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ircd::util: Optimizations for the bernstein constexpr string hasher.

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This commit is contained in:
Jason Volk 2020-12-11 13:33:15 -08:00
parent b798192c15
commit dac50c8da7
1 changed files with 30 additions and 13 deletions
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43
include/ircd/util/hash.h
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@ -11,29 +11,38 @@
#pragma once
#define HAVE_IRCD_UTIL_HASH_H
// constexpr bernstein string hasher suite; these functions will hash the
// string at compile time leaving an integer residue at runtime. Decent
// primes are at least 7681 and 5381.
namespace ircd {
inline namespace util
{
// constexpr bernstein string hasher suite; these functions will hash the
// string at compile time leaving an integer residue at runtime. Decent
// primes are at least 7681 and 5381.
template<size_t PRIME = 7681> constexpr size_t hash(const char16_t *const str, const size_t i = 0) noexcept;
template<size_t PRIME = 7681> constexpr size_t hash(const string_view str, const size_t i = 0) noexcept;
template<size_t PRIME = 7681>
constexpr size_t hash(const string_view str, size_t i = 0, size_t r = PRIME) noexcept;
template<size_t PRIME = 7681>
constexpr size_t hash(const char16_t *const str, size_t i = 0, size_t r = PRIME) noexcept;
// Note that at runtime this hash uses multiplication on every character
// which can consume many cycles...
template<size_t PRIME = 7681> size_t hash(const std::u16string &str, const size_t i = 0) noexcept;
template<size_t PRIME = 7681>
size_t hash(const std::u16string &str, size_t i = 0, size_t r = PRIME) noexcept;
}}
/// Runtime hashing of a std::u16string (for js). Non-cryptographic.
template<size_t PRIME>
[[gnu::pure]]
size_t
inline size_t
ircd::util::hash(const std::u16string &str,
const size_t i)
size_t i,
size_t r)
noexcept
{
return i >= str.size()? PRIME : (hash(str, i+1) * 33ULL) ^ str.at(i);
for(; i < str.size(); ++i)
r = str[i] ^ (r * 33ULL);
return r;
}
/// Runtime hashing of a string_view. Non-cryptographic.
@ -41,10 +50,14 @@ template<size_t PRIME>
[[gnu::pure]]
constexpr size_t
ircd::util::hash(const string_view str,
const size_t i)
size_t i,
size_t r)
noexcept
{
return i >= str.size()? PRIME : (hash(str, i+1) * 33ULL) ^ str.at(i);
for(; i < str.size(); ++i)
r = str[i] ^ (r * 33ULL);
return r;
}
/// Compile-time hashing of a wider string literal (for js). Non-cryptographic.
@ -52,8 +65,12 @@ template<size_t PRIME>
[[gnu::pure]]
constexpr size_t
ircd::util::hash(const char16_t *const str,
const size_t i)
size_t i,
size_t r)
noexcept
{
return !str[i]? PRIME : (hash(str, i+1) * 33ULL) ^ str[i];
for(assert(str); str[i]; ++i)
r = str[i] ^ (r * 33ULL);
return r;
}