/* Retrieve information about a FILE stream.
   Copyright (C) 2007-2023 Free Software Foundation, Inc.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file 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 Lesser General Public License for more details.

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

#include <config.h>

/* Specification.  */
#include "freadahead.h"

#include <stdlib.h>
#include "stdio-impl.h"

#if defined __DragonFly__
/* Defined in libc, but not declared in <stdio.h>.  */
extern size_t __sreadahead (FILE *);
#endif

/* This file is not used on systems that have the __freadahead function,
   namely musl libc.  */

size_t
freadahead (FILE *fp)
{
#if defined _IO_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
  /* GNU libc, BeOS, Haiku, Linux libc5 */
  if (fp->_IO_write_ptr > fp->_IO_write_base)
    return 0;
  return (fp->_IO_read_end - fp->_IO_read_ptr)
         + (fp->_flags & _IO_IN_BACKUP ? fp->_IO_save_end - fp->_IO_save_base :
            0);
#elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
  /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
  if ((fp_->_flags & __SWR) != 0 || fp_->_r < 0)
    return 0;
# if defined __DragonFly__
  return __sreadahead (fp);
# else
  return fp_->_r
         + (HASUB (fp) ? fp_->_ur : 0);
# endif
#elif defined __EMX__               /* emx+gcc */
  if ((fp->_flags & _IOWRT) != 0)
    return 0;
  /* Note: fp->_ungetc_count > 0 implies fp->_rcount <= 0,
           fp->_ungetc_count = 0 implies fp->_rcount >= 0.  */
  /* equivalent to
     (fp->_ungetc_count == 0 ? fp->_rcount : fp->_ungetc_count - fp->_rcount) */
  return (fp->_rcount > 0 ? fp->_rcount : fp->_ungetc_count - fp->_rcount);
#elif defined __minix               /* Minix */
  if ((fp_->_flags & _IOWRITING) != 0)
    return 0;
  return fp_->_count;
#elif defined _IOERR                /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
  if ((fp_->_flag & _IOWRT) != 0)
    return 0;
  return fp_->_cnt;
#elif defined __UCLIBC__            /* uClibc */
# ifdef __STDIO_BUFFERS
  if (fp->__modeflags & __FLAG_WRITING)
    return 0;
  return (fp->__bufread - fp->__bufpos)
         + (fp->__modeflags & __FLAG_UNGOT ? 1 : 0);
# else
  return 0;
# endif
#elif defined __QNX__               /* QNX */
  if ((fp->_Mode & 0x2000 /* _MWRITE */) != 0)
    return 0;
  /* fp->_Buf <= fp->_Next <= fp->_Rend,
     and fp->_Rend may be overridden by fp->_Rsave. */
  return ((fp->_Rsave ? fp->_Rsave : fp->_Rend) - fp->_Next)
         + (fp->_Mode & 0x4000 /* _MBYTE */
            ? (fp->_Back + sizeof (fp->_Back)) - fp->_Rback
            : 0);
#elif defined __MINT__              /* Atari FreeMiNT */
  if (!fp->__mode.__read)
    return 0;
  return (fp->__pushed_back
          ? fp->__get_limit - fp->__pushback_bufp + 1
          : fp->__get_limit - fp->__bufp);
#elif defined EPLAN9                /* Plan9 */
  if (fp->state == 4 /* WR */ || fp->rp >= fp->wp)
    return 0;
  return fp->wp - fp->rp;
#elif defined SLOW_BUT_NO_HACKS     /* users can define this */
  abort ();
  return 0;
#else
 #error "Please port gnulib freadahead.c to your platform! Look at the definition of fflush, fread, ungetc on your system, then report this to bug-gnulib."
#endif
}