Linux平台下基于BitTorrent应用层协议的下载软件开发---位图模块（bitfield.c）

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <malloc.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include "parse_metafile.h"
#include "bitfield.h"

extern int  pieces_length;
extern char *file_name;

Bitmap      *bitmap = NULL;         // 指向位图
int         download_piece_num = 0; // 当前已下载的piece数

// 如果存在一个位图文件,则读位图文件并把获取的内容保存到bitmap
// 如此一来,就可以实现断点续传,即上次下载的内容不至于丢失
int create_bitfield()
{
bitmap = (Bitmap *)malloc(sizeof(Bitmap));
if(bitmap == NULL) {
printf("allocate memory for bitmap fiailed\n");
return -1;
}

// pieces_length除以20即为总的piece数
bitmap->valid_length = pieces_length / 20;
bitmap->bitfield_length = pieces_length / 20 / 8;//总的piece数，如果用字节来表示的话，需要多少字节？一个字节为8位
if( (pieces_length/20) % 8 != 0 )  bitmap->bitfield_length++;

bitmap->bitfield = (unsigned char *)malloc(bitmap->bitfield_length);//分配多少个字节
if(bitmap->bitfield == NULL)  {
printf("allocate memory for bitmap->bitfield fiailed\n");
if(bitmap != NULL)  free(bitmap);
return -1;
}

char bitmapfile[64];
sprintf(bitmapfile,"%dbitmap",pieces_length);

int  i;
FILE *fp = fopen(bitmapfile,"rb");
if(fp == NULL) {  // 若打开文件失败,说明开始的是一个全新的下载
memset(bitmap->bitfield, 0, bitmap->bitfield_length);
} else {
fseek(fp,0,SEEK_SET);
for(i = 0; i < bitmap->bitfield_length; i++)
(bitmap->bitfield)[i] = fgetc(fp);
fclose(fp);
// 给download_piece_num赋新的初值
download_piece_num = get_download_piece_num();
}

return 0;
}

int get_bit_value(Bitmap *bitmap,int index)
{
int           ret;
int           byte_index;
unsigned char byte_value;
unsigned char inner_byte_index;

if(index >= bitmap->valid_length)  return -1;

byte_index = index / 8;
byte_value = bitmap->bitfield[byte_index];
inner_byte_index = index % 8;

byte_value = byte_value >> (7 - inner_byte_index);//假设这里index为3，那么inner_byte_index为3,这里就要右移4位。正好可以用2是否整除来判断
if(byte_value % 2 == 0) ret = 0;
else                    ret = 1;

return ret;
}

int set_bit_value(Bitmap *bitmap,int index,unsigned char v)
{
int           byte_index;
unsigned char inner_byte_index;

if(index >= bitmap->valid_length)  return -1;//每一个piece都是1位
if((v != 0) && (v != 1))   return -1;//这里v的取值不是0就是1.

byte_index = index / 8;
inner_byte_index = index % 8;

v = v << (7 - inner_byte_index);//还是假设要设置第三位的值，那么左移4位，使其为1.然后再用或求其值
bitmap->bitfield[byte_index] = bitmap->bitfield[byte_index] | v;

return 0;
}

int all_zero(Bitmap *bitmap)
{
if(bitmap->bitfield == NULL)  return -1;
memset(bitmap->bitfield,0,bitmap->bitfield_length);
return 0;
}

int all_set(Bitmap *bitmap)
{
if(bitmap->bitfield == NULL)  return -1;
memset(bitmap->bitfield,0xff,bitmap->bitfield_length);//memset都是以字节来赋值，所以0xff相当于将每一个字节都置为1.
return 0;
}

void release_memory_in_bitfield()
{
if(bitmap->bitfield != NULL) free(bitmap->bitfield);
if(bitmap != NULL)  free(bitmap);
}

int print_bitfield(Bitmap *bitmap)
{
int i;

for(i = 0; i < bitmap->bitfield_length; i++) {//将每一个字节的值都打印出来，保留2位小数。
printf("%.2X ",bitmap->bitfield[i]);
if( (i+1) % 16 == 0)  printf("\n");//每16个数打印一个换行符。从上面的程序中可以看到，打印的数字应该在0到255之间
}
printf("\n");

return 0;
}

int restore_bitmap()
{
int  fd;
char bitmapfile[64];

if( (bitmap == NULL) || (file_name == NULL) )  return -1;

sprintf(bitmapfile,"%dbitmap",pieces_length);
fd = open(bitmapfile,O_RDWR|O_CREAT|O_TRUNC,0666);//如果文件存在，则将文件清零，且原有的存储属性不发生变化。
if(fd < 0)  return -1;

write(fd,bitmap->bitfield,bitmap->bitfield_length);//将位图写入fd描述符所对应的文件中。
close(fd);

return 0;
}

int is_interested(Bitmap *dst,Bitmap *src)
{
unsigned char const_char[8] = { 0x80,0x40,0x20,0x10,0x08,0x04,0x02,0x01};
unsigned char c1, c2;
int           i, j;

if( dst==NULL || src==NULL )  return -1;
if( dst->bitfield==NULL || src->bitfield==NULL )  return -1;//这里说的一个问题是，如果两个位图不同，则不会发生比较
if( dst->bitfield_length!=src->bitfield_length ||         //这里不同就是位图的大小不一样，或者有效位不一样
dst->valid_length!=src->valid_length )
return -1;

for(i = 0; i < dst->bitfield_length-1; i++) { //如果dest中的某一位为1，而src为0，则说明src对des感兴趣
for(j = 0; j < 8; j++) {
c1 = (dst->bitfield)[i] & const_char[j];//这里大于0，但是不一定等于1
c2 = (src->bitfield)[i] & const_char[j];
if(c1>0 && c2==0) return 1;
}
}

j  = dst->valid_length % 8; //从上面可以看出，上面比较的是bitfield_length-1个字节，现在比较的是最后一个字节
c1 = dst->bitfield[dst->bitfield_length-1];
c2 = src->bitfield[src->bitfield_length-1];
for(i = 0; i < j; i++) {
if( (c1&const_char[i])>0 && (c2&const_char[i])==0 )
return 1;
}

return 0;
}
/*
以上函数的功能测试代码如下：
测试时可以交换map1.bitfield和map2.bitfield的值或赋其他值

Bitmap map1, map2;
unsigned char bf1[2] = { 0xa0, 0xa0 };
unsigned char bf2[2] = { 0xe0, 0xe0 };

map1.bitfield        = bf1;
map1.bitfield_length = 2;
map1.valid_length    = 11;
map2.bitfield        = bf2;
map2.bitfield_length = 2;
map2.valid_length    = 11;

int ret = is_interested(&map1,&map2);
printf("%d\n",ret);
*/

// 获取当前已下载到的总的piece数
int get_download_piece_num()
{
unsigned char const_char[8] = { 0x80,0x40,0x20,0x10,0x08,0x04,0x02,0x01};
int           i, j;

if(bitmap==NULL || bitmap->bitfield==NULL)  return 0;

download_piece_num =0;

for(i = 0; i < bitmap->bitfield_length-1; i++) {
for(j = 0; j < 8; j++) {
if( ((bitmap->bitfield)[i] & const_char[j]) != 0)
download_piece_num++; //获取前bitfield_length-1个字节
}
}

unsigned char c = (bitmap->bitfield)[i]; // c存放位图最后一个字节
j = bitmap->valid_length % 8;            // j是位图最后一个字节的有效位数
for(i = 0; i < j; i++) {
if( (c & const_char[i]) !=0 ) download_piece_num++;
}

return download_piece_num;
}