/*
 * geo.ts — spherical geo math for the route model (D-07).
 * --------------------------------------------------------
 * haversineM: great-circle distance in meters on the mean-earth sphere
 * (radius 6 371 000 m — at KL-metro scale the model error is far below the
 * fixture corpus's 200 m origin tolerance). Finite for finite inputs.
 *
 * deriveCumulativeDistanceM: the snapped-progress support field (D-07 — the
 * GUID-05/06 upstream obligation). Produces [0, d(p0→p1), d(p0→p1)+d(p1→p2),
 * …] — the same length as the input, [0] === 0, non-decreasing. Phase 6's
 * guidance snaps a live fix's progress along the line using this array
 * instead of re-deriving it per fix; the fixture source derives it once at
 * decode/load and memoizes (RESEARCH Finding 1).
 *
 * Pure: no I/O, no logging, no storage (the D-15 analog — nothing about a
 * request outlives the caller's frame).
 */

import type { RoutePoint } from "./types.ts";

const EARTH_RADIUS_M = 6_371_000;
const DEGREES_TO_RADIANS = Math.PI / 180;

/** Great-circle distance between two WGS84 points, in meters. */
export function haversineM(a: RoutePoint, b: RoutePoint): number {
  const lat1 = a.lat * DEGREES_TO_RADIANS;
  const lat2 = b.lat * DEGREES_TO_RADIANS;
  const dLat = (b.lat - a.lat) * DEGREES_TO_RADIANS;
  const dLng = (b.lng - a.lng) * DEGREES_TO_RADIANS;
  const sinLat = Math.sin(dLat / 2);
  const sinLng = Math.sin(dLng / 2);
  const h = sinLat * sinLat + Math.cos(lat1) * Math.cos(lat2) * sinLng * sinLng;
  return 2 * EARTH_RADIUS_M * Math.asin(Math.min(1, Math.sqrt(h)));
}

/** Meters walked along the polyline: same length as points, [0] === 0,
 *  non-decreasing. Empty -> []; single point -> [0]. */
export function deriveCumulativeDistanceM(points: readonly RoutePoint[]): number[] {
  if (points.length === 0) return [];
  const cumulative: number[] = [0];
  for (let i = 1; i < points.length; i++) {
    const prev = points[i - 1];
    const curr = points[i];
    const prevMeters = cumulative[i - 1];
    if (prev === undefined || curr === undefined || prevMeters === undefined) break;
    cumulative.push(prevMeters + haversineM(prev, curr));
  }
  return cumulative;
}