update brass dbcheck

xapian-core/backends/brass/brass_dbcheck.cc

@@ -617,37 +617,61 @@ check_brass_table(const char * tablename, string filename,
 	    }
 	    lastdid += did;
 	    bool bad = false;
+
+        // Read the data of skip-list format
 	    while (true) {
-		Xapian::termcount wdf;
-		if (!unpack_uint(&pos, end, &wdf)) {
-		    if (out)
-			*out << "Failed to unpack wdf" << endl;
-		    ++errors;
-		    bad = true;
-		    break;
-		}
-		++tf;
-		cf += wdf;
-
-		if (pos == end) break;
-
-		Xapian::docid inc;
-		if (!unpack_uint(&pos, end, &inc)) {
-		    if (out)
-			*out << "Failed to unpack docid increase" << endl;
-		    ++errors;
-		    bad = true;
-		    break;
-		}
-		++inc;
-		did += inc;
-		if (did > lastdid) {
-		    if (out)
-			*out << "docid " << did << " > last docid " << lastdid
-			     << endl;
-		    ++errors;
-		}
+            Xapian::docid inc = 0;
+            if (!unpack_uint(&pos, end, &inc)) {
+                if (out)
+                    *out << "Failed to unpack docid increase" << endl;
+                ++errors;
+                bad = true;
+                break;
+            }
+            while (inc == (unsigned)-1) {
+                if (!unpack_uint(&pos, end, &inc)) {
+                    if (out)
+                        *out << "Failed to unpack skip info" << endl;
+                    ++errors;
+                    bad = true;
+                    break;
+                }
+                if (!unpack_uint(&pos, end, &inc)) {
+                    if (out)
+                        *out << "Failed to unpack skip info" << endl;
+                    ++errors;
+                    bad = true;
+                    break;
+                }
+                if (!unpack_uint(&pos, end, &inc)) {
+                    if (out)
+                        *out << "Failed to unpack docid increase" << endl;
+                    ++errors;
+                    bad = true;
+                    break;
+                }
+            }
+            did += inc;
+            if (did > lastdid) {
+                if (out)
+                    *out << "docid " << did << " > last docid " << lastdid
+                    << endl;
+                ++errors;
+            }
+            Xapian::termcount wdf;
+            if (!unpack_uint(&pos, end, &wdf)) {
+                if (out)
+                    *out << "Failed to unpack wdf" << endl;
+                ++errors;
+                bad = true;
+                break;
+            }
+            ++tf;
+            cf += wdf;
+
+            if (pos == end) break;
 	    }
+
 	    if (bad) {
 		continue;
 	    }

xapian-core/backends/brass/brass_postlist.cc

@@ -1342,9 +1342,20 @@ SkipListWriter::merge_postlist_changes(const string& term)
  *  document ID of the first document in the chunk (encoded as length of
  *  representation in first byte, and then docid).
  *  
- *  there are two kinds of postlist, 
- *  one is for document length, another is normal postlist storing wdf.
- *  doc length postlist adopts fixed-width format while normal postlist adopts skip-list format.
+ *  There are two kinds of postlist:
+ *  One is for storing document length, its termname is always a null string "",
+ *  another is normal postlist storing wdf, it has normal termname.
+ *  Document length postlist adopts fixed-width format while normal postlist adopts skip-list format.
+ *  
+ *  A chunk (except for the first chunk) contains:
+ *
+ *  1)  bool - true if this is the last chunk.
+ *  2)  difference between final docid in chunk and first docid.
+ *  3)  data of fixed-width format or skip-list format
+ *
+ *  The first chunk begins with the number of entries, the collection
+ *  frequency, then the docid of the first document, then has the header of a
+ *  standard chunk.
  */
 BrassPostList::BrassPostList(intrusive_ptr<const BrassDatabase> this_db_,
 			     const string & term_,