Add connectivity tracking to fsck.

This shows that I've lost track of one commit already. Most likely
because I forgot to update the .dircache/HEAD file when doing a
commit, so that the next commit referenced not the top-of-tree, but
the one older commit.

Having dangling commits is fine (in fact, you should always have
at least _one_ dangling commit in the top-of-tree). But it's
good to know about them.

fsck-cache.c

@@ -3,18 +3,113 @@
 #include <sys/types.h>
 #include <dirent.h>
 
+struct needs {
+	unsigned char parent[20];
+	unsigned char needs[20];
+	char tag[10];
+};
+
+struct seen {
+	unsigned char sha1[20];
+	char tag[10];
+	unsigned needed;
+};
+
+static struct needs *needs;
+static struct seen *seen;
+
+static int nr_seen, alloc_seen, nr_needs, alloc_needs;
+
 /*
- * These two functions should build up a graph in memory about
+ * These two functions build up a graph in memory about
  * what objects we've referenced, and found, and types..
- *
- * Right now we don't do that kind of reachability checking. Yet.
  */
-static void mark_needs_sha1(unsigned char *parent, const char * type, unsigned char *child)
+static int compare_seen(const void *s1, const void *s2)
+{
+	return memcmp(s1, s2, 20);
+}
+
+static int lookup_seen(unsigned char *sha1, char *tag)
+{
+	int first = 0, last = nr_seen;
+
+	while (last > first) {
+		int next = (last + first) / 2;
+		struct seen *s = seen + next;
+		int cmp = memcmp(sha1, s->sha1, 20);
+
+		if (cmp < 0) {
+			last = next;
+			continue;
+		}
+		if (cmp > 0) {
+			first = next+1;
+			continue;
+		}
+		if (strcmp(tag, s->tag))
+			break;
+		s->needed++;
+		return 1;
+	}
+	return 0;
+}
+
+static void check_connectivity(void)
+{
+	int i;
+
+	/* Sort the "seen" tags for quicker lookup */
+	qsort(seen, nr_seen, sizeof(struct seen), compare_seen);
+
+	/* Look up all the requirements, warn about missing objects.. */
+	for (i = 0; i < nr_needs; i++) {
+		struct needs *n = needs + i;
+		char hex[60];
+
+		if (lookup_seen(n->needs, n->tag))
+			continue;
+		strcpy(hex, sha1_to_hex(n->parent));
+	}
+
+	/* Tell the user about things not referenced.. */
+	for (i = 0; i < nr_seen; i++) {
+		struct seen *s = seen + i;
+
+		if (s->needed)
+			continue;
+		printf("unreferenced %s: %s\n", s->tag, sha1_to_hex(s->sha1));
+	}
+}
+
+static void mark_needs_sha1(unsigned char *parent, const char * tag, unsigned char *child)
 {
+	struct needs *n;
+
+	if (nr_needs == alloc_needs) {
+		alloc_needs = alloc_nr(alloc_needs);
+		needs = realloc(needs, alloc_needs*sizeof(struct needs));
+	}
+	n = needs + nr_needs;
+	nr_needs++;
+	memcpy(n->parent, parent, 20);
+	memcpy(n->needs, child, 20);
+	strncpy(n->tag, tag, sizeof(n->tag));
 }
 
 static int mark_sha1_seen(unsigned char *sha1, char *tag)
 {
+	struct seen *s;
+
+	if (nr_seen == alloc_seen) {
+		alloc_seen = alloc_nr(alloc_seen);
+		seen = realloc(seen, alloc_seen*sizeof(struct seen));
+	}
+	s = seen + nr_seen;
+	memset(s, 0, sizeof(s));
+	nr_seen++;
+	memcpy(s->sha1, sha1, 20);
+	strncpy(s->tag, tag, sizeof(s->tag));
+	
 	return 0;
 }
 
@@ -150,5 +245,6 @@ int main(int argc, char **argv)
 		sprintf(dir, "%s/%02x", sha1_dir, i);
 		fsck_dir(i, dir);
 	}
+	check_connectivity();
 	return 0;
 }