Ticket #8910: compressed-save-seek.patch

backends/saves/compressed/compressed-saves.cpp

@@ -62 +62 @@
                 _stream.zfree = Z_NULL;
                 _stream.opaque = Z_NULL;
 
-                // Verify file header is correct once more
+                // Verify file header is correct
                 w->seek(0, SEEK_SET);
                 uint16 header = w->readUint16BE();
                 assert(header == 0x1F8B ||
@@ -133 +133 @@
         }
         void seek(int32 offset, int whence = SEEK_SET) {
                 int32 newPos = 0;
+                assert(whence != SEEK_END);     // SEEK_END not supported
                 switch(whence) {
-                case SEEK_END:
-                        newPos = size() - offset;
-                        break;
                 case SEEK_SET:
                         newPos = offset;
                         break;
                 case SEEK_CUR:
                         newPos = _pos + offset;
                 }
-                offset = newPos - _pos;
+                
+                assert(newPos >= 0);
 
-                if (offset < 0)
-                        error("Backward seeking not supported in compressed savefiles");
+                if ((uint32)newPos < _pos) {
+                        // To search backward, we have to restart the whole decompression
+                        // from the start of the file. A rather wasteful operation, best
+                        // to avoid it. :/
+#if DEBUG
+                        warning("Backward seeking in CompressedInSaveFile detected");
+#endif
+                        _pos = 0;
+                        _wrapped->seek(0, SEEK_SET);
+                        _zlibErr = inflateReset(&_stream);
+                        if (_zlibErr != Z_OK)
+                                return;
+                        _stream.next_in = _buf;
+                        _stream.avail_in = 0;
+                }
 
-                // We could implement backward seeking, but it is tricky to do efficiently.
-                // A simple solution would be to restart the whole decompression from the
-                // start of the file. Or we could decompress the whole file in one go
-                // in the constructor, and wrap it into a MemoryReadStream -- but that
-                // would be rather wasteful. As long as we don't need it, I'd rather not
-                // implement this at all. -- Fingolfin
+                offset = newPos - _pos;
 
                 // Skip the given amount of data (very inefficient if one tries to skip
                 // huge amounts of data, but usually client code will only skip a few