知識+ 位元陣列轉換

//===============================================================
// 20090616 知識 +
// http://tw.knowledge.yahoo.com/question/question?qid=1509061603511
// 發問者 :(pplgo ) http://tw.knowledge.yahoo.com/my/my?show=AD01409480
//===============================================================

這還是一個小程式, 是做位元陣列轉換的, 題目是將8bits的陣列, 轉換為5bits的陣列,
我寫了一種方法; 而東大也寫了一種方法, 因有參考價值所以一併收錄...

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// ADing法轉換 -------
unsigned char * BitsConvert(int bits, unsigned char *A, int *size)
{
  if (bits<1 || bits>8 || A==NULL || *size<1) return NULL;
  int i, a, b, s[8], len=*size;
  *size = (*size*8+bits-1)/bits;
  unsigned char * B = (unsigned char *) malloc(*size);
  unsigned char m = 0xFF>>(8-bits);
  for(b=0, i=8-bits; b<8; ++b, i=(i<=0)?i+8-bits: i-bits)
       s[b]=i;
  for(b=0; b<*size; ++b)
  {
     i=s[b%8];
     a=b*bits/8;
     if (i>=0) B[b] = (A[a]>>i)&m;
     else if(a+1>=len) B[b] = (A[a]<<-i)&m;
     else B[b] = ((A[a]<<-i)|(A[a+1]>>(8+i)))&m;
  }
  return B;
}
// 東大法轉換 --------
unsigned char * BitsTransfer(int bits, unsigned char *A, int *size)
{
  if (bits<1 || bits>8 || A==NULL || *size<1) return NULL;
  int i, j, a, b, d, len=*size;
  *size = (*size*8+bits-1)/bits;
  unsigned char * B = (unsigned char *) malloc(*size);
  unsigned char m = 0x80;
  for(a=b=d=j=0; a<len; ++a)
  {
    for(i=0; i<8; ++i)
    {
       d = (d<<1) + ((A[a]&m)? 1: 0);
       if((m>>=1)==0) m=0x80;
       if(++j==bits)
       {
         B[b++]=d;
         d=j=0;
       }
    }
  }
  if (j)  B[b++]=d<<(bits-j);
  return B;
}
//==================================
const int MAX = 32;    // 資料量
const int BIT = 5;     // 擷取bit數

// --- 資料顯示
void DisplayData( char *Msg, unsigned char *data, int len)
{
  int i;
  printf(Msg, len);
  for(i=0; i<len; )
  {
    printf("%02X ", data[i]);
    if(++i%20==0 || i==len) printf("\n ");
  }
}

// 主程式 =======
int main()
{
  unsigned char *a;
  int i, len;
  unsigned char *b;
  double runC, runT;
  clock_t startC, startT;
 
  a = (unsigned char *)malloc(MAX);
  srand((unsigned)time(NULL));
  for(i=0; i<MAX; i++)
     a[i] = (unsigned char)(rand()%0x100);
  DisplayData("顯示原始資料, 共%d筆:\n ", a, MAX);

  //-------------------------- BitsConvert()
  startC = clock();
  len = MAX;
  b = BitsConvert(BIT, a, &len);
  runC = (double)(clock()-startC)*1000/CLOCKS_PER_SEC;
  if (b!=NULL)
  {
    DisplayData("\n顯示用 BitsConvert()處理後的資料, 共%d筆:\n ", b, len);
    free(b);
  }
  printf("\n* BitsConvert() 的處理時間為 %.2fms\n\n", runC);
  //--------------------------

  //-------------------------- BitsTransfer()
  startT = clock();
  len = MAX;
  b = BitsTransfer(BIT, a, &len);
  runT = (double)(clock()-startT)*1000/CLOCKS_PER_SEC;
  if (b!=NULL)
  {
    DisplayData("\n顯示用 BitsConvert()處理後的資料, 共%d筆:\n ", b, len);
    free(b);
  }
  printf("\n* BitsTransfer() 的處理時間為 %.2fms\n\n", runT);
  //--------------------------

  free(a);
  return 0;
}
//==============================================