[compress_gzip]
| Link | compress_gzip |
| Author | Fletcher Sandbeck |
| Category | Technical |
| Version | 8.x |
| License | Public Domain |
| Posted | 30 Jun 2006 |
| Updated | 30 Jun 2006 |
| More by this author... | |
Description
This tag performs Gzip compression on some data and returns the result. Gzip compression is similar to the Zlib compression which Lasso's [compress] tag returns, but has a different wrapper around the compressed data.
The wrapper starts with a header that includes flags which identify the file as including Gzip compression, the date and time the data was compressed, and the optional name of the file. This is followed by data compressed using the "deflate" algorithm. The wrapper concludes with a 32-bit CRC checksum and the original size of the data before compression. These final values are used by the decompressor to check that the compressed data wasn't altered in transit.
Lasso's built-in [compress] tag performs Zlib compression which uses the same "deflate" algorithm as Gzip compression, but has a different wrapper. In order to get the compressed data for the Gzip compression we simply strip the Zlib wrapper off the output of the [compress] tag.
The 32-bit CRC checksum for the data is returned using the [Encode_CRC32] tag which is implemented natively in Lasso 8.1 and higher.
This tag was created as part of this article: Tip of the Week - Automatic Gzip Compression for Lasso 8.1
Sample Usage
var('zipped') = compress_gzip($data);
Source Code
Click the "Download" button below to retrieve a copy of this tag, including the complete documentation and sample usage shown on this page. Place the downloaded ".inc" file in your LassoStartup folder, restart Lasso, and you can begin using this tag immediately.
Define_Tag: 'Gzip', -Namespace='Compress_', -Required='data', -Optional='name'; Local: 'bytes' = Bytes; // ID 1 - Fixed value (31) #bytes->(Import8Bits: 0x1f); // ID 2 - Fixed value (139) #bytes->(Import8Bits: 0x8b); // CM - Compression Method (8 Deflate) #bytes->(Import8Bits: 0x08); // FLG - Flags (8 = FNAME, 0 = NONE) if: (local_defined: 'name'); #bytes->(Import8Bits: 0x08); else; #bytes->(Import8Bits: 0x00); /if; // MTIME - Compression Time (0 Unavailable) Local: 'time' = (integer: date); #bytes->(Import8Bits: #time % 256); #time = (#time - (#time % 256)) / 256; #bytes->(Import8Bits: #time % 256); #time = (#time - (#time % 256)) / 256; #bytes->(Import8Bits: #time % 256); #time = (#time - (#time % 256)) / 256; #bytes->(Import8Bits: #time % 256); // XFL - Extra Flags (0 None) #bytes->(Import8Bits: 0x00); // OS - Operating System (255 Unknown) #bytes->(Import8Bits: 0xFF); // FNAME if: (local_defined: 'name'); #bytes->(importstring: (string_replaceregexp: #name, -find='.*/', -replace=''), 'iso-8859-1'); #bytes->(import8bits: 0x00); /if; // DATA local: 'temp'= (Compress: #data); #bytes += #temp->(getrange: 7, #temp->size - 10); // CRC32 local: 'crc' = (Encode_CRC32: #data); #bytes->(Import8Bits: #crc % 256); #crc = (#crc - (#crc % 256)) / 256; #bytes->(Import8Bits: #crc % 256); #crc = (#crc - (#crc % 256)) / 256; #bytes->(Import8Bits: #crc % 256); #crc = (#crc - (#crc % 256)) / 256; #bytes->(Import8Bits: #crc % 256); // ISIZE Local: 'size' = #data->size; #bytes->(Import8Bits: #size % 256); #size = (#size - (#size % 256)) / 256; #bytes->(Import8Bits: #size % 256); #size = (#size - (#size % 256)) / 256; #bytes->(Import8Bits: #size % 256); #size = (#size - (#size % 256)) / 256; #bytes->(Import8Bits: #size % 256); Return: @#bytes; /Define_Tag;
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