leopard/LeopardFF8.cpp

/*
    Copyright (c) 2017 Christopher A. Taylor.  All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:

    * Redistributions of source code must retain the above copyright notice,
      this list of conditions and the following disclaimer.
    * Redistributions in binary form must reproduce the above copyright notice,
      this list of conditions and the following disclaimer in the documentation
      and/or other materials provided with the distribution.
    * Neither the name of Leopard-RS nor the names of its contributors may be
      used to endorse or promote products derived from this software without
      specific prior written permission.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
    AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
    ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
    LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
    CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
    SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
    INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
    CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
    ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
    POSSIBILITY OF SUCH DAMAGE.
*/

#include "LeopardFF8.h"

#ifdef LEO_HAS_FF8

#include <string.h>

#ifdef _MSC_VER
    #pragma warning(disable: 4752) // found Intel(R) Advanced Vector Extensions; consider using /arch:AVX
#endif

namespace leopard { namespace ff8 {


//------------------------------------------------------------------------------
// Datatypes and Constants

// Basis used for generating logarithm tables
static const ffe_t kCantorBasis[kBits] = {
    1, 214, 152, 146, 86, 200, 88, 230
};

// Using the Cantor basis {2} here enables us to avoid a lot of extra calculations
// when applying the formal derivative in decoding.


//------------------------------------------------------------------------------
// Field Operations

// z = x + y (mod kModulus)
static inline ffe_t AddMod(const ffe_t a, const ffe_t b)
{
    const unsigned sum = static_cast<unsigned>(a) + b;

    // Partial reduction step, allowing for kModulus to be returned
    return static_cast<ffe_t>(sum + (sum >> kBits));
}

// z = x - y (mod kModulus)
static inline ffe_t SubMod(const ffe_t a, const ffe_t b)
{
    const unsigned dif = static_cast<unsigned>(a) - b;

    // Partial reduction step, allowing for kModulus to be returned
    return static_cast<ffe_t>(dif + (dif >> kBits));
}


//------------------------------------------------------------------------------
// Fast Walsh-Hadamard Transform (FWHT) (mod kModulus)

// {a, b} = {a + b, a - b} (Mod Q)
static LEO_FORCE_INLINE void FWHT_2(ffe_t& LEO_RESTRICT a, ffe_t& LEO_RESTRICT b)
{
    const ffe_t sum = AddMod(a, b);
    const ffe_t dif = SubMod(a, b);
    a = sum;
    b = dif;
}

#if defined(LEO_FWHT_OPT)

static LEO_FORCE_INLINE void FWHT_4(ffe_t* data)
{
    ffe_t t0 = data[0];
    ffe_t t1 = data[1];
    ffe_t t2 = data[2];
    ffe_t t3 = data[3];
    FWHT_2(t0, t1);
    FWHT_2(t2, t3);
    FWHT_2(t0, t2);
    FWHT_2(t1, t3);
    data[0] = t0;
    data[1] = t1;
    data[2] = t2;
    data[3] = t3;
}

static LEO_FORCE_INLINE void FWHT_4(ffe_t* data, unsigned s)
{
    unsigned x = 0;
    ffe_t t0 = data[x];  x += s;
    ffe_t t1 = data[x];  x += s;
    ffe_t t2 = data[x];  x += s;
    ffe_t t3 = data[x];
    FWHT_2(t0, t1);
    FWHT_2(t2, t3);
    FWHT_2(t0, t2);
    FWHT_2(t1, t3);
    unsigned y = 0;
    data[y] = t0;  y += s;
    data[y] = t1;  y += s;
    data[y] = t2;  y += s;
    data[y] = t3;
}

// Decimation in time (DIT) version
static void FWHT(ffe_t* data, const unsigned bits)
{
    const unsigned n = (1UL << bits);

    if (n <= 2)
    {
        if (n == 2)
            FWHT_2(data[0], data[1]);
        return;
    }

    for (unsigned i = bits; i > 3; i -= 2)
    {
        unsigned m = (1UL << i);
        unsigned m4 = (m >> 2);
        for (unsigned r = 0; r < n; r += m)
            for (unsigned j = 0; j < m4; j++)
                FWHT_4(data + j + r, m4);
    }

    for (unsigned i0 = 0; i0 < n; i0 += 4)
        FWHT_4(data + i0);
}

#else // LEO_FWHT_OPT

// Reference implementation
void FWHT(ffe_t* data, const unsigned bits)
{
    const unsigned size = (unsigned)(1UL << bits);
    for (unsigned width = 1; width < size; width <<= 1)
        for (unsigned i = 0; i < size; i += (width << 1))
            for (unsigned j = i; j < (width + i); ++j)
                FWHT_2(data[j], data[j + width]);
}

#endif // LEO_FWHT_OPT

// Transform specialized for the finite field order
void FWHT(ffe_t data[kOrder])
{
    FWHT(data, kBits);
}


//------------------------------------------------------------------------------
// Logarithm Tables

static ffe_t LogLUT[kOrder];
static ffe_t ExpLUT[kOrder];


// Returns a * Log(b)
static ffe_t MultiplyLog(ffe_t a, ffe_t log_b)
{
    /*
        Note that this operation is not a normal multiplication in a finite
        field because the right operand is already a logarithm.  This is done
        because it moves K table lookups from the Decode() method into the
        initialization step that is less performance critical.  The LogWalsh[]
        table below contains precalculated logarithms so it is easier to do
        all the other multiplies in that form as well.
    */
    if (a == 0)
        return 0;
    return ExpLUT[AddMod(LogLUT[a], log_b)];
}


// Initialize LogLUT[], ExpLUT[]
static void InitializeLogarithmTables()
{
    // LFSR table generation:

    unsigned state = 1;
    for (unsigned i = 0; i < kModulus; ++i)
    {
        ExpLUT[state] = static_cast<ffe_t>(i);
        state <<= 1;
        if (state >= kOrder)
            state ^= kPolynomial;
    }
    ExpLUT[0] = kModulus;

    // Conversion to Cantor basis {2}:

    LogLUT[0] = 0;
    for (unsigned i = 0; i < kBits; ++i)
    {
        const ffe_t basis = kCantorBasis[i];
        const unsigned width = static_cast<unsigned>(1UL << i);

        for (unsigned j = 0; j < width; ++j)
            LogLUT[j + width] = LogLUT[j] ^ basis;
    }

    for (unsigned i = 0; i < kOrder; ++i)
        LogLUT[i] = ExpLUT[LogLUT[i]];

    // Generate Exp table from Log table:

    for (unsigned i = 0; i < kOrder; ++i)
        ExpLUT[LogLUT[i]] = i;

    // Note: Handles modulus wrap around with LUT
    ExpLUT[kModulus] = ExpLUT[0];
}


//------------------------------------------------------------------------------
// Multiplies

/*
    The multiplication algorithm used follows the approach outlined in {4}.
    Specifically section 6 outlines the algorithm used here for 8-bit fields.
*/

struct {
    LEO_ALIGNED LEO_M128 Value[2];
} static Multiply128LUT[kOrder];
#if defined(LEO_TRY_AVX2)
struct {
    LEO_ALIGNED LEO_M256 Value[2];
} static Multiply256LUT[kOrder];
#endif // LEO_TRY_AVX2


void InitializeMultiplyTables()
{
    // For each value we could multiply by:
    for (unsigned log_m = 0; log_m < kOrder; ++log_m)
    {
        // For each 4 bits of the finite field width in bits:
        for (unsigned i = 0, shift = 0; i < 2; ++i, shift += 4)
        {
            // Construct 16 entry LUT for PSHUFB
            uint8_t lut[16];
            for (ffe_t x = 0; x < 16; ++x)
                lut[x] = MultiplyLog(x << shift, static_cast<ffe_t>(log_m));

            // Store in 128-bit wide table
            const LEO_M128 *v_ptr = reinterpret_cast<const LEO_M128 *>(&lut[0]);
            const LEO_M128 value = _mm_loadu_si128(v_ptr);
            _mm_storeu_si128(&Multiply128LUT[log_m].Value[i], value);

            // Store in 256-bit wide table
#if defined(LEO_TRY_AVX2)
            if (CpuHasAVX2)
            {
                _mm256_storeu_si256(&Multiply256LUT[log_m].Value[i],
                    _mm256_broadcastsi128_si256(value));
            }
#endif // LEO_TRY_AVX2
        }
    }
}


void mul_mem(
    void * LEO_RESTRICT x, const void * LEO_RESTRICT y,
    ffe_t log_m, uint64_t bytes)
{
#if defined(LEO_TRY_AVX2)
    if (CpuHasAVX2)
    {
        const LEO_M256 table_lo_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[0]);
        const LEO_M256 table_hi_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[1]);

        const LEO_M256 clr_mask = _mm256_set1_epi8(0x0f);

        LEO_M256 * LEO_RESTRICT x32 = reinterpret_cast<LEO_M256 *>(x);
        const LEO_M256 * LEO_RESTRICT y32 = reinterpret_cast<const LEO_M256 *>(y);

        do
        {
#define LEO_MUL_256(x_ptr, y_ptr) { \
            LEO_M256 data = _mm256_loadu_si256(y_ptr); \
            LEO_M256 lo = _mm256_and_si256(data, clr_mask); \
            lo = _mm256_shuffle_epi8(table_lo_y, lo); \
            LEO_M256 hi = _mm256_srli_epi64(data, 4); \
            hi = _mm256_and_si256(hi, clr_mask); \
            hi = _mm256_shuffle_epi8(table_hi_y, hi); \
            _mm256_storeu_si256(x_ptr, _mm256_xor_si256(lo, hi)); }

            LEO_MUL_256(x32 + 1, y32 + 1);
            LEO_MUL_256(x32, y32);
            y32 += 2, x32 += 2;

            bytes -= 64;
        } while (bytes > 0);

        return;
    }
#endif // LEO_TRY_AVX2

    const LEO_M128 table_lo_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[0]);
    const LEO_M128 table_hi_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[1]);

    const LEO_M128 clr_mask = _mm_set1_epi8(0x0f);

    LEO_M128 * LEO_RESTRICT x16 = reinterpret_cast<LEO_M128 *>(x);
    const LEO_M128 * LEO_RESTRICT y16 = reinterpret_cast<const LEO_M128 *>(y);

    do
    {
#define LEO_MUL_128(x_ptr, y_ptr) { \
        LEO_M128 data = _mm_loadu_si128(y_ptr); \
        LEO_M128 lo = _mm_and_si128(data, clr_mask); \
        lo = _mm_shuffle_epi8(table_lo_y, lo); \
        LEO_M128 hi = _mm_srli_epi64(data, 4); \
        hi = _mm_and_si128(hi, clr_mask); \
        hi = _mm_shuffle_epi8(table_hi_y, hi); \
        _mm_storeu_si128(x_ptr, _mm_xor_si128(lo, hi)); }

        LEO_MUL_128(x16 + 3, y16 + 3);
        LEO_MUL_128(x16 + 2, y16 + 2);
        LEO_MUL_128(x16 + 1, y16 + 1);
        LEO_MUL_128(x16, y16);
        x16 += 4, y16 += 4;

        bytes -= 64;
    } while (bytes > 0);
}


//------------------------------------------------------------------------------
// FFT Operations

void fft_butterfly(
    void * LEO_RESTRICT x, void * LEO_RESTRICT y,
    ffe_t log_m, uint64_t bytes)
{
#if defined(LEO_TRY_AVX2)
    if (CpuHasAVX2)
    {
        const LEO_M256 table_lo_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[0]);
        const LEO_M256 table_hi_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[1]);

        const LEO_M256 clr_mask = _mm256_set1_epi8(0x0f);

        LEO_M256 * LEO_RESTRICT x32 = reinterpret_cast<LEO_M256 *>(x);
        LEO_M256 * LEO_RESTRICT y32 = reinterpret_cast<LEO_M256 *>(y);

        do
        {
#define LEO_FFTB_256(x_ptr, y_ptr) { \
            LEO_M256 y_data = _mm256_loadu_si256(y_ptr); \
            LEO_M256 lo = _mm256_and_si256(y_data, clr_mask); \
            lo = _mm256_shuffle_epi8(table_lo_y, lo); \
            LEO_M256 hi = _mm256_srli_epi64(y_data, 4); \
            hi = _mm256_and_si256(hi, clr_mask); \
            hi = _mm256_shuffle_epi8(table_hi_y, hi); \
            LEO_M256 x_data = _mm256_loadu_si256(x_ptr); \
            x_data = _mm256_xor_si256(x_data, _mm256_xor_si256(lo, hi)); \
            y_data = _mm256_xor_si256(y_data, x_data); \
            _mm256_storeu_si256(x_ptr, x_data); \
            _mm256_storeu_si256(y_ptr, y_data); }

            LEO_FFTB_256(x32 + 1, y32 + 1);
            LEO_FFTB_256(x32, y32);
            y32 += 2, x32 += 2;

            bytes -= 64;
        } while (bytes > 0);

        return;
    }
#endif // LEO_TRY_AVX2

    const LEO_M128 table_lo_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[0]);
    const LEO_M128 table_hi_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[1]);

    const LEO_M128 clr_mask = _mm_set1_epi8(0x0f);

    LEO_M128 * LEO_RESTRICT x16 = reinterpret_cast<LEO_M128 *>(x);
    LEO_M128 * LEO_RESTRICT y16 = reinterpret_cast<LEO_M128 *>(y);

    do
    {
#define LEO_FFTB_128(x_ptr, y_ptr) { \
        LEO_M128 y_data = _mm_loadu_si128(y_ptr); \
        LEO_M128 lo = _mm_and_si128(y_data, clr_mask); \
        lo = _mm_shuffle_epi8(table_lo_y, lo); \
        LEO_M128 hi = _mm_srli_epi64(y_data, 4); \
        hi = _mm_and_si128(hi, clr_mask); \
        hi = _mm_shuffle_epi8(table_hi_y, hi); \
        LEO_M128 x_data = _mm_loadu_si128(x_ptr); \
        x_data = _mm_xor_si128(x_data, _mm_xor_si128(lo, hi)); \
        y_data = _mm_xor_si128(y_data, x_data); \
        _mm_storeu_si128(x_ptr, x_data); \
        _mm_storeu_si128(y_ptr, y_data); }

        LEO_FFTB_128(x16 + 3, y16 + 3);
        LEO_FFTB_128(x16 + 2, y16 + 2);
        LEO_FFTB_128(x16 + 1, y16 + 1);
        LEO_FFTB_128(x16, y16);
        x16 += 4, y16 += 4;

        bytes -= 64;
    } while (bytes > 0);
}

#ifdef LEO_USE_VECTOR4_OPT

void fft_butterfly4(
    void * LEO_RESTRICT x_0, void * LEO_RESTRICT y_0,
    void * LEO_RESTRICT x_1, void * LEO_RESTRICT y_1,
    void * LEO_RESTRICT x_2, void * LEO_RESTRICT y_2,
    void * LEO_RESTRICT x_3, void * LEO_RESTRICT y_3,
    ffe_t log_m, uint64_t bytes)
{
#if defined(LEO_TRY_AVX2)
    if (CpuHasAVX2)
    {
        const LEO_M256 table_lo_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[0]);
        const LEO_M256 table_hi_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[1]);

        const LEO_M256 clr_mask = _mm256_set1_epi8(0x0f);

        LEO_M256 * LEO_RESTRICT x32_0 = reinterpret_cast<LEO_M256 *>(x_0);
        LEO_M256 * LEO_RESTRICT y32_0 = reinterpret_cast<LEO_M256 *>(y_0);
        LEO_M256 * LEO_RESTRICT x32_1 = reinterpret_cast<LEO_M256 *>(x_1);
        LEO_M256 * LEO_RESTRICT y32_1 = reinterpret_cast<LEO_M256 *>(y_1);
        LEO_M256 * LEO_RESTRICT x32_2 = reinterpret_cast<LEO_M256 *>(x_2);
        LEO_M256 * LEO_RESTRICT y32_2 = reinterpret_cast<LEO_M256 *>(y_2);
        LEO_M256 * LEO_RESTRICT x32_3 = reinterpret_cast<LEO_M256 *>(x_3);
        LEO_M256 * LEO_RESTRICT y32_3 = reinterpret_cast<LEO_M256 *>(y_3);

        do
        {
            LEO_FFTB_256(x32_0 + 1, y32_0 + 1);
            LEO_FFTB_256(x32_0, y32_0);
            y32_0 += 2, x32_0 += 2;

            LEO_FFTB_256(x32_1 + 1, y32_1 + 1);
            LEO_FFTB_256(x32_1, y32_1);
            y32_1 += 2, x32_1 += 2;

            LEO_FFTB_256(x32_2 + 1, y32_2 + 1);
            LEO_FFTB_256(x32_2, y32_2);
            y32_2 += 2, x32_2 += 2;

            LEO_FFTB_256(x32_3 + 1, y32_3 + 1);
            LEO_FFTB_256(x32_3, y32_3);
            y32_3 += 2, x32_3 += 2;

            bytes -= 64;
        } while (bytes > 0);

        return;
    }
#endif // LEO_TRY_AVX2

    const LEO_M128 table_lo_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[0]);
    const LEO_M128 table_hi_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[1]);

    const LEO_M128 clr_mask = _mm_set1_epi8(0x0f);

    LEO_M128 * LEO_RESTRICT x16_0 = reinterpret_cast<LEO_M128 *>(x_0);
    LEO_M128 * LEO_RESTRICT y16_0 = reinterpret_cast<LEO_M128 *>(y_0);
    LEO_M128 * LEO_RESTRICT x16_1 = reinterpret_cast<LEO_M128 *>(x_1);
    LEO_M128 * LEO_RESTRICT y16_1 = reinterpret_cast<LEO_M128 *>(y_1);
    LEO_M128 * LEO_RESTRICT x16_2 = reinterpret_cast<LEO_M128 *>(x_2);
    LEO_M128 * LEO_RESTRICT y16_2 = reinterpret_cast<LEO_M128 *>(y_2);
    LEO_M128 * LEO_RESTRICT x16_3 = reinterpret_cast<LEO_M128 *>(x_3);
    LEO_M128 * LEO_RESTRICT y16_3 = reinterpret_cast<LEO_M128 *>(y_3);

    do
    {
        LEO_FFTB_128(x16_0 + 3, y16_0 + 3);
        LEO_FFTB_128(x16_0 + 2, y16_0 + 2);
        LEO_FFTB_128(x16_0 + 1, y16_0 + 1);
        LEO_FFTB_128(x16_0, y16_0);
        x16_0 += 4, y16_0 += 4;

        LEO_FFTB_128(x16_1 + 3, y16_1 + 3);
        LEO_FFTB_128(x16_1 + 2, y16_1 + 2);
        LEO_FFTB_128(x16_1 + 1, y16_1 + 1);
        LEO_FFTB_128(x16_1, y16_1);
        x16_1 += 4, y16_1 += 4;

        LEO_FFTB_128(x16_2 + 3, y16_2 + 3);
        LEO_FFTB_128(x16_2 + 2, y16_2 + 2);
        LEO_FFTB_128(x16_2 + 1, y16_2 + 1);
        LEO_FFTB_128(x16_2, y16_2);
        x16_2 += 4, y16_2 += 4;

        LEO_FFTB_128(x16_3 + 3, y16_3 + 3);
        LEO_FFTB_128(x16_3 + 2, y16_3 + 2);
        LEO_FFTB_128(x16_3 + 1, y16_3 + 1);
        LEO_FFTB_128(x16_3, y16_3);
        x16_3 += 4, y16_3 += 4;

        bytes -= 64;
    } while (bytes > 0);
}

#endif // LEO_USE_VECTOR4_OPT


//------------------------------------------------------------------------------
// IFFT Operations

void ifft_butterfly(
    void * LEO_RESTRICT x, void * LEO_RESTRICT y,
    ffe_t log_m, uint64_t bytes)
{
#if defined(LEO_TRY_AVX2)
    if (CpuHasAVX2)
    {
        const LEO_M256 table_lo_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[0]);
        const LEO_M256 table_hi_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[1]);

        const LEO_M256 clr_mask = _mm256_set1_epi8(0x0f);

        LEO_M256 * LEO_RESTRICT x32 = reinterpret_cast<LEO_M256 *>(x);
        LEO_M256 * LEO_RESTRICT y32 = reinterpret_cast<LEO_M256 *>(y);

        do
        {
#define LEO_IFFTB_256(x_ptr, y_ptr) { \
            LEO_M256 x_data = _mm256_loadu_si256(x_ptr); \
            LEO_M256 y_data = _mm256_loadu_si256(y_ptr); \
            y_data = _mm256_xor_si256(y_data, x_data); \
            _mm256_storeu_si256(y_ptr, y_data); \
            LEO_M256 lo = _mm256_and_si256(y_data, clr_mask); \
            lo = _mm256_shuffle_epi8(table_lo_y, lo); \
            LEO_M256 hi = _mm256_srli_epi64(y_data, 4); \
            hi = _mm256_and_si256(hi, clr_mask); \
            hi = _mm256_shuffle_epi8(table_hi_y, hi); \
            x_data = _mm256_xor_si256(x_data, _mm256_xor_si256(lo, hi)); \
            _mm256_storeu_si256(x_ptr, x_data); }

            LEO_IFFTB_256(x32 + 1, y32 + 1);
            LEO_IFFTB_256(x32, y32);
            y32 += 2, x32 += 2;

            bytes -= 64;
        } while (bytes > 0);

        return;
    }
#endif // LEO_TRY_AVX2

    const LEO_M128 table_lo_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[0]);
    const LEO_M128 table_hi_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[1]);

    const LEO_M128 clr_mask = _mm_set1_epi8(0x0f);

    LEO_M128 * LEO_RESTRICT x16 = reinterpret_cast<LEO_M128 *>(x);
    LEO_M128 * LEO_RESTRICT y16 = reinterpret_cast<LEO_M128 *>(y);

    do
    {
#define LEO_IFFTB_128(x_ptr, y_ptr) { \
        LEO_M128 x_data = _mm_loadu_si128(x_ptr); \
        LEO_M128 y_data = _mm_loadu_si128(y_ptr); \
        y_data = _mm_xor_si128(y_data, x_data); \
        _mm_storeu_si128(y_ptr, y_data); \
        LEO_M128 lo = _mm_and_si128(y_data, clr_mask); \
        lo = _mm_shuffle_epi8(table_lo_y, lo); \
        LEO_M128 hi = _mm_srli_epi64(y_data, 4); \
        hi = _mm_and_si128(hi, clr_mask); \
        hi = _mm_shuffle_epi8(table_hi_y, hi); \
        x_data = _mm_xor_si128(x_data, _mm_xor_si128(lo, hi)); \
        _mm_storeu_si128(x_ptr, x_data); }

        LEO_IFFTB_128(x16 + 3, y16 + 3);
        LEO_IFFTB_128(x16 + 2, y16 + 2);
        LEO_IFFTB_128(x16 + 1, y16 + 1);
        LEO_IFFTB_128(x16, y16);
        x16 += 4, y16 += 4;

        bytes -= 64;
    } while (bytes > 0);
}

#ifdef LEO_USE_VECTOR4_OPT

void ifft_butterfly4(
    void * LEO_RESTRICT x_0, void * LEO_RESTRICT y_0,
    void * LEO_RESTRICT x_1, void * LEO_RESTRICT y_1,
    void * LEO_RESTRICT x_2, void * LEO_RESTRICT y_2,
    void * LEO_RESTRICT x_3, void * LEO_RESTRICT y_3,
    ffe_t log_m, uint64_t bytes)
{
#if defined(LEO_TRY_AVX2)
    if (CpuHasAVX2)
    {
        const LEO_M256 table_lo_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[0]);
        const LEO_M256 table_hi_y = _mm256_loadu_si256(&Multiply256LUT[log_m].Value[1]);

        const LEO_M256 clr_mask = _mm256_set1_epi8(0x0f);

        LEO_M256 * LEO_RESTRICT x32_0 = reinterpret_cast<LEO_M256 *>(x_0);
        LEO_M256 * LEO_RESTRICT y32_0 = reinterpret_cast<LEO_M256 *>(y_0);
        LEO_M256 * LEO_RESTRICT x32_1 = reinterpret_cast<LEO_M256 *>(x_1);
        LEO_M256 * LEO_RESTRICT y32_1 = reinterpret_cast<LEO_M256 *>(y_1);
        LEO_M256 * LEO_RESTRICT x32_2 = reinterpret_cast<LEO_M256 *>(x_2);
        LEO_M256 * LEO_RESTRICT y32_2 = reinterpret_cast<LEO_M256 *>(y_2);
        LEO_M256 * LEO_RESTRICT x32_3 = reinterpret_cast<LEO_M256 *>(x_3);
        LEO_M256 * LEO_RESTRICT y32_3 = reinterpret_cast<LEO_M256 *>(y_3);

        do
        {
            LEO_IFFTB_256(x32_0 + 1, y32_0 + 1);
            LEO_IFFTB_256(x32_0, y32_0);
            y32_0 += 2, x32_0 += 2;

            LEO_IFFTB_256(x32_1 + 1, y32_1 + 1);
            LEO_IFFTB_256(x32_1, y32_1);
            y32_1 += 2, x32_1 += 2;

            LEO_IFFTB_256(x32_2 + 1, y32_2 + 1);
            LEO_IFFTB_256(x32_2, y32_2);
            y32_2 += 2, x32_2 += 2;

            LEO_IFFTB_256(x32_3 + 1, y32_3 + 1);
            LEO_IFFTB_256(x32_3, y32_3);
            y32_3 += 2, x32_3 += 2;

            bytes -= 64;
        } while (bytes > 0);

        return;
    }
#endif // LEO_TRY_AVX2

    const LEO_M128 table_lo_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[0]);
    const LEO_M128 table_hi_y = _mm_loadu_si128(&Multiply128LUT[log_m].Value[1]);

    const LEO_M128 clr_mask = _mm_set1_epi8(0x0f);

    LEO_M128 * LEO_RESTRICT x16_0 = reinterpret_cast<LEO_M128 *>(x_0);
    LEO_M128 * LEO_RESTRICT y16_0 = reinterpret_cast<LEO_M128 *>(y_0);
    LEO_M128 * LEO_RESTRICT x16_1 = reinterpret_cast<LEO_M128 *>(x_1);
    LEO_M128 * LEO_RESTRICT y16_1 = reinterpret_cast<LEO_M128 *>(y_1);
    LEO_M128 * LEO_RESTRICT x16_2 = reinterpret_cast<LEO_M128 *>(x_2);
    LEO_M128 * LEO_RESTRICT y16_2 = reinterpret_cast<LEO_M128 *>(y_2);
    LEO_M128 * LEO_RESTRICT x16_3 = reinterpret_cast<LEO_M128 *>(x_3);
    LEO_M128 * LEO_RESTRICT y16_3 = reinterpret_cast<LEO_M128 *>(y_3);

    do
    {
        LEO_IFFTB_128(x16_0 + 3, y16_0 + 3);
        LEO_IFFTB_128(x16_0 + 2, y16_0 + 2);
        LEO_IFFTB_128(x16_0 + 1, y16_0 + 1);
        LEO_IFFTB_128(x16_0, y16_0);
        x16_0 += 4, y16_0 += 4;

        LEO_IFFTB_128(x16_1 + 3, y16_1 + 3);
        LEO_IFFTB_128(x16_1 + 2, y16_1 + 2);
        LEO_IFFTB_128(x16_1 + 1, y16_1 + 1);
        LEO_IFFTB_128(x16_1, y16_1);
        x16_1 += 4, y16_1 += 4;

        LEO_IFFTB_128(x16_2 + 3, y16_2 + 3);
        LEO_IFFTB_128(x16_2 + 2, y16_2 + 2);
        LEO_IFFTB_128(x16_2 + 1, y16_2 + 1);
        LEO_IFFTB_128(x16_2, y16_2);
        x16_2 += 4, y16_2 += 4;

        LEO_IFFTB_128(x16_3 + 3, y16_3 + 3);
        LEO_IFFTB_128(x16_3 + 2, y16_3 + 2);
        LEO_IFFTB_128(x16_3 + 1, y16_3 + 1);
        LEO_IFFTB_128(x16_3, y16_3);
        x16_3 += 4, y16_3 += 4;

        bytes -= 64;
    } while (bytes > 0);
}

#endif // LEO_USE_VECTOR4_OPT


//------------------------------------------------------------------------------
// FFT

// Twisted factors used in FFT
static ffe_t FFTSkew[kModulus];

// Factors used in the evaluation of the error locator polynomial
static ffe_t LogWalsh[kOrder];

static void FFTInitialize()
{
    ffe_t temp[kBits - 1];

    // Generate FFT skew vector {1}:

    for (unsigned i = 1; i < kBits; ++i)
        temp[i - 1] = static_cast<ffe_t>(1UL << i);

    for (unsigned m = 0; m < (kBits - 1); ++m)
    {
        const unsigned step = 1UL << (m + 1);

        FFTSkew[(1UL << m) - 1] = 0;

        for (unsigned i = m; i < (kBits - 1); ++i)
        {
            const unsigned s = (1UL << (i + 1));

            for (unsigned j = (1UL << m) - 1; j < s; j += step)
                FFTSkew[j + s] = FFTSkew[j] ^ temp[i];
        }

        temp[m] = kModulus - LogLUT[MultiplyLog(temp[m], LogLUT[temp[m] ^ 1])];

        for (unsigned i = m + 1; i < (kBits - 1); ++i)
        {
            const ffe_t sum = AddMod(LogLUT[temp[i] ^ 1], temp[m]);
            temp[i] = MultiplyLog(temp[i], sum);
        }
    }

    for (unsigned i = 0; i < kOrder; ++i)
        FFTSkew[i] = LogLUT[FFTSkew[i]];

    // Precalculate FWHT(Log[i]):

    for (unsigned i = 0; i < kOrder; ++i)
        LogWalsh[i] = LogLUT[i];
    LogWalsh[0] = 0;

    FWHT(LogWalsh, kBits);
}


//------------------------------------------------------------------------------
// Reed-Solomon Encode

/*
    Decimation in time IFFT:

    The decimation in time IFFT algorithm allows us to unroll 2 layers at a time,
    performing calculations on local registers and faster cache memory.

    Each ^___^ below indicates a butterfly between the associated indices.

    The ifft_butterfly(x, y) operation:

        if (log_m != kModulus)
            x[] ^= exp(log(y[]) + log_m)
        y[] ^= x[]

    Layer 0:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
        ^_^ ^_^ ^_^ ^_^ ^_^ ^_^ ^_^ ^_^

    Layer 1:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
        ^___^   ^___^   ^___^   ^___^
          ^___^   ^___^   ^___^   ^___^
  
    Layer 2:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 
        ^_______^       ^_______^
          ^_______^       ^_______^
            ^_______^       ^_______^
              ^_______^       ^_______^

    Layer 3:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
        ^_______________^
          ^_______________^
            ^_______________^
              ^_______________^
                ^_______________^
                  ^_______________^
                    ^_______________^
                      ^_______________^

    DIT layer 0-1 operations, grouped 4 at a time:
        {0-1, 2-3, 0-2, 1-3},
        {4-5, 6-7, 4-6, 5-7},

    DIT layer 1-2 operations, grouped 4 at a time:
        {0-2, 4-6, 0-4, 2-6},
        {1-3, 5-7, 1-5, 3-7},

    DIT layer 2-3 operations, grouped 4 at a time:
        {0-4, 0'-4', 0-0', 4-4'},
        {1-5, 1'-5', 1-1', 5-5'},
*/

// 4-way butterfly
static void IFFT_DIT4(
    const uint64_t bytes,
    void** work,
    unsigned dist,
    const ffe_t log_m01,
    const ffe_t log_m23,
    const ffe_t log_m02)
{
    // FIXME: Interleave these

    // First layer:
    if (log_m01 == kModulus)
        xor_mem(work[dist], work[0], bytes);
    else
        ifft_butterfly(work[0], work[dist], log_m01, bytes);

    if (log_m23 == kModulus)
        xor_mem(work[dist * 3], work[dist * 2], bytes);
    else
        ifft_butterfly(work[dist * 2], work[dist * 3], log_m23, bytes);

    // Second layer:
    if (log_m02 == kModulus)
    {
        xor_mem(work[dist * 2], work[0], bytes);
        xor_mem(work[dist * 3], work[dist], bytes);
    }
    else
    {
        ifft_butterfly(work[0], work[dist * 2], log_m02, bytes);
        ifft_butterfly(work[dist], work[dist * 3], log_m02, bytes);
    }
}

void IFFT_DIT(
    const uint64_t bytes,
    void* const* data,
    const unsigned m_truncated,
    void** work,
    void** xor_result,
    const unsigned m,
    const ffe_t* skewLUT)
{
    // FIXME: Roll into first layer
    if (data)
    {
        for (unsigned i = 0; i < m_truncated; ++i)
            memcpy(work[i], data[i], bytes);
        for (unsigned i = m_truncated; i < m; ++i)
            memset(work[i], 0, bytes);
    }

    // Decimation in time: Unroll 2 layers at a time
    unsigned dist = 1, dist4 = 4;
    for (; dist4 <= m; dist = dist4, dist4 <<= 2)
    {
        // FIXME: Walk this in reverse order every other pair of layers for better cache locality
        // FIXME: m_truncated

        // For each set of dist*4 elements:
        for (unsigned r = 0; r < m_truncated; r += dist4)
        {
            const ffe_t log_m01 = skewLUT[r + dist];
            const ffe_t log_m23 = skewLUT[r + dist * 3];
            const ffe_t log_m02 = skewLUT[r + dist * 2];

            // For each set of dist elements:
            for (unsigned i = r; i < r + dist; ++i)
            {
                IFFT_DIT4(
                    bytes,
                    work + i,
                    dist,
                    log_m01,
                    log_m23,
                    log_m02);
            }
        }

        data = nullptr;
    }

    // If there is one layer left:
    if (dist < m)
    {
        const ffe_t log_m = skewLUT[dist];

        if (log_m == kModulus)
        {
            for (unsigned i = 0; i < dist; ++i)
                VectorXOR(bytes, dist, work + dist, work);
        }
        else
        {
            for (unsigned i = 0; i < dist; ++i)
            {
                ifft_butterfly(
                    work[i],
                    work[i + dist],
                    log_m,
                    bytes);
            }
        }
    }

    // FIXME: Roll into last layer
    if (xor_result)
        for (unsigned i = 0; i < m; ++i)
            xor_mem(xor_result[i], work[i], bytes);
}

/*
    Decimation in time FFT:

    The decimation in time FFT algorithm allows us to unroll 2 layers at a time,
    performing calculations on local registers and faster cache memory.

    Each ^___^ below indicates a butterfly between the associated indices.

    The fft_butterfly(x, y) operation:

        y[] ^= x[]
        if (log_m != kModulus)
            x[] ^= exp(log(y[]) + log_m)

    Layer 0:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
        ^_______________^
          ^_______________^
            ^_______________^
              ^_______________^
                ^_______________^
                  ^_______________^
                    ^_______________^
                      ^_______________^

    Layer 1:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 
        ^_______^       ^_______^
          ^_______^       ^_______^
            ^_______^       ^_______^
              ^_______^       ^_______^
  
    Layer 2:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
        ^___^   ^___^   ^___^   ^___^
          ^___^   ^___^   ^___^   ^___^

    Layer 3:
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
        ^_^ ^_^ ^_^ ^_^ ^_^ ^_^ ^_^ ^_^

    DIT layer 0-1 operations, grouped 4 at a time:
        {0-0', 4-4', 0-4, 0'-4'},
        {1-1', 5-5', 1-5, 1'-5'},

    DIT layer 1-2 operations, grouped 4 at a time:
        {0-4, 2-6, 0-2, 4-6},
        {1-5, 3-7, 1-3, 5-7},

    DIT layer 2-3 operations, grouped 4 at a time:
        {0-2, 1-3, 0-1, 2-3},
        {4-6, 5-7, 4-5, 6-7},
*/

static void FFT_DIT4(
    const uint64_t bytes,
    void** work,
    const unsigned dist,
    const ffe_t log_m01,
    const ffe_t log_m23,
    const ffe_t log_m02)
{
    // FIXME: Interleave

    // First layer:
    if (log_m02 == kModulus)
    {
        xor_mem(work[dist * 2], work[0], bytes);
        xor_mem(work[dist * 3], work[dist], bytes);
    }
    else
    {
        fft_butterfly(work[0], work[dist * 2], log_m02, bytes);
        fft_butterfly(work[dist], work[dist * 3], log_m02, bytes);
    }

    // Second layer:
    if (log_m01 == kModulus)
        xor_mem(work[dist], work[0], bytes);
    else
        fft_butterfly(work[0], work[dist], log_m01, bytes);

    if (log_m23 == kModulus)
        xor_mem(work[dist * 3], work[dist * 2], bytes);
    else
        fft_butterfly(work[dist * 2], work[dist * 3], log_m23, bytes);
}

void FFT_DIT(
    const uint64_t bytes,
    void** work,
    const unsigned m_truncated,
    const unsigned m,
    const ffe_t* skewLUT)
{
    // Decimation in time: Unroll 2 layers at a time
    unsigned dist4 = m, dist = m >> 2;
    for (; dist != 0; dist4 = dist, dist >>= 2)
    {
        // FIXME: Walk this in reverse order every other pair of layers for better cache locality
        // FIXME: m_truncated

        // For each set of dist*4 elements:
        for (unsigned r = 0; r < m_truncated; r += dist4)
        {
            const ffe_t log_m01 = skewLUT[r + dist];
            const ffe_t log_m23 = skewLUT[r + dist * 3];
            const ffe_t log_m02 = skewLUT[r + dist * 2];

            // For each set of dist elements:
            for (unsigned i = r; i < r + dist; ++i)
            {
                FFT_DIT4(
                    bytes,
                    work + i,
                    dist,
                    log_m01,
                    log_m23,
                    log_m02);
            }
        }
    }

    // If there is one layer left:
    if (dist4 == 2)
    {
        for (unsigned r = 0; r < m_truncated; r += 2)
        {
            const ffe_t log_m = skewLUT[r + 1];

            if (log_m == kModulus)
                xor_mem(work[r + 1], work[r], bytes);
            else
            {
                fft_butterfly(
                    work[r],
                    work[r + 1],
                    log_m,
                    bytes);
            }
        }
    }
}

void ReedSolomonEncode(
    uint64_t buffer_bytes,
    unsigned original_count,
    unsigned recovery_count,
    unsigned m,
    void* const* data,
    void** work)
{
    // work <- IFFT(data, m, m)

    const ffe_t* skewLUT = FFTSkew + m - 1;

    IFFT_DIT(
        buffer_bytes,
        data,
        original_count < m ? original_count : m,
        work,
        nullptr, // No xor output
        m,
        skewLUT);

    if (m >= original_count)
        goto skip_body;

    // For sets of m data pieces:
    for (unsigned i = m; i + m <= original_count; i += m)
    {
        data += m;
        skewLUT += m;

        // work <- work xor IFFT(data + i, m, m + i)

        IFFT_DIT(
            buffer_bytes,
            data, // data source
            m,
            work + m, // temporary workspace
            work, // xor destination
            m,
            skewLUT);
    }

    // Handle final partial set of m pieces:
    const unsigned last_count = original_count % m;
    if (last_count != 0)
    {
        const unsigned i = original_count - last_count;

        data += m;
        skewLUT += m;

        // work <- work xor IFFT(data + i, m, m + i)

        IFFT_DIT(
            buffer_bytes,
            data, // data source
            last_count,
            work + m, // temporary workspace
            work, // xor destination
            m,
            skewLUT);
    }

skip_body:

    // work <- FFT(work, m, 0)

    FFT_DIT(
        buffer_bytes,
        work,
        recovery_count,
        m,
        FFTSkew - 1);
}


//------------------------------------------------------------------------------
// ErrorBitfield

#ifdef LEO_ERROR_BITFIELD_OPT

// Used in decoding to decide which final FFT operations to perform
class ErrorBitfield
{
    static const unsigned kWords = kOrder / 64;
    uint64_t Words[7][kWords] = {};

public:
    LEO_FORCE_INLINE void Set(unsigned i)
    {
        Words[0][i / 64] |= (uint64_t)1 << (i % 64);
    }

    void Prepare();

    LEO_FORCE_INLINE bool IsNeeded(unsigned mip_level, unsigned bit) const
    {
        if (mip_level >= 8)
            return true;
        return 0 != (Words[mip_level - 1][bit / 64] & ((uint64_t)1 << (bit % 64)));
    }
};

static const uint64_t kHiMasks[5] = {
    0xAAAAAAAAAAAAAAAAULL,
    0xCCCCCCCCCCCCCCCCULL,
    0xF0F0F0F0F0F0F0F0ULL,
    0xFF00FF00FF00FF00ULL,
    0xFFFF0000FFFF0000ULL,
};

void ErrorBitfield::Prepare()
{
    // First mip level is for final layer of FFT: pairs of data
    for (unsigned i = 0; i < kWords; ++i)
    {
        uint64_t w_i = Words[0][i];
        const uint64_t hi2lo0 = w_i | ((w_i & kHiMasks[0]) >> 1);
        const uint64_t lo2hi0 = ((w_i & (kHiMasks[0] >> 1)) << 1);
        Words[0][i] = w_i = hi2lo0 | lo2hi0;

        for (unsigned j = 1, bits = 2; j < 5; ++j, bits <<= 1)
        {
            const uint64_t hi2lo_j = w_i | ((w_i & kHiMasks[j]) >> bits);
            const uint64_t lo2hi_j = ((w_i & (kHiMasks[j] >> bits)) << bits);
            Words[j][i] = w_i = hi2lo_j | lo2hi_j;
        }
    }

    for (unsigned i = 0; i < kWords; ++i)
    {
        uint64_t w = Words[4][i];
        w |= w >> 32;
        w |= w << 32;
        Words[5][i] = w;
    }

    for (unsigned i = 0; i < kWords; i += 2)
        Words[6][i] = Words[6][i + 1] = Words[5][i] | Words[5][i + 1];
}

#endif // LEO_ERROR_BITFIELD_OPT


//------------------------------------------------------------------------------
// Reed-Solomon Decode

void VectorFFTButterfly(
    const uint64_t bytes,
    unsigned count,
    void** x,
    void** y,
    const ffe_t log_m)
{
    if (log_m == kModulus)
    {
        VectorXOR(bytes, count, y, x);
        return;
    }

#ifdef LEO_USE_VECTOR4_OPT
    while (count >= 4)
    {
        fft_butterfly4(
            x[0], y[0],
            x[1], y[1],
            x[2], y[2],
            x[3], y[3],
            log_m, bytes);
        x += 4, y += 4;
        count -= 4;
    }
#endif // LEO_USE_VECTOR4_OPT

    for (unsigned i = 0; i < count; ++i)
        fft_butterfly(x[i], y[i], log_m, bytes);
}

void VectorIFFTButterfly(
    const uint64_t bytes,
    unsigned count,
    void** x,
    void** y,
    const ffe_t log_m)
{
    if (log_m == kModulus)
    {
        VectorXOR(bytes, count, y, x);
        return;
    }

#ifdef LEO_USE_VECTOR4_OPT
    while (count >= 4)
    {
        ifft_butterfly4(
            x[0], y[0],
            x[1], y[1],
            x[2], y[2],
            x[3], y[3],
            log_m, bytes);
        x += 4, y += 4;
        count -= 4;
    }
#endif // LEO_USE_VECTOR4_OPT

    for (unsigned i = 0; i < count; ++i)
        ifft_butterfly(x[i], y[i], log_m, bytes);
}

void ReedSolomonDecode(
    uint64_t buffer_bytes,
    unsigned original_count,
    unsigned recovery_count,
    unsigned m, // NextPow2(recovery_count)
    unsigned n, // NextPow2(m + original_count) = work_count
    void* const * const original, // original_count entries
    void* const * const recovery, // recovery_count entries
    void** work) // n entries
{
    // Fill in error locations

#ifdef LEO_ERROR_BITFIELD_OPT
    ErrorBitfield ErrorBits;
#endif // LEO_ERROR_BITFIELD_OPT

    ffe_t ErrorLocations[kOrder] = {};
    for (unsigned i = 0; i < recovery_count; ++i)
        if (!recovery[i])
            ErrorLocations[i] = 1;
    for (unsigned i = recovery_count; i < m; ++i)
        ErrorLocations[i] = 1;
    for (unsigned i = 0; i < original_count; ++i)
    {
        if (!original[i])
        {
            ErrorLocations[i + m] = 1;
#ifdef LEO_ERROR_BITFIELD_OPT
            ErrorBits.Set(i + m);
#endif // LEO_ERROR_BITFIELD_OPT
        }
    }

#ifdef LEO_ERROR_BITFIELD_OPT
    ErrorBits.Prepare();
#endif // LEO_ERROR_BITFIELD_OPT

    // Evaluate error locator polynomial

    FWHT(ErrorLocations);

    for (unsigned i = 0; i < kOrder; ++i)
        ErrorLocations[i] = ((unsigned)ErrorLocations[i] * (unsigned)LogWalsh[i]) % kModulus;

    FWHT(ErrorLocations);

    // work <- recovery data

    for (unsigned i = 0; i < recovery_count; ++i)
    {
        if (recovery[i])
            mul_mem(work[i], recovery[i], ErrorLocations[i], buffer_bytes);
        else
            memset(work[i], 0, buffer_bytes);
    }
    for (unsigned i = recovery_count; i < m; ++i)
        memset(work[i], 0, buffer_bytes);

    // work <- original data

    for (unsigned i = 0; i < original_count; ++i)
    {
        if (original[i])
            mul_mem(work[m + i], original[i], ErrorLocations[m + i], buffer_bytes);
        else
            memset(work[m + i], 0, buffer_bytes);
    }
    for (unsigned i = m + original_count; i < n; ++i)
        memset(work[i], 0, buffer_bytes);

    // work <- IFFT(work, n, 0)

    IFFT_DIT(
        buffer_bytes,
        nullptr,
        n,
        work,
        nullptr,
        n,
        FFTSkew - 1);

    // work <- FormalDerivative(work, n)

    for (unsigned i = 1; i < n; ++i)
    {
        const unsigned width = ((i ^ (i - 1)) + 1) >> 1;

        VectorXOR(
            buffer_bytes,
            width,
            work + i - width,
            work + i);
    }

    // work <- FFT(work, n, 0) truncated to m + original_count

    unsigned mip_level = LastNonzeroBit32(n);
    const unsigned output_count = m + original_count;
    for (unsigned width = (n >> 1); width > 0; width >>= 1, --mip_level)
    {
        const ffe_t* skewLUT = FFTSkew + width - 1;
        const unsigned range = width << 1;

#ifdef LEO_SCHEDULE_OPT
        for (unsigned j = (m < range) ? 0 : m; j < output_count; j += range)
#else
        for (unsigned j = 0; j < n; j += range)
#endif
        {
#ifdef LEO_ERROR_BITFIELD_OPT
            if (!ErrorBits.IsNeeded(mip_level, j))
                continue;
#endif // LEO_ERROR_BITFIELD_OPT

            VectorFFTButterfly(
                buffer_bytes,
                width,
                work + j,
                work + j + width,
                skewLUT[j]);
        }
    }

    // Reveal erasures

    for (unsigned i = 0; i < original_count; ++i)
        if (!original[i])
            mul_mem(work[i], work[i + m], kModulus - ErrorLocations[i + m], buffer_bytes);
}


//------------------------------------------------------------------------------
// API

static bool IsInitialized = false;

bool Initialize()
{
    if (IsInitialized)
        return true;

    if (!CpuHasSSSE3)
        return false;

    InitializeLogarithmTables();
    InitializeMultiplyTables();
    FFTInitialize();

    IsInitialized = true;
    return true;
}


}} // namespace leopard::ff8

#endif // LEO_HAS_FF8