Am 24.10.2017 um 20:59 schrieb Stefan Beller:
> This will be useful shortly.
>
> Signed-off-by: Stefan Beller <sbeller@xxxxxxxxxx>
> ---
> hashmap.c | 7 ++++++-
> hashmap.h | 3 +++
> 2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/hashmap.c b/hashmap.c
> index d42f01ff5a..d103eb1fd2 100644
> --- a/hashmap.c
> +++ b/hashmap.c
> @@ -26,13 +26,18 @@ unsigned int strihash(const char *str)
> return hash;
> }
>
> +unsigned int memhash_feed(unsigned int hash_seed, const unsigned char next)
Why is the second parameter const and the first one isn't? (We tend
not to bother with const for value types.)
> +{
> + return (hash_seed * FNV32_PRIME) ^ next;
> +}
> +
> unsigned int memhash(const void *buf, size_t len)
> {
> unsigned int hash = FNV32_BASE;
> unsigned char *ucbuf = (unsigned char *) buf;
> while (len--) {
> unsigned int c = *ucbuf++;
> - hash = (hash * FNV32_PRIME) ^ c;
> + hash = memhash_feed(hash, c);
I guess compilers inline a copy of the function here with -O2. My
knee-jerk reaction, however, is horror in the face of adding a function
call to the inner loop of a hash function. Do you have performance
test results, ideally also with -O0? And why not make memhash_feed()
an inline function or macro to sidestep that issue?
> }
> return hash;
> }
> diff --git a/hashmap.h b/hashmap.h
> index 7cb29a6aed..c2464385ed 100644
> --- a/hashmap.h
> +++ b/hashmap.h
> @@ -105,10 +105,13 @@
> * `strihash` and `memihash` are case insensitive versions.
> * `memihash_cont` is a variant of `memihash` that allows a computation to be
> * continued with another chunk of data.
> + * `memhash_feed` takes just one character and returns the hash based off
> + * a previous hash.
> */
> extern unsigned int strhash(const char *buf);
> extern unsigned int strihash(const char *buf);
> extern unsigned int memhash(const void *buf, size_t len);
> +extern unsigned int memhash_feed(unsigned int hash_seed, const unsigned char next);
> extern unsigned int memihash(const void *buf, size_t len);
> extern unsigned int memihash_cont(unsigned int hash_seed, const void *buf, size_t len);
>
>
