namespace OcrDaemon;

static class SignalProcessing
{
    /// <summary>
    /// Find the dominant period in a signal using autocorrelation.
    /// Returns (period, score) where score is the autocorrelation strength.
    /// </summary>
    public static (int period, double score) FindPeriodWithScore(double[] signal, int minPeriod, int maxPeriod)
    {
        int n = signal.Length;
        if (n < minPeriod * 3) return (-1, 0);

        double mean = signal.Average();
        double variance = 0;
        for (int i = 0; i < n; i++)
            variance += (signal[i] - mean) * (signal[i] - mean);
        if (variance < 1.0) return (-1, 0);

        int maxLag = Math.Min(maxPeriod, n / 3);
        double[] ac = new double[maxLag + 1];
        for (int lag = minPeriod; lag <= maxLag; lag++)
        {
            double sum = 0;
            for (int i = 0; i < n - lag; i++)
                sum += (signal[i] - mean) * (signal[i + lag] - mean);
            ac[lag] = sum / variance;
        }

        // Find the first significant peak — this is the fundamental period.
        // Using "first" avoids picking harmonics (2x, 3x) or unrelated larger patterns.
        for (int lag = minPeriod + 1; lag < maxLag; lag++)
        {
            if (ac[lag] > 0.01 && ac[lag] >= ac[lag - 1] && ac[lag] >= ac[lag + 1])
                return (lag, ac[lag]);
        }

        return (-1, 0);
    }

    /// <summary>
    /// Find contiguous segments where values are ABOVE threshold.
    /// Used to find grid panel regions by density of very dark pixels.
    /// Allows brief gaps (up to 5px) to handle grid borders.
    /// </summary>
    public static List<(int start, int end)> FindDarkDensitySegments(double[] profile, double threshold, int minLength)
    {
        var segments = new List<(int start, int end)>();
        int n = profile.Length;
        int curStart = -1;
        int maxGap = 5;
        int gapCount = 0;

        for (int i = 0; i < n; i++)
        {
            if (profile[i] >= threshold)
            {
                if (curStart < 0) curStart = i;
                gapCount = 0;
            }
            else
            {
                if (curStart >= 0)
                {
                    gapCount++;
                    if (gapCount > maxGap)
                    {
                        int end = i - gapCount;
                        if (end - curStart >= minLength)
                            segments.Add((curStart, end));
                        curStart = -1;
                        gapCount = 0;
                    }
                }
            }
        }
        if (curStart >= 0)
        {
            int end = gapCount > 0 ? n - gapCount : n;
            if (end - curStart >= minLength)
                segments.Add((curStart, end));
        }

        return segments;
    }

    /// <summary>
    /// Find the extent of the grid in a 1D profile using local autocorrelation
    /// at the specific detected period. Only regions where the signal actually
    /// repeats at the given period will score high — much more precise than variance.
    /// </summary>
    public static (int start, int end) FindGridExtent(double[] signal, int period)
    {
        int n = signal.Length;
        int halfWin = period * 2; // window radius: 2 periods each side
        if (n < halfWin * 2 + period) return (-1, -1);

        // Compute local AC at the specific lag=period in a sliding window
        double[] localAc = new double[n];
        for (int center = halfWin; center < n - halfWin; center++)
        {
            int wStart = center - halfWin;
            int wEnd = center + halfWin;
            int count = wEnd - wStart;

            // Local mean
            double sum = 0;
            for (int i = wStart; i < wEnd; i++)
                sum += signal[i];
            double mean = sum / count;

            // Local variance
            double varSum = 0;
            for (int i = wStart; i < wEnd; i++)
                varSum += (signal[i] - mean) * (signal[i] - mean);

            if (varSum < 1.0) continue;

            // AC at the specific lag=period
            double acSum = 0;
            for (int i = wStart; i < wEnd - period; i++)
                acSum += (signal[i] - mean) * (signal[i + period] - mean);

            localAc[center] = Math.Max(0, acSum / varSum);
        }

        // Find the longest contiguous run above threshold
        double maxAc = 0;
        for (int i = 0; i < n; i++)
            if (localAc[i] > maxAc) maxAc = localAc[i];
        if (maxAc < 0.02) return (-1, -1);

        double threshold = maxAc * 0.25;

        int bestStart = -1, bestEnd = -1, bestLen = 0;
        int curStartPos = -1;
        for (int i = 0; i < n; i++)
        {
            if (localAc[i] > threshold)
            {
                if (curStartPos < 0) curStartPos = i;
            }
            else
            {
                if (curStartPos >= 0)
                {
                    int len = i - curStartPos;
                    if (len > bestLen)
                    {
                        bestLen = len;
                        bestStart = curStartPos;
                        bestEnd = i;
                    }
                    curStartPos = -1;
                }
            }
        }
        // Handle run extending to end of signal
        if (curStartPos >= 0)
        {
            int len = n - curStartPos;
            if (len > bestLen)
            {
                bestStart = curStartPos;
                bestEnd = n;
            }
        }

        if (bestStart < 0) return (-1, -1);

        // Small extension to include cell borders at edges
        bestStart = Math.Max(0, bestStart - period / 4);
        bestEnd = Math.Min(n - 1, bestEnd + period / 4);

        return (bestStart, bestEnd);
    }
}