[patch 02/12] rfc: 2fsprogs update

Alexandre Ratchov
2006-09-26T14:45:50Z

Signed-off-by: Alexandre Ratchov <alexandre.ratchov@bull.net>

Index: e2fsprogs-1.39/lib/e2p/percent.c
===================================================================
--- e2fsprogs-1.39.orig/lib/e2p/percent.c 2006-09-20 12:07:05.000000000 +0200
+++ e2fsprogs-1.39/lib/e2p/percent.c 2006-09-20 12:36:32.000000000 +0200
@@ -7,25 +7,50 @@
* Public License
*/

+/*
#include "e2p.h"
+*/

#include <stdlib.h>

/*
* We work really hard to calculate this accurately, while avoiding
* an overflow. "Is there a hyphen in anal-retentive?" :-)
+ *
+ * -- "Yes there is, as in hair-splitting and nit-picking"
*/
unsigned int e2p_percent(int percent, unsigned int base)
{
- unsigned int mask = ~((1 << (sizeof(unsigned int) - 1) * 8) - 1);
+ unsigned hi, lo, q, r;

- if (100 % percent == 0)
- return base / (100 / percent);
- if (mask & base)
- return (base / 100) * percent;
- return base * percent / 100;
+ /*
+ * in order to avoid overflow we write 'base' as:
+ *
+ * base = hi * 2^16 + lo
+ *
+ * then we do all computations separately on 'hi' and 'lo'.
+ * By using the definition of division:
+ *
+ * precent * base = result * 100 + reminder
+ *
+ * (reminder < 100), we obtain the exact value of 'result'
+ * as follows:
+ */
+#define BITS 16
+#define MASK ((1 << BITS) - 1)
+
+ hi = percent * (base >> BITS);
+ lo = percent * (base & MASK);
+
+ q = ((hi / 100) << BITS) + lo / 100;
+ r = ((hi % 100) << BITS) + lo % 100;
+
+ return q + r / 100;
+#undef BITS
+#undef MASK
}

+
#ifdef DEBUG
#include <unistd.h>
#include <stdio.h>