《编程珠玑》一段代码看不懂，“a[i>>SHIFT] |= (1<<(i & MASK));”

/* 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

#define N 10000000

int a[1 + N/BITSPERWORD];



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);

	return 0;

}