Simple LZ, TK uLZ 0

TK uLZ
* Small Huffman-coded LZ77 variant.
* Aim is to be smaller/simpler to decode than Deflate.
* Will still be Huffman coded, unlike LZ4.
* Will not (necessarily) aim for speed, but should not be overly slow.


Bitstream
* LSB first.
* Huffman symbols are 1-12 bits.
** Rather than 1-15 as in Deflate.
** This allows simplifying the table encoding some.
** Also allows for a single-stage lookup table to be smaller.


=== Top Level Bitstream ===

Top level bitstream will consist of a series of 4 bit tags.
* 0: End of Stream.
* 1: Raw Uncompressed Data
* 2: LZ Compressed Data
* 3: Reserved
* 4: Packed Huffman Table, Tag
* 5: Packed Huffman Table, Literal
* 6: Packed Huffman Table, Distance
* 7: Reserved
* 8: Reserved
* 9: Raw Uncompressed Data, Stream Ends.
* A: LZ Packed Data, Stream Ends.
* B..F: Reserved

Raw Uncompressed Data:
* 2 bit prefixed length:
** 0: 12 bit length (0 to 4K)
** 1: 16 bit length (0 to 64K)
** 2: 20 bit length (0 to 1M)
** 3: 24 bit length (0 to 16M)
* Bitstream then realigns to a byte boundary.
** This many raw bytes follows.


=== Packed Huffman Table ===

Huffman tables will encode a series of lengths, but the tables will not be entropy coded. Each table will represent up to 256 lengths.

Table contents will be encoded via 4 bit codes:
* 0: Non-encoded symbol
* 1..D: Symbol Length, 1..13 bits.
** Length 13: Reserved for now.
* E: 3..18 zeroes (4 bits follow).
* F: Escaped Run, 2 bit code follows:
** 0: 19..82 zeroes (6 bits).
** 1: 4..66 repeats of prior length (6 bits).
*** Bias is 3, with 0 encoding an "End of Table".
** 2: Reserved
** 3: Reserved
** Rest of table is filled with zeroes.

A table does not necessarily end with an End of Table marker, but may also end upon reaching its total number of lengths. A table may not encode more than 256 lengths.

If an End of Table is seen, any remaining lengths are set to zero.


Major Tables:
* Tag(T): Run Prefix Tags
* Literal(L): Literal Bytes
* Distance(D): Distance Prefix

Symbols will be assigned code patterns such that:
* Shorter symbols precede longer ones;
* Symbols with the same length will be be ordered by symbol value.

Symbols will be effectively transposed in the bitstream, such that the high-order bit of the symbol's codeword appears in LSB position in the bitstream.


=== LZ Compressed Data ===

Runs will be encoded as:
* Tag Prefix
* If Needed, Overflowed Literal Length
* If Needed, Overflowed Match Length
* Match Distance
* If Special Case, Extra Distance
* Zero or more literal symbols
* (Match copy happens here)

The Tag prefix will be encoded as:
* High 4 bits: Literal Length, or 15 for overflow.
* Low 4 bits: Match Length (3..17), or 15 for overflow.

In the overflow cases, the length will be encoded using a distance.
* The overflow length will add a bias, 15 for literal, and 18 for match.


Lengths and distances will be encoded as a prefix followed by zero or more "extra bits".

Distance Prefix:
* High 5 bits: Exponent (ExtraBits-1)
* Low 3 bits: Fraction

Thus (Prefix, Range, Extra Bits):
* 00..07:     0..    7 (0)
* 08..0F:     8..   15 (0)
* 10..17:    16..   31 (1)
* 18..1F:    32..   63 (2)
* 20..27:    64..  127 (3)
* 28..2F:   128..  255 (4)
* 30..37:   256..  511 (5)
* 38..3F:   512.. 1023 (6)
* 40..47:  1024.. 2047 (7)
* 48..4F:  2048.. 4095 (8)
* 50..57:  4096.. 8191 (9)
* 58..5F:  8192..16383 (10)
* 60..67: 16384..32767 (11)
* 68..6F: 32768..65535 (12)
* ...


If the LZ match distance is zero, this will encode an escape case.
What happens next will depend on the match length.
* 3(0): End of Block
* 4(1): Predicted match length and distance.
* 5(2): Predicted match length (match distance given as ExtraDistance).
* 6(3): Predicted distance (match length given as ExtraDistance+3).
* Other values are reserved.

If a predicted value is used, the previously encoded value will be used as a predictor. At the start of a block these values are undefined and may not be used.