/*
 * Copyright (c) 2000-2002, 2005 Sendmail, Inc. and its suppliers.
 *      All rights reserved.
 * Copyright (c) 1990, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Chris Torek.
 *
 * By using this file, you agree to the terms and conditions set
 * forth in the LICENSE file which can be found at the top level of
 * the sendmail distribution.
 */

#include "sm/generic.h"
SM_RCSID("@(#)$Id: fseek.c,v 1.13 2006/07/14 14:42:25 ca Exp $")
#include "sm/assert.h"
#include "sm/types.h"
#include "sm/stat.h"
#include "sm/fcntl.h"
#include "sm/error.h"
#include "sm/time.h"
#include "sm/io.h"
#include "io-int.h"

#define POS_ERR	(-(off_t)1)

/*
**  SM_IO_SEEK -- position the file pointer
**
**	Parameters:
**		fp -- the file pointer to be seek'd
**		offset -- seek offset based on 'whence'
**		whence -- indicates where seek is relative from.
**			One of SM_IO_SEEK_{CUR,SET,END}.
**	Returns:
**		usual sm_error code
*/

sm_ret_T
sm_io_seek(sm_file_T *fp, long offset, int whence)
{
	bool havepos;
	off_t target, curoff;
	size_t n;
	struct stat st;
	int ret;
	seek_F *seekfn;

	SM_IS_FP(fp);

	/* Have to be able to seek. */
	seekfn = f_seek(*fp);
	if (seekfn == NULL)
		return sm_error_perm(SM_EM_IO, ESPIPE);
		/* historic practice */

	/*
	**  Change any SM_IO_SEEK_CUR to SM_IO_SEEK_SET, and check `whence'
	**  argument. After this, whence is either SM_IO_SEEK_SET or
	**  SM_IO_SEEK_END.
	*/

	switch (whence)
	{
	  case SM_IO_SEEK_CUR:

		/*
		**  In order to seek relative to the current stream offset,
		**  we have to first find the current stream offset a la
		**  ftell (see ftell for details).
		*/

		/* may adjust seek offset on append stream */
		sm_flush(fp);
		if (f_flags(*fp) & SMOFF)
			curoff = fp->f_lseekoff;
		else
		{
			curoff = (*seekfn)(fp, (off_t) 0, SM_IO_SEEK_CUR);
			if (curoff == -1L)
			{
				ret = -1;
				goto clean;
			}
		}
		if (f_flags(*fp) & SMRD)
			curoff -= f_r(*fp);
		else if (f_flags(*fp) & SMWR && f_p(*fp) != NULL)
			curoff += f_p(*fp) - f_bfbase(*fp);

		offset += curoff;
		whence = SM_IO_SEEK_SET;
		havepos = true;
		break;

	  case SM_IO_SEEK_SET:
	  case SM_IO_SEEK_END:
		curoff = 0;		/* just to keep gcc quiet */
		havepos = false;
		break;

	  default:
		errno = EINVAL;
		return -1;
	}

	/*
	**  Can only optimise if:
	**	reading (and not reading-and-writing);
	**	not unbuffered; and
	**	this is a `regular' Unix file (and hence seekfn==sm_stdseek).
	**  We must check SMNBF first, because it is possible to have SMNBF
	**  and SMOPT both set.
	*/

	if (f_bfbase(*fp) == NULL)
		sm_makefilebuf(fp);
	if (f_flags(*fp) & (SMWR | SMRW | SMNBF | SMNPT))
		goto dumb;
	if ((f_flags(*fp) & SMOPT) == 0)
	{
		if (seekfn != sm_stdseek ||
		    f_fd(*fp) < 0 || fstat(f_fd(*fp), &st) ||
		    (st.st_mode & S_IFMT) != S_IFREG)
		{
			f_flags(*fp) |= SMNPT;
			goto dumb;
		}
#if WIN32
		fp->f_blksize = 4096; /* XXX temp */
#else
		fp->f_blksize = st.st_blksize;
#endif
		f_flags(*fp) |= SMOPT;
	}

	/*
	**  We are reading; we can try to optimise.
	**  Figure out where we are going and where we are now.
	*/

	if (whence == SM_IO_SEEK_SET)
		target = offset;
	else
	{
		if (fstat(f_fd(*fp), &st))
			goto dumb;
		target = st.st_size + offset;
	}

	if (!havepos)
	{
		if (f_flags(*fp) & SMOFF)
			curoff = fp->f_lseekoff;
		else
		{
			curoff = (*seekfn)(fp, (off_t) 0, SM_IO_SEEK_CUR);
			if (curoff == POS_ERR)
				goto dumb;
		}
		curoff -= f_r(*fp);
	}

	/*
	**  Compute the number of bytes in the input buffer (pretending
	**  that any ungetc() input has been discarded).  Adjust current
	**  offset backwards by this count so that it represents the
	**  file offset for the first byte in the current input buffer.
	*/

	n = f_p(*fp) - f_bfbase(*fp);
	curoff -= n;
	n += f_r(*fp);

	/*
	**  If the target offset is within the current buffer,
	**  simply adjust the pointers, clear SMFEOF, undo ungetc(),
	**  and return.  (If the buffer was modified, we have to
	**  skip this; see getln in fget.c.)
	*/

	if (target >= curoff && target < curoff + (off_t) n)
	{
		int o;

		o = target - curoff;
		f_p(*fp) = f_bfbase(*fp) + o;
		f_r(*fp) = n - o;
		f_flags(*fp) &= ~SMFEOF;
		ret = 0;
		goto clean;
	}

	/*
	**  The place we want to get to is not within the current buffer,
	**  but we can still be kind to the kernel copyout mechanism.
	**  By aligning the file offset to a block boundary, we can let
	**  the kernel use the VM hardware to map pages instead of
	**  copying bytes laboriously.  Using a block boundary also
	**  ensures that we only read one block, rather than two.
	*/

	curoff = target & ~(fp->f_blksize - 1);
	if ((*seekfn)(fp, curoff, SM_IO_SEEK_SET) == POS_ERR)
		goto dumb;
	f_r(*fp) = 0;
	f_p(*fp) = f_bfbase(*fp);
	f_flags(*fp) &= ~SMFEOF;
	n = target - curoff;
	if (n)
	{
		if (sm_refill(fp) || f_r(*fp) < (int) n)
			goto dumb;
		f_p(*fp) += n;
		f_r(*fp) -= n;
	}

	ret = 0;
clean:
	return ret;
dumb:
	/*
	**  We get here if we cannot optimise the seek ... just
	**  do it.  Allow the seek function to change f_bfbase(*fp).
	*/

	if (sm_flush(fp) != 0 ||
	    (*seekfn)(fp, (off_t) offset, whence) == POS_ERR)
	{
		ret = -1;
		goto clean;
	}

	/* success: clear SMFEOF indicator */
	f_p(*fp) = f_bfbase(*fp);
	f_r(*fp) = 0;
	f_flags(*fp) &= ~SMFEOF;
	ret = 0;
	goto clean;
}