Re: [PATCH 2/2] packfile: refactor hash search with fanout table
Am 02.02.2018 um 23:36 schrieb Jonathan Tan:
Show 53 quoted lines
> Subsequent patches will introduce file formats that make use of a fanout
> array and a sorted table containing hashes, just like packfiles.
> Refactor the hash search in packfile.c into its own function, so that
> those patches can make use of it as well.
>
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
> packfile.c | 19 +++++--------------
> sha1-lookup.c | 24 ++++++++++++++++++++++++
> sha1-lookup.h | 21 +++++++++++++++++++++
> 3 files changed, 50 insertions(+), 14 deletions(-)
>
> diff --git a/packfile.c b/packfile.c
> index 58bdced3b..29f5dc239 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -1712,7 +1712,8 @@ off_t find_pack_entry_one(const unsigned char *sha1,
> {
> const uint32_t *level1_ofs = p->index_data;
> const unsigned char *index = p->index_data;
> - unsigned hi, lo, stride;
> + unsigned stride;
> + int ret;
>
> if (!index) {
> if (open_pack_index(p))
> @@ -1725,8 +1726,6 @@ off_t find_pack_entry_one(const unsigned char *sha1,
> index += 8;
> }
> index += 4 * 256;
> - hi = ntohl(level1_ofs[*sha1]);
> - lo = ((*sha1 == 0x0) ? 0 : ntohl(level1_ofs[*sha1 - 1]));
> if (p->index_version > 1) {
> stride = 20;
> } else {
> @@ -1734,17 +1733,9 @@ off_t find_pack_entry_one(const unsigned char *sha1,
> index += 4;
> }
>
> - while (lo < hi) {
> - unsigned mi = lo + (hi - lo) / 2;
> - int cmp = hashcmp(index + mi * stride, sha1);
> -
> - if (!cmp)
> - return nth_packed_object_offset(p, mi);
> - if (cmp > 0)
> - hi = mi;
> - else
> - lo = mi+1;
> - }
> + ret = bsearch_hash(sha1, level1_ofs, index, stride);
> + if (ret >= 0)
> + return nth_packed_object_offset(p, ret);Going from unsigned to signed int means the patch breaks support for more than 2G pack entries, which was put with 326bf39677 (Use uint32_t for all packed object counts.) in 2007.
Show 16 quoted lines
> return 0;
> }
>
> diff --git a/sha1-lookup.c b/sha1-lookup.c
> index 4cf3ebd92..d11c5e526 100644
> --- a/sha1-lookup.c
> +++ b/sha1-lookup.c
> @@ -99,3 +99,27 @@ int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
> } while (lo < hi);
> return -lo-1;
> }
> +
> +int bsearch_hash(const unsigned char *sha1, const void *fanout_,
> + const void *table_, size_t stride)
> +{
> + const uint32_t *fanout = fanout_;Why hide the type? It doesn't make the function more generic.
Show 50 quoted lines
> + const unsigned char *table = table_;
> + int hi, lo;
> +
> + hi = ntohl(fanout[*sha1]);
> + lo = ((*sha1 == 0x0) ? 0 : ntohl(fanout[*sha1 - 1]));
> +
> + while (lo < hi) {
> + unsigned mi = lo + (hi - lo) / 2;
> + int cmp = hashcmp(table + mi * stride, sha1);
> +
> + if (!cmp)
> + return mi;
> + if (cmp > 0)
> + hi = mi;
> + else
> + lo = mi + 1;
> + }
> + return -lo - 1;
> +}
> diff --git a/sha1-lookup.h b/sha1-lookup.h
> index cf5314f40..3c59e9cb1 100644
> --- a/sha1-lookup.h
> +++ b/sha1-lookup.h
> @@ -7,4 +7,25 @@ extern int sha1_pos(const unsigned char *sha1,
> void *table,
> size_t nr,
> sha1_access_fn fn);
> +
> +/*
> + * Searches for sha1 in table, using the given fanout table to determine the
> + * interval to search, then using binary search. Returns the element index of
> + * the position found if successful, -i-1 if not (where i is the index of the
> + * least element that is greater than sha1).
> + *
> + * Takes the following parameters:
> + *
> + * - sha1: the hash to search for
> + * - fanout: a 256-element array of NETWORK-order 32-bit integers; the integer
> + * at position i represents the number of elements in table whose first byte
> + * is less than or equal to i
> + * - table: a sorted list of hashes with optional extra information in between
> + * - stride: distance between two consecutive elements in table (should be
> + * GIT_MAX_RAWSZ or greater)
> + *
> + * This function does not verify the validity of the fanout table.
> + */
> +extern int bsearch_hash(const unsigned char *sha1, const void *fanout,
> + const void *table, size_t stride);
> #endif
> Why not use sha1_pos()? I guess because it avoids the overhead of the accessor function, right? And I wonder how much of difference it makes.
A binary search function for embedded hashes just needs the key, a pointer to the first hash in the array, the stride and the number of elements. It can then be used with or without a fanout table, making it more versatile. Just a thought.
René
