/*
 * Copyright (c) 1988 Mark Nudleman
 * Copyright (c) 1988, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#ifndef lint
static char sccsid[] = "@(#)ch.c	8.1 (Berkeley) 6/6/93";
#endif /* not lint */

#ifndef lint
static const char rcsid[] =
  "$FreeBSD$";
#endif /* not lint */

/*
 * Low level character input from the input file.
 * We use these special purpose routines which optimize moving
 * both forward and backward from the current read pointer.
 */

#include <sys/types.h>
#include <sys/file.h>
#include <unistd.h>
#include <stdio.h>
#include <less.h>

int file = -1;		/* File descriptor of the input file */

/*
 * Pool of buffers holding the most recently used blocks of the input file.
 */
struct buf {
	struct buf *next, *prev;
	long block;
	int datasize;
	char data[BUFSIZ];
};
int nbufs;

/*
 * The buffer pool is kept as a doubly-linked circular list.  For the ispipe
 * case, this list will always be ordered from highest-numbered block downto
 * lowest-numbered block, skipping no blocks.  For the !ispipe case,
 * it may become disordered.  It is not clear that this is a feature.
 */
#define	END_OF_CHAIN	((struct buf *)&buf_anchor)
#define	buf_head	buf_anchor.next
#define	buf_tail	buf_anchor.prev

static struct {
	struct buf *next, *prev;
	long block;  /* this is never changed from -1 */
} buf_anchor = { END_OF_CHAIN, END_OF_CHAIN, (long)-1 };

/*
 * The last buffer in the circular list that was accessed, and correspondingly
 * the most likely to be accessed in the future.
 */
static struct buf *buf_lastacc = END_OF_CHAIN;

extern int ispipe, cbufs, sigs;

/*
 * Current position in file.
 * Stored as a block number and an offset into the block.
 */
static long ch_block;
static int ch_offset;

/* Length of file, needed if input is a pipe. */
static off_t ch_fsize;

/* Number of bytes read, if input is standard input (a pipe). */
static off_t last_piped_pos;

/*
 * Get the character pointed to by the read pointer.  ch_get() is a macro
 * which is more efficient to call than fch_get (the function), in the usual
 * case that the block desired is at the head of the chain.
 */
#define	ch_get() \
	((buf_lastacc->block == ch_block && \
	    ch_offset < buf_lastacc->datasize) ? \
	    (unsigned char)buf_lastacc->data[ch_offset] : fch_get())

static
fch_get()
{
	register struct buf *bp;
	register char *p, *t;
	int n, gofor;
	off_t pos, lseek();

	/*
	 * look for a buffer holding the desired block.
	 */
	if (abs(buf_lastacc->next->block - ch_block) <
	    abs(buf_lastacc->prev->block - ch_block))
		gofor = 1;  /* Look forwards through the buffer queue */
	else
		gofor = 0;  /* Look backwards through the buffer queue */

	bp = buf_lastacc;
	do {
		if (bp->block == ch_block) {
			buf_lastacc = bp;
			if (ch_offset >= bp->datasize)
				goto read_more;
			return((unsigned char)bp->data[ch_offset]);
		}
		if (gofor)
			bp = bp->next;
		else
			bp = bp->prev;
	} while (bp != buf_lastacc);

	/*
	 * Block is not in a buffer.  Take the buffer from the tail and
	 * read the desired block into it.  If the input is a pipe, we try
	 * to buffer as much input as possible since the input will be
	 * permanently lost if we throw it from the buffer queue.
	 */
	if (ispipe && buf_tail->block != (long)(-1))
		(void)ch_addbuf(1);
	bp = buf_tail;
	bp->block = ch_block;
	bp->datasize = 0;

read_more:
	pos = (ch_block * BUFSIZ) + bp->datasize;
	if (ispipe) {
		/*
		 * The data requested should be immediately after
		 * the last data read from the pipe.
		 */
		if (pos != last_piped_pos) {
			error("pipe error");
			quit();
		}
	} else
		(void)lseek(file, pos, L_SET);

	/*
	 * Read the block.
	 *
	 * If we read less than a full block, we just return the
	 * partial block and pick up the rest next time.
	 */
	n = iread(file, &bp->data[bp->datasize], BUFSIZ - bp->datasize);
	if (n == READ_INTR)
		return (EOI);
	if (n < 0) {
		error("read error");
		quit();
	}
	if (ispipe)
		last_piped_pos += n;

	bp->datasize += n;

	if (n == 0) {
		ch_fsize = pos;
		bp->data[bp->datasize++] = EOI;
	}

	/*
	 * Turn other EOI (nul) chars into 0200 since EOI has special meaning.
	 */
	for (p = &bp->data[bp->datasize]; --n >= 0;) {
		--p;
		if (*p == EOI)
			*p = 0200;
	}

found:
	if (buf_head != bp) {
		/*
		 * Move the buffer to the head of the buffer chain.  This
		 * ensures correct order for the ispipe case and prevents
		 * needless buffer thrashing for the !ispipe case.  It's not
		 * clear that buffer thrashing isn't desirable in this latter
		 * case, since the VM should probably be handling the file
		 * buffer...
		 */
		bp->next->prev = bp->prev;
		bp->prev->next = bp->next;

		bp->next = buf_head;
		bp->prev = END_OF_CHAIN;
		buf_head->prev = bp;
		buf_head = bp;
	}

	if (ch_offset >= bp->datasize)
		/*
		 * After all that, we still don't have enough data.
		 * Go back and try again.
		 */
		goto read_more;

	return((unsigned char)bp->data[ch_offset]);
}

/*
 * Determine if a specific block is currently in one of the buffers.
 *
 * In general, this function is only called for the ispipe case.  For the
 * !ispipe case, ch.c generally assumes that any given block is accessible
 * through ch_get(), even though ch_get() may not have it buffered.
 */
static
buffered(block)
	long block;
{
	register struct buf *bp;

	/* For the ispipe case, we know that the buffer queue is sequentially
	 * ordered from tail to head. */
	if (ispipe && (block <= buf_head->block && block >= buf_tail->block))
		return(1);

	/*
	 * XXX This is dead code.
	 */
	for (bp = buf_head; bp != END_OF_CHAIN; bp = bp->next)
		if (bp->block == block)
			return(1);
	return(0);
}

/*
 * Seek to a specified position in the file.
 * Return 0 if successful, non-zero if can't seek there.
 */
ch_seek(pos)
	register off_t pos;
{
	long new_block;

	new_block = pos / BUFSIZ;
	if (!ispipe || pos == last_piped_pos || buffered(new_block)) {
		/*
		 * Set read pointer.
		 */
		ch_block = new_block;
		ch_offset = pos % BUFSIZ;
		return(0);
	}
	return(1);
}

/*
 * Seek to the end of the file.
 */
ch_end_seek()
{
	off_t ch_length();

	if (!ispipe)
		return(ch_seek(ch_length()));

	/*
	 * Do it the slow way: read till end of data.
	 */
	while (ch_forw_get() != EOI)
		if (sigs)
			return(1);
	return(0);
}

/*
 * Seek to the beginning of the file, or as close to it as we can get.
 * We may not be able to seek there if input is a pipe and the
 * beginning of the pipe is no longer buffered.
 */
ch_beg_seek()
{
	register struct buf *bp, *firstbp;

	/*
	 * Try a plain ch_seek first.
	 */
	if (ch_seek((off_t)0) == 0)
		return(0);

	/*
	 * Can't get to position 0.
	 * Look thru the buffers for the one closest to position 0.
	 *
	 * This should use the obvious optimization that applies for the
	 * ispipe case (which is also the only case under which this
	 * code will be executed, ie. the only case under which ch_seek()
	 * will fail).
	 */
	firstbp = bp = buf_head;
	if (bp == END_OF_CHAIN)
		return(1);
	while ((bp = bp->next) != END_OF_CHAIN)
		if (bp->block < firstbp->block)
			firstbp = bp;
	ch_block = firstbp->block;
	ch_offset = 0;
	return(0);
}

/*
 * Return the length of the file, if known.
 */
off_t
ch_length()
{
	off_t lseek();

	if (ispipe)
		return(ch_fsize);
	return((off_t)(lseek(file, (off_t)0, L_XTND)));
}

/*
 * Return the current position in the file.
 */
off_t
ch_tell()
{
	return(ch_block * BUFSIZ + ch_offset);
}

/*
 * Get the current char and post-increment the read pointer.
 */
ch_forw_get()
{
	register int c;

	c = ch_get();
	if (c != EOI && ++ch_offset >= BUFSIZ) {
		ch_offset = 0;
		++ch_block;
	}
	return(c);
}

/*
 * Pre-decrement the read pointer and get the new current char.
 */
ch_back_get()
{
	if (--ch_offset < 0) {
		if (ch_block <= 0 || (ispipe && !buffered(ch_block-1))) {
			ch_offset = 0;
			return(EOI);
		}
		ch_offset = BUFSIZ - 1;
		ch_block--;
	}
	return(ch_get());
}

/*
 * Allocate buffers.
 * Caller wants us to have a total of at least want_nbufs buffers.
 * keep==1 means keep the data in the current buffers;
 * otherwise discard the old data.
 */
ch_init(want_nbufs, keep)
	int want_nbufs;
	int keep;
{
	register struct buf *bp;
	char message[80];

	cbufs = nbufs;
	if (nbufs < want_nbufs && ch_addbuf(want_nbufs - nbufs)) {
		/*
		 * Cannot allocate enough buffers.
		 * If we don't have ANY, then quit.
		 * Otherwise, just report the error and return.
		 */
		(void)snprintf(message, sizeof(message),
		    "cannot allocate %d buffers", want_nbufs - nbufs);
		error(message);
		if (nbufs == 0)
			quit();
		return;
	}

	if (keep)
		return;

	/*
	 * We don't want to keep the old data,
	 * so initialize all the buffers now.
	 */
	for (bp = buf_head;  bp != END_OF_CHAIN;  bp = bp->next)
		bp->block = (long)(-1);
	last_piped_pos = (off_t)0;
	ch_fsize = NULL_POSITION;
	(void)ch_seek((off_t)0);
}

/*
 * Allocate some new buffers.
 * The buffers are added to the tail of the buffer chain.
 */
ch_addbuf(nnew)
	int nnew;
{
	register struct buf *bp;
	register struct buf *newbufs;
	char *calloc();

	/*
	 * We don't have enough buffers.
	 * Allocate some new ones.
	 */
	newbufs = (struct buf *)calloc((u_int)nnew, sizeof(struct buf));
	if (newbufs == NULL)
		return(1);

	/*
	 * Initialize the new buffers and link them together.
	 * Link them all onto the tail of the buffer list.
	 */
	nbufs += nnew;
	cbufs = nbufs;
	for (bp = &newbufs[0];  bp < &newbufs[nnew];  bp++) {
		bp->next = bp + 1;
		bp->prev = bp - 1;
		bp->block = (long)(-1);
	}
	newbufs[nnew-1].next = END_OF_CHAIN;
	newbufs[0].prev = buf_tail;
	buf_tail->next = &newbufs[0];
	buf_tail = &newbufs[nnew-1];
	return(0);
}