File: enc_int.c

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/*
**      SWISH++
**      enc_int.c
**
**      Copyright (C) 2003  Paul J. Lucas
**
**      This program is free software; you can redistribute it and/or modify
**      it under the terms of the GNU General Public License as published by
**      the Free Software Foundation; either version 2 of the License, or
**      (at your option) any later version.
**
**      This program is distributed in the hope that it will be useful,
**      but WITHOUT ANY WARRANTY; without even the implied warranty of
**      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
**      GNU General Public License for more details.
**
**      You should have received a copy of the GNU General Public License
**      along with this program; if not, write to the Free Software
**      Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

// standard
#include <iostream>

// local
#include "enc_int.h"
#include "platform.h"

using namespace std;

//*****************************************************************************
//
// SYNOPSIS
//
        int dec_int( register unsigned char const *&p )
//
// DESCRIPTION
//
//      Decode an integer from an encoded byte sequence.  See the comment for
//      enc_int() for details of the encoding scheme.
//
// PARAMETERS
//
//      p   A pointer to the start of the encoded integer.  After an integer is
//          decoded, it is left one past the last byte.
//
// RETURN VALUE
//
//      The integer.
//
//*****************************************************************************
{
    register unsigned n = 0;
    do {
        n = (n << 7) | (*p & 0x7Fu);
    } while ( *p++ & 0x80u );
    return n;
}

//*****************************************************************************
//
// SYNOPSIS
//
        ostream& enc_int( ostream &o, register unsigned n )
//
// DESCRIPTION
//
//      Write an unsigned integer to the given ostream in an encoded format.
//      The format uses a varying number of bytes.  For a given byte, only the
//      lower 7 bits are used for data; the high bit, if set, is used to
//      indicate whether the integer continues into the next byte.  The encoded
//      integer is written to the given ostream starting with the most
//      significant byte.
//
// PARAMETERS
//
//      o   The ostream to write to.
//
//      n   The integer to be written.
//
// RETURN VALUE
//
//      Returns the passed-in ostream.
//
//*****************************************************************************
{
    unsigned char buf[ 20 ];
    //
    // Encode the integer (in reverse because it's easier) just like atoi().
    //
    register unsigned char *p = buf + sizeof buf;
    do {
        *--p = 0x80u | (n & 0x7Fu);
    } while ( n >>= 7 );
    buf[ sizeof buf - 1 ] &= 0x7Fu;     // clear last "continuation bit"

    return o.write( reinterpret_cast<char*>( p ), buf + sizeof buf - p );
}
/* vim:set et sw=4 ts=4: */