More work. Much clearer now... not yet complete though.

common/exp_attrdll.c

@@ -53,10 +53,36 @@ static void linkInsertBefore(struct attrlist *alist, struct attr *what, struct a
 	a->prev->next = a;
 }
 
+static int attrHasPos(struct attr *a, size_t pos)
+{
+	if (pos < a->start)
+		return 0;
+	return pos < a->end;
+}
+
+// returns 1 if there was an intersection and 0 otherwise
+static int attrRangeIntersect(struct attr *a, size_t *start, size_t *end)
+{
+	// is the range outside a entirely?
+	if (*start >= a->end)
+		return 0;
+	if (*end < a->start)
+		return 0;
+
+	// okay, so there is an overlap
+	// compute the intersection
+	if (*start < a->start)
+		*start = a->start;
+	if (*end > a->end)
+		*end = a->end;
+	return 1;
+}
+
+#if 0
 void attrlistInsertAt(struct attrlist *alist, uiAttribute type, uintptr_t val, size_t start, size_t end)
 {
 	struct attr *a;
-	struct attr *after;
+	struct attr *before;
 
 	a = uiNew(struct attr);
 	a->type = type;
@@ -65,10 +91,81 @@ void attrlistInsertAt(struct attrlist *alist, uiAttribute type, uintptr_t val, s
 	a->end = end;
 
 	// place a before the first element that starts after a does
-	for (before = alist->first; before != NULL; before = before->next)
+	for (before = alist->first; before != NULL; before = before->next) {
+		size_t lstart, lend;
+
 		if (before->start > a->start)
 			break;
-	// TODO this does not handle cases where the attribute overwrites an existing attribute
+		// if this attribute overrides another, we have to split
+		lstart = start;
+		lend = end;
+		if (!attrRangeIntersect(before, &lstart, &lend))
+			continue;
+		if (before->type != type)
+			continue;
+		if (before->val == val)
+			continue;
+		// TODO now split around start/end and drop the overlap
+	}
 	linkInsertBefore(alist, a, before);
 	// TODO see if adding this attribute leads to a fragmented contiguous run
 }
+#endif
+
+void attrlistInsertAt(struct attrlist *alist, uiAttribute type, uintptr_t val, size_t start, size_t end)
+{
+	struct attr *a;
+	struct attr *before;
+	struct attr *tail = NULL;
+
+	// first, figure out where in the list this should go
+	// in addition, if this attribute overrides one that already exists, split that one apart so this one can take over
+	before = alist->first;
+	while (before != NULL) {
+		size_t lstart, lend;
+
+		// once we get to the first point after start, we know where to insert
+		if (before->start > start)
+			break;
+
+		// if we have already split a prior instance of this attribute, don't bother doing it again
+		if (tail != NULL)
+			goto next;
+
+		// should we split this?
+		if (before->type != type)
+			goto next;
+		lstart = start;
+		lend = end;
+		if (!attrRangeIntersects(before, &lstart, &lend))
+			goto next;
+
+		// okay so this might conflict; if the val is the same as the one we want, we need to expand the existing attribute, not fragment anything
+		// TODO this might cause problems with system specific attributes, if we support those; maybe also user-specific?
+		if (before->val == val) {
+			// TODO
+		}
+		// okay the values are different; we need to split apart
+		// TODO
+
+	next:
+		before = before->next;
+	}
+
+	// if we got here, we know we have to add the attribute before before
+	a = uiNew(struct attr);
+	a->type = type;
+	a->val = val;
+	a->start = start;
+	a->end = end;
+	linkInsertBefore(alist, a, before);
+
+	// and finally, if we split, insert the remainder
+	if (tail == NULL)
+		return;
+	// note we start at before; it won't be inserted before that by the sheer nature of how the code above works
+	for (; before != NULL; before = before->next)
+		if (before->start > tail->start)
+			break;
+	linkInsertBefore(alist, tail, before);
+}