www.pudn.com > gzip.rar > bits.c

/* bits.c -- output variable-length bit strings 
 * Copyright (C) 1992-1993 Jean-loup Gailly 
 * This is free software; you can redistribute it and/or modify it under the 
 * terms of the GNU General Public License, see the file COPYING. 
 */ 
 
/* 
 *  PURPOSE 
 * 
 *      Output variable-length bit strings. Compression can be done 
 *      to a file or to memory. (The latter is not supported in this version.) 
 * 
 *  DISCUSSION 
 * 
 *      The PKZIP "deflate" file format interprets compressed file data 
 *      as a sequence of bits.  Multi-bit strings in the file may cross 
 *      byte boundaries without restriction. 
 * 
 *      The first bit of each byte is the low-order bit. 
 * 
 *      The routines in this file allow a variable-length bit value to 
 *      be output right-to-left (useful for literal values). For 
 *      left-to-right output (useful for code strings from the tree routines), 
 *      the bits must have been reversed first with bi_reverse(). 
 * 
 *      For in-memory compression, the compressed bit stream goes directly 
 *      into the requested output buffer. The input data is read in blocks 
 *      by the mem_read() function. The buffer is limited to 64K on 16 bit 
 *      machines. 
 * 
 *  INTERFACE 
 * 
 *      void bi_init (FILE *zipfile) 
 *          Initialize the bit string routines. 
 * 
 *      void send_bits (int value, int length) 
 *          Write out a bit string, taking the source bits right to 
 *          left. 
 * 
 *      int bi_reverse (int value, int length) 
 *          Reverse the bits of a bit string, taking the source bits left to 
 *          right and emitting them right to left. 
 * 
 *      void bi_windup (void) 
 *          Write out any remaining bits in an incomplete byte. 
 * 
 *      void copy_block(char *buf, unsigned len, int header) 
 *          Copy a stored block to the zip file, storing first the length and 
 *          its one's complement if requested. 
 * 
 */ 
 
#include "gzip.h" 
 
/* =========================================================================== 
 * Local data used by the "bit string" routines. 
 */ 
 
local unsigned short bi_buf = 0; 
/* Output buffer. bits are inserted starting at the bottom (least significant 
 * bits). 
 */ 
 
#define Buf_size (8 * 2*sizeof(char)) 
/* Number of bits used within bi_buf. (bi_buf might be implemented on 
 * more than 16 bits on some systems.) 
 */ 
 
local int bi_valid = 0; 
/* Number of valid bits in bi_buf.  All bits above the last valid bit 
 * are always zero. 
 */ 
 
int (* read_buf ) OF(( char * buf, unsigned size )); 
/* Current input function. Set to mem_read for in-memory compression */ 
 
/* =========================================================================== 
 * Initialize the bit string routines. 
 */ 
void bi_init ( void ) 
{ 
    bi_buf = 0; 
    bi_valid = 0; 
 
    /* Set the defaults for file compression. They are set by memcompress 
     * for in-memory compression. 
     */ 
    read_buf  = mem_read; 
} 
 
/* =========================================================================== 
 * Send a value on a given number of bits. 
 * IN assertion: length <= 16 and value fits in length bits. 
 */ 
void send_bits( value, length ) 
    int value;  /* value to send */ 
int length; /* number of bits */ 
{ 
    /* If not enough room in bi_buf, use (valid) bits from bi_buf and 
     * (16 - bi_valid) bits from value, leaving (width - (16-bi_valid)) 
     * unused bits in value. 
     */ 
    if ( bi_valid > ( int )Buf_size - length ) { 
        bi_buf |= ( value << bi_valid ); 
        put_short( bi_buf ); 
        bi_buf = ( ush )value >> ( Buf_size - bi_valid ); 
        bi_valid += length - Buf_size; 
    } else { 
        bi_buf |= value << bi_valid; 
        bi_valid += length; 
    } 
} 
 
/* =========================================================================== 
 * Reverse the first len bits of a code, using straightforward code (a faster 
 * method would use a table) 
 * IN assertion: 1 <= len <= 15 
 */ 
unsigned bi_reverse( unsigned code, int len ) 
{ 
    register unsigned res = 0; 
    do 
    { 
        res |= code & 1; 
        code >>= 1, res <<= 1; 
    } while (-- len > 0 ); 
    return res >> 1; 
} 
 
/* =========================================================================== 
 * Write out any remaining bits in an incomplete byte. 
 */ 
void bi_windup() 
{ 
    if ( bi_valid > 8 )/* 是否大于short长度 */ 
    { 
        put_short( bi_buf ); 
    } 
    else if ( bi_valid > 0 ) 
    { 
        put_byte( bi_buf ); 
    } 
    bi_buf = 0; 
    bi_valid = 0; 
} 
 
/* =========================================================================== 
 * Copy a stored block to the zip file, storing first the length and its 
 * one's complement if requested. 
 */ 
void copy_block( char * buf, unsigned len, int header ) 
{ 
    bi_windup();              /* align on byte boundary */ 
 
    if ( header ) 
    { 
        put_short(( ush )len ); 
        put_short(( ush )~ len ); 
    } 
    while ( len --) 
    { 
        put_byte(* buf ++); 
    } 
}