bitfield.c

//
// bitfield.c - extract and set bit fields
//
// Written by Eryk Vershen (eryk@apple.com)
//
// See comments in bitfield.h
//

/*
 * Copyright 1996, 1997 by Apple Computer, Inc.
 *              All Rights Reserved 
 *  
 * Permission to use, copy, modify, and distribute this software and 
 * its documentation for any purpose and without fee is hereby granted, 
 * provided that the above copyright notice appears in all copies and 
 * that both the copyright notice and this permission notice appear in 
 * supporting documentation. 
 *  
 * APPLE COMPUTER DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE 
 * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS 
 * FOR A PARTICULAR PURPOSE. 
 *  
 * IN NO EVENT SHALL APPLE COMPUTER BE LIABLE FOR ANY SPECIAL, INDIRECT, OR 
 * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM 
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN ACTION OF CONTRACT, 
 * NEGLIGENCE, OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION 
 * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 
 */

#include "bitfield.h"

//
// Global Constants
//
static const uint32_t masks[] = {
    0x00000000,
    0x00000001, 0x00000003, 0x00000007, 0x0000000F,
    0x0000001F, 0x0000003F, 0x0000007F, 0x000000FF,
    0x000001FF, 0x000003FF, 0x000007FF, 0x00000FFF,
    0x00001FFF, 0x00003FFF, 0x00007FFF, 0x0000FFFF,
    0x0001FFFF, 0x0003FFFF, 0x0007FFFF, 0x000FFFFF,
    0x001FFFFF, 0x003FFFFF, 0x007FFFFF, 0x00FFFFFF,
    0x01FFFFFF, 0x03FFFFFF, 0x07FFFFFF, 0x0FFFFFFF,
    0x1FFFFFFF, 0x3FFFFFFF, 0x7FFFFFFF, 0xFFFFFFFF
};

uint32_t
bitfield_set(uint32_t *bf, int base, int length, uint32_t value)
{
    // compute shift & mask, coerce value to correct number of bits,
    // zap the old bits and stuff the new value
    // return the masked value in case someone wants it.
    int s = (base + 1) - length;
    uint32_t m = masks[length];
    uint32_t t = value & m;
    *bf = (*bf & ~(m << s)) | (t << s);
    return t;
}


uint32_t
bitfield_get(uint32_t bf, int base, int length)
{
    // compute shift & mask
    // return the correct number of bits (shifted to low end)
    int s = (base + 1) - length;
    uint32_t m = masks[length];
    return ((bf >> s) & m);
}