Re: [PATCH 3/4] xdiff: use stronger hash function internally
- From
René Scharfe <l.s.r@web.de>
- Date
- Oct 24, 2017, 20:23 UTC
- Message-ID
- <16718615-c5df-24dd-fa62-2b42f5d83a02@web.de>
- In-Reply-To
- <20171024185917.20515-4-sbeller@google.com>
Am 24.10.2017 um 20:59 schrieb Stefan Beller:
> Instead of using the hash seeded with 5381, and updated via > `(hash << 5) ^ new_byte`, use the FNV-1 primitives as offered by > hashmap.h, which is seeded with 0x811c9dc5 and computed as > `(hash * 0x01000193) ^ new_byte`.
The hash function you're replacing is called DJB2; I think that's worth mentioning.
Performance test results would be nice. No idea how to find edge cases, though, or better: demonstrate a lack thereof.
Show 17 quoted lines
> > Signed-off-by: Stefan Beller <sbeller@google.com> > --- > xdiff/xutils.c | 19 ++++++++----------- > 1 file changed, 8 insertions(+), 11 deletions(-) > > diff --git a/xdiff/xutils.c b/xdiff/xutils.c > index 04d7b32e4e..a58a28c687 100644 > --- a/xdiff/xutils.c > +++ b/xdiff/xutils.c > @@ -24,7 +24,8 @@ > #include <assert.h> > #include "xinclude.h" > > - > +#include "cache.h" > +#include "hashmap.h"
Ouch. Defining FNV32_BASE and FNV32_PRIME here would be much easier overall. And if that's too much duplication then those definitions could be extracted into a new header file (fnv32.h?) included by both hashmap.h and xutils.c.
Show 19 quoted lines
>
>
> long xdl_bogosqrt(long n) {
> @@ -228,7 +229,7 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
>
> static unsigned long xdl_hash_record_with_whitespace(char const **data,
> char const *top, long flags) {
> - unsigned long ha = 5381;
> + unsigned long ha = memhash(NULL, 0);
> char const *ptr = *data;
>
> for (; ptr < top && *ptr != '\n'; ptr++) {
> @@ -243,21 +244,18 @@ static unsigned long xdl_hash_record_with_whitespace(char const **data,
> ; /* already handled */
> else if (flags & XDF_IGNORE_WHITESPACE_CHANGE
> && !at_eol) {
> - ha += (ha << 5);
> - ha ^= (unsigned long) ' ';
> + ha = memhash_feed(ha, (unsigned char) ' ');All the memhash_feed() callers in this file cast to unsigned char. A macro or a function (possibly inline) defined at the top could do that for them.
Show 37 quoted lines
> }
> else if (flags & XDF_IGNORE_WHITESPACE_AT_EOL
> && !at_eol) {
> while (ptr2 != ptr + 1) {
> - ha += (ha << 5);
> - ha ^= (unsigned long) *ptr2;
> + ha = memhash_feed(ha, (unsigned char) *ptr2);
> ptr2++;
> }
> }
> continue;
> }
> - ha += (ha << 5);
> - ha ^= (unsigned long) *ptr;
> + ha = memhash_feed(ha, (unsigned char) *ptr);
> }
> *data = ptr < top ? ptr + 1: ptr;
>
> @@ -265,15 +263,14 @@ static unsigned long xdl_hash_record_with_whitespace(char const **data,
> }
>
> unsigned long xdl_hash_record(char const **data, char const *top, long flags) {
> - unsigned long ha = 5381;
> + unsigned long ha = memhash(NULL, 0);
> char const *ptr = *data;
>
> if (flags & XDF_WHITESPACE_FLAGS)
> return xdl_hash_record_with_whitespace(data, top, flags);
>
> for (; ptr < top && *ptr != '\n'; ptr++) {
> - ha += (ha << 5);
> - ha ^= (unsigned long) *ptr;
> + ha = memhash_feed(ha, (unsigned char) *ptr);
> }
> *data = ptr < top ? ptr + 1: ptr;
>
>