// SPDX-License-Identifier: WTFPL

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

#include <sys/param.h>
#include <stdlib.h>
#include <sys/stat.h>

char **_argv;

static int _cmdname_sub(char *out, const char *path, const char *name)
{
	int path_len, name_len;
	char tmp[PATH_MAX];
	struct stat sb;

	path_len = (path == NULL) ? 0 : (strlen(path) + 1); /* with '/' */
	name_len = (name == NULL) ? 0 : strlen(name);

	if (path_len + name_len >= sizeof(tmp))
		abort();
	if (path_len) {
		memcpy(tmp, path, path_len - 1);
		tmp[path_len - 1] = '/';
	}
	if (name_len)
		memcpy(tmp + path_len, name, name_len);
	tmp[path_len + name_len] = '\0';

	if (tmp[0] == '/')
		memcpy(out, tmp, path_len + name_len + 1);
	else
		realpath(tmp, out);
	return stat(out, &sb);
}

static char *_cmdname_tokenize(char *str, char **next)
{
	char *p;

	if (*str == '\0')
		return NULL;

	if ((p = strchr(str, ':')) == NULL) {
		*next = str + strlen(str);
	} else {
		*p = '\0';
		*next = p + 1;
	}

	return str;
}

char *_cmdname(char *name)
{
	int save_errno, envpath_len;
	char *path, *envpath, *result = NULL;
	char *p, *pp;

	save_errno = errno;

	if ((*_argv[0] == '.' || *_argv[0] == '/') &&
	    !_cmdname_sub(name, NULL, _argv[0])) {
		result = name;
		goto fin;
	}

	if ((envpath = getenv("PATH")) == NULL)
		goto fin;
	envpath_len = strlen(envpath) + 1;
	path = alloca(envpath_len);
	memcpy(path, envpath, envpath_len);

	for (p = _cmdname_tokenize(path, &pp); p != NULL;
	     p = _cmdname_tokenize(pp, &pp)) {
		if (!_cmdname_sub(name, *p == '\0' ? "." : p,
				  getprogname())) {
			result = name;
			goto fin;
		}
	}
fin:
	errno = save_errno;
	return result;
}

char *_cmdname_nbsd(char *name)
{
	int save_errno;
	int result;
	
	save_errno = errno;
	result = readlink("/proc/self/exe", name, PATH_MAX);
	if (result == -1) {
		/* try another way for BSD */
		result = readlink("/proc/curproc/exe", name, PATH_MAX);
	}
	errno = save_errno;

	/* fall back to argv[0] if readlink doesn't work */
	if (result == -1 || result == PATH_MAX)
		return strcpy(name, _argv[0]);

	/* readlink does not add a NUL so we need to do it ourselves */
	name[result] = '\0';
	return name;
}

int main(int argc, char *argv[])
{
	char name0[PATH_MAX], name1[PATH_MAX];

	_argv = argv;
  
	printf("%s\n", _cmdname_nbsd(name1));
	printf("%s\n", _cmdname(name0));

	return 0;
}