palMetroHash.h

/*
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/**
 ***********************************************************************************************************************
 * @file  palMetroHash.h
 * @brief PAL utility collection MetroHash namespace declarations.
 ***********************************************************************************************************************
 */

#pragma once

#include "metrohash.h"
#include "palAssert.h"
#include "palUtil.h"

namespace Util
{

/// Namespace containing functions that provide support for MetroHash.
namespace MetroHash
{

/// 128-bit hash structure
struct Hash
{
    union
    {
        uint32 dwords[4]; ///< Output hash in dwords.
        uint64 qwords[2]; ///< Output hash in qwords.
        uint8  bytes[16]; ///< Output hash in bytes.
    };
};

/// Compacts a 128-bit hash into a 64-bit one by XOR'ing the low and high 64-bits together
/// and then swapping the two 32-bit words.
///
/// @param [in] pHash 128-bit hash to be compacted.
///
/// @returns 64-bit hash value based on the inputted 128-bit hash.
constexpr uint64 Compact64(
    const Hash* pHash)
{
    return (static_cast<uint64>(pHash->dwords[3] ^ pHash->dwords[1]) |
           (static_cast<uint64>(pHash->dwords[2] ^ pHash->dwords[0]) << 32));
}

/// Compacts a 64-bit hash checksum into a 32-bit one by XOR'ing each 32-bit chunk together.
///
/// @param [in] pHash 128-bit hash to be compacted.
///
/// @returns 32-bit hash value based on the inputted 128-bit hash.
constexpr uint32 Compact32(
    const Hash* pHash)
{
    return pHash->dwords[3] ^ pHash->dwords[2] ^ pHash->dwords[1] ^ pHash->dwords[0];
}

/// Compacts a 64-bit hash checksum into a 32-bit one by XOR'ing each 32-bit chunk together.
///
/// @param [in] hash 64-bit hash to be compacted.
///
/// @returns 32-bit hash value based on the inputted 64-bit hash.
constexpr uint32 Compact32(
    uint64 hash)
{
    return static_cast<uint32>(hash) ^ static_cast<uint32>(hash >> 32);
}

/// Hash functor.
///
/// Helpful when a 128-bit hash is being used as a key in another container.
template<typename T>
struct HashFunc
{
    /// Hashes the specified 128-bit key value by XOR'ing each 32-bit chunk.
    ///
    /// @param [in] pVoidKey Pointer to the key to be hashed.
    /// @param [in] keyLen   Amount of data at pVoidKey to hash, in bytes.
    ///
    /// @returns 32-bit uint hash value.
    uint32 operator()(const void* pVoidKey, uint32 keyLen) const
    {
        PAL_ASSERT(keyLen == sizeof(Hash));
        return Compact32(static_cast<const Hash*>(pVoidKey));
    }

    /// No init job. Defined to be compatible with default hash func.
    void Init(uint32) const { }
};

} // MetroHash
} // Util