Skip to content

Instantly share code, notes, and snippets.

Embed
What would you like to do?
FNV1a c++11 constexpr compile time hash functions, 32 and 64 bit
#pragma once
#include <stdint.h>
//fnv1a 32 and 64 bit hash functions
// key is the data to hash, len is the size of the data (or how much of it to hash against)
// code license: public domain or equivalent
// post: https://notes.underscorediscovery.com/constexpr-fnv1a/
inline const uint32_t hash_32_fnv1a(const void* key, const uint32_t len) {
const char* data = (char*)key;
uint32_t hash = 0x811c9dc5;
uint32_t prime = 0x1000193;
for(int i = 0; i < len; ++i) {
uint8_t value = data[i];
hash = hash ^ value;
hash *= prime;
}
return hash;
} //hash_32_fnv1a
inline const uint64_t hash_64_fnv1a(const void* key, const uint64_t len) {
const char* data = (char*)key;
uint64_t hash = 0xcbf29ce484222325;
uint64_t prime = 0x100000001b3;
for(int i = 0; i < len; ++i) {
uint8_t value = data[i];
hash = hash ^ value;
hash *= prime;
}
return hash;
} //hash_64_fnv1a
#pragma once
#include <stdint.h>
// FNV1a c++11 constexpr compile time hash functions, 32 and 64 bit
// str should be a null terminated string literal, value should be left out
// e.g hash_32_fnv1a_const("example")
// code license: public domain or equivalent
// post: https://notes.underscorediscovery.com/constexpr-fnv1a/
constexpr uint32_t val_32_const = 0x811c9dc5;
constexpr uint32_t prime_32_const = 0x1000193;
constexpr uint64_t val_64_const = 0xcbf29ce484222325;
constexpr uint64_t prime_64_const = 0x100000001b3;
inline constexpr uint32_t hash_32_fnv1a_const(const char* const str, const uint32_t value = val_32_const) noexcept {
return (str[0] == '\0') ? value : hash_32_fnv1a_const(&str[1], (value ^ uint32_t(str[0])) * prime_32_const);
}
inline constexpr uint64_t hash_64_fnv1a_const(const char* const str, const uint64_t value = val_64_const) noexcept {
return (str[0] == '\0') ? value : hash_64_fnv1a_const(&str[1], (value ^ uint64_t(str[0])) * prime_64_const);
}
@Lijmer

This comment has been minimized.

Copy link

commented Jan 20, 2018

Is there any license on this code? I would love to use this in a little graphics demo that I am building. It's for a non-commercial purpose, just for fun and learning :).

@epicabsol

This comment has been minimized.

Copy link

commented Feb 24, 2018

How would one use this to hash wstrings?
With std::wstring text = L"Hello There", I get different results from hash_64_fnv1a_const(L"Hello There") and hash_64_fnv1a(text.c_str(), string.size() * 2 + 1).

@nshtg

This comment has been minimized.

Copy link

commented Oct 13, 2018

@epicabsol
c_str() adds a \0 byte at the end.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.