JavaScriptでdeflate強化

組み込み関数のCompressionStreamの圧縮力があまりにお粗末なのでzopfli級の最適化を施そうと目論んだ次第です。
C言語版のzopfliもどきを手作業でJavaScriptに移植するだけの超簡単ではない程度お仕事です。
本家は圧縮力0～9を指定できますが、それだけでは能力を十分に発揮できていません。JavaScript版ではもっと細かい設定ができるようにしていきます。それでは動作検証

See the Pen deflate/zopfli compression + inflate by xezz (@xezz) on CodePen.

設定項目

まずC言語版の構造体では18個の設定項目があります。圧縮力0～9はこれらをある決まった値に設定するだけの仕様です。

typedef struct ZopfliOptions{
	/*
	Maximum amount of times to rerun forward and backward pass to optimize LZ77
	compression cost. Good values: 10, 15 for small files, 5 for files over
	several MB in size or it will be too slow.
	*/
	int numiterations;

	unsigned filter_style;

	//Don't try to use dynamic block under this size
	unsigned skipdynamic;

	//Try static block if dynamic block is smaller than this. Needs to be much higher than skipdynamic
	unsigned trystatic;

	//Don't blocksplit under this size
	unsigned noblocksplit;

	// Don't blocksplit under this size (LZ77'd data)
	unsigned noblocksplitlz;

	//Number of parallel block split point searches
	unsigned num;

	/* Search longer for best huffman header compression:
		Value outstream important less important
		0:    1         1         1
		1:    10        10        1
		2:    32        32        32
	*/
	unsigned searchext;

	unsigned reuse_costmodel;

	//When using more than one iteration, this will save the found matches on the first run so they don't need to be found again. Uses large amounts of memory
	unsigned useCache;

	//Use per block multithreading
	unsigned multithreading;

	//Use tuning for PNG files
	unsigned isPNG;

	//Replace short lengths with literals if that improves compression. Higher numbers mean more aggressive behaviour
	unsigned replaceCodes;

	//Block split twice
	unsigned twice;

	//Sorry, can't figure out a better name. Leads to better final LZ costmodel
	unsigned ultra;

	//Use greedy search instead of lazy search above this value
	unsigned greed;

	//Use shannon entropy instead of real code lengths in blocksplitting
	unsigned entropysplit;

	//Use advanced huffman and header optimizations
	unsigned advanced;
}

JavaScript(CodePen)での仕様

/* deflate圧縮関数
@set: object. 設定を格納
@last: 最後のbitを最後のdeflate blockに設定するかどうかの真偽
@A: 圧縮元配列
@z: Aの大きさ
@O: 圧縮先配列
*/
function zopfliDeflate(set,last,A,z,O)

/* deflate-raw的な関数. zopfliDeflate包括関数
@A: 圧縮元配列
@set: object. 設定を格納
@done: 後処理用関数(省略可)
@rate: 保留(省略可)
@return: Aを圧縮した配列
*/
function rawComp(A,set,done,rate)

@setの構造

{
	x:0-255	// Maximum amount of times to rerun forward and backward pass to optimize LZ77 compression cost
	skipdynamic: 0-511	// Don't try to use dynamic block under this size
	trystatic: 0-4095	// Try static block if dynamic block is smaller than this. Needs to be much higher than skipdynamic
	noblocksplit: 0-2047	// Don't blocksplit under this size
	noblocksplitlz: 0-2047	// Don't blocksplit under this size (LZ77's data)
	num: 0-15	// [2-9?] Number of parallel block split point searches
	searchext: 0-2	// [0-2] Search longer for best huffman header compression
	reuseModel: 0-1	// numiterations must be >1
	useCache: 0-1	// When using more than one iteration, this will save the found matches on the first run so they don't need to be found again. Uses large amounts of memory
	multithreading: 0	// Use per block multithreading
	isPNG: 0-1	// Use tuning for PNG files
	replaceCodes: 0-2047	// Replace short lengths with literals if that improves compression. Higher numbers mean more aggressive behaviour
	twice: 0-7	// [0-7] Block split twice
	ultra: 0-3	// [0-3] Leads to better final LZ costmodel
	greed: 0-258	// [0-258] Use greedy search instead of lazy search above this value
	entropysplit: 0-1	// Use shannon entropy instead of real code lengths in blocksplitting
	advanced: 0-1	// Use advanced huffman and header optimizations
	bpm:0-1	// Use Boundary Package Merge for huffman code
}

ちなみにbpmという項目は本家にはありません。その実体ですがCodePenではBoundary PMという項目に該当します。有効にすると本家同様にBounded package merge algorithmと呼ばれる手法でhuffman符号を構築。無効の場合は7zipで採用されている高速な手法で構築。圧縮率が同じになるとは限りません。
isPNGを有効にすると、内部でPNG画像用の閾値を設定します。別にPNG画像を作るわけではありません。
CodePenではlevelという項目があり、これが本家の圧縮力0-9に該当します(多分)