/* tac - concatenate and print files in reverse
   Copyright (C) 1988-2022 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* Written by Jay Lepreau (lepreau@cs.utah.edu).
   GNU enhancements by David MacKenzie (djm@gnu.ai.mit.edu). */

/* Copy each FILE, or the standard input if none are given or when a
   FILE name of "-" is encountered, to the standard output with the
   order of the records reversed.  The records are separated by
   instances of a string, or a newline if none is given.  By default, the
   separator string is attached to the end of the record that it
   follows in the file.

   Options:
   -b, --before			The separator is attached to the beginning
                                of the record that it precedes in the file.
   -r, --regex			The separator is a regular expression.
   -s, --separator=separator	Use SEPARATOR as the record separator.

   To reverse a file byte by byte, use (in bash, ksh, or sh):
tac -r -s '.\|
' file */

#include <config.h>

#include <stdio.h>
#include <getopt.h>
#include <sys/types.h>
#include "system.h"

#include <regex.h>

#include "die.h"
#include "error.h"
#include "filenamecat.h"
#include "safe-read.h"
#include "stdlib--.h"
#include "xbinary-io.h"

/* The official name of this program (e.g., no 'g' prefix).  */
#define PROGRAM_NAME "tac"

#define AUTHORS \
  proper_name ("Jay Lepreau"), \
  proper_name ("David MacKenzie")

#if defined __MSDOS__ || defined _WIN32
/* Define this to non-zero on systems for which the regular mechanism
   (of unlinking an open file and expecting to be able to write, seek
   back to the beginning, then reread it) doesn't work.  E.g., on Windows
   and DOS systems.  */
# define DONT_UNLINK_WHILE_OPEN 1
#endif


#ifndef DEFAULT_TMPDIR
# define DEFAULT_TMPDIR "/tmp"
#endif

/* The number of bytes per atomic read. */
#define INITIAL_READSIZE 8192

/* The number of bytes per atomic write. */
#define WRITESIZE 8192

/* The string that separates the records of the file. */
static char const *separator;

/* True if we have ever read standard input.  */
static bool have_read_stdin = false;

/* If true, print 'separator' along with the record preceding it
   in the file; otherwise with the record following it. */
static bool separator_ends_record;

/* 0 if 'separator' is to be matched as a regular expression;
   otherwise, the length of 'separator', used as a sentinel to
   stop the search. */
static size_t sentinel_length;

/* The length of a match with 'separator'.  If 'sentinel_length' is 0,
   'match_length' is computed every time a match succeeds;
   otherwise, it is simply the length of 'separator'. */
static size_t match_length;

/* The input buffer. */
static char *G_buffer;

/* The number of bytes to read at once into 'buffer'. */
static size_t read_size;

/* The size of 'buffer'.  This is read_size * 2 + sentinel_length + 2.
   The extra 2 bytes allow 'past_end' to have a value beyond the
   end of 'G_buffer' and 'match_start' to run off the front of 'G_buffer'. */
static size_t G_buffer_size;

/* The compiled regular expression representing 'separator'. */
static struct re_pattern_buffer compiled_separator;
static char compiled_separator_fastmap[UCHAR_MAX + 1];
static struct re_registers regs;

static struct option const longopts[] =
{
  {"before", no_argument, NULL, 'b'},
  {"regex", no_argument, NULL, 'r'},
  {"separator", required_argument, NULL, 's'},
  {GETOPT_HELP_OPTION_DECL},
  {GETOPT_VERSION_OPTION_DECL},
  {NULL, 0, NULL, 0}
};

void
usage (int status)
{
  if (status != EXIT_SUCCESS)
    emit_try_help ();
  else
    {
      printf (_("\
Usage: %s [OPTION]... [FILE]...\n\
"),
              program_name);
      fputs (_("\
Write each FILE to standard output, last line first.\n\
"), stdout);

      emit_stdin_note ();
      emit_mandatory_arg_note ();

      fputs (_("\
  -b, --before             attach the separator before instead of after\n\
  -r, --regex              interpret the separator as a regular expression\n\
  -s, --separator=STRING   use STRING as the separator instead of newline\n\
"), stdout);
      fputs (HELP_OPTION_DESCRIPTION, stdout);
      fputs (VERSION_OPTION_DESCRIPTION, stdout);
      emit_ancillary_info (PROGRAM_NAME);
    }
  exit (status);
}

/* Print the characters from START to PAST_END - 1.
   If START is NULL, just flush the buffer. */

static void
output (char const *start, char const *past_end)
{
  static char buffer[WRITESIZE];
  static size_t bytes_in_buffer = 0;
  size_t bytes_to_add = past_end - start;
  size_t bytes_available = WRITESIZE - bytes_in_buffer;

  if (start == 0)
    {
      fwrite (buffer, 1, bytes_in_buffer, stdout);
      bytes_in_buffer = 0;
      return;
    }

  /* Write out as many full buffers as possible. */
  while (bytes_to_add >= bytes_available)
    {
      memcpy (buffer + bytes_in_buffer, start, bytes_available);
      bytes_to_add -= bytes_available;
      start += bytes_available;
      fwrite (buffer, 1, WRITESIZE, stdout);
      bytes_in_buffer = 0;
      bytes_available = WRITESIZE;
    }

  memcpy (buffer + bytes_in_buffer, start, bytes_to_add);
  bytes_in_buffer += bytes_to_add;
}

/* Print in reverse the file open on descriptor FD for reading FILE.
   The file is already positioned at FILE_POS, which should be near its end.
   Return true if successful.  */

static bool
tac_seekable (int input_fd, char const *file, off_t file_pos)
{
  /* Pointer to the location in 'G_buffer' where the search for
     the next separator will begin. */
  char *match_start;

  /* Pointer to one past the rightmost character in 'G_buffer' that
     has not been printed yet. */
  char *past_end;

  /* Length of the record growing in 'G_buffer'. */
  size_t saved_record_size;

  /* True if 'output' has not been called yet for any file.
     Only used when the separator is attached to the preceding record. */
  bool first_time = true;
  char first_char = *separator;	/* Speed optimization, non-regexp. */
  char const *separator1 = separator + 1; /* Speed optimization, non-regexp. */
  size_t match_length1 = match_length - 1; /* Speed optimization, non-regexp. */

  /* Arrange for the first read to lop off enough to leave the rest of the
     file a multiple of 'read_size'.  Since 'read_size' can change, this may
     not always hold during the program run, but since it usually will, leave
     it here for i/o efficiency (page/sector boundaries and all that).
     Note: the efficiency gain has not been verified. */
  size_t remainder = file_pos % read_size;
  if (remainder != 0)
    {
      file_pos -= remainder;
      if (lseek (input_fd, file_pos, SEEK_SET) < 0)
        error (0, errno, _("%s: seek failed"), quotef (file));
    }

  /* Scan backward, looking for end of file.  This caters to proc-like
     file systems where the file size is just an estimate.  */
  while ((saved_record_size = safe_read (input_fd, G_buffer, read_size)) == 0
         && file_pos != 0)
    {
      off_t rsize = read_size;
      if (lseek (input_fd, -rsize, SEEK_CUR) < 0)
        error (0, errno, _("%s: seek failed"), quotef (file));
      file_pos -= read_size;
    }

  /* Now scan forward, looking for end of file.  */
  while (saved_record_size == read_size)
    {
      size_t nread = safe_read (input_fd, G_buffer, read_size);
      if (nread == 0)
        break;
      saved_record_size = nread;
      if (saved_record_size == SAFE_READ_ERROR)
        break;
      file_pos += nread;
    }

  if (saved_record_size == SAFE_READ_ERROR)
    {
      error (0, errno, _("%s: read error"), quotef (file));
      return false;
    }

  match_start = past_end = G_buffer + saved_record_size;
  /* For non-regexp search, move past impossible positions for a match. */
  if (sentinel_length)
    match_start -= match_length1;

  while (true)
    {
      /* Search backward from 'match_start' - 1 to 'G_buffer' for a match
         with 'separator'; for speed, use strncmp if 'separator' contains no
         metacharacters.
         If the match succeeds, set 'match_start' to point to the start of
         the match and 'match_length' to the length of the match.
         Otherwise, make 'match_start' < 'G_buffer'. */
      if (sentinel_length == 0)
        {
          size_t i = match_start - G_buffer;
          regoff_t ri = i;
          regoff_t range = 1 - ri;
          regoff_t ret;

          if (1 < range)
            die (EXIT_FAILURE, 0, _("record too large"));

          if (range == 1
              || ((ret = re_search (&compiled_separator, G_buffer,
                                    i, i - 1, range, &regs))
                  == -1))
            match_start = G_buffer - 1;
          else if (ret == -2)
            {
              die (EXIT_FAILURE, 0,
                   _("error in regular expression search"));
            }
          else
            {
              match_start = G_buffer + regs.start[0];
              match_length = regs.end[0] - regs.start[0];
            }
        }
      else
        {
          /* 'match_length' is constant for non-regexp boundaries. */
          while (*--match_start != first_char
                 || (match_length1 && !STREQ_LEN (match_start + 1, separator1,
                                                  match_length1)))
            /* Do nothing. */ ;
        }

      /* Check whether we backed off the front of 'G_buffer' without finding
         a match for 'separator'. */
      if (match_start < G_buffer)
        {
          if (file_pos == 0)
            {
              /* Hit the beginning of the file; print the remaining record. */
              output (G_buffer, past_end);
              return true;
            }

          saved_record_size = past_end - G_buffer;
          if (saved_record_size > read_size)
            {
              /* 'G_buffer_size' is about twice 'read_size', so since
                 we want to read in another 'read_size' bytes before
                 the data already in 'G_buffer', we need to increase
                 'G_buffer_size'. */
              char *newbuffer;
              size_t offset = sentinel_length ? sentinel_length : 1;
              size_t old_G_buffer_size = G_buffer_size;

              read_size *= 2;
              G_buffer_size = read_size * 2 + sentinel_length + 2;
              if (G_buffer_size < old_G_buffer_size)
                xalloc_die ();
              newbuffer = xrealloc (G_buffer - offset, G_buffer_size);
              newbuffer += offset;
              G_buffer = newbuffer;
            }

          /* Back up to the start of the next bufferfull of the file.  */
          if (file_pos >= read_size)
            file_pos -= read_size;
          else
            {
              read_size = file_pos;
              file_pos = 0;
            }
          if (lseek (input_fd, file_pos, SEEK_SET) < 0)
            error (0, errno, _("%s: seek failed"), quotef (file));

          /* Shift the pending record data right to make room for the new.
             The source and destination regions probably overlap.  */
          memmove (G_buffer + read_size, G_buffer, saved_record_size);
          past_end = G_buffer + read_size + saved_record_size;
          /* For non-regexp searches, avoid unnecessary scanning. */
          if (sentinel_length)
            match_start = G_buffer + read_size;
          else
            match_start = past_end;

          if (safe_read (input_fd, G_buffer, read_size) != read_size)
            {
              error (0, errno, _("%s: read error"), quotef (file));
              return false;
            }
        }
      else
        {
          /* Found a match of 'separator'. */
          if (separator_ends_record)
            {
              char *match_end = match_start + match_length;

              /* If this match of 'separator' isn't at the end of the
                 file, print the record. */
              if (!first_time || match_end != past_end)
                output (match_end, past_end);
              past_end = match_end;
              first_time = false;
            }
          else
            {
              output (match_start, past_end);
              past_end = match_start;
            }

          /* For non-regex matching, we can back up.  */
          if (sentinel_length > 0)
            match_start -= match_length - 1;
        }
    }
}

#if DONT_UNLINK_WHILE_OPEN

/* FIXME-someday: remove all of this DONT_UNLINK_WHILE_OPEN junk.
   Using atexit like this is wrong, since it can fail
   when called e.g. 32 or more times.
   But this isn't a big deal, since the code is used only on WOE/DOS
   systems, and few people invoke tac on that many nonseekable files.  */

static char const *file_to_remove;
static FILE *fp_to_close;

static void
unlink_tempfile (void)
{
  fclose (fp_to_close);
  unlink (file_to_remove);
}

static void
record_or_unlink_tempfile (char const *fn, FILE *fp)
{
  if (!file_to_remove)
    {
      file_to_remove = fn;
      fp_to_close = fp;
      atexit (unlink_tempfile);
    }
}

#else

static void
record_or_unlink_tempfile (char const *fn, MAYBE_UNUSED FILE *fp)
{
  unlink (fn);
}

#endif

/* A wrapper around mkstemp that gives us both an open stream pointer,
   FP, and the corresponding FILE_NAME.  Always return the same FP/name
   pair, rewinding/truncating it upon each reuse.  */
static bool
temp_stream (FILE **fp, char **file_name)
{
  static char *tempfile = NULL;
  static FILE *tmp_fp;
  if (tempfile == NULL)
    {
      char const *t = getenv ("TMPDIR");
      char const *tempdir = t ? t : DEFAULT_TMPDIR;
      tempfile = mfile_name_concat (tempdir, "tacXXXXXX", NULL);
      if (tempdir == NULL)
        {
          error (0, 0, _("memory exhausted"));
          return false;
        }

      /* FIXME: there's a small window between a successful mkstemp call
         and the unlink that's performed by record_or_unlink_tempfile.
         If we're interrupted in that interval, this code fails to remove
         the temporary file.  On systems that define DONT_UNLINK_WHILE_OPEN,
         the window is much larger -- it extends to the atexit-called
         unlink_tempfile.
         FIXME: clean up upon fatal signal.  Don't block them, in case
         $TMPFILE is a remote file system.  */

      int fd = mkstemp (tempfile);
      if (fd < 0)
        {
          error (0, errno, _("failed to create temporary file in %s"),
                 quoteaf (tempdir));
          goto Reset;
        }

      tmp_fp = fdopen (fd, (O_BINARY ? "w+b" : "w+"));
      if (! tmp_fp)
        {
          error (0, errno, _("failed to open %s for writing"),
                 quoteaf (tempfile));
          close (fd);
          unlink (tempfile);
        Reset:
          free (tempfile);
          tempfile = NULL;
          return false;
        }

      record_or_unlink_tempfile (tempfile, tmp_fp);
    }
  else
    {
      clearerr (tmp_fp);
      if (fseeko (tmp_fp, 0, SEEK_SET) < 0
          || ftruncate (fileno (tmp_fp), 0) < 0)
        {
          error (0, errno, _("failed to rewind stream for %s"),
                 quoteaf (tempfile));
          return false;
        }
    }

  *fp = tmp_fp;
  *file_name = tempfile;
  return true;
}

/* Copy from file descriptor INPUT_FD (corresponding to the named FILE) to
   a temporary file, and set *G_TMP and *G_TEMPFILE to the resulting stream
   and file name.  Return the number of bytes copied, or -1 on error.  */

static off_t
copy_to_temp (FILE **g_tmp, char **g_tempfile, int input_fd, char const *file)
{
  FILE *fp;
  char *file_name;
  uintmax_t bytes_copied = 0;
  if (!temp_stream (&fp, &file_name))
    return -1;

  while (true)
    {
      size_t bytes_read = safe_read (input_fd, G_buffer, read_size);
      if (bytes_read == 0)
        break;
      if (bytes_read == SAFE_READ_ERROR)
        {
          error (0, errno, _("%s: read error"), quotef (file));
          return -1;
        }

      if (fwrite (G_buffer, 1, bytes_read, fp) != bytes_read)
        {
          error (0, errno, _("%s: write error"), quotef (file_name));
          return -1;
        }

      /* Implicitly <= OFF_T_MAX due to preceding fwrite(),
         but unsigned type used to avoid compiler warnings
         not aware of this fact.  */
      bytes_copied += bytes_read;
    }

  if (fflush (fp) != 0)
    {
      error (0, errno, _("%s: write error"), quotef (file_name));
      return -1;
    }

  *g_tmp = fp;
  *g_tempfile = file_name;
  return bytes_copied;
}

/* Copy INPUT_FD to a temporary, then tac that file.
   Return true if successful.  */

static bool
tac_nonseekable (int input_fd, char const *file)
{
  FILE *tmp_stream;
  char *tmp_file;
  off_t bytes_copied = copy_to_temp (&tmp_stream, &tmp_file, input_fd, file);
  if (bytes_copied < 0)
    return false;

  bool ok = tac_seekable (fileno (tmp_stream), tmp_file, bytes_copied);
  return ok;
}

/* Print FILE in reverse, copying it to a temporary
   file first if it is not seekable.
   Return true if successful.  */

static bool
tac_file (char const *filename)
{
  bool ok;
  off_t file_size;
  int fd;
  bool is_stdin = STREQ (filename, "-");

  if (is_stdin)
    {
      have_read_stdin = true;
      fd = STDIN_FILENO;
      filename = _("standard input");
      xset_binary_mode (STDIN_FILENO, O_BINARY);
    }
  else
    {
      fd = open (filename, O_RDONLY | O_BINARY);
      if (fd < 0)
        {
          error (0, errno, _("failed to open %s for reading"),
                 quoteaf (filename));
          return false;
        }
    }

  file_size = lseek (fd, 0, SEEK_END);

  ok = (file_size < 0 || isatty (fd)
        ? tac_nonseekable (fd, filename)
        : tac_seekable (fd, filename, file_size));

  if (!is_stdin && close (fd) != 0)
    {
      error (0, errno, _("%s: read error"), quotef (filename));
      ok = false;
    }
  return ok;
}

int
main (int argc, char **argv)
{
  char const *error_message;	/* Return value from re_compile_pattern. */
  int optc;
  bool ok;
  size_t half_buffer_size;

  /* Initializer for file_list if no file-arguments
     were specified on the command line.  */
  static char const *const default_file_list[] = {"-", NULL};
  char const *const *file;

  initialize_main (&argc, &argv);
  set_program_name (argv[0]);
  setlocale (LC_ALL, "");
  bindtextdomain (PACKAGE, LOCALEDIR);
  textdomain (PACKAGE);

  atexit (close_stdout);

  separator = "\n";
  sentinel_length = 1;
  separator_ends_record = true;

  while ((optc = getopt_long (argc, argv, "brs:", longopts, NULL)) != -1)
    {
      switch (optc)
        {
        case 'b':
          separator_ends_record = false;
          break;
        case 'r':
          sentinel_length = 0;
          break;
        case 's':
          separator = optarg;
          break;
        case_GETOPT_HELP_CHAR;
        case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);
        default:
          usage (EXIT_FAILURE);
        }
    }

  if (sentinel_length == 0)
    {
      if (*separator == 0)
        die (EXIT_FAILURE, 0, _("separator cannot be empty"));

      compiled_separator.buffer = NULL;
      compiled_separator.allocated = 0;
      compiled_separator.fastmap = compiled_separator_fastmap;
      compiled_separator.translate = NULL;
      error_message = re_compile_pattern (separator, strlen (separator),
                                          &compiled_separator);
      if (error_message)
        die (EXIT_FAILURE, 0, "%s", (error_message));
    }
  else
    match_length = sentinel_length = *separator ? strlen (separator) : 1;

  read_size = INITIAL_READSIZE;
  while (sentinel_length >= read_size / 2)
    {
      if (SIZE_MAX / 2 < read_size)
        xalloc_die ();
      read_size *= 2;
    }
  half_buffer_size = read_size + sentinel_length + 1;
  G_buffer_size = 2 * half_buffer_size;
  if (! (read_size < half_buffer_size && half_buffer_size < G_buffer_size))
    xalloc_die ();
  G_buffer = xmalloc (G_buffer_size);
  if (sentinel_length)
    {
      memcpy (G_buffer, separator, sentinel_length + 1);
      G_buffer += sentinel_length;
    }
  else
    {
      ++G_buffer;
    }

  file = (optind < argc
          ? (char const *const *) &argv[optind]
          : default_file_list);

  xset_binary_mode (STDOUT_FILENO, O_BINARY);

  {
    ok = true;
    for (size_t i = 0; file[i]; ++i)
      ok &= tac_file (file[i]);
  }

  /* Flush the output buffer. */
  output ((char *) NULL, (char *) NULL);

  if (have_read_stdin && close (STDIN_FILENO) < 0)
    {
      error (0, errno, "-");
      ok = false;
    }

  main_exit (ok ? EXIT_SUCCESS : EXIT_FAILURE);
}