Jon Bentley in Column 1 of his book programming pearls introduces a technique for sorting a sequence of non-zero positive integers using bit vectors.
I have taken the program bitsort.c from [here][1] and pasted it below:
/* 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
#define BITSPERWORD 32
#define SHIFT 5
#define MASK 0x1F
#define N 10000000
int a[1 + N/BITSPERWORD];
void set(int i)
{
int sh = i>>SHIFT;
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;
}
I understand what the functions clr, set and test are doing and explain them below: ( please correct me if I am wrong here ).
- clr clears the ith bit
- set sets the ith bit
- test returns the value at the ith bit
Now, I don't understand how the functions do what they do. I am unable to figure out all the bit manipulation happening in those three functions.
[1]: https://web.archive.org/web/20080725054521/http://www.cs.bell-labs.com/cm/cs/pearls/code.html I will accept Laurence's answer as it helped me understand about the mechanics of the program at a fundamental level.
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