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you keep saying LIST_END. I do not think it means what you think it
means.

If we hit an OOM condition, mg started to try and dump the older undo buffer
entries in order to be able to continue. OTOH, it was grabbing this entry with
LIST_END, which like all *_END() list macros evaluates to NULL.

Do what we actually want and switch that list to a TAILQ and use
TAILQ_LAST to grab the last entry.

Wrote this a loooooong time ago after a mail from Matthew Dempsky on
bugs@. ok kjell@, beck@ also looked at this months ago and thought it
was alright.

/* $OpenBSD: undo.c,v 1.50 2010/06/30 19:12:54 oga Exp $ */
/*
 * This file is in the public domain
 */

#include "def.h"
#include "kbd.h"

#define MAX_FREE_RECORDS	32

/*
 * Local variables
 */
static struct undoq		 undo_free;
static int			 undo_free_num;
static int			 boundary_flag = TRUE;
static int			 undo_enable_flag = TRUE;

/*
 * Local functions
 */
static int find_dot(struct line *, int);
static int find_lo(int, struct line **, int *, int *);
static struct undo_rec *new_undo_record(void);
static int drop_oldest_undo_record(void);

/*
 * find_dot, find_lo()
 *
 * Find an absolute dot in the buffer from a line/offset pair, and vice-versa.
 *
 * Since lines can be deleted while they are referenced by undo record, we
 * need to have an absolute dot to have something reliable.
 */
static int
find_dot(struct line *lp, int off)
{
	int	 count = 0;
	struct line	*p;

	for (p = curbp->b_headp; p != lp; p = lforw(p)) {
		if (count != 0) {
			if (p == curbp->b_headp) {
				ewprintf("Error: Undo stuff called with a"
				    "nonexistent line");
				return (FALSE);
			}
		}
		count += llength(p) + 1;
	}
	count += off;

	return (count);
}

static int
find_lo(int pos, struct line **olp, int *offset, int *lnum)
{
	struct line *p;
	int lineno;

	p = curbp->b_headp;
	lineno = 0;
	while (pos > llength(p)) {
		pos -= llength(p) + 1;
		if ((p = lforw(p)) == curbp->b_headp) {
			*olp = NULL;
			*offset = 0;
			return (FALSE);
		}
		lineno++;
	}
	*olp = p;
	*offset = pos;
	*lnum = lineno;

	return (TRUE);
}

static struct undo_rec *
new_undo_record(void)
{
	struct undo_rec *rec;

	rec = TAILQ_FIRST(&undo_free);
	if (rec != NULL) {
		/* Remove it from the free-list */
		TAILQ_REMOVE(&undo_free, rec, next);
		undo_free_num--;
	} else {
		if ((rec = malloc(sizeof(*rec))) == NULL)
			panic("Out of memory in undo code (record)");
	}
	memset(rec, 0, sizeof(struct undo_rec));

	return (rec);
}

void
free_undo_record(struct undo_rec *rec)
{
	static int initialised = 0;

	/*
	 * On the first run, do initialisation of the free list.
	 */
	if (initialised == 0) {
		TAILQ_INIT(&undo_free);
		initialised = 1;
	}
	if (rec->content != NULL) {
		free(rec->content);
		rec->content = NULL;
	}
	if (undo_free_num >= MAX_FREE_RECORDS) {
		free(rec);
		return;
	}
	undo_free_num++;

	TAILQ_INSERT_HEAD(&undo_free, rec, next);
}

/*
 * Drop the oldest undo record in our list. Return 1 if we could remove it,
 * 0 if the undo list was empty.
 */
static int
drop_oldest_undo_record(void)
{
	struct undo_rec *rec;

	rec = TAILQ_LAST(&curbp->b_undo, undoq);
	if (rec != NULL) {
		undo_free_num--;
		TAILQ_REMOVE(&curbp->b_undo, rec, next);
		free_undo_record(rec);
		return (1);
	}
	return (0);
}

static int
lastrectype(void)
{
	struct undo_rec *rec;

	if ((rec = TAILQ_FIRST(&curbp->b_undo)) != NULL)
		return (rec->type);
	return (0);
}

/*
 * Returns TRUE if undo is enabled, FALSE otherwise.
 */
int
undo_enabled(void)
{
	return (undo_enable_flag);
}

/*
 * undo_enable: toggle undo_enable.
 * Returns the previous value of the flag.
 */
int
undo_enable(int f, int n)
{
	int pon = undo_enable_flag;

	if (f & (FFARG | FFRAND))
		undo_enable_flag = n > 0;
	else
		undo_enable_flag = !undo_enable_flag;

	if (!(f & FFRAND))
		ewprintf("Undo %sabled", undo_enable_flag ? "en" : "dis");

	return (pon);
}

/*
 * If undo is enabled, then:
 *  Toggle undo boundary recording.
 *  If called with an argument, (n > 0) => enable. Otherwise disable.
 * In either case, add an undo boundary
 * If undo is disabled, this function has no effect.
 */
int
undo_boundary_enable(int f, int n)
{
	int bon = boundary_flag;

	if (!undo_enable_flag)
		return (FALSE);

	undo_add_boundary(FFRAND, 1);

	if (f & (FFARG | FFRAND))
		boundary_flag = n > 0;
	else
		boundary_flag = !boundary_flag;

	if (!(f & FFRAND))
		ewprintf("Undo boundaries %sabled",
		    boundary_flag ? "en" : "dis");

	return (bon);
}

/*
 * Record an undo boundary, unless boundary_flag == FALSE.
 * Does nothing if previous undo entry is already a boundary or 'modified' flag.
 */
int
undo_add_boundary(int f, int n)
{
	struct undo_rec *rec;
	int last;

	if (boundary_flag == FALSE)
		return (FALSE);

	last = lastrectype();
	if (last == BOUNDARY || last == MODIFIED)
		return (TRUE);

	rec = new_undo_record();
	rec->type = BOUNDARY;

	TAILQ_INSERT_HEAD(&curbp->b_undo, rec, next);

	return (TRUE);
}

/*
 * Record an undo "modified" boundary
 */
void
undo_add_modified(void)
{
	struct undo_rec *rec;

	rec = new_undo_record();
	rec->type = MODIFIED;

	TAILQ_INSERT_HEAD(&curbp->b_undo, rec, next);

	return;
}

int
undo_add_insert(struct line *lp, int offset, int size)
{
	struct region	reg;
	struct	undo_rec *rec;
	int	pos;

	if (!undo_enable_flag)
		return (TRUE);
	reg.r_linep = lp;
	reg.r_offset = offset;
	reg.r_size = size;

	pos = find_dot(lp, offset);

	/*
	 * We try to reuse the last undo record to `compress' things.
	 */
	rec = TAILQ_FIRST(&curbp->b_undo);
	if (rec != NULL && rec->type == INSERT) {
		if (rec->pos + rec->region.r_size == pos) {
			rec->region.r_size += reg.r_size;
			return (TRUE);
		}
	}

	/*
	 * We couldn't reuse the last undo record, so prepare a new one.
	 */
	rec = new_undo_record();
	rec->pos = pos;
	rec->type = INSERT;
	memmove(&rec->region, &reg, sizeof(struct region));
	rec->content = NULL;

	undo_add_boundary(FFRAND, 1);

	TAILQ_INSERT_HEAD(&curbp->b_undo, rec, next);

	return (TRUE);
}

/*
 * This of course must be done _before_ the actual deletion is done.
 */
int
undo_add_delete(struct line *lp, int offset, int size, int isreg)
{
	struct region	reg;
	struct	undo_rec *rec;
	int	pos;

	if (!undo_enable_flag)
		return (TRUE);

	reg.r_linep = lp;
	reg.r_offset = offset;
	reg.r_size = size;

	pos = find_dot(lp, offset);

	if (offset == llength(lp))	/* if it's a newline... */
		undo_add_boundary(FFRAND, 1);
	else if ((rec = TAILQ_FIRST(&curbp->b_undo)) != NULL) {
		/*
		 * Separate this command from the previous one if we're not
		 * just before the previous record...
		 */
		if (!isreg && rec->type == DELETE) {
			if (rec->pos - rec->region.r_size != pos)
				undo_add_boundary(FFRAND, 1);
		}
	}
	rec = new_undo_record();
	rec->pos = pos;
	if (isreg)
		rec->type = DELREG;
	else
		rec->type = DELETE;
	memmove(&rec->region, &reg, sizeof(struct region));
	do {
		rec->content = malloc(reg.r_size + 1);
	} while ((rec->content == NULL) && drop_oldest_undo_record());

	if (rec->content == NULL)
		panic("Out of memory");

	region_get_data(&reg, rec->content, reg.r_size);

	if (isreg || lastrectype() != DELETE)
		undo_add_boundary(FFRAND, 1);

	TAILQ_INSERT_HEAD(&curbp->b_undo, rec, next);

	return (TRUE);
}

/*
 * This of course must be called before the change takes place.
 */
int
undo_add_change(struct line *lp, int offset, int size)
{
	if (!undo_enable_flag)
		return (TRUE);
	undo_add_boundary(FFRAND, 1);
	boundary_flag = FALSE;
	undo_add_delete(lp, offset, size, 0);
	undo_add_insert(lp, offset, size);
	boundary_flag = TRUE;
	undo_add_boundary(FFRAND, 1);

	return (TRUE);
}

/*
 * Show the undo records for the current buffer in a new buffer.
 */
/* ARGSUSED */
int
undo_dump(int f, int n)
{
	struct	 undo_rec *rec;
	struct buffer	*bp;
	struct mgwin	*wp;
	char	 buf[4096], tmp[1024];
	int	 num;

	/*
	 * Prepare the buffer for insertion.
	 */
	if ((bp = bfind("*undo*", TRUE)) == NULL)
		return (FALSE);
	bp->b_flag |= BFREADONLY;
	bclear(bp);
	popbuf(bp, WNONE);

	for (wp = wheadp; wp != NULL; wp = wp->w_wndp) {
		if (wp->w_bufp == bp) {
			wp->w_dotp = bp->b_headp;
			wp->w_doto = 0;
		}
	}

	num = 0;
	TAILQ_FOREACH(rec, &curbp->b_undo, next) {
		num++;
		snprintf(buf, sizeof(buf),
		    "%d:\t %s at %d ", num,
		    (rec->type == DELETE) ? "DELETE":
		    (rec->type == DELREG) ? "DELREGION":
		    (rec->type == INSERT) ? "INSERT":
		    (rec->type == BOUNDARY) ? "----" :
		    (rec->type == MODIFIED) ? "MODIFIED": "UNKNOWN",
		    rec->pos);

		if (rec->content) {
			(void)strlcat(buf, "\"", sizeof(buf));
			snprintf(tmp, sizeof(tmp), "%.*s", rec->region.r_size,
			    rec->content);
			(void)strlcat(buf, tmp, sizeof(buf));
			(void)strlcat(buf, "\"", sizeof(buf));
		}
		snprintf(tmp, sizeof(tmp), " [%d]", rec->region.r_size);
		if (strlcat(buf, tmp, sizeof(buf)) >= sizeof(buf)) {
			ewprintf("Undo record too large. Aborted.");
			return (FALSE);
		}
		addlinef(bp, "%s", buf);
	}
	return (TRUE);
}

/*
 * After the user did action1, then action2, then action3:
 *
 *	[action3] <--- Undoptr
 *	[action2]
 *	[action1]
 *	 ------
 *	 [undo]
 *
 * After undo:
 *
 *	[undo of action3]
 *	[action2] <--- Undoptr
 *	[action1]
 *	 ------
 *	 [undo]
 *
 * After another undo:
 *
 *
 *	[undo of action2]
 *	[undo of action3]
 *	[action1]  <--- Undoptr
 *	 ------
 *	 [undo]
 *
 * Note that the "undo of actionX" have no special meaning. Only when
 * we undo a deletion, the insertion will be recorded just as if it
 * was typed on the keyboard. Resulting in the inverse operation being
 * saved in the list.
 *
 * If undoptr reaches the bottom of the list, or if we moved between
 * two undo actions, we make it point back at the topmost record. This is
 * how we handle redoing.
 */
/* ARGSUSED */
int
undo(int f, int n)
{
	struct undo_rec	*ptr, *nptr;
	int		 done, rval;
	struct line	*lp;
	int		 offset, save, dot;
	static int	 nulled = FALSE;
	int		 lineno;

	if (n < 0)
		return (FALSE);

	dot = find_dot(curwp->w_dotp, curwp->w_doto);

	ptr = curbp->b_undoptr;

	/* first invocation, make ptr point back to the top of the list */
	if ((ptr == NULL && nulled == TRUE) ||  rptcount == 0) {
		ptr = TAILQ_FIRST(&curbp->b_undo);
		nulled = TRUE;
	}

	rval = TRUE;
	while (n--) {
		/* if we have a spurious boundary, free it and move on.... */
		while (ptr && ptr->type == BOUNDARY) {
			nptr = TAILQ_NEXT(ptr, next);
			TAILQ_REMOVE(&curbp->b_undo, ptr, next);
			free_undo_record(ptr);
			ptr = nptr;
		}
		/*
		 * Ptr is NULL, but on the next run, it will point to the
		 * top again, redoing all stuff done in the buffer since
		 * its creation.
		 */
		if (ptr == NULL) {
			ewprintf("No further undo information");
			rval = FALSE;
			nulled = TRUE;
			break;
		}
		nulled = FALSE;

		/*
		 * Loop while we don't get a boundary specifying we've
		 * finished the current action...
		 */

		undo_add_boundary(FFRAND, 1);

		save = boundary_flag;
		boundary_flag = FALSE;

		done = 0;
		do {
			/*
			 * Move to where this has to apply
			 *
			 * Boundaries (and the modified flag)  are put as
			 * position 0 (to save lookup time in find_dot)
			 * so we must not move there...
			 */
			if (ptr->type != BOUNDARY && ptr->type != MODIFIED) {
				if (find_lo(ptr->pos, &lp,
				    &offset, &lineno) == FALSE) {
					ewprintf("Internal error in Undo!");
					rval = FALSE;
					break;
				}
				curwp->w_dotp = lp;
				curwp->w_doto = offset;
				curwp->w_markline = curwp->w_dotline;
				curwp->w_dotline = lineno;
			}

			/*
			 * Do operation^-1
			 */
			switch (ptr->type) {
			case INSERT:
				ldelete(ptr->region.r_size, KNONE);
				break;
			case DELETE:
				lp = curwp->w_dotp;
				offset = curwp->w_doto;
				region_put_data(ptr->content,
				    ptr->region.r_size);
				curwp->w_dotp = lp;
				curwp->w_doto = offset;
				break;
			case DELREG:
				region_put_data(ptr->content,
				    ptr->region.r_size);
				break;
			case BOUNDARY:
				done = 1;
				break;
			case MODIFIED:
				curbp->b_flag &= ~BFCHG;
				break;
			default:
				break;
			}

			/* And move to next record */
			ptr = TAILQ_NEXT(ptr, next);
		} while (ptr != NULL && !done);

		boundary_flag = save;
		undo_add_boundary(FFRAND, 1);

		ewprintf("Undo!");
	}
	/*
	 * Record where we are. (we have to save our new position at the end
	 * since we change the dot when undoing....)
	 */
	curbp->b_undoptr = ptr;

	curbp->b_undopos = find_dot(curwp->w_dotp, curwp->w_doto);

	return (rval);
}