thwack.h

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

typedef struct Thwack		Thwack;
typedef struct Unthwack		Unthwack;
typedef struct ThwBlock		ThwBlock;
typedef struct UnthwBlock	UnthwBlock;

enum
{
	ThwStats	= 8,
	ThwErrLen	= 64,		/* max length of error message from thwack or unthwack */
	ThwMaxBlock	= 1600,		/* max size of compressible block */

	HashLog		= 12,
	HashSize	= 1<<HashLog,
	HashMask	= HashSize - 1,

	MinMatch	= 3,		/* shortest match possible */

	MaxOff		= 8,
	OffBase		= 6,

	MinDecode	= 8,		/* minimum bits to decode a match or lit; >= 8 */

	CompBlocks	= 10,		/* max blocks used to encode data */
	EWinBlocks	= 64,		/* blocks held in encoder window */
	DWinBlocks	= EWinBlocks,	/* blocks held in decoder window */

	MaxSeqMask	= 8,		/* number of bits in coding block mask */
	MaxSeqStart	= 256		/* max offset of initial coding block */
};

struct ThwBlock
{
	ulong	seq;			/* sequence number for this data */
	uchar	acked;			/* ok to use this block; the decoder has it */
	ushort	begin;			/* time of first byte in hash */
	uchar	*edata;			/* last byte of valid data */
	ushort	maxoff;			/* time of last valid hash entry */
	ushort	*hash;
	uchar	*data;
};

struct Thwack
{
	QLock		acklock;	/* locks slot, blocks[].(acked|seq) */
	int		slot;		/* next block to use */
	ThwBlock	blocks[EWinBlocks];
	ushort		hash[EWinBlocks][HashSize];
	Block		*data[EWinBlocks];
};

struct UnthwBlock
{
	ulong	seq;			/* sequence number for this data */
	ushort	maxoff;			/* valid data in each block */
	uchar	*data;
};

struct Unthwack
{
	int		slot;		/* next block to use */
	char		err[ThwErrLen];
	UnthwBlock	blocks[DWinBlocks];
	uchar		data[DWinBlocks][ThwMaxBlock];
};

void	thwackinit(Thwack*);
void	thwackcleanup(Thwack *tw);
void	unthwackinit(Unthwack*);
int	thwack(Thwack*, int mustadd, uchar *dst, int ndst, Block *bsrc, ulong seq, ulong stats[ThwStats]);
void	thwackack(Thwack*, ulong seq, ulong mask);
int	unthwack(Unthwack*, uchar *dst, int ndst, uchar *src, int nsrc, ulong seq);
ulong	unthwackstate(Unthwack *ut, uchar *mask);
int	unthwackadd(Unthwack *ut, uchar *src, int nsrc, ulong seq);