Storing and Retrieving compressed texts as BLOB from Sqlite via FMDB - ios

Question

I am trying to store text data as compressed blobs.
[db executeUpdate:#"create table blobTable (a text, b blob)"];
[db executeUpdate:#"insert into blobTable (a, b) values (?, compressMe('random text string'))", #"lord of the rings"];
And then I try to query them using:
FMResultSet *rs = [db executeQuery:#"select uncompressMe(b) as k from blobTable where a = ?", #"lord of the rings"];
Where compressMe and uncompressMe are defined as:
FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:dbPath];
[queue inDatabase:^(FMDatabase *adb) {
[adb makeFunctionNamed:#"compressMe" maximumArguments:1 withBlock:^(sqlite3_context *context, int aargc, sqlite3_value **aargv) {
if (sqlite3_value_type(aargv[0]) == SQLITE_TEXT) {
#autoreleasepool {
const char *c = (const char *)sqlite3_value_text(aargv[0]);
NSString *s = [NSString stringWithUTF8String:c];
NSLog(#"string to compress: %#", s);
NSData *compressedBody = [self gzipDeflate:[s dataUsingEncoding:NSUTF8StringEncoding]];
sqlite3_result_blob(context, (__bridge const void *)(compressedBody), [compressedBody length], nil);
}
}
else {
NSLog(#"Unknown formart for StringStartsWithH (%d) %s:%d", sqlite3_value_type(aargv[0]), __FUNCTION__, __LINE__);
sqlite3_result_null(context);
}
}];
}];
[queue inDatabase:^(FMDatabase *adb) {
[adb makeFunctionNamed:#"uncompressMe" maximumArguments:1 withBlock:^(sqlite3_context *context, int aargc, sqlite3_value **aargv) {
if (sqlite3_value_type(aargv[0]) == SQLITE_BLOB) {
#autoreleasepool {
NSLog(#"inside uncompressMe");
NSUInteger len = sqlite3_value_bytes(aargv[0]);
Byte *byteData = (Byte*)malloc(len);
memcpy(byteData, sqlite3_value_blob(aargv[0]), len);
NSData *data = [[NSData alloc] initWithBytes:byteData length:len];
NSData *deflatedBody = [self gzipInflate:data];
NSString *deflatedString = [[NSString alloc] initWithData:deflatedBody encoding:NSUTF8StringEncoding];
sqlite3_result_text(context, (__bridge const void *)(deflatedString), -1, SQLITE_UTF8);
}
}
else {
NSLog(#"Unknown formart for StringStartsWithH (%d) %s:%d", sqlite3_value_type(aargv[0]), __FUNCTION__, __LINE__);
sqlite3_result_null(context);
}
}];
}];
For the sake of completeness, the zip functions are defined as such:
- (NSData *)gzipInflate:(NSData*)data
{
if ([data length] == 0) return data;
unsigned full_length = [data length];
unsigned half_length = [data length] / 2;
NSMutableData *decompressed = [NSMutableData dataWithLength: full_length + half_length];
BOOL done = NO;
int status;
z_stream strm;
strm.next_in = (Bytef *)[data bytes];
strm.avail_in = [data length];
strm.total_out = 0;
strm.zalloc = Z_NULL;
strm.zfree = Z_NULL;
if (inflateInit2(&strm, (15+32)) != Z_OK) return nil;
while (!done)
{
// Make sure we have enough room and reset the lengths.
if (strm.total_out >= [decompressed length])
[decompressed increaseLengthBy: half_length];
strm.next_out = [decompressed mutableBytes] + strm.total_out;
strm.avail_out = [decompressed length] - strm.total_out;
// Inflate another chunk.
status = inflate (&strm, Z_SYNC_FLUSH);
if (status == Z_STREAM_END) done = YES;
else if (status != Z_OK) break;
}
if (inflateEnd (&strm) != Z_OK) return nil;
// Set real length.
if (done)
{
[decompressed setLength: strm.total_out];
return [NSData dataWithData: decompressed];
}
else return nil;
}
- (NSData *)gzipDeflate:(NSData*)data
{
if ([data length] == 0) return data;
z_stream strm;
strm.zalloc = Z_NULL;
strm.zfree = Z_NULL;
strm.opaque = Z_NULL;
strm.total_out = 0;
strm.next_in=(Bytef *)[data bytes];
strm.avail_in = [data length];
// Compresssion Levels:
// Z_NO_COMPRESSION
// Z_BEST_SPEED
// Z_BEST_COMPRESSION
// Z_DEFAULT_COMPRESSION
if (deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED, (15+16), 8, Z_DEFAULT_STRATEGY) != Z_OK) return nil;
NSMutableData *compressed = [NSMutableData dataWithLength:16384]; // 16K chunks for expansion
do {
if (strm.total_out >= [compressed length])
[compressed increaseLengthBy: 16384];
strm.next_out = [compressed mutableBytes] + strm.total_out;
strm.avail_out = [compressed length] - strm.total_out;
deflate(&strm, Z_FINISH);
} while (strm.avail_out == 0);
deflateEnd(&strm);
[compressed setLength: strm.total_out];
return [NSData dataWithData:compressed];
}
However, the uncompressMe function doesnt seem to succeed. It fails in the Inflate method, which always returns a nil. Any thoughts what I might be doing wrong? I have checked to make sure that the blob column does get populated.

Answer 1

I feel like issue is from below code:
NSUInteger len = sqlite3_value_bytes(aargv[0]);
Byte *byteData = (Byte*)malloc(len);
memcpy(byteData, sqlite3_value_blob(aargv[0]), len);
NSData *data = [[NSData alloc] initWithBytes:byteData length:len];
Try this to load NSData from database instead of creating byte variable and copying it.
const void *bytes = sqlite3_column_blob(statement, 3);
NSData *data = [[NSData alloc] initWithBytes:bytes length:size];