编程珠玑chapter1

 

/* Copyright (C) 1999 Lucent Technologies */
/* From 'Programming Pearls' by Jon Bentley */

/* bitsort.c -- bitmap sort from Column 1
 *   Sort distinct integers in the range [0..N-1]
 */

#include <stdio.h>

#define BITSPERWORD 32
#define SHIFT 5
#define MASK 0x1F // MASK = 31
#define N 1000000
int a[1 + N/BITSPERWORD];
//i >> SHIFT -> i >> 5 -> i/32 -> 数字i所对应的数组的index
//i & MASK -> i & 31 -> i % 32 -> 对应数组元素的bit位
//a[i >> SHIFT] -> 指的是i所在的位所在的数组位置
//1<<(i & MASK)
void set(int i) {        a[i>>SHIFT] |=  (1<<(i & MASK)); }
void clr(int i) {        a[i>>SHIFT] &= ~(1<<(i & MASK)); }
int  test(int i){ return a[i>>SHIFT] &   (1<<(i & MASK)); }

int main()
{ int i;
  for (i = 0; i < N; i++)
    clr(i);
/*  Replace above 2 lines with below 3 for word-parallel init
  int top = 1 + N/BITSPERWORD;
  for (i = 0; i < top; i++)
    a[i] = 0;
 */
  while (scanf("%d", &i) != EOF)
    set(i);
  for (i = 0; i < N; i++)
    if (test(i))
      printf("%d\n", i);
}