Added comment.

diff --git a/re2c/src/ir/dfa/tag_indexing.cc b/re2c/src/ir/dfa/tag_indexing.cc
index 916a26cf28b7a0bbc684b3141c2f5644245f864c..e50e3f69fff6200588a5e3662e75de265eff1357 100644 (file)
--- a/re2c/src/ir/dfa/tag_indexing.cc
+++ b/re2c/src/ir/dfa/tag_indexing.cc
@@ -8,6 +8,23 @@ namespace re2c
 template<typename cmd_t> struct cmp_t;
 template<typename cmd_t> void doindex(cmd_t **pcmd, lookup_t<cmd_t*> &index);
 
+/* note [indexing tag commands]
+ *
+ * After optimizations different commands may become equal up to
+ * reordering and removing duplicates. We want such commands to have
+ * identical representation and compare as equal: this may enable
+ * further optimizations like minimization, tag hoisting, tunnelling,
+ * etc. These optimizations need fast (constant time) comparison of
+ * two commands (especially Moore's minimization, which compares sets
+ * of transitions at once).
+ *
+ * So we bring each command to some 'normal form' and insert it into
+ * common index. Then we can address and compare commands by index,
+ * which is indeed constant time.
+ *
+ * However, after indexing different commands may share representation
+ * in memory, so they must not be modified.
+ */
 void tag_indexing(dfa_t &dfa)
 {
        const size_t
@@ -31,13 +48,6 @@ void tag_indexing(dfa_t &dfa)
        }
 }
 
-// After renaming different tags may map to the same version,
-// thus one set of versions may have different representations.
-// Transition tags are treated as sets and should be unified.
-
-// After renaming different lists may become identical
-// up to reordering and deleting duplicates.
-
 template<typename cmd_t>
 void doindex(cmd_t **pcmd, lookup_t<cmd_t*> &index)
 {