// Copyright 2020 Mobilinkd LLC.

#pragma once

#include <array>
#include <algorithm>
#include <cassert>

namespace mobilinkd
{

/**
 * Estimate the phase of a sample by estimating the
 * tangent of the sample point.  This is done by computing
 * the magnitude difference of the previous and following
 * samples.  We do not correct for 0-crossing errors because
 * these errors have not affected the performance of clock
 * recovery.
 */
template <typename FloatType>
struct PhaseEstimator
{
    using samples_t = std::array<FloatType, 3>;    // 3 samples in length

    FloatType dx_;

    PhaseEstimator(FloatType sample_rate, FloatType symbol_rate)
    : dx_(2.0 * symbol_rate / sample_rate)
    {
        assert(dx_ > 0.0);
    }

    /**
     * This performs a rolling estimate of the phase.
     * 
     * @param samples are three samples centered around the current sample point
     *  (t-1, t, t+1).
     */
    FloatType operator()(const samples_t& samples)
    {
        auto ratio = ((samples.at(2) - samples.at(0)) / 3.0f) / dx_;
        // Clamp +/-5.
        ratio = std::min(FloatType(5.0), ratio);
        ratio = std::max(FloatType(-5.0), ratio);
        
        return ratio;
    }
};

} // mobilinkd
