JUCE/modules/juce_dsp/maths/juce_LookupTable.cpp

/*
  ==============================================================================

   This file is part of the JUCE 6 technical preview.
   Copyright (c) 2020 - Raw Material Software Limited

   You may use this code under the terms of the GPL v3
   (see www.gnu.org/licenses).

   For this technical preview, this file is not subject to commercial licensing.

   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.

  ==============================================================================
*/

namespace juce
{
namespace dsp
{

template <typename FloatType>
LookupTable<FloatType>::LookupTable()
{
    data.resize (1);
}

template <typename FloatType>
LookupTable<FloatType>::LookupTable (const std::function<FloatType (size_t)>& functionToApproximate,
                                     size_t numPointsToUse)
{
    initialise (functionToApproximate, numPointsToUse);
}

//==============================================================================
template <typename FloatType>
void LookupTable<FloatType>::initialise (const std::function<FloatType (size_t)>& functionToApproximate,
                                         size_t numPointsToUse)
{
    data.resize (static_cast<int> (getRequiredBufferSize (numPointsToUse)));

    for (size_t i = 0; i < numPointsToUse; ++i)
    {
        auto value = functionToApproximate (i);

        jassert (! std::isnan (value));
        jassert (! std::isinf (value));
        // Make sure functionToApproximate returns a sensible value for the entire specified range.
        // E.g., this won't work for zero:  [] (size_t i) { return 1.0f / i; }

        data.getReference (static_cast<int> (i)) = value;
    }

    prepare();
}

template <typename FloatType>
void LookupTable<FloatType>::prepare() noexcept
{
    auto guardIndex = static_cast<int> (getGuardIndex());
    data.getReference (guardIndex) = data.getUnchecked (guardIndex - 1);
}

template <typename FloatType>
void LookupTableTransform<FloatType>::initialise (const std::function<FloatType (FloatType)>& functionToApproximate,
                                                  FloatType minInputValueToUse,
                                                  FloatType maxInputValueToUse,
                                                  size_t numPoints)
{
    jassert (maxInputValueToUse > minInputValueToUse);

    minInputValue = minInputValueToUse;
    maxInputValue = maxInputValueToUse;
    scaler = FloatType (numPoints - 1) / (maxInputValueToUse - minInputValueToUse);
    offset = -minInputValueToUse * scaler;

    const auto initFn = [functionToApproximate, minInputValueToUse, maxInputValueToUse, numPoints] (size_t i)
    {
        return functionToApproximate (
            jlimit (
                minInputValueToUse, maxInputValueToUse,
                jmap (FloatType (i), FloatType (0), FloatType (numPoints - 1), minInputValueToUse, maxInputValueToUse))
            );
    };

    lookupTable.initialise (initFn, numPoints);
}

//==============================================================================
template <typename FloatType>
double LookupTableTransform<FloatType>::calculateMaxRelativeError (const std::function<FloatType (FloatType)>& functionToApproximate,
                                                                   FloatType minInputValue,
                                                                   FloatType maxInputValue,
                                                                   size_t numPoints,
                                                                   size_t numTestPoints)
{
    jassert (maxInputValue > minInputValue);

    if (numTestPoints == 0)
        numTestPoints = 100 * numPoints;    // use default

    LookupTableTransform transform (functionToApproximate, minInputValue, maxInputValue, numPoints);

    double maxError = 0;

    for (size_t i = 0; i < numTestPoints; ++i)
    {
        auto inputValue = jmap (FloatType (i), FloatType (0), FloatType (numTestPoints - 1), minInputValue, maxInputValue);
        auto approximatedOutputValue = transform.processSample (inputValue);
        auto referenceOutputValue = functionToApproximate (inputValue);

        maxError = jmax (maxError, calculateRelativeDifference ((double) referenceOutputValue, (double) approximatedOutputValue));
    }

    return maxError;
}

//==============================================================================
template <typename FloatType>
double LookupTableTransform<FloatType>::calculateRelativeDifference (double x, double y) noexcept
{
    static const auto eps = std::numeric_limits<double>::min();

    auto absX = std::abs (x);
    auto absY = std::abs (y);
    auto absDiff = std::abs (x - y);

    if (absX < eps)
    {
        if (absY >= eps)
            return absDiff / absY;

        return absDiff;    // return the absolute error if both numbers are too close to zero
    }

    return absDiff / std::min (absX, absY);
}

//==============================================================================
template class LookupTable<float>;
template class LookupTable<double>;

template class LookupTableTransform<float>;
template class LookupTableTransform<double>;

} // namespace dsp
} // namespace juce