Can someone explain fs/ext2/bitmap.c to me?

afei@jhu.edu
Sun, 12 Dec 1999 03:11:29 -0500 (EST)

what is this nibblemap? how is it determined?
The algorithm to calculate sum is so weird. I do not have a clue what it
means although I know this function calculates the number of free blocks
based on the bitmap.

/*
* linux/fs/ext2/bitmap.c
*
* Copyright (C) 1992, 1993, 1994, 1995
* Remy Card (card@masi.ibp.fr)
* Laboratoire MASI - Institut Blaise Pascal
* Universite Pierre et Marie Curie (Paris VI)
*/

#include <linux/fs.h>
#include <linux/ext2_fs.h>

static int nibblemap[] = {4, 3, 3, 2, 3, 2, 2, 1, 3, 2, 2, 1, 2, 1, 1, 0};

unsigned long ext2_count_free (struct buffer_head * map, unsigned int
numchars)
{
unsigned int i;
unsigned long sum = 0;

if (!map)
return (0);
for (i = 0; i < numchars; i++)
sum += nibblemap[map->b_data[i] & 0xf] +
nibblemap[(map->b_data[i] >> 4) & 0xf];
return (sum);
}

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