palHashLiteralString.h

/*
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/**
 ***********************************************************************************************************************
 * @file  palHashLiteralString.h
 * @brief PAL utility collection implementation of the HashLiteralString constexpr template function.
 ***********************************************************************************************************************
 */

#pragma once

#include "palInlineFuncs.h"

namespace Util
{
/// @internal FNV-1a hash offset used by HashLiteralString().
static constexpr uint32 Fnv1aOffset = 2166136261u;
/// FNV-1a hash prime used by HashLiteralString().
static constexpr uint32 Fnv1aPrime  = 16777619u;

/// @internal Template metafunction which can be used to generate a compile-time hash of a string literal.  This relies
///           on recursively applying the template to smaller and smaller subsections of the original string.
/// @note     MSVC requires index to be provided as a template argument to avoid having the input string appear in the
///           compiled binary.  This cannot safely be implemented as a non-templated constexpr function.
template <size_t N, uint32 I>
class Fnv1aHashHelper
{
public:
    /// @internal Recursively compute a hash value.
    ///
    /// @param [in] string C-style string to be hashed.
    ///
    /// @returns Hash of the specified substring.
    static constexpr uint32 Hash(
        const char (&string)[N])
    {
        return
            LowPart(static_cast<uint64>(Fnv1aHashHelper<N, I - 1>::Hash(string) ^ uint8(string[I - 1])) * Fnv1aPrime);
    }
};

/// @internal Partial specialization of the above template metafunction. This is needed to provide a base case which
///           terminates further recursion.
template <size_t N>
class Fnv1aHashHelper<N, 1>
{
public:
    /// @internal Recursively compute a hash value.
    ///
    /// @param [in] string C-style string to be hashed.
    ///
    /// @returns Hash of the specified substring.
    static constexpr uint32 Hash(
        const char (&string)[N])
    {
        return LowPart(static_cast<uint64>(Fnv1aOffset ^ uint8(string[0])) * Fnv1aPrime);
    }
};

/// Generates a compile-time hash of the specified literal C-style string using the FNV-1a hash algorithm.
///
/// @param [in] string C-style string literal to be hashed.
///
/// @returns 32-bit hash value for the specified string.
template <size_t N>
constexpr uint32 HashLiteralString(
    const char (&string)[N])
{
    // The "-1" on the second template parameter is necessary to avoid hashing the string's null-terminator!
    return Fnv1aHashHelper<N, N - 1>::Hash(string);
}

} // Util