sum.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

#include <u.h>
#include <libc.h>

typedef ulong	Sumfn(ulong, void*, uvlong);
extern Sumfn	sumr, sum5, sum32;
char		*sumfile(char*, Sumfn*);

void
usage(void)
{
	fprint(2, "Usage: %s [-r5] [files]\n", argv0);
	exits("usage");
}

void
main(int argc, char **argv)
{
	Sumfn *fn = sum32;
	char *exitstr=0, *s;

	ARGBEGIN{
	case 'r':
		fn = sumr;
		break;
	case '5':
		fn = sum5;
		break;
	default:
		usage();
		break;
	}ARGEND
	if(*argv){
		while(*argv)
			if(s = sumfile(*argv++, fn))	/* assign = */
				exitstr = s;
	}else
		exitstr = sumfile(0, fn);
	exits(exitstr);
}

char*
sumfile(char *file, Sumfn *fn)
{
	int fd;
	int n;
	ulong sum;
	uvlong fsize;
	char buf[8*1024];

	if(file){
		if((fd = open(file, OREAD)) < 0){
			errstr(buf, sizeof buf);
			fprint(2, "%s: %s: %s\n", argv0, file, buf);
			return "can't open";
		}
	}else
		fd = 0;
	fsize = 0;
	sum = 0;
	while((n=read(fd, buf, sizeof buf)) > 0){
		fsize += n;
		sum = (*fn)(sum, buf, n);
	}
	if(n < 0){
		errstr(buf, sizeof buf);
		fprint(2, "%s: %s: read error: %s\n", argv0, file? file:"<stdin>", buf);
		if(file)
			close(fd);
		return "read error";
	}
	if(file)
		close(fd);
	(*fn)(sum, (char*)0, fsize);
	if(file)
		print(" %s", file);
	print("\n");
	return 0;
}

#define	VBSIZE		512		/* system v */

ulong
sum5(ulong sum, void *buf, uvlong uvn)
{
	uchar *s, *send;
	int n;

	if(buf == 0){
		sum = ((sum>>16)+sum) & 0xFFFF;
		print("%.5lud%6lld", sum, (uvn+(VBSIZE-1))/VBSIZE);
		return 0;
	}
	n = uvn;
	for(s=buf, send=s+n; s<send; s++)
		sum += 0xffff & *s;
	return sum;
}

#define	RBSIZE		1024		/* research */

ulong
sumr(ulong sum, void *buf, uvlong uvn)
{
	uchar *s, *send;
	int n;

	if(buf == 0){
		sum &= 0xFFFF;
		print("%.5lud%6lld", sum, (uvn+(RBSIZE-1))/RBSIZE);
		return 0;
	}
	n = uvn;
	for(s=buf, send=s+n; s<send; s++)
		if(sum & 1)
			sum = 0xffff & ((sum>>1)+*s+0x8000);
		else
			sum = 0xffff & ((sum>>1)+*s);
	return sum;
}

extern ulong crc_table[256];

ulong
sum32(ulong lcrc, void *buf, uvlong uvn)
{
	uchar *s = buf;
	ulong crc = lcrc;
	int n;

	n = uvn;
	if(buf == 0){
		char x[4];

		x[0] = (n>>24)^0xCC;	/* encode the length but make n==0 not 0 */
		x[1] = (n>>16)^0x55;
		x[2] = (n>>8)^0xCC;
		x[3] = (n)^0x55;
		crc = sum32(lcrc, x, 4);
		print("%.8lux %6lld", crc, uvn);
		return 0;
	}
	while(n-- > 0)
		crc = crc_table[(crc^*s++)&0xff] ^ (crc>>8);
	return crc;
}

/*
 *	CRC 035556101440
 */
ulong crc_table[256] = {
	0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
	0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
	0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
	0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
	0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
	0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
	0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
	0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
	0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
	0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
	0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
	0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
	0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
	0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
	0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
	0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
	0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
	0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
	0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
	0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
	0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
	0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
	0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
	0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
	0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
	0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
	0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
	0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
	0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
	0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
	0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
	0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
	0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
	0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
	0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
	0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
	0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
	0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
	0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
	0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
	0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
	0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
	0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
	0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
	0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
	0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
	0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
	0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
	0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
	0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
	0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
	0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
	0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
	0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
	0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
	0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
	0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
	0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
	0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
	0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
	0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
	0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
	0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
	0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d,
};