A compressed data set consists of a series of blocks, corresponding
to successive blocks of input data. The block sizes are arbitrary,
except that non-compressible blocks are limited to 65,535 bytes.
Each block is compressed using a combination of the LZ77 algorithm
and Huffman coding. The Huffman trees for each block are independent
of those for previous or subsequent blocks; the LZ77 algorithm may
use a reference to a duplicated string occurring in a previous block,
up to 32K input bytes before.
Each block consists of two parts: a pair of Huffman code trees that
describe the representation of the compressed data part, and a
compressed data part. (The Huffman trees themselves are compressed
using Huffman encoding.) The compressed data consists of a series of
elements of two types: literal bytes (of strings that have not been
detected as duplicated within the previous 32K input bytes), and
pointers to duplicated strings, where a pointer is represented as a
pair <length, backward distance>. The representation used in the
"deflate" format limits distances to 32K bytes and lengths to 258
bytes, but does not limit the size of a block, except for
uncompressible blocks, which are limited as noted above.
Each type of value (literals, distances, and lengths) in the
compressed data is represented using a Huffman code, using one code
tree for literals and lengths and a separate code tree for distances.
The code trees for each block appear in a compact form just before
the compressed data for that block.