tree-index-opt: add comments.

mod/box/box.h

@@ -42,14 +42,26 @@ enum
 };
 
 struct space {
+	Index *index[BOX_INDEX_MAX];
 	int n;
-	bool enabled;
 	int cardinality;
-	Index *index[BOX_INDEX_MAX];
 
-	/* inferred data */
+	/**
+	 * Inferred data: max field no which participates in an
+	 * index. Each tuple in this space must have, therefore, at
+	 * least indexed_field_count fields.
+	 */
 	int field_count;
-	int *field_types;
+	/**
+	 * Field types of indexed fields. This is an array of size
+	 * indexed_field_count. If there are gaps, i.e. fields
+	 * which do not participate in an index and for which we
+	 * thus can't infer field type, respective array member
+	 * has value UNKNOWN. XXX: right now UNKNOWN is also
+	 * set for fields which types in two indexes contradict.
+	 */
+	enum field_data_type *field_types;
+	bool enabled;
 };
 
 extern struct space *space;

mod/box/index.h

@@ -131,7 +131,7 @@ struct iterator {
 	void (*free)(struct iterator *);
 };
 
-struct box_tuple * iterator_first_equal(struct iterator *it);
+struct box_tuple *iterator_first_equal(struct iterator *it);
 
 #define foreach_index(n, index_var)					\
 	Index *index_var;						\