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*                            Open Watcom Project
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* Description:  WHEN YOU FIGURE OUT WHAT THIS FILE DOES, PLEASE
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****************************************************************************/


/*
COMPRESS:  WinHelp-style LZ77 compression
*/

#include "compress.h"
#include <string.h>

#define HOLD_SIZE   4096
#define HOLD_BUF    (2*HOLD_SIZE)
#define WRITE_SIZE  8
#define READ_SIZE   18
#define MIN_READ    3
#define HTABLE_SIZE 65537
#define HTABLE_NIL  -1
#define RARE_BYTE   0xF1


//  LocalHash   --Hash a 3-character string for lookup purposes.

int LocalHash( uint_8 values[] )
{
    int result = 0;
    for( int i=0; i<MIN_READ; i++ ){
    result *= 0x10;
    result += values[i];
    result %= HTABLE_SIZE;
    }
    return result;
}


//  CompWriter::CompWriter

CompWriter::CompWriter()
    : _numBytes( 0 ),
      _numTokens( 0 ),
      _bitMask( 0 )
{
    _buffer = new uint_8[2*WRITE_SIZE];
}


//  CompWriter::~CompWriter

CompWriter::~CompWriter()
{
    delete[] _buffer;
}


//  CompWriter::dump    --For the base class, throw out the buffers.

void CompWriter::dump()
{
    _numBytes = 0;
    _numTokens = 0;
    _bitMask = 0;
}


//  CompWriter::putChr  --Add a raw character to output.

int CompWriter::putChr( uint_8 c )
{
    int result = 1;
    if( _numTokens == 0 ){
    ++result;
    }
    _numTokens++;
    _buffer[_numBytes++] = c;
    if( _numTokens == WRITE_SIZE ){
    dump();
    }
    return result;
}


//  CompWriter::putCode --Add a distance/length code to output.

int CompWriter::putCode( int distance, int length )
{
    int result = 2;
    if( _numTokens == 0 ){
    ++result;
    }

    // Convert the distance/length to a two-byte code.
    // See "compress.doc" for the description of Microsoft-style
    // LZ77 compression.

    _buffer[_numBytes++] = (uint_8) ((distance-1) % 256);
    _buffer[_numBytes++] = (uint_8) ((length-3)*16 + (distance-1)/256);

    // Update the current bitmask.
    uint_8 mask = 1;
    mask <<= _numTokens++;
    _bitMask |= mask;

    if( _numTokens == WRITE_SIZE ){
    dump();
    }
    return result;
}


//  CompOutFile::dump   --Send the current set of codes to the output file.

void CompOutFile::dump()
{
    if( _numTokens > 0 ){
    _dest->writech( _bitMask );
    _dest->writebuf( _buffer, 1, _numBytes );
    _numBytes = _numTokens = 0;
    }
    _bitMask = 0;
}


//  CompReader::CompReader

CompReader::CompReader( CompWriter *riter )
    : _buffer(HOLD_BUF),
      _indices(HOLD_BUF),
      _htable(HTABLE_SIZE),
      _dest( riter ),
      _last( 0 ),
      _first( 0 ),
      _current( 0 )
{
    // Set all of the hash table entries to -1.
    memset( &(_htable[0]), (uint_8) HTABLE_NIL, HTABLE_SIZE * sizeof( short ) );

    _indices[0] = HTABLE_NIL;
}


//  CompReader::flush   --Throw out the buffered text and
//            start compressing from scratch.

void CompReader::flush( int nodump )
{
    _last = 0;
    _first = 0;
    _current = 0;

    // Set all of the hash table entries to -1.
    memset( &(_htable[0]), (uint_8) HTABLE_NIL, HTABLE_SIZE * sizeof( short ) );

    _indices[0] = HTABLE_NIL;

    if( !nodump ){
    _dest->dump();
    }
    return;
}


//  CompReader::reset   --Switch to a new CompWriter.

void CompReader::reset( CompWriter *riter, int nodump )
{
    flush( nodump );
    _dest = riter;
}


//  CompReader::shuffle --Make room in the buffer for new text.

void CompReader::shuffle()
{
    memmove( _buffer, _buffer+_first, _last-_first );
    HCTick();

    for( int i=0; i<_last-_first && i<_current; ++i ){
    if( _indices[i+_first] < _first ){
        _indices[i] = HTABLE_NIL;
    } else {
        _indices[i] = (short) (_indices[i+_first]-_first);
    }
    }

    for( i=0; i<HTABLE_SIZE; ++i ){
    if( _htable[i] < _first ){
        _htable[i] = HTABLE_NIL;
    } else {
        _htable[i] -= _first;
    }
    }

    _last -= _first;
    _current -= _first;
    _first = 0;
}


//  CompReader::compress    --Compress a block of text.

int CompReader::compress( char const source[], int amount )
{
    int result = 0;
    if( _last + amount > HOLD_BUF ){
    shuffle();
    }

    memcpy( _buffer+_last, source, amount );
    _last += amount;

    int hash_value;
    int key_size, old_key_size;
    short   offset;
    short   best_match;
    int     limit;
    uint_8  *p1, *p2;

    while( _current+MIN_READ <= _last ){

    // Find the linked list corresponding to the current string.

    hash_value = LocalHash( _buffer+_current );
    old_key_size = MIN_READ-1;
    offset = _htable[hash_value];

    _indices[_current] = offset;
    _htable[hash_value] = _current;

    limit = READ_SIZE;
    if( limit > _last-_current ){
        limit = _last-_current;
    }

    while( offset >= _first ){
        // Find the length of the longest common string
        // starting at offset and current.  Optimized
        // for a little extra speed.

        p1 = _buffer+offset;
        p2 = _buffer+_current;

        // Compare four bytes at a time.
        for( key_size = 0; key_size<limit; key_size+=4 ){
        if( *(uint_32*)(p1+key_size) != *(uint_32*)(p2+key_size) ){
            break;
        }
        }
        if( key_size > limit ){
        key_size = limit;
        } else while( key_size<limit &&
                      _buffer[offset+key_size]==_buffer[_current+key_size]){
        key_size += 1;
        }

        if( key_size > old_key_size ){
        old_key_size = key_size;
        best_match = offset;
        if( old_key_size == limit ) break;
        }
        offset = _indices[offset];
    }

    // See if we found a match of usable size.

    if( old_key_size < MIN_READ ){
        result += _dest->putChr( _buffer[_current] );
        _current += 1;
    } else {
        result += _dest->putCode( _current-best_match, old_key_size );
        _current += old_key_size;
    }
    if( _current > HOLD_SIZE ){
        _first = (short) (_current - HOLD_SIZE);
    }
    }

    // If there's text left over, dump it to output.

    while( _current < _last ){
    result += _dest->putChr( _buffer[_current++] );
    }
    if( _current > HOLD_SIZE ){
    _first = (short) (_current - HOLD_SIZE);
    }

    return result;
}


//  CompReader::add --Add more text to buffer, but
//            pretend it's uncompressible.

int CompReader::add( char const source[], int amount )
{
    int result = 0;
    if( _last + amount > HOLD_BUF ){
    shuffle();
    }

    // There is a danger of the compressor referring to
    // this part of the buffer later to compress other text
    // (which would be inconsistent with skip().) so we fill
    // the buffer with a "rare" byte.
    memset( _buffer+_current, RARE_BYTE, amount );

    for( int i=0; i<amount; ++i ){
    result += _dest->putChr( source[i] );
    }
    _last += amount;
    _current += amount;
    if( _current > HOLD_SIZE ){
    _first = (short) (_current - HOLD_SIZE);
    }
    return result;
}


//  CompReader::skip    --Pretend to add uncompressible text to output.

int CompReader::skip( int amount )
{
    int result = 0;
    if( _last + amount > HOLD_BUF ){
    shuffle();
    }

    // There is a danger of the the compressor referring to
    // this part of the buffer later to compress other text.
    // So we fill the buffer with a "rare" byte.
    memset( _buffer+_current, RARE_BYTE, amount );

    for( int i=0; i<amount; ++i ){
    result += _dest->putChr( 0 );
    }
    _last += amount;
    _current += amount;
    if( _current > HOLD_SIZE ){
    _first = (short) (_current - HOLD_SIZE);
    }
    return result;
}
