/*
 * Copyright (C) 1984-2012  Mark Nudelman
 * Modified for use with illumos by Garrett D'Amore.
 * Copyright 2014 Garrett D'Amore <garrett@damore.org>
 *
 * 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.
 */

/*
 * Code to handle displaying line numbers.
 *
 * Finding the line number of a given file position is rather tricky.
 * We don't want to just start at the beginning of the file and
 * count newlines, because that is slow for large files (and also
 * wouldn't work if we couldn't get to the start of the file; e.g.
 * if input is a long pipe).
 *
 * So we use the function add_lnum to cache line numbers.
 * We try to be very clever and keep only the more interesting
 * line numbers when we run out of space in our table.  A line
 * number is more interesting than another when it is far from
 * other line numbers.   For example, we'd rather keep lines
 * 100,200,300 than 100,101,300.  200 is more interesting than
 * 101 because 101 can be derived very cheaply from 100, while
 * 200 is more expensive to derive from 100.
 *
 * The function currline() returns the line number of a given
 * position in the file.  As a side effect, it calls add_lnum
 * to cache the line number.  Therefore currline is occasionally
 * called to make sure we cache line numbers often enough.
 */

#include <sys/time.h>

#include <time.h>

#include "less.h"

/*
 * Structure to keep track of a line number and the associated file position.
 * A doubly-linked circular list of line numbers is kept ordered by line number.
 */
struct linenum_info {
	struct linenum_info *next;	/* Link to next in the list */
	struct linenum_info *prev;	/* Line to previous in the list */
	off_t pos;			/* File position */
	off_t gap;			/* Gap between prev and next */
	off_t line;			/* Line number */
};
/*
 * "gap" needs some explanation: the gap of any particular line number
 * is the distance between the previous one and the next one in the list.
 * ("Distance" means difference in file position.)  In other words, the
 * gap of a line number is the gap which would be introduced if this
 * line number were deleted.  It is used to decide which one to replace
 * when we have a new one to insert and the table is full.
 */

#define	NPOOL	200			/* Size of line number pool */

#define	LONGTIME	(2)		/* In seconds */

static struct linenum_info anchor;	/* Anchor of the list */
static struct linenum_info *freelist;	/* Anchor of the unused entries */
static struct linenum_info pool[NPOOL];	/* The pool itself */
static struct linenum_info *spare;	/* We always keep one spare entry */

extern int linenums;
extern int sc_height;
extern int screen_trashed;

/*
 * Initialize the line number structures.
 */
void
clr_linenum(void)
{
	struct linenum_info *p;

	/*
	 * Put all the entries on the free list.
	 * Leave one for the "spare".
	 */
	for (p = pool; p < &pool[NPOOL-2]; p++)
		p->next = p+1;
	pool[NPOOL-2].next = NULL;
	freelist = pool;

	spare = &pool[NPOOL-1];

	/*
	 * Initialize the anchor.
	 */
	anchor.next = anchor.prev = &anchor;
	anchor.gap = 0;
	anchor.pos = 0;
	anchor.line = 1;
}

/*
 * Calculate the gap for an entry.
 */
static void
calcgap(struct linenum_info *p)
{
	/*
	 * Don't bother to compute a gap for the anchor.
	 * Also don't compute a gap for the last one in the list.
	 * The gap for that last one should be considered infinite,
	 * but we never look at it anyway.
	 */
	if (p == &anchor || p->next == &anchor)
		return;
	p->gap = p->next->pos - p->prev->pos;
}

/*
 * Add a new line number to the cache.
 * The specified position (pos) should be the file position of the
 * FIRST character in the specified line.
 */
void
add_lnum(off_t linenum, off_t pos)
{
	struct linenum_info *p;
	struct linenum_info *new;
	struct linenum_info *nextp;
	struct linenum_info *prevp;
	off_t mingap;

	/*
	 * Find the proper place in the list for the new one.
	 * The entries are sorted by position.
	 */
	for (p = anchor.next; p != &anchor && p->pos < pos; p = p->next)
		if (p->line == linenum)
			/* We already have this one. */
			return;
	nextp = p;
	prevp = p->prev;

	if (freelist != NULL) {
		/*
		 * We still have free (unused) entries.
		 * Use one of them.
		 */
		new = freelist;
		freelist = freelist->next;
	} else {
		/*
		 * No free entries.
		 * Use the "spare" entry.
		 */
		new = spare;
		spare = NULL;
	}

	/*
	 * Fill in the fields of the new entry,
	 * and insert it into the proper place in the list.
	 */
	new->next = nextp;
	new->prev = prevp;
	new->pos = pos;
	new->line = linenum;

	nextp->prev = new;
	prevp->next = new;

	/*
	 * Recalculate gaps for the new entry and the neighboring entries.
	 */
	calcgap(new);
	calcgap(nextp);
	calcgap(prevp);

	if (spare == NULL) {
		/*
		 * We have used the spare entry.
		 * Scan the list to find the one with the smallest
		 * gap, take it out and make it the spare.
		 * We should never remove the last one, so stop when
		 * we get to p->next == &anchor.  This also avoids
		 * looking at the gap of the last one, which is
		 * not computed by calcgap.
		 */
		mingap = anchor.next->gap;
		for (p = anchor.next; p->next != &anchor; p = p->next) {
			if (p->gap <= mingap) {
				spare = p;
				mingap = p->gap;
			}
		}
		spare->next->prev = spare->prev;
		spare->prev->next = spare->next;
	}
}

static int loopcount;
static struct timespec timeout;

static void
timeout_set(int seconds)
{
	clock_gettime(CLOCK_MONOTONIC, &timeout);
	timeout.tv_sec += seconds;
}

static int
timeout_elapsed(void)
{
	struct timespec now;

	clock_gettime(CLOCK_MONOTONIC, &now);
	return timespeccmp(&now, &timeout, >=);
}

static void
longish(void)
{
	if (loopcount >= 0 && ++loopcount > 100) {
		loopcount = 0;
		if (timeout_elapsed()) {
			ierror("Calculating line numbers", NULL);
			loopcount = -1;
		}
	}
}

/*
 * Turn off line numbers because the user has interrupted
 * a lengthy line number calculation.
 */
static void
abort_long(void)
{
	if (linenums == OPT_ONPLUS)
		/*
		 * We were displaying line numbers, so need to repaint.
		 */
		screen_trashed = 1;
	linenums = 0;
	error("Line numbers turned off", NULL);
}

/*
 * Find the line number associated with a given position.
 * Return 0 if we can't figure it out.
 */
off_t
find_linenum(off_t pos)
{
	struct linenum_info *p;
	off_t linenum;
	off_t cpos;

	if (!linenums)
		/*
		 * We're not using line numbers.
		 */
		return (0);
	if (pos == -1)
		/*
		 * Caller doesn't know what he's talking about.
		 */
		return (0);
	if (pos <= ch_zero())
		/*
		 * Beginning of file is always line number 1.
		 */
		return (1);

	/*
	 * Find the entry nearest to the position we want.
	 */
	for (p = anchor.next; p != &anchor && p->pos < pos; p = p->next)
		continue;
	if (p->pos == pos)
		/* Found it exactly. */
		return (p->line);

	/*
	 * This is the (possibly) time-consuming part.
	 * We start at the line we just found and start
	 * reading the file forward or backward till we
	 * get to the place we want.
	 *
	 * First decide whether we should go forward from the
	 * previous one or backwards from the next one.
	 * The decision is based on which way involves
	 * traversing fewer bytes in the file.
	 */
	timeout_set(LONGTIME);
	if (p == &anchor || pos - p->prev->pos < p->pos - pos) {
		/*
		 * Go forward.
		 */
		p = p->prev;
		if (ch_seek(p->pos))
			return (0);
		loopcount = 0;
		for (linenum = p->line, cpos = p->pos; cpos < pos; linenum++) {
			/*
			 * Allow a signal to abort this loop.
			 */
			cpos = forw_raw_line(cpos, NULL, NULL);
			if (abort_sigs()) {
				abort_long();
				return (0);
			}
			if (cpos == -1)
				return (0);
			longish();
		}
		/*
		 * We might as well cache it.
		 */
		add_lnum(linenum, cpos);
		/*
		 * If the given position is not at the start of a line,
		 * make sure we return the correct line number.
		 */
		if (cpos > pos)
			linenum--;
	} else {
		/*
		 * Go backward.
		 */
		if (ch_seek(p->pos))
			return (0);
		loopcount = 0;
		for (linenum = p->line, cpos = p->pos; cpos > pos; linenum--) {
			/*
			 * Allow a signal to abort this loop.
			 */
			cpos = back_raw_line(cpos, NULL, NULL);
			if (abort_sigs()) {
				abort_long();
				return (0);
			}
			if (cpos == -1)
				return (0);
			longish();
		}
		/*
		 * We might as well cache it.
		 */
		add_lnum(linenum, cpos);
	}

	return (linenum);
}

/*
 * Find the position of a given line number.
 * Return -1 if we can't figure it out.
 */
off_t
find_pos(off_t linenum)
{
	struct linenum_info *p;
	off_t cpos;
	off_t clinenum;

	if (linenum <= 1)
		/*
		 * Line number 1 is beginning of file.
		 */
		return (ch_zero());

	/*
	 * Find the entry nearest to the line number we want.
	 */
	for (p = anchor.next; p != &anchor && p->line < linenum; p = p->next)
		continue;
	if (p->line == linenum)
		/* Found it exactly. */
		return (p->pos);

	if (p == &anchor || linenum - p->prev->line < p->line - linenum) {
		/*
		 * Go forward.
		 */
		p = p->prev;
		if (ch_seek(p->pos))
			return (-1);
		for (clinenum = p->line, cpos = p->pos;
		    clinenum < linenum;
		    clinenum++) {
			/*
			 * Allow a signal to abort this loop.
			 */
			cpos = forw_raw_line(cpos, NULL, NULL);
			if (abort_sigs())
				return (-1);
			if (cpos == -1)
				return (-1);
		}
	} else {
		/*
		 * Go backward.
		 */
		if (ch_seek(p->pos))
			return (-1);
		for (clinenum = p->line, cpos = p->pos;
		    clinenum > linenum;
		    clinenum--) {
			/*
			 * Allow a signal to abort this loop.
			 */
			cpos = back_raw_line(cpos, (char **)NULL, (int *)NULL);
			if (abort_sigs())
				return (-1);
			if (cpos == -1)
				return (-1);
		}
	}
	/*
	 * We might as well cache it.
	 */
	add_lnum(clinenum, cpos);
	return (cpos);
}

/*
 * Return the line number of the "current" line.
 * The argument "where" tells which line is to be considered
 * the "current" line (e.g. TOP, BOTTOM, MIDDLE, etc).
 */
off_t
currline(int where)
{
	off_t pos;
	off_t len;
	off_t linenum;

	pos = position(where);
	len = ch_length();
	while (pos == -1 && where >= 0 && where < sc_height)
		pos = position(++where);
	if (pos == -1)
		pos = len;
	linenum = find_linenum(pos);
	if (pos == len)
		linenum--;
	return (linenum);
}