Keep a linked list sorted

A general linked list function, llist_reorder(), is introduced that will reorder
a list after a list element has changed.

Some refactoring to avoid code dupliction.

Background

The four linked lists of appointment panel items (appointments, recurring
appointments, events, recurring events) are kept sorted by inserting elements
in order, either when they are first loaded from disk or when new are added.
The ordering is by start time (numerical) and description (alphabetical).

The user is allowed to change start time as well as description. A change is
committed directly to the list item (unlike cut/paste where an item is deleted
and then inserted). This may break the order.

The order property is used when events are loaded from the evenlist into the
day_item vector, see LLIST_FIND_FOREACH_CONT, and when looking for the next
upcoming appointment, see apoint_check_next().

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/llist.c

@@ -108,6 +108,34 @@ llist_item_t *llist_next(llist_item_t * i)
 	return i ? i->next : NULL;
 }
 
+/*
+ * Return the predecessor of a list item or, if head, the list item itself,
+ * or if not in the list, NULL.
+ * The list item may be supplied either directly (i) or as a pointer to
+ * the contents (data); the first case takes precedence.
+ */
+static llist_item_t *llist_prev(llist_t *l, llist_item_t *i, void *data)
+{
+	llist_item_t *j;
+
+	if (!i && !data)
+		return NULL;
+
+	if (l->head == i || l->head->data == data)
+		return l->head;
+
+	if (i) {
+		for (j = l->head; j; j = j->next)
+			if (j->next == i)
+				return j;
+	} else {
+		for (j = l->head; j && j->next; j = j->next)
+			if (j->next->data == data)
+				return j;
+	}
+	return NULL;
+}
+
 /*
  * Return the successor of a list item if it is matched by some filter
  * callback. Return NULL otherwise.
@@ -149,35 +177,95 @@ void llist_add(llist_t * l, void *data)
 	}
 }
 
+/*
+ * Insert an existing item in a sorted list.
+ */
+static void llist_relink(llist_t *l, llist_item_t *i, llist_fn_cmp_t fn_cmp)
+{
+	llist_item_t *j;
+
+	if (!i)
+		return;
+
+	i->next = NULL;
+	if (!l->head) {
+		l->head = l->tail = i;
+	} else if (fn_cmp(i->data, l->tail->data) >= 0) {
+		l->tail->next = i;
+		l->tail = i;
+	} else if (fn_cmp(i->data, l->head->data) < 0) {
+		i->next = l->head;
+		l->head = i;
+	} else {
+		j = l->head;
+		while (j->next && fn_cmp(i->data, j->next->data) >= 0)
+			j = j->next;
+		i->next = j->next;
+		j->next = i;
+	}
+}
+
+/*
+ * Unlink an item from a list and return it.
+ */
+static llist_item_t *llist_unlink(llist_t *l, llist_item_t *i)
+{
+	llist_item_t *p;
+
+	if (!i)
+		return NULL;
+
+	p = llist_prev(l, i, NULL);
+	if (!p)
+		return NULL;
+
+	if (i == l->tail)
+		l->tail = (i == l->head) ? NULL : p;
+	if (i == l->head)
+		l->head = i->next;
+	else
+		p->next = i->next;
+	i->next = NULL;
+	return i;
+}
+
+/*
+ * Find an item matched by some filter callback; start from a specified item.
+ */
+static llist_item_t *llist_find_from(llist_item_t *i, void *data,
+				     llist_fn_match_t fn_match)
+{
+	if (!i)
+		return NULL;
+
+	if (fn_match) {
+		for (; i; i = i->next) {
+			if (fn_match(i->data, data))
+				return i;
+		}
+	} else {
+		for (; i; i = i->next) {
+			if (i->data == data)
+				return i;
+		}
+	}
+
+	return NULL;
+}
+
 /*
  * Add an item to a sorted list.
  */
 void llist_add_sorted(llist_t * l, void *data, llist_fn_cmp_t fn_cmp)
 {
 	llist_item_t *o = mem_malloc(sizeof(llist_item_t));
-	llist_item_t *i;
 
 	if (o) {
 		o->data = data;
 		o->next = NULL;
-
-		if (!l->head) {
-			l->head = l->tail = o;
-		} else if (fn_cmp(o->data, l->tail->data) >= 0) {
-			l->tail->next = o;
-			l->tail = o;
-		} else if (fn_cmp(o->data, l->head->data) < 0) {
-			o->next = l->head;
-			l->head = o;
-		} else {
-			i = l->head;
-			while (i->next
-			       && fn_cmp(o->data, i->next->data) >= 0)
-				i = i->next;
-			o->next = i->next;
-			i->next = o;
-		}
 	}
+
+	llist_relink(l, o, fn_cmp);
 }
 
 /*
@@ -209,21 +297,7 @@ void llist_remove(llist_t * l, llist_item_t * i)
 llist_item_t *llist_find_first(llist_t * l, void *data,
 			       llist_fn_match_t fn_match)
 {
-	llist_item_t *i;
+	return l ? llist_find_from(l->head, data, fn_match) : NULL;
-
-	if (fn_match) {
-		for (i = l->head; i; i = i->next) {
-			if (fn_match(i->data, data))
-				return i;
-		}
-	} else {
-		for (i = l->head; i; i = i->next) {
-			if (i->data == data)
-				return i;
-		}
-	}
-
-	return NULL;
 }
 
 /*
@@ -232,22 +306,7 @@ llist_item_t *llist_find_first(llist_t * l, void *data,
 llist_item_t *llist_find_next(llist_item_t * i, void *data,
 			      llist_fn_match_t fn_match)
 {
-	if (i) {
+	return i ? llist_find_from(i->next, data, fn_match) : NULL;
-		i = i->next;
-		if (fn_match) {
-			for (; i; i = i->next) {
-				if (fn_match(i->data, data))
-					return i;
-			}
-		} else {
-			for (; i; i = i->next) {
-				if (i->data == data)
-					return i;
-			}
-		}
-	}
-
-	return NULL;
 }
 
 /*
@@ -261,17 +320,42 @@ llist_item_t *llist_find_nth(llist_t * l, int n, void *data,
 	if (n < 0)
 		return NULL;
 
-	if (fn_match) {
+	for (i = l->head; i; i = i->next, n--) {
-		for (i = l->head; i; i = i->next) {
+		i = llist_find_from(i, data, fn_match);
-			if (fn_match(i->data, data) && (n-- == 0))
+		if (!i || !n)
-				return i;
+			return i;
-		}
-	} else {
-		for (i = l->head; i; i = i->next) {
-			if ((i->data == data) && (n-- == 0))
-				return i;
-		}
 	}
 
 	return NULL;
 }
+
+/*
+ * Reorder a sorted linked list when an item has changed.
+ */
+void llist_reorder(llist_t *l, void *data, llist_fn_cmp_t fn_cmp)
+{
+	llist_item_t *o, *p;
+
+	if (!(p = llist_prev(l, NULL, data)))
+		return;
+
+	/* List head? */
+	if (p->data == data)
+		o = p;
+	else
+		o = p->next;
+
+	/* Sorted already?
+	 * Note: p is either the previous element or o itself.
+	 */
+	if (o->next &&
+	    fn_cmp(p->data, o->data) <= 0 &&
+	    fn_cmp(o->data, o->next->data) <= 0)
+		return;
+	if (!o->next &&
+	    fn_cmp(p->data, o->data) <= 0)
+		return;
+
+	/* Link manipulation only. */
+	llist_relink(l, llist_unlink(l, o), fn_cmp);
+}

src/llist.h

@@ -99,8 +99,11 @@ void *llist_get_data(llist_item_t *);
 void llist_add(llist_t *, void *);
 void llist_add_sorted(llist_t *, void *, llist_fn_cmp_t);
 void llist_remove(llist_t *, llist_item_t *);
+void llist_reorder(llist_t *, void *, llist_fn_cmp_t);
 
 #define LLIST_ADD(l, data) llist_add(l, data)
 #define LLIST_ADD_SORTED(l, data, fn_cmp)                                     \
   llist_add_sorted(l, data, (llist_fn_cmp_t)fn_cmp)
 #define LLIST_REMOVE(l, i) llist_remove(l, i)
+#define LLIST_REORDER(l, data, fn_cmp)                                        \
+  llist_reorder(l, data, (llist_fn_cmp_t)fn_cmp)

src/llist_ts.h

@@ -90,3 +90,5 @@ struct llist_ts {
 #define LLIST_TS_REMOVE(l_ts, i) llist_remove ((llist_t *)l_ts, i)
 #define LLIST_TS_ADD_SORTED(l_ts, data, fn_cmp)                               \
   llist_add_sorted ((llist_t *)l_ts, data, (llist_fn_cmp_t)fn_cmp)
+#define LLIST_TS_REORDER(l_ts, data, fn_cmp)                                  \
+  llist_reorder((llist_t *)l_ts, data, (llist_fn_cmp_t)fn_cmp)