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Deflate Decompression

Deflate Decompression#

Job Structure Settings#

Fixed, Dynamic, and Static Blocks#

Assuming that a single job is used (see Compression and Decompression across Multiple Jobs), the job structure for fixed, dynamic, and static decompression all can be set using:

job->op            = qpl_op_decompress;
job->next_in_ptr   = <input_buffer>;
job->available_in  = <input_size>;
job->next_out_ptr  = <output_buffer>;
job->available_out = <output_size>;
job->flags         = QPL_FLAG_FIRST | QPL_FLAG_LAST;

Canned#

The job structure for canned decompression can be set using:

job->op            = qpl_op_compress;
job->next_in_ptr   = <input_buffer>;
job->available_in  = <input_size>;
job->next_out_ptr  = <output_buffer>;
job->available_out = <output_size>;
job->flags         = QPL_FLAG_FIRST | QPL_FLAG_LAST | QPL_FLAG_CANNED_MODE;
job->huffman_table = huffman_table;

The Huffman table object huffman_table can be allocated by qpl_deflate_huffman_table_create() and initialized with literal/length and distance histograms through qpl_huffman_table_init_with_histogram(). Refer to the Huffman Table Objects section for more information.

Attention

The current implementation does not support canned decompression across multiple jobs, thus canned decompression jobs must specify the flags QPL_FLAG_FIRST | QPL_FLAG_LAST.

Getting QPL_STS_MORE_OUTPUT_NEEDED after Decompression#

In general, the application may not know how much data a given compressed stream will decompress to. If the output buffer fills up before the input is completely processed, the library returns the status QPL_STS_MORE_OUTPUT_NEEDED, which means that the decompressor stopped early to avoid overflowing the output buffer. In that case, only partial data is written to the output, and internal state is now stored in the job structure. This allows for the decompression operation to resume from the same place where it stopped.

The job structure would be updated as in the result of successful execution, meaning, for example, that the size of the partial data written to the output is available via qpl_job.total_out, and input buffer parameters qpl_job.next_in_ptr and qpl_job.available_in are updated by the library to reflect how many input bytes were processed successfully.

Note

It is possible to have qpl_job.available_in returned as 0 and get the QPL_STS_MORE_OUTPUT_NEEDED error. This case means that all the input bytes were processed, but the output buffer was smaller than required, and resubmission will happen with the input buffer of zero length.

In order to submit job for continuation:

  • The user must not reset input qpl_job.next_in_ptr and qpl_job.available_in parameters, as they were updated by the library and already reflect the correct offset.

  • If the job was marked previously with QPL_FLAG_FIRST, this flag must be unset.

  • The user must provide an output buffer for continuation and reset qpl_job.next_out_ptr to its value and qpl_job.available_out to its size. The user could choose, for instance, to allocate a bigger output buffer or to reuse the same memory. In the latter case, previously decompressed data must be stored somewhere else, as it would be overwritten otherwise.

Attention

If returned values in qpl_job.available_in and qpl_job.available_out fields are the same when error QPL_STS_MORE_OUTPUT_NEEDED occurred, this means that the library was not able to make any forward progress. In this case, it is most probably that the output buffer size is too small, and the user must allocate a bigger chunk of memory for an output.

Checksums#

Decompression jobs return CRC-32 and 16-bit XOR checksums of the decompressed data in qpl_job.crc and qpl_job.xor_checksum. By default, the CRC-32 uses the polynomial 0x104c11db7, which follows ITU-T Recommendation V.42. If the flag QPL_FLAG_CRC32C is specified in qpl_job.flags, then the polynomial 0x11edc6f41 is used, which follows RFC 3720. To compute the 16-bit XOR checksum, the data is treated as 16-bit words. If the data has an odd number of bytes, the final byte is zero-extended to 16 bits.

Warning

The implementation of QPL_FLAG_CRC32C is in progress.

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