// darn.h - written and placed in public domain by Jeffrey Walton
//          DARN requires POWER9/ISA 3.0.

// At the moment only GCC 7.0 (and above) seems to support __builtin_darn()
// and __builtin_darn_32(). Clang 7.0 does not provide them, but it does
// support assembly instructions. XLC is unknown, but there are no hits when
// searching IBM's site. To cover more platforms we provide GCC inline
// assembly like we do with RDRAND and RDSEED. Platforms that don't support
// GCC inline assembly or the builtin will fail the compile. Also see
// https://gcc.gnu.org/onlinedocs/gcc/Basic-PowerPC-Built-in-Functions-Available-on-ISA-3_002e0.html

/// \file darn.h
/// \brief Classes for DARN RNG
/// \sa <A HREF="https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0">Power
///   ISA Version 3.0B</A>
/// \since Crypto++ 8.0

#ifndef CRYPTOPP_DARN_H
#define CRYPTOPP_DARN_H

#include "cryptlib.h"

NAMESPACE_BEGIN(CryptoPP)

/// \brief Exception thrown when a DARN generator encounters
///    a generator related error.
/// \since Crypto++ 8.0
class DARN_Err : public Exception
{
public:
    DARN_Err(const std::string &operation)
        : Exception(OTHER_ERROR, "DARN: " + operation + " operation failed") {}
};

/// \brief Hardware generated random numbers using DARN instruction
/// \details DARN() provides access to Power9's random number generator. The
///   Crypto++ implementation provides conditioned random numbers from the
///   generator as opposed to raw random numbers. According to Power ISA 3.0B
///   manual, a conditioned random number has been processed by hardware to
///   reduce bias. A raw random number is unconditioned noise source output.
/// \details According to Power ISA 3.0B manual, the random number generator
///   provided by the <tt>darn</tt> instruction is NIST SP800-90B and SP800-90C
///   compliant to the extent possible given the completeness of the standards
///   at the time the hardware is designed. The random number generator provides
///   a minimum of 0.5 bits of entropy per bit.
/// \par Wraps
///   darn instruction
/// \sa <A HREF="https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0">Power
///   ISA Version 3.0B</A>, MaurerRandomnessTest() for random bit generators
/// \since Crypto++ 8.0
class DARN : public RandomNumberGenerator
{
public:
    CRYPTOPP_STATIC_CONSTEXPR const char* StaticAlgorithmName() { return "DARN"; }

    virtual ~DARN() {}

    /// \brief Construct a DARN generator
     /// \throws DARN_Err if the random number generator is not available
    DARN();

    /// \brief Generate random array of bytes
    /// \param output the byte buffer
    /// \param size the length of the buffer, in bytes
    virtual void GenerateBlock(byte *output, size_t size);

    /// \brief Generate and discard n bytes
    /// \param n the number of bytes to generate and discard
    /// \details the RDSEED generator discards words, not bytes. If n is
    ///   not a multiple of a machine word, then it is rounded up to
    ///   that size.
    virtual void DiscardBytes(size_t n);

    /// \brief Update RNG state with additional unpredictable values
    /// \param input unused
    /// \param length unused
    /// \details The operation is a nop for this generator.
    virtual void IncorporateEntropy(const byte *input, size_t length)
    {
        // Override to avoid the base class' throw.
        CRYPTOPP_UNUSED(input); CRYPTOPP_UNUSED(length);
    }

    std::string AlgorithmProvider() const {
        return "Power9";
    }
};

NAMESPACE_END

#endif // CRYPTOPP_DARN_H