/****************************************************************************
 * libs/libc/misc/lib_envpath.c
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.  The
 * ASF licenses this file to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the
 * License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
 * License for the specific language governing permissions and limitations
 * under the License.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>

#include <sys/types.h>
#include <sys/stat.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>

#include <nuttx/envpath.h>

#include "libc.h"

#if defined(CONFIG_LIBC_ENVPATH)

/****************************************************************************
 * Private Types
 ****************************************************************************/

struct envpath_s
{
  FAR char *next; /* Pointer to the next (unterminated) value in the PATH variable */
  char path[1];
};

#define SIZEOF_ENVPATH_S(n) (sizeof(struct envpath_s) + (n) - 1)

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name: envpath_init
 *
 * Description:
 *   Initialize for the traversal of each value in the PATH variable.  The
 *   usage is sequence is as follows:
 *
 *   1) Call envpath_init() to initialize for the traversal.  envpath_init()
 *      will return an opaque handle that can then be provided to
 *      envpath_next() and envpath_release().
 *   2) Call envpath_next() repeatedly to examine every file that lies
 *      in the directories of the PATH variable
 *   3) Call envpath_release() to free resources set aside by envpath_init().
 *
 * Input Parameters:
 *   None
 *
 * Returned Value:
 *   On success, envpath_init() return a non-NULL, opaque handle that may
 *   subsequently be used in calls to envpath_next() and envpath_release().
 *   On error, a NULL handle value will be returned.  The most likely cause
 *   of an error would be that there is no value associated with the PATH
 *   variable.
 *
 ****************************************************************************/

ENVPATH_HANDLE envpath_init(FAR const char *name)
{
  FAR struct envpath_s *envpath;
  FAR char *path;
  size_t size;

  /* Get the value of the PATH variable */

  path = getenv(name);
  if (!path)
    {
      /* getenv() will return a NULL value if the PATH variable does not
       * exist in the environment.
       */

      return NULL;
    }

  /* Allocate a container for the PATH variable contents */

  size = strlen(path) + 1;
  envpath = (FAR struct envpath_s *)
    lib_malloc(SIZEOF_ENVPATH_S(size));

  if (!envpath)
    {
      /* Ooops.. we are out of memory */

      return NULL;
    }

  /* Populate the container */

  strlcpy(envpath->path, path, size);
  envpath->next = envpath->path;

  /* And return the containing cast to an opaque handle */

  return envpath;
}

/****************************************************************************
 * Name: envpath_next
 *
 * Description:
 *   Traverse all possible values in the PATH variable in attempt to find
 *   the full path to an envcutable file when only a relative path is
 *   provided.
 *
 * Input Parameters:
 *   handle - The handle value returned by envpath_init
 *   relpath - The relative path to the file to be found.
 *
 * Returned Value:
 *   On success, a non-NULL pointer to a null-terminated string is provided.
 *   This is the full path to a file that exists in the file system.  This
 *   function will verify that the file exists (but will not verify that it
 *   is marked envcutable).
 *
 *   NOTE: The string pointer return in the success case points to allocated
 *   memory.  This memory must be freed by the called by calling lib_free().
 *
 *   NULL is returned if no path is found to any file with the provided
 *   'relpath' from any absolute path in the PATH variable.  In this case,
 *   there is no point in calling envpath_next() further; envpath_release()
 *   must be called to release resources set aside by expath_init().
 *
 ****************************************************************************/

FAR char *envpath_next(ENVPATH_HANDLE handle, FAR const char *relpath)
{
  FAR struct envpath_s *envpath = (FAR struct envpath_s *)handle;
  struct stat buf;
  FAR char *endptr;
  FAR char *path;
  FAR char *fullpath;
  int pathlen;
  int ret;

  /* Verify that a value handle and relative path were provided */

  DEBUGASSERT(envpath && relpath);
  DEBUGASSERT(relpath[0] != '\0' && relpath[0] != '/');

  /* Loop until (1) we find a file with this relative path from one of the
   * absolute paths in the PATH variable, or (2) all of the absolute paths
   * in the PATH variable have been considered.
   */

  for (; ; )
    {
      /* Make sure that envpath->next points to the beginning of a string */

      path = envpath->next;
      if (*path == '\0')
        {
          /* If it points to a NULL it means that either (1) the PATH
           * variable is empty, or (2) we have already examined all of the
           * paths in the path variable.
           */

          return NULL;
        }

      /* Okay... 'path' points to the beginning of the string.  The string
       * may be terminated either with (1) ':' which separates the path from
       * the next path in the list, or (2) NUL which marks the end of the
       * list.
       */

      endptr = strchr(path, ':');
      if (endptr == NULL)
        {
          /* If strchr returns NUL it means that ':' does not appear in the
           * string.  Therefore, this must be the final path in the PATH
           * variable content.
           */

          endptr = &path[strlen(path)];
          envpath->next = endptr;
          DEBUGASSERT(*endptr == '\0');
        }
      else
        {
          DEBUGASSERT(*endptr == ':');
          envpath->next = endptr + 1;
          *endptr = '\0';
        }

      pathlen  = strlen(path) + strlen(relpath) + 2;
      fullpath = (FAR char *)lib_malloc(pathlen);
      if (fullpath == NULL)
        {
          /* Failed to allocate memory */

          return NULL;
        }

      /* Construct the full path */

      snprintf(fullpath, pathlen, "%s/%s", path, relpath);

      /* Verify that a regular file exists at this path */

      ret = stat(fullpath, &buf);
      if (ret == OK && S_ISREG(buf.st_mode))
        {
          return fullpath;
        }

      /* Failed to stat the file.  Just free the allocated memory and
       * continue to try the next path.
       */

       lib_free(fullpath);
    }

  /* We will not get here */
}

/****************************************************************************
 * Name: envpath_release
 *
 * Description:
 *   Release all resources set aside by envpath_init() when the handle value
 *   was created.  The handle value is invalid on return from this function.
 *   Attempts to all envpath_next() or envpath_release() with such a 'stale'
 *   handle will result in undefined (i.e., not good) behavior.
 *
 * Input Parameters:
 *   handle - The handle value returned by envpath_init
 *
 * Returned Value:
 *   None
 *
 ****************************************************************************/

void envpath_release(ENVPATH_HANDLE handle)
{
  lib_free(handle);
}

#endif /* CONFIG_LIBC_ENVPATH */