diff options
Diffstat (limited to 'src/corelib/tools/qhash.cpp')
| -rw-r--r-- | src/corelib/tools/qhash.cpp | 25 |
1 files changed, 17 insertions, 8 deletions
diff --git a/src/corelib/tools/qhash.cpp b/src/corelib/tools/qhash.cpp index 5320ea1fbf..a5d14a3535 100644 --- a/src/corelib/tools/qhash.cpp +++ b/src/corelib/tools/qhash.cpp @@ -337,20 +337,29 @@ uint qt_hash(const QStringRef &key) Q_DECL_NOTHROW } /* - The prime_deltas array is a table of selected prime values, even - though it doesn't look like one. The primes we are using are 1, - 2, 5, 11, 17, 37, 67, 131, 257, ..., i.e. primes in the immediate - surrounding of a power of two. + The prime_deltas array contains the difference between a power + of two and the next prime number: - The primeForNumBits() function returns the prime associated to a - power of two. For example, primeForNumBits(8) returns 257. + prime_deltas[i] = nextprime(2^i) - 2^i + + Basically, it's sequence A092131 from OEIS, assuming: + - nextprime(1) = 1 + - nextprime(2) = 2 + and + - left-extending it for the offset 0 (A092131 starts at i=1) + - stopping the sequence at i = 28 (the table is big enough...) */ static const uchar prime_deltas[] = { - 0, 0, 1, 3, 1, 5, 3, 3, 1, 9, 7, 5, 3, 9, 25, 3, - 1, 21, 3, 21, 7, 15, 9, 5, 3, 29, 15, 0, 0, 0, 0, 0 + 0, 0, 1, 3, 1, 5, 3, 3, 1, 9, 7, 5, 3, 17, 27, 3, + 1, 29, 3, 21, 7, 17, 15, 9, 43, 35, 15, 0, 0, 0, 0, 0 }; +/* + The primeForNumBits() function returns the prime associated to a + power of two. For example, primeForNumBits(8) returns 257. +*/ + static inline int primeForNumBits(int numBits) { return (1 << numBits) + prime_deltas[numBits]; |