/*
 * Copyright (C) 1984-2023  Mark Nudelman
 *
 * You may distribute under the terms of either the GNU General Public
 * License or the Less License, as specified in the README file.
 *
 * For more information, see the README file.
 */


/*
 * 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 "less.h"
#if MSDOS_COMPILER==WIN32C
#include <errno.h>
#include <windows.h>
#endif

#if HAVE_PROCFS
#include <sys/statfs.h>
#if HAVE_LINUX_MAGIC_H
#include <linux/magic.h>
#endif
#endif

typedef POSITION BLOCKNUM;

public int ignore_eoi;

/*
 * Pool of buffers holding the most recently used blocks of the input file.
 * The buffer pool is kept as a doubly-linked circular list,
 * in order from most- to least-recently used.
 * The circular list is anchored by the file state "thisfile".
 */
struct bufnode {
	struct bufnode *next, *prev;
	struct bufnode *hnext, *hprev;
};

#define LBUFSIZE        8192
struct buf {
	struct bufnode node;
	BLOCKNUM block;
	unsigned int datasize;
	unsigned char data[LBUFSIZE];
};
#define bufnode_buf(bn)  ((struct buf *) bn)

/*
 * The file state is maintained in a filestate structure.
 * A pointer to the filestate is kept in the ifile structure.
 */
#define BUFHASH_SIZE    1024
struct filestate {
	struct bufnode buflist;
	struct bufnode hashtbl[BUFHASH_SIZE];
	int file;
	int flags;
	POSITION fpos;
	int nbufs;
	BLOCKNUM block;
	unsigned int offset;
	POSITION fsize;
};

#define ch_bufhead      thisfile->buflist.next
#define ch_buftail      thisfile->buflist.prev
#define ch_nbufs        thisfile->nbufs
#define ch_block        thisfile->block
#define ch_offset       thisfile->offset
#define ch_fpos         thisfile->fpos
#define ch_fsize        thisfile->fsize
#define ch_flags        thisfile->flags
#define ch_file         thisfile->file

#define END_OF_CHAIN    (&thisfile->buflist)
#define END_OF_HCHAIN(h) (&thisfile->hashtbl[h])
#define BUFHASH(blk)    ((blk) & (BUFHASH_SIZE-1))

/*
 * Macros to manipulate the list of buffers in thisfile->buflist.
 */
#define FOR_BUFS(bn) \
	for (bn = ch_bufhead;  bn != END_OF_CHAIN;  bn = bn->next)

#define BUF_RM(bn) \
	(bn)->next->prev = (bn)->prev; \
	(bn)->prev->next = (bn)->next;

#define BUF_INS_HEAD(bn) \
	(bn)->next = ch_bufhead; \
	(bn)->prev = END_OF_CHAIN; \
	ch_bufhead->prev = (bn); \
	ch_bufhead = (bn);

#define BUF_INS_TAIL(bn) \
	(bn)->next = END_OF_CHAIN; \
	(bn)->prev = ch_buftail; \
	ch_buftail->next = (bn); \
	ch_buftail = (bn);

/*
 * Macros to manipulate the list of buffers in thisfile->hashtbl[n].
 */
#define FOR_BUFS_IN_CHAIN(h,bn) \
	for (bn = thisfile->hashtbl[h].hnext;  \
	     bn != END_OF_HCHAIN(h);  bn = bn->hnext)

#define BUF_HASH_RM(bn) \
	(bn)->hnext->hprev = (bn)->hprev; \
	(bn)->hprev->hnext = (bn)->hnext;

#define BUF_HASH_INS(bn,h) \
	(bn)->hnext = thisfile->hashtbl[h].hnext; \
	(bn)->hprev = END_OF_HCHAIN(h); \
	thisfile->hashtbl[h].hnext->hprev = (bn); \
	thisfile->hashtbl[h].hnext = (bn);

static struct filestate *thisfile;
static int ch_ungotchar = -1;
static int maxbufs = -1;

extern int autobuf;
extern int sigs;
extern int secure;
extern int screen_trashed;
extern int follow_mode;
extern int waiting_for_data;
extern constant char helpdata[];
extern constant int size_helpdata;
extern IFILE curr_ifile;
#if LOGFILE
extern int logfile;
extern char *namelogfile;
#endif

static int ch_addbuf();


/*
 * Get the character pointed to by the read pointer.
 */
static int ch_get(void)
{
	struct buf *bp;
	struct bufnode *bn;
	int n;
	int read_again;
	int h;
	POSITION pos;
	POSITION len;

	if (thisfile == NULL)
		return (EOI);

	/*
	 * Quick check for the common case where 
	 * the desired char is in the head buffer.
	 */
	if (ch_bufhead != END_OF_CHAIN)
	{
		bp = bufnode_buf(ch_bufhead);
		if (ch_block == bp->block && ch_offset < bp->datasize)
			return bp->data[ch_offset];
	}

	/*
	 * Look for a buffer holding the desired block.
	 */
	waiting_for_data = FALSE;
	h = BUFHASH(ch_block);
	FOR_BUFS_IN_CHAIN(h, bn)
	{
		bp = bufnode_buf(bn);
		if (bp->block == ch_block)
		{
			if (ch_offset >= bp->datasize)
				/*
				 * Need more data in this buffer.
				 */
				break;
			goto found;
		}
	}
	if (bn == END_OF_HCHAIN(h))
	{
		/*
		 * Block is not in a buffer.  
		 * Take the least recently used buffer 
		 * and read the desired block into it.
		 * If the LRU buffer has data in it, 
		 * then maybe allocate a new buffer.
		 */
		if (ch_buftail == END_OF_CHAIN || 
			bufnode_buf(ch_buftail)->block != -1)
		{
			/*
			 * There is no empty buffer to use.
			 * Allocate a new buffer if:
			 * 1. We can't seek on this file and -b is not in effect; or
			 * 2. We haven't allocated the max buffers for this file yet.
			 */
			if ((autobuf && !(ch_flags & CH_CANSEEK)) ||
				(maxbufs < 0 || ch_nbufs < maxbufs))
				if (ch_addbuf())
					/*
					 * Allocation failed: turn off autobuf.
					 */
					autobuf = OPT_OFF;
		}
		bn = ch_buftail;
		bp = bufnode_buf(bn);
		BUF_HASH_RM(bn); /* Remove from old hash chain. */
		bp->block = ch_block;
		bp->datasize = 0;
		BUF_HASH_INS(bn, h); /* Insert into new hash chain. */
	}

	for (;;)
	{
		pos = (ch_block * LBUFSIZE) + bp->datasize;
		if ((len = ch_length()) != NULL_POSITION && pos >= len)
			/*
			 * At end of file.
			 */
			return (EOI);

		if (pos != ch_fpos)
		{
			/*
			 * Not at the correct position: must seek.
			 * If input is a pipe, we're in trouble (can't seek on a pipe).
			 * Some data has been lost: just return "?".
			 */
			if (!(ch_flags & CH_CANSEEK))
				return ('?');
			if (lseek(ch_file, (off_t)pos, SEEK_SET) == BAD_LSEEK)
			{
				error("seek error", NULL_PARG);
				clear_eol();
				return (EOI);
			}
			ch_fpos = pos;
		}

		/*
		 * Read the block.
		 * If we read less than a full block, that's ok.
		 * We use partial block and pick up the rest next time.
		 */
		if (ch_ungotchar != -1)
		{
			bp->data[bp->datasize] = ch_ungotchar;
			n = 1;
			ch_ungotchar = -1;
		} else if (ch_flags & CH_HELPFILE)
		{
			bp->data[bp->datasize] = helpdata[ch_fpos];
			n = 1;
		} else
		{
			n = iread(ch_file, &bp->data[bp->datasize], 
				(unsigned int)(LBUFSIZE - bp->datasize));
		}

		read_again = FALSE;
		if (n == READ_INTR)
		{
			ch_fsize = pos;
			return (EOI);
		}
		if (n == READ_AGAIN)
		{
			read_again = TRUE;
			n = 0;
		}
		if (n < 0)
		{
#if MSDOS_COMPILER==WIN32C
			if (errno != EPIPE)
#endif
			{
				error("read error", NULL_PARG);
				clear_eol();
			}
			n = 0;
		}

#if LOGFILE
		/*
		 * If we have a log file, write the new data to it.
		 */
		if (!secure && logfile >= 0 && n > 0)
			write(logfile, (char *) &bp->data[bp->datasize], n);
#endif

		ch_fpos += n;
		bp->datasize += n;

		if (n == 0)
		{
			/* Either end of file or no data available.
			 * read_again indicates the latter. */
			if (!read_again)
				ch_fsize = pos;
			if (ignore_eoi || read_again)
			{
				/* Wait a while, then try again. */
				if (!waiting_for_data)
				{
					PARG parg;
					parg.p_string = wait_message();
					ixerror("%s", &parg);
					waiting_for_data = TRUE;
				}
				sleep_ms(50); /* Reduce system load */
			}
			if (ignore_eoi && follow_mode == FOLLOW_NAME && curr_ifile_changed())
			{
				/* screen_trashed=2 causes make_display to reopen the file. */
				screen_trashed = 2;
				return (EOI);
			}
			if (sigs)
				return (EOI);
		}

		found:
		if (ch_bufhead != bn)
		{
			/*
			 * Move the buffer to the head of the buffer chain.
			 * This orders the buffer chain, most- to least-recently used.
			 */
			BUF_RM(bn);
			BUF_INS_HEAD(bn);

			/*
			 * Move to head of hash chain too.
			 */
			BUF_HASH_RM(bn);
			BUF_HASH_INS(bn, h);
		}

		if (ch_offset < bp->datasize)
			break;
		/*
		 * After all that, we still don't have enough data.
		 * Go back and try again.
		 */
	}
	return (bp->data[ch_offset]);
}

/*
 * ch_ungetchar is a rather kludgy and limited way to push 
 * a single char onto an input file descriptor.
 */
public void ch_ungetchar(int c)
{
	if (c != -1 && ch_ungotchar != -1)
		error("ch_ungetchar overrun", NULL_PARG);
	ch_ungotchar = c;
}

#if LOGFILE
/*
 * Close the logfile.
 * If we haven't read all of standard input into it, do that now.
 */
public void end_logfile(void)
{
	static int tried = FALSE;

	if (logfile < 0)
		return;
	if (!tried && ch_fsize == NULL_POSITION)
	{
		tried = TRUE;
		ierror("Finishing logfile", NULL_PARG);
		while (ch_forw_get() != EOI)
			if (ABORT_SIGS())
				break;
	}
	close(logfile);
	logfile = -1;
	free(namelogfile);
	namelogfile = NULL;
}

/*
 * Start a log file AFTER less has already been running.
 * Invoked from the - command; see toggle_option().
 * Write all the existing buffered data to the log file.
 */
public void sync_logfile(void)
{
	struct buf *bp;
	struct bufnode *bn;
	int warned = FALSE;
	BLOCKNUM block;
	BLOCKNUM nblocks;

	nblocks = (ch_fpos + LBUFSIZE - 1) / LBUFSIZE;
	for (block = 0;  block < nblocks;  block++)
	{
		int wrote = FALSE;
		FOR_BUFS(bn)
		{
			bp = bufnode_buf(bn);
			if (bp->block == block)
			{
				write(logfile, (char *) bp->data, bp->datasize);
				wrote = TRUE;
				break;
			}
		}
		if (!wrote && !warned)
		{
			error("Warning: log file is incomplete",
				NULL_PARG);
			warned = TRUE;
		}
	}
}

#endif

/*
 * Determine if a specific block is currently in one of the buffers.
 */
static int buffered(BLOCKNUM block)
{
	struct buf *bp;
	struct bufnode *bn;
	int h;

	h = BUFHASH(block);
	FOR_BUFS_IN_CHAIN(h, bn)
	{
		bp = bufnode_buf(bn);
		if (bp->block == block)
			return (TRUE);
	}
	return (FALSE);
}

/*
 * Seek to a specified position in the file.
 * Return 0 if successful, non-zero if can't seek there.
 */
public int ch_seek(POSITION pos)
{
	BLOCKNUM new_block;
	POSITION len;

	if (thisfile == NULL)
		return (0);

	len = ch_length();
	if (pos < ch_zero() || (len != NULL_POSITION && pos > len))
		return (1);

	new_block = pos / LBUFSIZE;
	if (!(ch_flags & CH_CANSEEK) && pos != ch_fpos && !buffered(new_block))
	{
		if (ch_fpos > pos)
			return (1);
		while (ch_fpos < pos)
		{
			if (ch_forw_get() == EOI)
				return (1);
			if (ABORT_SIGS())
				return (1);
		}
		return (0);
	}
	/*
	 * Set read pointer.
	 */
	ch_block = new_block;
	ch_offset = pos % LBUFSIZE;
	return (0);
}

/*
 * Seek to the end of the file.
 */
public int ch_end_seek(void)
{
	POSITION len;

	if (thisfile == NULL)
		return (0);

	if (ch_flags & CH_CANSEEK)
		ch_fsize = filesize(ch_file);

	len = ch_length();
	if (len != NULL_POSITION)
		return (ch_seek(len));

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

/*
 * Seek to the last position in the file that is currently buffered.
 */
public int ch_end_buffer_seek(void)
{
	struct buf *bp;
	struct bufnode *bn;
	POSITION buf_pos;
	POSITION end_pos;

	if (thisfile == NULL || (ch_flags & CH_CANSEEK))
		return (ch_end_seek());

	end_pos = 0;
	FOR_BUFS(bn)
	{
		bp = bufnode_buf(bn);
		buf_pos = (bp->block * LBUFSIZE) + bp->datasize;
		if (buf_pos > end_pos)
			end_pos = buf_pos;
	}

	return (ch_seek(end_pos));
}

/*
 * 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.
 */
public int ch_beg_seek(void)
{
	struct bufnode *bn;
	struct bufnode *firstbn;

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

	/*
	 * Can't get to position 0.
	 * Look thru the buffers for the one closest to position 0.
	 */
	firstbn = ch_bufhead;
	if (firstbn == END_OF_CHAIN)
		return (1);
	FOR_BUFS(bn)
	{
		if (bufnode_buf(bn)->block < bufnode_buf(firstbn)->block)
			firstbn = bn;
	}
	ch_block = bufnode_buf(firstbn)->block;
	ch_offset = 0;
	return (0);
}

/*
 * Return the length of the file, if known.
 */
public POSITION ch_length(void)
{
	if (thisfile == NULL)
		return (NULL_POSITION);
	if (ignore_eoi)
		return (NULL_POSITION);
	if (ch_flags & CH_HELPFILE)
		return (size_helpdata);
	if (ch_flags & CH_NODATA)
		return (0);
	return (ch_fsize);
}

/*
 * Return the current position in the file.
 */
public POSITION ch_tell(void)
{
	if (thisfile == NULL)
		return (NULL_POSITION);
	return (ch_block * LBUFSIZE) + ch_offset;
}

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

	if (thisfile == NULL)
		return (EOI);
	c = ch_get();
	if (c == EOI)
		return (EOI);
	if (ch_offset < LBUFSIZE-1)
		ch_offset++;
	else
	{
		ch_block ++;
		ch_offset = 0;
	}
	return (c);
}

/*
 * Pre-decrement the read pointer and get the new current char.
 */
public int ch_back_get(void)
{
	if (thisfile == NULL)
		return (EOI);
	if (ch_offset > 0)
		ch_offset --;
	else
	{
		if (ch_block <= 0)
			return (EOI);
		if (!(ch_flags & CH_CANSEEK) && !buffered(ch_block-1))
			return (EOI);
		ch_block--;
		ch_offset = LBUFSIZE-1;
	}
	return (ch_get());
}

/*
 * Set max amount of buffer space.
 * bufspace is in units of 1024 bytes.  -1 mean no limit.
 */
public void ch_setbufspace(int bufspace)
{
	if (bufspace < 0)
		maxbufs = -1;
	else
	{
		int lbufk = LBUFSIZE / 1024;
		maxbufs = bufspace / lbufk + (bufspace % lbufk != 0);
		if (maxbufs < 1)
			maxbufs = 1;
	}
}

/*
 * Flush (discard) any saved file state, including buffer contents.
 */
public void ch_flush(void)
{
	struct bufnode *bn;

	if (thisfile == NULL)
		return;

	if (!(ch_flags & CH_CANSEEK))
	{
		/*
		 * If input is a pipe, we don't flush buffer contents,
		 * since the contents can't be recovered.
		 */
		ch_fsize = NULL_POSITION;
		return;
	}

	/*
	 * Initialize all the buffers.
	 */
	FOR_BUFS(bn)
	{
		bufnode_buf(bn)->block = -1;
	}

	/*
	 * Figure out the size of the file, if we can.
	 */
	ch_fsize = filesize(ch_file);

	/*
	 * Seek to a known position: the beginning of the file.
	 */
	ch_fpos = 0;
	ch_block = 0; /* ch_fpos / LBUFSIZE; */
	ch_offset = 0; /* ch_fpos % LBUFSIZE; */

#if HAVE_PROCFS
	/*
	 * This is a kludge to workaround a Linux kernel bug: files in
	 * /proc have a size of 0 according to fstat() but have readable 
	 * data.  They are sometimes, but not always, seekable.
	 * Force them to be non-seekable here.
	 */
	if (ch_fsize == 0)
	{
		struct statfs st;
		if (fstatfs(ch_file, &st) == 0)
		{
			if (st.f_type == PROC_SUPER_MAGIC)
			{
				ch_fsize = NULL_POSITION;
				ch_flags &= ~CH_CANSEEK;
			}
		}
	}
#endif

	if (lseek(ch_file, (off_t)0, SEEK_SET) == BAD_LSEEK)
	{
		/*
		 * Warning only; even if the seek fails for some reason,
		 * there's a good chance we're at the beginning anyway.
		 * {{ I think this is bogus reasoning. }}
		 */
		error("seek error to 0", NULL_PARG);
	}
}

/*
 * Allocate a new buffer.
 * The buffer is added to the tail of the buffer chain.
 */
static int ch_addbuf(void)
{
	struct buf *bp;
	struct bufnode *bn;

	/*
	 * Allocate and initialize a new buffer and link it 
	 * onto the tail of the buffer list.
	 */
	bp = (struct buf *) calloc(1, sizeof(struct buf));
	if (bp == NULL)
		return (1);
	ch_nbufs++;
	bp->block = -1;
	bn = &bp->node;

	BUF_INS_TAIL(bn);
	BUF_HASH_INS(bn, 0);
	return (0);
}

/*
 *
 */
static void init_hashtbl(void)
{
	int h;

	for (h = 0;  h < BUFHASH_SIZE;  h++)
	{
		thisfile->hashtbl[h].hnext = END_OF_HCHAIN(h);
		thisfile->hashtbl[h].hprev = END_OF_HCHAIN(h);
	}
}

/*
 * Delete all buffers for this file.
 */
static void ch_delbufs(void)
{
	struct bufnode *bn;

	while (ch_bufhead != END_OF_CHAIN)
	{
		bn = ch_bufhead;
		BUF_RM(bn);
		free(bufnode_buf(bn));
	}
	ch_nbufs = 0;
	init_hashtbl();
}

/*
 * Is it possible to seek on a file descriptor?
 */
public int seekable(int f)
{
#if MSDOS_COMPILER
	extern int fd0;
	if (f == fd0 && !isatty(fd0))
	{
		/*
		 * In MS-DOS, pipes are seekable.  Check for
		 * standard input, and pretend it is not seekable.
		 */
		return (0);
	}
#endif
	return (lseek(f, (off_t)1, SEEK_SET) != BAD_LSEEK);
}

/*
 * Force EOF to be at the current read position.
 * This is used after an ignore_eof read, during which the EOF may change.
 */
public void ch_set_eof(void)
{
	if (ch_fsize != NULL_POSITION && ch_fsize < ch_fpos)
		ch_fsize = ch_fpos;
}


/*
 * Initialize file state for a new file.
 */
public void ch_init(int f, int flags)
{
	/*
	 * See if we already have a filestate for this file.
	 */
	thisfile = (struct filestate *) get_filestate(curr_ifile);
	if (thisfile == NULL)
	{
		/*
		 * Allocate and initialize a new filestate.
		 */
		thisfile = (struct filestate *) 
				ecalloc(1, sizeof(struct filestate));
		thisfile->buflist.next = thisfile->buflist.prev = END_OF_CHAIN;
		thisfile->nbufs = 0;
		thisfile->flags = flags;
		thisfile->fpos = 0;
		thisfile->block = 0;
		thisfile->offset = 0;
		thisfile->file = -1;
		thisfile->fsize = NULL_POSITION;
		init_hashtbl();
		/*
		 * Try to seek; set CH_CANSEEK if it works.
		 */
		if ((flags & CH_CANSEEK) && !seekable(f))
			ch_flags &= ~CH_CANSEEK;
		set_filestate(curr_ifile, (void *) thisfile);
	}
	if (thisfile->file == -1)
		thisfile->file = f;
	ch_flush();
}

/*
 * Close a filestate.
 */
public void ch_close(void)
{
	int keepstate = FALSE;

	if (thisfile == NULL)
		return;

	if ((ch_flags & (CH_CANSEEK|CH_POPENED|CH_HELPFILE)) && !(ch_flags & CH_KEEPOPEN))
	{
		/*
		 * We can seek or re-open, so we don't need to keep buffers.
		 */
		ch_delbufs();
	} else
		keepstate = TRUE;
	if (!(ch_flags & CH_KEEPOPEN))
	{
		/*
		 * We don't need to keep the file descriptor open
		 * (because we can re-open it.)
		 * But don't really close it if it was opened via popen(),
		 * because pclose() wants to close it.
		 */
		if (!(ch_flags & (CH_POPENED|CH_HELPFILE)))
			close(ch_file);
		ch_file = -1;
	} else
		keepstate = TRUE;
	if (!keepstate)
	{
		/*
		 * We don't even need to keep the filestate structure.
		 */
		free(thisfile);
		thisfile = NULL;
		set_filestate(curr_ifile, (void *) NULL);
	}
}

/*
 * Return ch_flags for the current file.
 */
public int ch_getflags(void)
{
	if (thisfile == NULL)
		return (0);
	return (ch_flags);
}

#if 0
static void ch_dump(struct filestate *fs)
{
	struct buf *bp;
	struct bufnode *bn;
	unsigned char *s;

	if (fs == NULL)
	{
		printf(" --no filestate\n");
		return;
	}
	printf(" file %d, flags %x, fpos %x, fsize %x, blk/off %x/%x\n",
		fs->file, fs->flags, fs->fpos, 
		fs->fsize, fs->block, fs->offset);
	printf(" %d bufs:\n", fs->nbufs);
	for (bn = fs->next; bn != &fs->buflist;  bn = bn->next)
	{
		bp = bufnode_buf(bn);
		printf("%x: blk %x, size %x \"",
			bp, bp->block, bp->datasize);
		for (s = bp->data;  s < bp->data + 30;  s++)
			if (*s >= ' ' && *s < 0x7F)
				printf("%c", *s);
			else
				printf(".");
		printf("\"\n");
	}
}
#endif