package com.rippr.stats

import com.rippr.Telemetry
import com.rippr.data.Segment
import com.rippr.data.TrackPoint
import com.rippr.geo.Geo
import kotlin.math.abs
import kotlin.math.max
import kotlin.math.min
import kotlin.math.roundToInt

data class RideSummary(
    val distanceM: Double = 0.0,
    /** Wall clock, first fix to last. */
    val elapsedMillis: Long = 0,
    /** Time spent above the speed noise floor. */
    val movingMillis: Long = 0,
    val maxSpeedKmh: Float = 0f,
    /** Distance ÷ moving time — not the mean of the speed samples. */
    val avgMovingSpeedKmh: Float = 0f,
    val elevationGainM: Double = 0.0,
    val elevationLossM: Double = 0.0,
    val pointCount: Int = 0,
) {
    val stoppedMillis: Long get() = max(0L, elapsedMillis - movingMillis)

    companion object {
        val EMPTY = RideSummary()
    }
}

data class SpeedBucket(val fromKmh: Int, val toKmh: Int, val millis: Long) {
    val label: String get() = "$fromKmh–$toKmh"
}

data class ElevationSample(val distanceM: Double, val altitudeM: Double)

/**
 * Batch statistics over a stored ride.
 *
 * This is the authoritative computation. The recorder accumulates the same values live
 * as points arrive, but re-runs this on trip completion so a mid-ride process kill
 * cannot leave permanently skewed totals. Both paths must agree, which is why they share
 * the constants below rather than duplicating magic numbers.
 */
object RideStatistics {

    /**
     * A GPS dropout leaves a large gap between consecutive fixes. Without a cap, a
     * two-minute tunnel counts as two minutes of moving time at the last known speed.
     */
    const val MAX_SAMPLE_GAP_MILLIS = 10_000L

    /**
     * Raw GPS altitude wanders by ±5–10 m even sitting still. Summing every positive
     * delta turns a flat ride into thousands of metres of climbing — the classic bug in
     * this calculation. A climb only counts once it exceeds this much in one direction.
     */
    const val ELEVATION_HYSTERESIS_M = 3.0

    fun compute(points: List<TrackPoint>, segments: List<Segment> = emptyList()): RideSummary {
        if (points.isEmpty()) return RideSummary.EMPTY

        var distanceM = 0.0
        var movingMillis = 0L
        var maxSpeedKmh = 0f

        val elevation = ElevationAccumulator()

        // Grouping by segment is what keeps a pause from inventing distance: points
        // either side of a gas-station stop can be kilometres apart.
        for (segmentPoints in points.groupBy { it.segmentId }.values) {
            var previous: TrackPoint? = null
            for (point in segmentPoints) {
                maxSpeedKmh = max(maxSpeedKmh, point.speedKmh)
                elevation.add(point.altitudeM)

                previous?.let { prev ->
                    distanceM += Geo.haversineMeters(
                        prev.latitude, prev.longitude, point.latitude, point.longitude,
                    )
                    val dt = point.timestamp - prev.timestamp
                    if (dt in 1..MAX_SAMPLE_GAP_MILLIS &&
                        point.speedKmh >= Telemetry.SPEED_NOISE_FLOOR_KMH
                    ) {
                        movingMillis += dt
                    }
                }
                previous = point
            }
        }

        elevation.finish()

        val elapsedMillis = elapsedFor(points, segments)

        // Guard the divide: a ride that never moved would otherwise produce NaN, which
        // Compose happily renders as the literal text "NaN".
        val avgMovingSpeedKmh = if (movingMillis > 0) {
            (distanceM / 1000.0 / (movingMillis / 3_600_000.0)).toFloat()
        } else {
            0f
        }

        return RideSummary(
            distanceM = distanceM,
            elapsedMillis = elapsedMillis,
            movingMillis = movingMillis,
            maxSpeedKmh = maxSpeedKmh,
            avgMovingSpeedKmh = avgMovingSpeedKmh,
            elevationGainM = elevation.gain,
            elevationLossM = elevation.loss,
            pointCount = points.size,
        )
    }

    /**
     * Prefers segment boundaries over point timestamps: they capture the time between a
     * segment's last fix and the pause itself, which point timestamps cannot see.
     */
    private fun elapsedFor(points: List<TrackPoint>, segments: List<Segment>): Long {
        val closed = segments.mapNotNull { s -> s.endedAt?.let { it - s.startedAt } }
        if (closed.isNotEmpty() && closed.size == segments.size) {
            return closed.sum()
        }
        val timestamps = points.map { it.timestamp }
        return max(0L, (timestamps.maxOrNull() ?: 0L) - (timestamps.minOrNull() ?: 0L))
    }

    /**
     * Time spent in each speed band. Buckets are keyed on the *interval* between fixes,
     * so the result is a time distribution rather than a sample count — a bike that sits
     * idle at 2 Hz would otherwise dominate purely by producing more samples.
     */
    fun speedHistogram(points: List<TrackPoint>, bucketKmh: Int = 10): List<SpeedBucket> {
        if (points.size < 2 || bucketKmh <= 0) return emptyList()

        val millisByBucket = sortedMapOf<Int, Long>()
        for (segmentPoints in points.groupBy { it.segmentId }.values) {
            for (i in 1 until segmentPoints.size) {
                val dt = segmentPoints[i].timestamp - segmentPoints[i - 1].timestamp
                if (dt !in 1..MAX_SAMPLE_GAP_MILLIS) continue
                val bucket = (segmentPoints[i].speedKmh / bucketKmh).toInt()
                millisByBucket[bucket] = (millisByBucket[bucket] ?: 0L) + dt
            }
        }

        return millisByBucket.map { (bucket, millis) ->
            SpeedBucket(bucket * bucketKmh, (bucket + 1) * bucketKmh, millis)
        }
    }

    /**
     * Altitude against distance travelled, downsampled for charting.
     *
     * Sampled by distance along the path rather than by index, so a long stop does not
     * flatten the interesting part of the profile into a few pixels.
     */
    fun elevationProfile(points: List<TrackPoint>, maxSamples: Int = 200): List<ElevationSample> {
        if (points.isEmpty()) return emptyList()
        if (points.size == 1) return listOf(ElevationSample(0.0, points[0].altitudeM))

        val full = ArrayList<ElevationSample>(points.size)
        var cumulative = 0.0
        var previous: TrackPoint? = null
        var previousSegment = points.first().segmentId

        for (point in points) {
            previous?.let { prev ->
                // Distance only accrues within a segment, matching compute().
                if (point.segmentId == previousSegment) {
                    cumulative += Geo.haversineMeters(
                        prev.latitude, prev.longitude, point.latitude, point.longitude,
                    )
                }
            }
            full += ElevationSample(cumulative, point.altitudeM)
            previous = point
            previousSegment = point.segmentId
        }

        if (full.size <= maxSamples) return full

        val step = full.size.toDouble() / maxSamples
        return (0 until maxSamples).map { full[(it * step).roundToInt().coerceAtMost(full.lastIndex)] } +
            full.last()
    }
}

/**
 * Elevation gain/loss accumulator that survives GPS altitude noise.
 *
 * Two mechanisms, because one is not enough:
 *
 *  1. **A moving-average window.** Raw GPS altitude wanders by ±5–10 m while completely
 *     stationary. Averaging over [windowSize] samples cuts the noise by roughly
 *     sqrt(windowSize), bringing it under the threshold below.
 *  2. **Reversal hysteresis.** A climb is only banked once the altitude turns back down
 *     by more than [thresholdM] from its peak. Simply summing every delta that exceeds a
 *     threshold does *not* work — noise crosses any small threshold constantly, and a
 *     parked bike accumulates well over a kilometre of imaginary climbing. That was
 *     measured, not assumed.
 *
 * Streaming rather than batch so the recorder can accumulate live and the batch
 * computation can reuse the identical code path.
 */
class ElevationAccumulator(
    private val windowSize: Int = SMOOTHING_WINDOW,
    private val thresholdM: Double = RideStatistics.ELEVATION_HYSTERESIS_M,
) {
    private val window = ArrayDeque<Double>(windowSize)
    private var windowSum = 0.0

    private var lastRaw: Double = 0.0
    private var lastCommitted: Double? = null
    private var extreme: Double = 0.0
    private var direction = 0 // 0 unknown, +1 climbing, -1 descending

    var gain: Double = 0.0
        private set
    var loss: Double = 0.0
        private set

    fun add(altitudeM: Double) {
        if (!altitudeM.isFinite()) return
        lastRaw = altitudeM

        window.addLast(altitudeM)
        windowSum += altitudeM
        if (window.size > windowSize) windowSum -= window.removeFirst()
        val smoothed = windowSum / window.size

        val committed = lastCommitted
        if (committed == null) {
            lastCommitted = smoothed
            extreme = smoothed
            return
        }

        when (direction) {
            0 -> when {
                smoothed > committed + thresholdM -> { direction = 1; extreme = smoothed }
                smoothed < committed - thresholdM -> { direction = -1; extreme = smoothed }
            }
            1 -> if (smoothed > extreme) {
                extreme = smoothed
            } else if (smoothed < extreme - thresholdM) {
                gain += extreme - committed
                lastCommitted = extreme
                direction = -1
                extreme = smoothed
            }
            else -> if (smoothed < extreme) {
                extreme = smoothed
            } else if (smoothed > extreme + thresholdM) {
                loss += committed - extreme
                lastCommitted = extreme
                direction = 1
                extreme = smoothed
            }
        }
    }

    /**
     * Gain including the run still in progress, without mutating state.
     *
     * Safe to poll while recording continues — [finish] would end the run, which is wrong
     * mid-ride, but reading only [gain] would report zero for a climb that has not yet
     * turned back down.
     */
    fun gainIncludingPending(): Double {
        val committed = lastCommitted ?: return gain
        val tip = if (direction == 1) max(extreme, lastRaw) else extreme
        return if (direction == 1 && tip > committed) gain + (tip - committed) else gain
    }

    /**
     * Banks the run still in progress. Must be called once the last point is added, or a
     * steady climb to the summit with no descent afterwards reports zero gain.
     */
    fun finish() {
        val committed = lastCommitted ?: return
        // The moving average lags the true altitude by about half a window, so the final
        // smoothed value clips the tail of a climb (a 100 m ascent measured 93 m before
        // this). Reconcile against the last raw reading to recover it.
        when (direction) {
            1 -> extreme = max(extreme, lastRaw)
            -1 -> extreme = min(extreme, lastRaw)
        }
        when {
            direction == 1 && extreme > committed -> gain += extreme - committed
            direction == -1 && extreme < committed -> loss += committed - extreme
        }
        lastCommitted = extreme
        direction = 0
    }

    private companion object {
        /** ~7 s at 2 Hz: long enough to suppress wander, short enough to keep real terrain. */
        const val SMOOTHING_WINDOW = 15
    }
}