			}
		} else if (r[j] == '/' && a[i] == '/') continue;
		a[++i] = r[j];
	}
	/* terminate a */
	if (a[i] == '/' && i > 0) --i;
	a[++i] = 0;

	return i;
}

char *realpath(const char *restrict filename, char *restrict resolved)
{
	int fd;
	ssize_t r;
	size_t i;
	struct stat st1, st2;
	char buf[15+3*sizeof(int)];
	char tmp[PATH_MAX], link[PATH_MAX];

	if (!filename) {
		errno = EINVAL;
		return 0;
	}

	fd = sys_open(filename, O_PATH|O_NONBLOCK|O_CLOEXEC);
	if (fd < 0) return 0;
	__procfdname(buf, fd);

	r = readlink(buf, tmp, sizeof tmp - 1);
	if (r < 0) goto noproc;
	tmp[r] = 0;

	fstat(fd, &st1);
	r = stat(tmp, &st2);
	if (r<0 || st1.st_dev != st2.st_dev || st1.st_ino != st2.st_ino) {
		if (!r) errno = ELOOP;
		goto err;
	}

end:
	__syscall(SYS_close, fd);
	return resolved ? strcpy(resolved, tmp) : strdup(tmp);

noproc:
	if (*filename != '/')
		if (syscall(SYS_getcwd, tmp, sizeof(tmp)) < 0) goto err;

	/* concatenate cwd and target */
	r = mergepaths(tmp, filename);

	if (lstat(tmp, &st1) < 0) goto err;
	/* resolve link chain if any */
	while (S_ISLNK(st1.st_mode)) {
		if ((r = readlink(tmp, link, sizeof(link))) < 0) goto err;
		link[r] = 0;

		/* remove link name */
		i = strlen(tmp) - 1;
		while (i > 0 && tmp[--i] != '/');
		tmp[i] = 0;

		/* merge path and link target */
		mergepaths(tmp, link);
		if (lstat(tmp, &st1) < 0) goto err;
	}
	goto end;

err:
	__syscall(SYS_close, fd);
	return 0;
}