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Use identity hash for integers and pointers 

This is appropriate because HashArray in tanya.container.entry uses
prime numbers instead of powers of 2 for its number of buckets so there
is no pitfall if the hashes are all multiples of some power of 2.
This commit is contained in:
Nathan Sashihara 2018-08-02 12:08:32 -04:00
parent 900a7172bf
commit ceb8e6a113
1 changed files with 21 additions and 6 deletions
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27
source/tanya/hash/lookup.d
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@ -89,7 +89,10 @@ private struct FNV
* }
* ---
*
* For scalar types FNV-1a (Fowler-Noll-Vo) hash function is used internally.
* For pointers and for scalar types implicitly convertible to `size_t` this
* is an identity operation (i.e. the value is cast to `size_t` and returned
* unaltered). Integer types wider than `size_t` are XOR folded down to
* `size_t`. Other scalar types use the FNV-1a (Fowler-Noll-Vo) hash function.
* If the type provides a `toHash`-function, only `toHash()` is called and its
* result is returned.
*
@ -110,6 +113,19 @@ size_t hash(T)(auto ref T key)
{
return key.toHash();
}
else static if ((isIntegral!T || isSomeChar!T || isBoolean!T)
&& T.sizeof <= size_t.sizeof)
{
return cast(size_t) key;
}
else static if (isIntegral!T && T.sizeof > size_t.sizeof)
{
return cast(size_t) (key ^ (key >>> (size_t.sizeof * 8)));
}
else static if (isPointer!T || is(T : typeof(null)))
{
return (() @trusted => cast(size_t) key)();
}
else
{
FNV fnv;
@ -177,30 +193,29 @@ version (unittest)
// Tests that work for any hash size
@nogc nothrow pure @safe unittest
{
assert(hash(null) == FNV.offsetBasis);
assert(hash(null) == 0);
assert(hash(ToHash()) == 0U);
assert(hash('a') == 'a');
}
static if (size_t.sizeof == 4) @nogc nothrow pure @safe unittest
{
assert(hash('a') == 0xe40c292cU);
assert(hash(HashRange()) == 0x6222e842U);
assert(hash(ToHashRange()) == 1268118805U);
}
static if (size_t.sizeof == 8) @nogc nothrow pure @safe unittest
{
assert(hash('a') == 0xaf63dc4c8601ec8cUL);
assert(hash(HashRange()) == 0x08985907b541d342UL);
assert(hash(ToHashRange()) == 12161962213042174405UL);
}
static if (size_t.sizeof == 4) @nogc nothrow pure @system unittest
{
assert(hash(cast(void*) 0x6e6f6863) == 0xac297727U);
assert(hash(cast(void*) 0x6e6f6863) == 0x6e6f6863);
}
static if (size_t.sizeof == 8) @nogc nothrow pure @system unittest
{
assert(hash(cast(void*) 0x77206f676e6f6863) == 0xd1edd10b507344d0UL);
assert(hash(cast(void*) 0x77206f676e6f6863) == 0x77206f676e6f6863);
}
/*