Add some source code comments.

src/patricia.c

@@ -41,6 +41,23 @@
 
 #include "patricia.h"
 
+/*
+ * A trie is comprised by a tree of pt_y nodes and an array of strings,
+ * both of which are owned by a VMOD object. Both are passed in to the
+ * PT_* functions.
+ *
+ * idx is an index to the strings array, and the region of the string
+ * represented at a node is strings[idx][off] to strings[idx][len],
+ * inclusive.
+ *
+ * Terminating nulls form a part of the target string. So "foo\0" and
+ * "foobar\0" differ at the fourth byte, and the critical bit at that byte
+ * is the most significant 1-bit in 'b'.
+ *
+ * bitmask is set at the critical bit in the differing byte, and
+ * determines whether searches continue into leaf[0] or leaf[1].
+ */
+
 struct pt_y {
 	unsigned	magic;
 #define PT_Y_MAGIC	0xfa564d14
@@ -176,7 +193,7 @@ PT_Insert(struct pt_y * * restrict root, unsigned idx,
 		if (i == y->len) {
 			/*
 			 * The string to be inserted has a prefix that is
-			 * already in the tree.
+			 * already in the trie.
 			 */
 			bit = (y->bitmask & *c) != 0;
 			assert(bit < 2);