bit count in c (1)-优快云博客

本文详细探讨了四种高性能位计数算法的实现原理、优化策略及其在不同场景下的应用效果。通过对比分析，展示了如何在有限时间内高效地统计整数中1的个数，为计算机科学领域的性能优化提供了宝贵的思路。

=============================运行结果

4296000 -> 10000011000110101000000

a1_bitcount: num of 1=7, time=782 ms

a2_bitcount: num of 1=7, time=265 ms

a3_bitcount: num of 1=7, time=938 ms

a4a_bitcount: num of 1=7, time=78 ms

a4b_bitcount: num of 1=7, time=62 ms

=============================运行结果end

Playing

=============================文件列表

src/main.c

src/lib/bitCount.c

src/lib/showBinary.c

src/lib/ut.h

src/lib/pre16.h

src/lib/pre8.h

=============================代码

src/main.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "lib/ut.h"

typedef int (*FunctionCallback)(unsigned int);
FunctionCallback functions[] = { &a1_bitcount, &a2_bitcount, &a3_bitcount
    , &a4a_bitcount, &a4b_bitcount};
char * names[]={"a1_bitcount", "a2_bitcount", "a3_bitcount"
    , "a4a_bitcount", "a4b_bitcount"};

void testFun(int fIdx, unsigned int num) {
  int rlt;
  long i;
  clock_t start = clock();
  //注意，这里多运行n次，只是为了测试时间方便。
  for (i = 0; i < 10000000; i++) {
    rlt = functions[fIdx](num);
  }
  double time = ((double) clock() - start); // / CLOCKS_PER_SEC
  printf("  %s: num of 1=%d, time=%.0f ms\n", names[fIdx], rlt, time);
}

int main(void) {
  //unsigned int num = 4294967295;
  unsigned int num = 4296000;
  //unsigned int num = -1;
  int funSize = sizeof(functions) / sizeof(FunctionCallback);
  showBinary(num);
  int i;
  for(i=0;i<funSize;i++){
    testFun(i, num);
  }
  return EXIT_SUCCESS;
}

src/lib/bitCount.c

/*
 * bitCount.c
 * http://gurmeet.net/puzzles/fast-bit-counting-routines/
 */
#include "ut.h"
#include "pre8.h"
#include "pre16.h"

//1. Iterated Count
int a1_bitcount(unsigned int n) {
  int count = 0;
  while (n) {
    count += n & 0x1u;
    n >>= 1;
  }
  return count;
}

//2. Sparse Ones
int a2_bitcount(unsigned int n) {
  int count = 0;
  while (n) {
    count++;
    n &= (n - 1);
  }
  return count;
}

//3. Dense Ones
int a3_bitcount(unsigned int n) {
  int count = 8 * sizeof(int);
  n ^= (unsigned int) -1;
  while (n) {
    count--;
    n &= (n - 1);
  }
  return count;
}

//4a. Precompute-8bit
int a4a_bitcount(unsigned int n) {
  // works only for 32-bit ints
  return bits_in_char[n & 0xffu]
       + bits_in_char[(n >> 8) & 0xffu]
       + bits_in_char[(n >> 16) & 0xffu]
       + bits_in_char[(n >> 24) & 0xffu];
}

//4b. Precompute-16bit
int a4b_bitcount (unsigned int n)  {
   // works only for 32-bit ints
   return bits_in_16bits [n         & 0xffffu]
       +  bits_in_16bits [(n >> 16) & 0xffffu] ;
}

src/lib/showBinary.c

/*
 *  http://stackoverflow.com/questions/699968/display-the-binary-representation-of-a-number-in-c
 */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "ut.h"
#define SZ 32

/* Create a string of binary digits based on the input value.
 Input:
 val:  value to convert.
 buff: buffer to write to must be >= sz+1 chars.
 sz:   size of buffer.
 Returns address of string or NULL if not enough space provided.
 */
static char *binrep(unsigned int val, char *buff, int sz) {
  char *pbuff = buff;

  /* Must be able to store one character at least. */
  if (sz < 1)
    return NULL;

  /* Special case for zero to ensure some output. */
  if (val == 0) {
    *pbuff++ = '0';
    *pbuff = '\0';
    return buff;
  }

  /* Work from the end of the buffer back. */
  pbuff += sz;
  *pbuff-- = '\0';

  /* For each bit (going backwards) store character. */
  while (val != 0) {
    if (sz-- == 0)
      return NULL;
    *pbuff-- = ((val & 1) == 1) ? '1' : '0';

    /* Get next bit. */
    val >>= 1;
  }
  return pbuff + 1;
}

void showBinary(unsigned int num) {
  char buff[SZ + 1];
  printf("%d -> %s\n", num, binrep(num, buff, SZ));
}

int main_showBinary(void) {
  char *argv[] = { "", "4" };
  int argc = sizeof(argv) / sizeof(char *);
  int i;
  int n;
  for (i = 1; i < argc; i++) {
    n = atoi(argv[i]);
    showBinary(n);
  }
  return 0;
}

src/lib/ut.h

#ifndef H1_H_
#define H1_H_

// here goes all the code
void showBinary(unsigned int num);
int a1_bitcount(unsigned int n);
int a2_bitcount(unsigned int n);
int a3_bitcount(unsigned int n);
int a4a_bitcount(unsigned int n);
int a4b_bitcount(unsigned int n);

#endif /* H1_H_ */

src/lib/pre8.h

#ifndef PRE8_H_
#define PRE8_H_

static int bits_in_char [256]={
0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5
, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6
, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6
, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7
, 1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6
, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7
, 2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7
, 3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7, 4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};

#endif /* PRE8_H_ */

src/lib/pre8.h

src/lib/pre16.h

生成上述二文件的代码在bit count in c (2) -- precompute