regex-from-trie: correctly handle empty trie.

* filter.c (regex_from_trie): An empty trie matches nothing,
so we must return the t regex syntax (match nothing), not
nil (match empty string). A hash-based trie matches nothing
if it is empty; but if it has user data, then it matches
the empty string.

* tests/015/trie.tl: Test cases added.

2 files changed, 6 insertions, 2 deletions

diff --git a/filter.c b/filter.c
index a8f0b787..348a697c 100644
--- a/filter.c
+++ b/filter.c
@@ -121,7 +121,7 @@ static val regex_from_trie(val trie)
 {
   switch (type(trie)) {
   case NIL:
-    return nil;
+    return t;
   case CONS:
     {
       val a = car(trie);
@@ -145,7 +145,7 @@ static val regex_from_trie(val trie)
   case COBJ:
     if (trie->co.cls == hash_s) {
       if (zerop(hash_count(trie))) {
-        return nil;
+        return tnil(!get_hash_userdata(trie));
       } else {
         val out = nil;
         val cell;
diff --git a/tests/015/trie.tl b/tests/015/trie.tl
index de88a797..c145c060 100644
--- a/tests/015/trie.tl
+++ b/tests/015/trie.tl
@@ -52,3 +52,7 @@
 (mvtest
   (build (each ((d dat)) (add [rx1 d]))) dat
   (build (each ((n ndt)) (add [rx1 n]))) (repeat '(nil) (len dat)))
+
+(mtest
+  (regex-from-trie (make-trie)) t
+  (regex-from-trie (trie-compress (make-trie))) t)