From: Julian Andres Klode <jak@debian.org>
Date: Tue, 29 Dec 2015 01:40:18 +0000 (+0100)
Subject: Switch to DJB hashing and use prime number as table size
X-Git-Tag: 1.1.10~8
X-Git-Url: https://git.saurik.com/apt.git/commitdiff_plain/0748f03aca4db260ad964b44519e0971647d1e9d

Switch to DJB hashing and use prime number as table size

On my testing system, consisting of unstable and experimental,
this reduces the average chain from 6.5 to 4.5, and the longest
chain from 17 to 15.
---

diff --git a/apt-pkg/pkgcache.cc b/apt-pkg/pkgcache.cc
index 23b75a640..dd25fc51d 100644
--- a/apt-pkg/pkgcache.cc
+++ b/apt-pkg/pkgcache.cc
@@ -57,7 +57,7 @@ pkgCache::Header::Header()
    /* Whenever the structures change the major version should be bumped,
       whenever the generator changes the minor version should be bumped. */
    APT_HEADER_SET(MajorVersion, 10);
-   APT_HEADER_SET(MinorVersion, 2);
+   APT_HEADER_SET(MinorVersion, 3);
    APT_HEADER_SET(Dirty, false);
 
    APT_HEADER_SET(HeaderSz, sizeof(pkgCache::Header));
@@ -93,7 +93,7 @@ pkgCache::Header::Header()
    VerSysName = 0;
    Architecture = 0;
    SetArchitectures(0);
-   SetHashTableSize(_config->FindI("APT::Cache-HashTableSize", 10 * 1048));
+   SetHashTableSize(_config->FindI("APT::Cache-HashTableSize", 15013));
    memset(Pools,0,sizeof(Pools));
 
    CacheFileSize = 0;
@@ -203,17 +203,17 @@ bool pkgCache::ReMap(bool const &Errorchecks)
    table (480 used items) */
 map_id_t pkgCache::sHash(const string &Str) const
 {
-   uint32_t Hash = 0;
+   uint32_t Hash = 5381;
    for (string::const_iterator I = Str.begin(); I != Str.end(); ++I)
-      Hash = 41 * Hash + tolower_ascii(*I);
+      Hash = 33 * Hash + tolower_ascii(*I);
    return Hash % HeaderP->GetHashTableSize();
 }
 
 map_id_t pkgCache::sHash(const char *Str) const
 {
-   uint32_t Hash = tolower_ascii(*Str);
-   for (const char *I = Str + 1; *I != 0; ++I)
-      Hash = 41 * Hash + tolower_ascii(*I);
+   uint32_t Hash = 5381;
+   for (const char *I = Str; *I != 0; ++I)
+      Hash = 33 * Hash + tolower_ascii(*I);
    return Hash % HeaderP->GetHashTableSize();
 }
 									/*}}}*/