ross@spam.ua.oz.au (Ross Williams) (06/30/91)
/******************************************************************************/
/* */
/* LZRW3.C */
/* */
/******************************************************************************/
/* */
/* Author : Ross Williams. */
/* Date : 30-Jun-1991. */
/* Release : 1. */
/* */
/******************************************************************************/
/* */
/* This file contains an implementation of the LZRW3 data compression */
/* algorithm in C. */
/* */
/* The algorithm is a general purpose compression algorithm that runs fast */
/* and gives reasonable compression. The algorithm is a member of the Lempel */
/* Ziv family of algorithms and bases its compression on the presence in the */
/* data of repeated substrings. */
/* */
/* This algorithm is unpatented and the code is public domain. As the */
/* algorithm is based on the LZ77 class of algorithms, it is unlikely to be */
/* the subject of a patent challenge. */
/* */
/* Unlike the LZRW1 and LZRW1-A algorithms, the LZRW3 algorithm is */
/* deterministic and is guaranteed to yield the same compressed */
/* representation for a given file each time it is run. */
/* */
/* The LZRW3 algorithm was originally designed and implemented */
/* by Ross Williams on 31-Dec-1990. */
/* */
/* Here are the results of applying this code, compiled under THINK C 4.0 */
/* and running on a Mac-SE (8MHz 68000), to the standard calgary corpus. */
/* */
/* +----------------------------------------------------------------+ */
/* | DATA COMPRESSION TEST | */
/* | ===================== | */
/* | Time of run : Sun 30-Jun-1991 09:31PM | */
/* | Timing accuracy : One part in 100 | */
/* | Context length : 262144 bytes (= 256.0000K) | */
/* | Test suite : Calgary Corpus Suite | */
/* | Files in suite : 14 | */
/* | Algorithm : LZRW3 | */
/* | Note: All averages are calculated from the un-rounded values. | */
/* +----------------------------------------------------------------+ */
/* | File Name Length CxB ComLen %Remn Bits Com K/s Dec K/s | */
/* | ---------- ------ --- ------ ----- ---- ------- ------- | */
/* | rpus:Bib.D 111261 1 55033 49.5 3.96 19.46 32.27 | */
/* | us:Book1.D 768771 3 467962 60.9 4.87 17.03 31.07 | */
/* | us:Book2.D 610856 3 317102 51.9 4.15 19.39 34.15 | */
/* | rpus:Geo.D 102400 1 82424 80.5 6.44 11.65 18.18 | */
/* | pus:News.D 377109 2 205670 54.5 4.36 17.14 27.47 | */
/* | pus:Obj1.D 21504 1 13027 60.6 4.85 13.40 18.95 | */
/* | pus:Obj2.D 246814 1 116286 47.1 3.77 19.31 30.10 | */
/* | s:Paper1.D 53161 1 27522 51.8 4.14 18.60 31.15 | */
/* | s:Paper2.D 82199 1 45160 54.9 4.40 18.45 32.84 | */
/* | rpus:Pic.D 513216 2 122388 23.8 1.91 35.29 51.05 | */
/* | us:Progc.D 39611 1 19669 49.7 3.97 18.87 30.64 | */
/* | us:Progl.D 71646 1 28247 39.4 3.15 24.34 40.66 | */
/* | us:Progp.D 49379 1 19377 39.2 3.14 23.91 39.23 | */
/* | us:Trans.D 93695 1 33481 35.7 2.86 25.48 40.37 | */
/* +----------------------------------------------------------------+ */
/* | Average 224401 1 110953 50.0 4.00 20.17 32.72 | */
/* +----------------------------------------------------------------+ */
/* */
/******************************************************************************/
/* INCLUDE FILES */
/* ============= */
#include "port.h" /* Defines symbols for the non portable stuff. */
#include "compress.h" /* Defines single exported function "compress". */
#include "fast_copy.h" /* Fast memory copy routine. */
/******************************************************************************/
/* The following structure is returned by the "compress" function below when */
/* the user asks the function to return identifying information. */
/* The most important field in the record is the working memory field which */
/* tells the calling program how much working memory should be passed to */
/* "compress" when it is called to perform a compression or decompression. */
/* LZRW3 uses the same amount of memory during compression and decompression. */
/* For more information on this structure see "compress.h". */
#define U(X) ((ULONG) X)
#define SIZE_P_BYTE (U(sizeof(UBYTE *)))
#define SIZE_WORD (U(sizeof(UWORD )))
#define ALIGNMENT_FUDGE (U(16))
#define MEM_REQ ( U(4096)*(SIZE_P_BYTE) + ALIGNMENT_FUDGE )
static struct compress_identity identity =
{
U(0x032DDEA8), /* Algorithm identification number. */
MEM_REQ, /* Working memory (bytes) required. */
"LZRW3", /* Name of algorithm. */
"1.0", /* Version number of algorithm. */
"31-Dec-1990", /* Date of algorithm. */
"Public Domain", /* Copyright notice. */
"Ross N. Williams", /* Author of algorithm. */
"Renaissance Software", /* Affiliation of author. */
"Public Domain" /* Vendor of algorithm. */
};
void compress_compress (UBYTE *,UBYTE *,ULONG,UBYTE *,ULONG *);
void compress_decompress(UBYTE *,UBYTE *,ULONG,UBYTE *,ULONG *);
/******************************************************************************/
/* This function is the only function exported by this module. */
/* Depending on its first parameter, the function can be requested to */
/* compress a block of memory, decompress a block of memory, or to identify */
/* itself. For more information, see the specification file "compress.h". */
EXPORT void compress(action,wrk_mem,src_adr,src_len,dst_adr,p_dst_len)
UWORD action; /* Action to be performed. */
UBYTE *wrk_mem; /* Address of working memory we can use. */
UBYTE *src_adr; /* Address of input data. */
ULONG src_len; /* Length of input data. */
UBYTE *dst_adr; /* Address to put output data. */
ULONG *p_dst_len; /* Address of longword for length of output data. */
{
switch (action)
{
case COMPRESS_ACTION_IDENTITY:
*p_dst_len=(ULONG) &identity;
break;
case COMPRESS_ACTION_COMPRESS:
compress_compress(wrk_mem,src_adr,src_len,dst_adr,p_dst_len);
break;
case COMPRESS_ACTION_DECOMPRESS:
compress_decompress(wrk_mem,src_adr,src_len,dst_adr,p_dst_len);
break;
}
}
/******************************************************************************/
/* */
/* BRIEF DESCRIPTION OF THE LZRW3 ALGORITHM */
/* ======================================== */
/* The LZRW3 algorithm is identical to the LZRW1-A algorithm except that */
/* instead of transmitting history offsets, it transmits hash table indexes. */
/* In order to decode the indexes, the decompressor must maintain an */
/* identical hash table. Copy items are straightforward:when the decompressor */
/* receives a copy item, it simply looks up the hash table to translate the */
/* index into a pointer into the data already decompressed. To update the */
/* hash table, it replaces the same table entry with a pointer to the start */
/* of the newly decoded phrase. The tricky part is with literal items, for at */
/* the time that the decompressor receives a literal item the decompressor */
/* does not have the three bytes in the Ziv (that the compressor has) to */
/* perform the three-byte hash. To solve this problem, in LZRW3, both the */
/* compressor and decompressor are wired up so that they "buffer" these */
/* literals and update their hash tables only when three bytes are available. */
/* This makes the maximum buffering 2 bytes. */
/* */
/* Replacement of offsets by hash table indexes yields a few percent extra */
/* compression at the cost of some speed. LZRW3 is slower than LZRW1, LZRW1-A */
/* and LZRW2, but yields better compression. */
/* */
/* Extra compression could be obtained by using a hash table of depth two. */
/* However, increasing the depth above one incurs a significant decrease in */
/* compression speed which was not considered worthwhile. Another reason for */
/* keeping the depth down to one was to allow easy comparison with the */
/* LZRW1-A and LZRW2 algorithms so as to demonstrate the exact effect of the */
/* use of direct hash indexes. */
/* */
/* +---+ */
/* |___|4095 */
/* |___| */
/* +---------------------*_|<---+ /----+---\ */
/* | |___| +---|Hash | */
/* | |___| |Function| */
/* | |___| \--------/ */
/* | |___|0 ^ */
/* | +---+ | */
/* | Hash +-----+ */
/* | Table | */
/* | --- */
/* v ^^^ */
/* +-------------------------------------|----------------+ */
/* |||||||||||||||||||||||||||||||||||||||||||||||||||||||| */
/* +-------------------------------------|----------------+ */
/* | |1......18| | */
/* |<------- Lempel=History ------------>|<--Ziv-->| | */
/* | (=bytes already processed) |<-Still to go-->| */
/* |<-------------------- INPUT BLOCK ------------------->| */
/* */
/* The diagram above for LZRW3 looks almost identical to the diagram for */
/* LZRW1. The difference is that in LZRW3, the compressor transmits hash */
/* table indices instead of Lempel offsets. For this to work, the */
/* decompressor must maintain a hash table as well as the compressor and both */
/* compressor and decompressor must "buffer" literals, as the decompressor */
/* cannot hash phrases commencing with a literal until another two bytes have */
/* arrived. */
/* */
/* LZRW3 Algorithm Execution Summary */
/* --------------------------------- */
/* 1. Hash the first three bytes of the Ziv to yield a hash table index h. */
/* 2. Look up the hash table yielding history pointer p. */
/* 3. Match where p points with the Ziv. If there is a match of three or */
/* more bytes, code those bytes (in the Ziv) as a copy item, otherwise */
/* code the next byte in the Ziv as a literal item. */
/* 4. Update the hash table as possible subject to the constraint that only */
/* phrases commencing three bytes back from the Ziv can be hashed and */
/* entered into the hash table. (This enables the decompressor to keep */
/* pace). See the description and code for more details. */
/* */
/******************************************************************************/
/* */
/* DEFINITION OF COMPRESSED FILE FORMAT */
/* ==================================== */
/* * A compressed file consists of a COPY FLAG followed by a REMAINDER. */
/* * The copy flag CF uses up four bytes with the first byte being the */
/* least significant. */
/* * If CF=1, then the compressed file represents the remainder of the file */
/* exactly. Otherwise CF=0 and the remainder of the file consists of zero */
/* or more GROUPS, each of which represents one or more bytes. */
/* * Each group consists of two bytes of CONTROL information followed by */
/* sixteen ITEMs except for the last group which can contain from one */
/* to sixteen items. */
/* * An item can be either a LITERAL item or a COPY item. */
/* * Each item corresponds to a bit in the control bytes. */
/* * The first control byte corresponds to the first 8 items in the group */
/* with bit 0 corresponding to the first item in the group and bit 7 to */
/* the eighth item in the group. */
/* * The second control byte corresponds to the second 8 items in the group */
/* with bit 0 corresponding to the ninth item in the group and bit 7 to */
/* the sixteenth item in the group. */
/* * A zero bit in a control word means that the corresponding item is a */
/* literal item. A one bit corresponds to a copy item. */
/* * A literal item consists of a single byte which represents itself. */
/* * A copy item consists of two bytes that represent from 3 to 18 bytes. */
/* * The first byte in a copy item will be denoted C1. */
/* * The second byte in a copy item will be denoted C2. */
/* * Bits will be selected using square brackets. */
/* For example: C1[0..3] is the low nibble of the first control byte. */
/* of copy item C1. */
/* * The LENGTH of a copy item is defined to be C1[0..3]+3 which is a number */
/* in the range [3,18]. */
/* * The INDEX of a copy item is defined to be C1[4..7]*256+C2[0..8] which */
/* is a number in the range [0,4095]. */
/* * A copy item represents the sequence of bytes */
/* text[POS-OFFSET..POS-OFFSET+LENGTH-1] where */
/* text is the entire text of the uncompressed string. */
/* POS is the index in the text of the character following the */
/* string represented by all the items preceeding the item */
/* being defined. */
/* OFFSET is obtained from INDEX by looking up the hash table. */
/* */
/******************************************************************************/
/* The following #define defines the length of the copy flag that appears at */
/* the start of the compressed file. The value of four bytes was chosen */
/* because the fast_copy routine on my Macintosh runs faster if the source */
/* and destination blocks are relatively longword aligned. */
/* The actual flag data appears in the first byte. The rest are zeroed so as */
/* to normalize the compressed representation (i.e. not non-deterministic). */
#define FLAG_BYTES 4
/* The following #defines define the meaning of the values of the copy */
/* flag at the start of the compressed file. */
#define FLAG_COMPRESS 0 /* Signals that output was result of compression. */
#define FLAG_COPY 1 /* Signals that output was simply copied over. */
/* The 68000 microprocessor (on which this algorithm was originally developed */
/* is fussy about non-aligned arrays of words. To avoid these problems the */
/* following macro can be used to "waste" from 0 to 3 bytes so as to align */
/* the argument pointer. */
#define ULONG_ALIGN_UP(X) ((((ULONG)X)+3)&~3)
/* The following constant defines the maximum length of an uncompressed item. */
/* This definition must not be changed; its value is hardwired into the code. */
/* The longest number of bytes that can be spanned by a single item is 18 */
/* for the longest copy item. */
#define MAX_RAW_ITEM (18)
/* The following constant defines the maximum length of an uncompressed group.*/
/* This definition must not be changed; its value is hardwired into the code. */
/* A group contains at most 16 items which explains this definition. */
#define MAX_RAW_GROUP (16*MAX_RAW_ITEM)
/* The following constant defines the maximum length of a compressed group. */
/* This definition must not be changed; its value is hardwired into the code. */
/* A compressed group consists of two control bytes followed by up to 16 */
/* compressed items each of which can have a maximum length of two bytes. */
#define MAX_CMP_GROUP (2+16*2)
/* The following constant defines the number of entries in the hash table. */
/* This definition must not be changed; its value is hardwired into the code. */
#define HASH_TABLE_LENGTH (4096)
/* LZRW3, unlike LZRW1(-A), must initialize its hash table so as to enable */
/* the compressor and decompressor to stay in step maintaining identical hash */
/* tables. In an early version of the algorithm, the tables were simply */
/* initialized to zero and a check for zero was included just before the */
/* matching code. However, this test costs time. A better solution is to */
/* initialize all the entries in the hash table to point to a constant */
/* string. The decompressor does the same. This solution requires no extra */
/* test. The contents of the string do not matter so long as the string is */
/* the same for the compressor and decompressor and contains at least */
/* MAX_RAW_ITEM bytes. I chose consecutive decimal digits because they do not */
/* have white space problems (e.g. there is no chance that the compiler will */
/* replace more than one space by a TAB) and because they make the length of */
/* the string obvious by inspection. */
#define START_STRING_18 ((UBYTE *) "123456789012345678")
/* In this algorithm, hash values have to be calculated at more than one */
/* point. The following macro neatens the code up for this. */
#define HASH(PTR) \
(((40543*(((*(PTR))<<8)^((*((PTR)+1))<<4)^(*((PTR)+2))))>>4) & 0xFFF)
/******************************************************************************/
LOCAL void compress_compress
(p_wrk_mem,p_src_first,src_len,p_dst_first,p_dst_len)
/* Input : Hand over the required amount of working memory in p_wrk_mem. */
/* Input : Specify input block using p_src_first and src_len. */
/* Input : Point p_dst_first to the start of the output zone (OZ). */
/* Input : Point p_dst_len to a ULONG to receive the output length. */
/* Input : Input block and output zone must not overlap. */
/* Output : Length of output block written to *p_dst_len. */
/* Output : Output block in Mem[p_dst_first..p_dst_first+*p_dst_len-1]. May */
/* Output : write in OZ=Mem[p_dst_first..p_dst_first+src_len+MAX_CMP_GROUP-1].*/
/* Output : Upon completion guaranteed *p_dst_len<=src_len+FLAG_BYTES. */
UBYTE *p_wrk_mem;
UBYTE *p_src_first;
ULONG src_len;
UBYTE *p_dst_first;
ULONG *p_dst_len;
{
/* p_src and p_dst step through the source and destination blocks. */
register UBYTE *p_src = p_src_first;
register UBYTE *p_dst = p_dst_first;
/* The following variables are never modified and are used in the */
/* calculations that determine when the main loop terminates. */
UBYTE *p_src_post = p_src_first+src_len;
UBYTE *p_dst_post = p_dst_first+src_len;
UBYTE *p_src_max1 = p_src_first+src_len-MAX_RAW_ITEM;
UBYTE *p_src_max16 = p_src_first+src_len-MAX_RAW_ITEM*16;
/* The variables 'p_control' and 'control' are used to buffer control bits. */
/* Before each group is processed, the next two bytes of the output block */
/* are set aside for the control word for the group about to be processed. */
/* 'p_control' is set to point to the first byte of that word. Meanwhile, */
/* 'control' buffers the control bits being generated during the processing */
/* of the group. Instead of having a counter to keep track of how many items */
/* have been processed (=the number of bits in the control word), at the */
/* start of each group, the top word of 'control' is filled with 1 bits. */
/* As 'control' is shifted for each item, the 1 bits in the top word are */
/* absorbed or destroyed. When they all run out (i.e. when the top word is */
/* all zero bits, we know that we are at the end of a group. */
#define TOPWORD 0xFFFF0000
UBYTE *p_control;
register ULONG control=TOPWORD;
/* THe variable 'hash' always points to the first element of the hash table. */
UBYTE **hash= (UBYTE **) ULONG_ALIGN_UP(p_wrk_mem);
/* The following two variables represent the literal buffer. p_h1 points to */
/* the hash table entry corresponding to the youngest literal. p_h2 points */
/* to the hash table entry corresponding to the second youngest literal. */
/* Note: p_h1=0=>p_h2=0 because zero values denote absence of a pending */
/* literal. The variables are initialized to zero meaning an empty "buffer". */
UBYTE **p_h1=0;
UBYTE **p_h2=0;
/* To start, we write the flag bytes. Being optimistic, we set the flag to */
/* FLAG_COMPRESS. The remaining flag bytes are zeroed so as to keep the */
/* algorithm deterministic. */
*p_dst++=FLAG_COMPRESS;
{UWORD i; for (i=2;i<=FLAG_BYTES;i++) *p_dst++=0;}
/* Reserve the first word of output as the control word for the first group. */
/* Note: This is undone at the end if the input block is empty. */
p_control=p_dst; p_dst+=2;
/* Initialize all elements of the hash table to point to a constant string. */
/* Use of an unrolled loop speeds this up considerably. */
{UWORD i; UBYTE **p_h=hash;
#define ZH *p_h++=START_STRING_18
for (i=0;i<256;i++) /* 256=HASH_TABLE_LENGTH/16. */
{ZH;ZH;ZH;ZH;
ZH;ZH;ZH;ZH;
ZH;ZH;ZH;ZH;
ZH;ZH;ZH;ZH;}
}
/* The main loop processes either 1 or 16 items per iteration. As its */
/* termination logic is complicated, I have opted for an infinite loop */
/* structure containing 'break' and 'goto' statements. */
while (TRUE)
{/* Begin main processing loop. */
/* Note: All the variables here except unroll should be defined within */
/* the inner loop. Unfortunately the loop hasn't got a block. */
register UBYTE *p; /* Scans through targ phrase during matching. */
register UBYTE *p_ziv; /* Points to first byte of current Ziv. */
register UWORD unroll; /* Loop counter for unrolled inner loop. */
register UWORD index; /* Index of current hash table entry. */
register UBYTE **p_h0; /* Pointer to current hash table entry. */
/* Test for overrun and jump to overrun code if necessary. */
if (p_dst>p_dst_post)
goto overrun;
/* The following cascade of if statements efficiently catches and deals */
/* with varying degrees of closeness to the end of the input block. */
/* When we get very close to the end, we stop updating the table and */
/* code the remaining bytes as literals. This makes the code simpler. */
unroll=16;
if (p_src>p_src_max16)
{
unroll=1;
if (p_src>p_src_max1)
{
if (p_src==p_src_post)
break;
else
goto literal;
}
}
/* This inner unrolled loop processes 'unroll' (whose value is either 1 */
/* or 16) items. I have chosen to implement this loop with labels and */
/* gotos to heighten the ease with which the loop may be implemented with */
/* a single decrement and branch instruction in assembly language and */
/* also because the labels act as highly readable place markers. */
/* (Also because we jump into the loop for endgame literals (see above)). */
begin_unrolled_loop:
/* To process the next phrase, we hash the next three bytes and use */
/* the resultant hash table index to look up the hash table. A pointer */
/* to the entry is stored in p_h0 so as to avoid an array lookup. The */
/* hash table entry *p_h0 is looked up yielding a pointer p to a */
/* potential match of the Ziv in the history. */
index=HASH(p_src);
p_h0=&hash[index];
p=*p_h0;
/* Having looked up the candidate position, we are in a position to */
/* attempt a match. The match loop has been unrolled using the PS */
/* macro so that failure within the first three bytes automatically */
/* results in the literal branch being taken. The coding is simple. */
/* p_ziv saves p_src so we can let p_src wander. */
#define PS *p++!=*p_src++
p_ziv=p_src;
if (PS || PS || PS)
{
/* Literal. */
/* Code the literal byte as itself and a zero control bit. */
p_src=p_ziv; literal: *p_dst++=*p_src++; control&=0xFFFEFFFF;
/* We have just coded a literal. If we had two pending ones, that */
/* makes three and we can update the hash table. */
if (p_h2!=0)
{*p_h2=p_ziv-2;}
/* In any case, rotate the hash table pointers for next time. */
p_h2=p_h1; p_h1=p_h0;
}
else
{
/* Copy */
/* Match up to 15 remaining bytes using an unrolled loop and code. */
PS || PS || PS || PS || PS || PS || PS || PS ||
PS || PS || PS || PS || PS || PS || PS || p_src++;
*p_dst++=((index&0xF00)>>4)|(--p_src-p_ziv-3);
*p_dst++=index&0xFF;
/* As we have just coded three bytes, we are now in a position to */
/* update the hash table with the literal bytes that were pending */
/* upon the arrival of extra context bytes. */
if (p_h1!=0)
{
if (p_h2!=0)
{*p_h2=p_ziv-2; p_h2=0;}
*p_h1=p_ziv-1; p_h1=0;
}
/* In any case, we can update the hash table based on the current */
/* position as we just coded at least three bytes in a copy items. */
*p_h0=p_ziv;
}
control>>=1;
/* This loop is all set up for a decrement and jump instruction! */
end_unrolled_loop: if (--unroll) goto begin_unrolled_loop;
/* At this point it will nearly always be the end of a group in which */
/* case, we have to do some control-word processing. However, near the */
/* end of the input block, the inner unrolled loop is only executed once. */
/* This necessitates the 'if' test. */
if ((control&TOPWORD)==0)
{
/* Write the control word to the place we saved for it in the output. */
*p_control++= control &0xFF;
*p_control = (control>>8) &0xFF;
/* Reserve the next word in the output block for the control word */
/* for the group about to be processed. */
p_control=p_dst; p_dst+=2;
/* Reset the control bits buffer. */
control=TOPWORD;
}
} /* End main processing loop. */
/* After the main processing loop has executed, all the input bytes have */
/* been processed. However, the control word has still to be written to the */
/* word reserved for it in the output at the start of the most recent group. */
/* Before writing, the control word has to be shifted so that all the bits */
/* are in the right place. The "empty" bit positions are filled with 1s */
/* which partially fill the top word. */
while(control&TOPWORD) control>>=1;
*p_control++= control &0xFF;
*p_control++=(control>>8) &0xFF;
/* If the last group contained no items, delete the control word too. */
if (p_control==p_dst) p_dst-=2;
/* Write the length of the output block to the dst_len parameter and return. */
*p_dst_len=p_dst-p_dst_first;
return;
/* Jump here as soon as an overrun is detected. An overrun is defined to */
/* have occurred if p_dst>p_dst_first+src_len. That is, the moment the */
/* length of the output written so far exceeds the length of the input block.*/
/* The algorithm checks for overruns at least at the end of each group */
/* which means that the maximum overrun is MAX_CMP_GROUP bytes. */
/* Once an overrun occurs, the only thing to do is to set the copy flag and */
/* copy the input over. */
overrun:
*p_dst_first=FLAG_COPY;
fast_copy(p_src_first,p_dst_first+FLAG_BYTES,src_len);
*p_dst_len=src_len+FLAG_BYTES;
}
/******************************************************************************/
LOCAL void compress_decompress
(p_wrk_mem,p_src_first,src_len,p_dst_first,p_dst_len)
/* Input : Hand over the required amount of working memory in p_wrk_mem. */
/* Input : Specify input block using p_src_first and src_len. */
/* Input : Point p_dst_first to the start of the output zone. */
/* Input : Point p_dst_len to a ULONG to receive the output length. */
/* Input : Input block and output zone must not overlap. User knows */
/* Input : upperbound on output block length from earlier compression. */
/* Input : In any case, maximum expansion possible is nine times. */
/* Output : Length of output block written to *p_dst_len. */
/* Output : Output block in Mem[p_dst_first..p_dst_first+*p_dst_len-1]. */
/* Output : Writes only in Mem[p_dst_first..p_dst_first+*p_dst_len-1]. */
UBYTE *p_wrk_mem;
UBYTE *p_src_first;
ULONG src_len;
UBYTE *p_dst_first;
ULONG *p_dst_len;
{
/* Byte pointers p_src and p_dst scan through the input and output blocks. */
register UBYTE *p_src = p_src_first+FLAG_BYTES;
register UBYTE *p_dst = p_dst_first;
/* The following two variables are never modified and are used to control */
/* the main loop. */
UBYTE *p_src_post = p_src_first+src_len;
UBYTE *p_src_max16 = p_src_first+src_len-(MAX_CMP_GROUP-2);
/* The hash table is the only resident of the working memory. The hash table */
/* contains HASH_TABLE_LENGTH=4096 pointers to positions in the history. To */
/* keep Macintoshes happy, it is longword aligned. */
UBYTE **hash = (UBYTE **) ULONG_ALIGN_UP(p_wrk_mem);
/* The variable 'control' is used to buffer the control bits which appear in */
/* groups of 16 bits (control words) at the start of each compressed group. */
/* When each group is read, bit 16 of the register is set to one. Whenever */
/* a new bit is needed, the register is shifted right. When the value of the */
/* register becomes 1, we know that we have reached the end of a group. */
/* Initializing the register to 1 thus instructs the code to follow that it */
/* should read a new control word immediately. */
register ULONG control=1;
/* The value of 'literals' is always in the range 0..3. It is the number of */
/* consecutive literal items just seen. We have to record this number so as */
/* to know when to update the hash table. When literals gets to 3, there */
/* have been three consecutive literals and we can update at the position of */
/* the oldest of the three. */
register UWORD literals=0;
/* Check the leading copy flag to see if the compressor chose to use a copy */
/* operation instead of a compression operation. If a copy operation was */
/* used, then all we need to do is copy the data over, set the output length */
/* and return. */
if (*p_src_first==FLAG_COPY)
{
fast_copy(p_src_first+FLAG_BYTES,p_dst_first,src_len-FLAG_BYTES);
*p_dst_len=src_len-FLAG_BYTES;
return;
}
/* Initialize all elements of the hash table to point to a constant string. */
/* Use of an unrolled loop speeds this up considerably. */
{UWORD i; UBYTE **p_h=hash;
#define ZJ *p_h++=START_STRING_18
for (i=0;i<256;i++) /* 256=HASH_TABLE_LENGTH/16. */
{ZJ;ZJ;ZJ;ZJ;
ZJ;ZJ;ZJ;ZJ;
ZJ;ZJ;ZJ;ZJ;
ZJ;ZJ;ZJ;ZJ;}
}
/* The outer loop processes either 1 or 16 items per iteration depending on */
/* how close p_src is to the end of the input block. */
while (p_src!=p_src_post)
{/* Start of outer loop */
register UWORD unroll; /* Counts unrolled loop executions. */
/* When 'control' has the value 1, it means that the 16 buffered control */
/* bits that were read in at the start of the current group have all been */
/* shifted out and that all that is left is the 1 bit that was injected */
/* into bit 16 at the start of the current group. When we reach the end */
/* of a group, we have to load a new control word and inject a new 1 bit. */
if (control==1)
{
control=0x10000|*p_src++;
control|=(*p_src++)<<8;
}
/* If it is possible that we are within 16 groups from the end of the */
/* input, execute the unrolled loop only once, else process a whole group */
/* of 16 items by looping 16 times. */
unroll= p_src<=p_src_max16 ? 16 : 1;
/* This inner loop processes one phrase (item) per iteration. */
while (unroll--)
{ /* Begin unrolled inner loop. */
/* Process a literal or copy item depending on the next control bit. */
if (control&1)
{
/* Copy item. */
register UBYTE *p; /* Points to place from which to copy. */
register UWORD lenmt; /* Length of copy item minus three. */
register UBYTE **p_hte; /* Pointer to current hash table entry.*/
register UBYTE *p_ziv=p_dst; /* Pointer to start of current Ziv. */
/* Read and dismantle the copy word. Work out from where to copy. */
lenmt=*p_src++;
p_hte=&hash[((lenmt&0xF0)<<4)|*p_src++];
p=*p_hte;
lenmt&=0xF;
/* Now perform the copy using a half unrolled loop. */
*p_dst++=*p++;
*p_dst++=*p++;
*p_dst++=*p++;
while (lenmt--)
*p_dst++=*p++;
/* Because we have just received 3 or more bytes in a copy item */
/* (whose bytes we have just installed in the output), we are now */
/* in a position to flush all the pending literal hashings that had */
/* been postponed for lack of bytes. */
if (literals>0)
{
register UBYTE *r=p_ziv-literals;;
hash[HASH(r)]=r;
if (literals==2)
{r++; hash[HASH(r)]=r;}
literals=0;
}
/* In any case, we can immediately update the hash table with the */
/* current position. We don't need to do a HASH(...) to work out */
/* where to put the pointer, as the compressor just told us!!! */
*p_hte=p_ziv;
}
else
{
/* Literal item. */
/* Copy over the literal byte. */
*p_dst++=*p_src++;
/* If we now have three literals waiting to be hashed into the hash */
/* table, we can do one of them now (because there are three). */
if (++literals == 3)
{register UBYTE *p=p_dst-3; hash[HASH(p)]=p; literals=2;}
}
/* Shift the control buffer so the next control bit is in bit 0. */
control>>=1;
} /* End unrolled inner loop. */
} /* End of outer loop */
/* Write the length of the decompressed data before returning. */
*p_dst_len=p_dst-p_dst_first;
}
/******************************************************************************/
/* End of LZRW3.C */
/******************************************************************************/