bits.c

   1 /* bits.c -- output variable-length bit strings
   2  * Copyright (C) 1992-1993 Jean-loup Gailly
   3  * This is free software; you can redistribute it and/or modify it under the
   4  * terms of the GNU General Public License, see the file COPYING.
   5  */
   6
   7
   8 /*
   9  *  PURPOSE
  10  *
  11  *      Output variable-length bit strings. Compression can be done
  12  *      to a file or to memory. (The latter is not supported in this version.)
  13  *
  14  *  DISCUSSION
  15  *
  16  *      The PKZIP "deflate" file format interprets compressed file data
  17  *      as a sequence of bits.  Multi-bit strings in the file may cross
  18  *      byte boundaries without restriction.
  19  *
  20  *      The first bit of each byte is the low-order bit.
  21  *
  22  *      The routines in this file allow a variable-length bit value to
  23  *      be output right-to-left (useful for literal values). For
  24  *      left-to-right output (useful for code strings from the tree routines),
  25  *      the bits must have been reversed first with bi_reverse().
  26  *
  27  *      For in-memory compression, the compressed bit stream goes directly
  28  *      into the requested output buffer. The input data is read in blocks
  29  *      by the mem_read() function. The buffer is limited to 64K on 16 bit
  30  *      machines.
  31  *
  32  *  INTERFACE
  33  *
  34  *      void bi_init (FILE *zipfile)
  35  *          Initialize the bit string routines.
  36  *
  37  *      void send_bits (int value, int length)
  38  *          Write out a bit string, taking the source bits right to
  39  *          left.
  40  *
  41  *      int bi_reverse (int value, int length)
  42  *          Reverse the bits of a bit string, taking the source bits left to
  43  *          right and emitting them right to left.
  44  *
  45  *      void bi_windup (void)
  46  *          Write out any remaining bits in an incomplete byte.
  47  *
  48  *      void copy_block(char *buf, unsigned len, int header)
  49  *          Copy a stored block to the zip file, storing first the length and
  50  *          its one's complement if requested.
  51  *
  52  */
  53
  54 #include "tailor.h"
  55 #include "gzip.h"
  56 #include "crypt.h"
  57
  58 #ifdef DEBUG
  59 #  include <stdio.h>
  60 #endif
  61
  62 #ifdef RCSID
  63 static char rcsid[] = "$Id$";
  64 #endif
  65
  66 /* ===========================================================================
  67  * Local data used by the "bit string" routines.
  68  */
  69
  70 local file_t zfile; /* output gzip file */
  71
  72 local unsigned short bi_buf;
  73 /* Output buffer. bits are inserted starting at the bottom (least significant
  74  * bits).
  75  */
  76
  77 #define Buf_size (8 * 2*sizeof(char))
  78 /* Number of bits used within bi_buf. (bi_buf might be implemented on
  79  * more than 16 bits on some systems.)
  80  */
  81
  82 local int bi_valid;
  83 /* Number of valid bits in bi_buf.  All bits above the last valid bit
  84  * are always zero.
  85  */
  86
  87 int (*read_buf) OF((char *buf, unsigned size));
  88 /* Current input function. Set to mem_read for in-memory compression */
  89
  90 #ifdef DEBUG
  91   ulg bits_sent;   /* bit length of the compressed data */
  92 #endif
  93
  94 /* ===========================================================================
  95  * Initialize the bit string routines.
  96  */
  97 void bi_init (zipfile)
  98     file_t zipfile; /* output zip file, NO_FILE for in-memory compression */
  99 {
 100     zfile  = zipfile;
 101     bi_buf = 0;
 102     bi_valid = 0;
 103 #ifdef DEBUG
 104     bits_sent = 0L;
 105 #endif
 106
 107     /* Set the defaults for file compression. They are set by memcompress
 108      * for in-memory compression.
 109      */
 110     if (zfile != NO_FILE) {
 111         read_buf  = file_read;
 112     }
 113 }
 114
 115 /* ===========================================================================
 116  * Send a value on a given number of bits.
 117  * IN assertion: length <= 16 and value fits in length bits.
 118  */
 119 void send_bits(value, length)
 120     int value;  /* value to send */
 121     int length; /* number of bits */
 122 {
 123 #ifdef DEBUG
 124     Tracev((stderr," l %2d v %4x ", length, value));
 125     Assert(length > 0 && length <= 15, "invalid length");
 126     bits_sent += (ulg)length;
 127 #endif
 128     /* If not enough room in bi_buf, use (valid) bits from bi_buf and
 129      * (16 - bi_valid) bits from value, leaving (width - (16-bi_valid))
 130      * unused bits in value.
 131      */
 132     if (bi_valid > (int)Buf_size - length) {
 133         bi_buf |= (value << bi_valid);
 134         put_short(bi_buf);
 135         bi_buf = (ush)value >> (Buf_size - bi_valid);
 136         bi_valid += length - Buf_size;
 137     } else {
 138         bi_buf |= value << bi_valid;
 139         bi_valid += length;
 140     }
 141 }
 142
 143 /* ===========================================================================
 144  * Reverse the first len bits of a code, using straightforward code (a faster
 145  * method would use a table)
 146  * IN assertion: 1 <= len <= 15
 147  */
 148 unsigned bi_reverse(code, len)
 149     unsigned code; /* the value to invert */
 150     int len;       /* its bit length */
 151 {
 152     register unsigned res = 0;
 153     do {
 154         res |= code & 1;
 155         code >>= 1, res <<= 1;
 156     } while (--len > 0);
 157     return res >> 1;
 158 }
 159
 160 /* ===========================================================================
 161  * Write out any remaining bits in an incomplete byte.
 162  */
 163 void bi_windup()
 164 {
 165     if (bi_valid > 8) {
 166         put_short(bi_buf);
 167     } else if (bi_valid > 0) {
 168         put_byte(bi_buf);
 169     }
 170     bi_buf = 0;
 171     bi_valid = 0;
 172 #ifdef DEBUG
 173     bits_sent = (bits_sent+7) & ~7;
 174 #endif
 175 }
 176
 177 /* ===========================================================================
 178  * Copy a stored block to the zip file, storing first the length and its
 179  * one's complement if requested.
 180  */
 181 void copy_block(buf, len, header)
 182     char     *buf;    /* the input data */
 183     unsigned len;     /* its length */
 184     int      header;  /* true if block header must be written */
 185 {
 186     bi_windup();              /* align on byte boundary */
 187
 188     if (header) {
 189         put_short((ush)len);
 190         put_short((ush)~len);
 191 #ifdef DEBUG
 192         bits_sent += 2*16;
 193 #endif
 194     }
 195 #ifdef DEBUG
 196     bits_sent += (ulg)len<<3;
 197 #endif
 198     while (len--) {
 199 #ifdef CRYPT
 200         int t;
 201         if (key) zencode(*buf, t);
 202 #endif
 203         put_byte(*buf++);
 204     }
 205 }