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package com.example.androidapp.routing
import org.terst.nav.data.model.BoatPolars
import org.terst.nav.data.model.WindForecast
import kotlin.math.asin
import kotlin.math.atan2
import kotlin.math.cos
import kotlin.math.pow
import kotlin.math.sin
import kotlin.math.sqrt
/**
* Isochrone-based weather routing engine (Section 3.4).
*
* Algorithm:
* 1. Start from a single point; expand a fan of headings at each time step.
* 2. For each candidate heading, compute BSP from [BoatPolars] at the local forecast wind.
* 3. Advance position by BSP × Δt using the spherical-Earth destination-point formula.
* 4. Check whether the destination has been reached (within [arrivalRadiusM]).
* 5. Prune candidates: for each angular sector around the start, keep only the point that
* advanced furthest (removes dominated points).
* 6. Repeat until the destination is reached or [maxSteps] is exhausted.
* 7. Backtrace parent pointers to produce the optimal path.
*/
object IsochroneRouter {
private const val EARTH_RADIUS_M = 6_371_000.0
internal const val NM_TO_M = 1_852.0
private const val KT_TO_M_PER_S = NM_TO_M / 3600.0
const val DEFAULT_HEADING_STEP_DEG = 5.0
const val DEFAULT_ARRIVAL_RADIUS_M = 1_852.0 // 1 NM
const val DEFAULT_PRUNE_SECTORS = 72 // 5° sectors
const val DEFAULT_MAX_STEPS = 200
/**
* Compute an optimised route from start to destination.
*
* @param startLat Start latitude (decimal degrees).
* @param startLon Start longitude (decimal degrees).
* @param destLat Destination latitude (decimal degrees).
* @param destLon Destination longitude (decimal degrees).
* @param startTimeMs Departure time as UNIX timestamp (ms).
* @param stepMs Time increment per isochrone step (ms). Typical: 1–3 hours.
* @param polars Boat polar table.
* @param windAt Function returning [WindForecast] for a given position and time.
* @param headingStepDeg Angular resolution of the heading fan (degrees). Default 5°.
* @param arrivalRadiusM Distance threshold to consider destination reached (metres).
* @param maxSteps Maximum number of isochrone expansions before giving up.
* @return [IsochroneResult] with the optimal path and ETA, or null if unreachable.
*/
fun route(
startLat: Double,
startLon: Double,
destLat: Double,
destLon: Double,
startTimeMs: Long,
stepMs: Long,
polars: BoatPolars,
windAt: (lat: Double, lon: Double, timeMs: Long) -> WindForecast,
headingStepDeg: Double = DEFAULT_HEADING_STEP_DEG,
arrivalRadiusM: Double = DEFAULT_ARRIVAL_RADIUS_M,
maxSteps: Int = DEFAULT_MAX_STEPS
): IsochroneResult? {
val start = RoutePoint(startLat, startLon, startTimeMs)
var isochrone = listOf(start)
repeat(maxSteps) { step ->
val nextTimeMs = startTimeMs + (step + 1).toLong() * stepMs
val candidates = mutableListOf<RoutePoint>()
for (point in isochrone) {
var heading = 0.0
while (heading < 360.0) {
val wind = windAt(point.lat, point.lon, point.timestampMs)
val twa = ((heading - wind.twdDeg + 360.0) % 360.0)
val bspKt = polars.bsp(twa, wind.twsKt)
if (bspKt > 0.0) {
val distM = bspKt * KT_TO_M_PER_S * (stepMs / 1000.0)
val (newLat, newLon) = destinationPoint(point.lat, point.lon, heading, distM)
val newPoint = RoutePoint(newLat, newLon, nextTimeMs, parent = point)
if (haversineM(newLat, newLon, destLat, destLon) <= arrivalRadiusM) {
return IsochroneResult(
path = backtrace(newPoint),
etaMs = nextTimeMs
)
}
candidates.add(newPoint)
}
heading += headingStepDeg
}
}
if (candidates.isEmpty()) return null
isochrone = prune(candidates, startLat, startLon, DEFAULT_PRUNE_SECTORS)
}
return null
}
/** Walk parent pointers from destination back to start, then reverse. */
internal fun backtrace(dest: RoutePoint): List<RoutePoint> {
val path = mutableListOf<RoutePoint>()
var current: RoutePoint? = dest
while (current != null) {
path.add(current)
current = current.parent
}
path.reverse()
return path
}
/**
* Angular-sector pruning: divide the plane into [sectors] equal angular sectors around the
* start. Within each sector keep only the candidate that is furthest from the start.
*/
internal fun prune(
candidates: List<RoutePoint>,
startLat: Double,
startLon: Double,
sectors: Int
): List<RoutePoint> {
val sectorSize = 360.0 / sectors
val best = mutableMapOf<Int, RoutePoint>()
for (point in candidates) {
val bearing = bearingDeg(startLat, startLon, point.lat, point.lon)
val sector = (bearing / sectorSize).toInt().coerceIn(0, sectors - 1)
val existing = best[sector]
if (existing == null ||
haversineM(startLat, startLon, point.lat, point.lon) >
haversineM(startLat, startLon, existing.lat, existing.lon)
) {
best[sector] = point
}
}
return best.values.toList()
}
/** Haversine great-circle distance in metres. */
internal fun haversineM(lat1: Double, lon1: Double, lat2: Double, lon2: Double): Double {
val dLat = Math.toRadians(lat2 - lat1)
val dLon = Math.toRadians(lon2 - lon1)
val a = sin(dLat / 2).pow(2) +
cos(Math.toRadians(lat1)) * cos(Math.toRadians(lat2)) * sin(dLon / 2).pow(2)
return 2.0 * EARTH_RADIUS_M * asin(sqrt(a))
}
/** Initial bearing from point 1 to point 2 (degrees, 0 = North, clockwise). */
internal fun bearingDeg(lat1Deg: Double, lon1Deg: Double, lat2Deg: Double, lon2Deg: Double): Double {
val lat1 = Math.toRadians(lat1Deg)
val lat2 = Math.toRadians(lat2Deg)
val dLon = Math.toRadians(lon2Deg - lon1Deg)
val y = sin(dLon) * cos(lat2)
val x = cos(lat1) * sin(lat2) - sin(lat1) * cos(lat2) * cos(dLon)
return (Math.toDegrees(atan2(y, x)) + 360.0) % 360.0
}
/** Spherical-Earth destination-point given start, bearing, and distance. */
internal fun destinationPoint(
lat1Deg: Double,
lon1Deg: Double,
bearingDeg: Double,
distM: Double
): Pair<Double, Double> {
val lat1 = Math.toRadians(lat1Deg)
val lon1 = Math.toRadians(lon1Deg)
val brng = Math.toRadians(bearingDeg)
val d = distM / EARTH_RADIUS_M
val lat2 = asin(sin(lat1) * cos(d) + cos(lat1) * sin(d) * cos(brng))
val lon2 = lon1 + atan2(sin(brng) * sin(d) * cos(lat1), cos(d) - sin(lat1) * sin(lat2))
return Pair(Math.toDegrees(lat2), Math.toDegrees(lon2))
}
}
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