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author | Giuseppe D'Angelo <dangelog@gmail.com> | 2012-01-25 22:12:43 +0000 |
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committer | Qt by Nokia <qt-info@nokia.com> | 2012-01-27 22:48:12 +0100 |
commit | e1149349c158364854bcfece3c6d80ebccb26f28 (patch) | |
tree | 06dff5c4acb10879f6c5a3e4b949f59826f46fca | |
parent | 071561f0eba5c92fc6363d54e8f7d4508a42df45 (diff) |
QHash benchmark: improve Java's hash
Added a bit of documentation to the Java-like hashing function.
Change-Id: I3f44eee305d91b76f0f89cd1acf21f6430b9482b
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
-rw-r--r-- | tests/benchmarks/corelib/tools/qhash/outofline.cpp | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/tests/benchmarks/corelib/tools/qhash/outofline.cpp b/tests/benchmarks/corelib/tools/qhash/outofline.cpp index 88fd9c144d..21740536dd 100644 --- a/tests/benchmarks/corelib/tools/qhash/outofline.cpp +++ b/tests/benchmarks/corelib/tools/qhash/outofline.cpp @@ -87,6 +87,17 @@ uint qHash(const String &str) return h; } +// The Java's hashing algorithm for strings is a variation of D. J. Bernstein +// hashing algorithm appeared here http://cr.yp.to/cdb/cdb.txt +// and informally known as DJB33XX - DJB's 33 Times Xor. +// Java uses DJB31XA, that is, 31 Times Add. +// The original algorithm was a loop around "(h << 5) + h ^ c", +// which is indeed "h * 33 ^ c"; it was then changed to +// "(h << 5) - h ^ c", so "h * 31 ^ c", and the XOR changed to a sum: +// "(h << 5) - h + c", which can save some assembly instructions. +// Still, we can avoid writing the multiplication as "(h << 5) - h" +// -- the compiler will turn it into a shift and an addition anyway +// (for instance, gcc 4.4 does that even at -O0). uint qHash(const JavaString &str) { const unsigned short *p = (unsigned short *)str.constData(); |