/* Copyright (C) 2002-2003 Free Software Foundation, Inc.
   Contributed by Andy Vaught

This file is part of the GNU Fortran 95 runtime library (libgfortran).

Libgfortran 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 2, or (at your option)
any later version.

In addition to the permissions in the GNU General Public License, the
Free Software Foundation gives you unlimited permission to link the
compiled version of this file into combinations with other programs,
and to distribute those combinations without any restriction coming
from the use of this file.  (The General Public License restrictions
do apply in other respects; for example, they cover modification of
the file, and distribution when not linked into a combine
executable.)

Libgfortran 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 Libgfortran; see the file COPYING.  If not, write to
the Free Software Foundation, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */

#include "config.h"
#include "libgfortran.h"
#include "io.h"

/* backspace.c -- Implement the BACKSPACE statement */

/* formatted_backspace(void)-- Move the file back one line.  The
 * current position is after the newline that terminates the previous
 * record, and we have to sift backwards to find the newline before
 * that or the start of the file, whichever comes first. */

#define READ_CHUNK 4096

static void
formatted_backspace (void)
{
  gfc_offset base;
  char *p;
  int n;

  base = file_position (current_unit->s) - 1;

  do
    {
      n = (base < READ_CHUNK) ? base : READ_CHUNK;
      base -= n;

      p = salloc_r_at (current_unit->s, &n, base);
      if (p == NULL)
	goto io_error;

      /* Because we've moved backwords from the current position, it
       * should not be possible to get a short read.  Because it isn't
       * clear what to do about such thing, we ignore the possibility. */

      /* There is no memrchr() in the C library, so we have to do it
       * ourselves. */

      n--;
      while (n >= 0)
	{
	  if (p[n] == '\n')
	    {
	      base += n + 1;
	      goto done;
	    }

	  n--;
	}

    }
  while (base != 0);

  /* base is the new pointer.  Seek to it exactly */
 done:
  if (sseek (current_unit->s, base) == FAILURE)
    goto io_error;
  current_unit->last_record--;
  current_unit->endfile = NO_ENDFILE;

  return;

 io_error:
  generate_error (ERROR_OS, NULL);
}


/* unformatted_backspace()-- Move the file backwards for an
 * unformatted sequential file.  We are guaranteed to be between
 * records on entry and we have to shift to the previous record.  */

static void
unformatted_backspace (void)
{
  gfc_offset *p, new;
  int length;

  length = sizeof (gfc_offset);

  p = (gfc_offset *) salloc_r_at (current_unit->s, &length,
				file_position (current_unit->s) - length);
  if (p == NULL)
    goto io_error;

  new = file_position (current_unit->s) - *p - length;
  if (sseek (current_unit->s, new) == FAILURE)
    goto io_error;

  current_unit->last_record--;
  return;

 io_error:
  generate_error (ERROR_OS, NULL);
}


extern void st_backspace (void);
export_proto(st_backspace);

void
st_backspace (void)
{
  gfc_unit *u;

  library_start ();

  u = find_unit (ioparm.unit);
  if (u == NULL)
    {
      generate_error (ERROR_BAD_UNIT, NULL);
      goto done;
    }

  current_unit = u;

  /* Ignore direct access.  Non-advancing I/O is only allowed for
   * formatted sequential I/O and the next direct access transfer
   * repositions the file anyway. */

  if (u->flags.access == ACCESS_DIRECT)
    goto done;

  /* Check for special cases involving the ENDFILE record first */

  if (u->endfile == AFTER_ENDFILE)
    u->endfile = AT_ENDFILE;
  else
    {
      if (u->current_record)
	next_record (1);

      if (file_position (u->s) == 0)
	goto done;		/* Common special case */

      if (u->flags.form == FORM_FORMATTED)
	formatted_backspace ();
      else
	unformatted_backspace ();
    }

 done:
  library_end ();
}