shadow-zone / files / src / shadowZone.ts
shadowZone.ts
| 1 | import type { EarthModel } from './earth'; |
| 2 | import { traceRay } from './rayTracer'; |
| 3 | import { R_EARTH } from './earth'; |
| 4 | |
| 5 | export interface ShadowZoneResult { |
| 6 | start: number; // degrees |
| 7 | end: number; // degrees |
| 8 | } |
| 9 | |
| 10 | export function findShadowZone( |
| 11 | model: EarthModel, |
| 12 | wave: 'P' | 'S', |
| 13 | ): ShadowZoneResult[] { |
| 14 | const N = 360; |
| 15 | const vSurf = model.vAt(R_EARTH - 0.1, wave); |
| 16 | if (vSurf === 0) return []; |
| 17 | |
| 18 | const distances: number[] = []; |
| 19 | |
| 20 | for (let i = 0; i < N; i++) { |
| 21 | const angleDeg = 1 + (i / (N - 1)) * 88; |
| 22 | const angleRad = (angleDeg * Math.PI) / 180; |
| 23 | const p = (R_EARTH * Math.sin(angleRad)) / vSurf; |
| 24 | const ray = traceRay(model, p, wave, angleDeg); |
| 25 | if (ray.epicentralDistance !== null && !ray.blocked) { |
| 26 | // Normalize to [0°, 180°]: a ray arriving at >180° has wrapped past the |
| 27 | // antipode; the receiver sits at 360°−dist on the near arc. |
| 28 | let d = ray.epicentralDistance % 360; |
| 29 | if (d > 180) d = 360 - d; |
| 30 | distances.push(d); |
| 31 | } |
| 32 | } |
| 33 | |
| 34 | if (distances.length === 0) return []; |
| 35 | |
| 36 | distances.sort((a, b) => a - b); |
| 37 | |
| 38 | // S-waves are blocked by the liquid outer core and never arrive beyond ~105°. |
| 39 | // Without an endpoint, the gap-finder never sees the 105°→180° shadow zone. |
| 40 | if (wave === 'S') distances.push(180); |
| 41 | |
| 42 | // Collect ALL gaps ≥ 3°. |
| 43 | const MIN_GAP = 3; |
| 44 | const zones: ShadowZoneResult[] = []; |
| 45 | |
| 46 | for (let i = 1; i < distances.length; i++) { |
| 47 | const gap = distances[i] - distances[i - 1]; |
| 48 | if (gap >= MIN_GAP) { |
| 49 | zones.push({ start: distances[i - 1], end: distances[i] }); |
| 50 | } |
| 51 | } |
| 52 | |
| 53 | // Merge zones whose inter-zone gap is ≤ MERGE_THRESH so that physically |
| 54 | // contiguous shadow regions (e.g. the many small gaps a water core creates) |
| 55 | // appear as one arc instead of many overlapping ones. |
| 56 | return mergeZones(zones, 5); |
| 57 | } |
| 58 | |
| 59 | function mergeZones(zones: ShadowZoneResult[], threshDeg: number): ShadowZoneResult[] { |
| 60 | if (zones.length <= 1) return zones; |
| 61 | const sorted = [...zones].sort((a, b) => a.start - b.start); |
| 62 | const merged: ShadowZoneResult[] = []; |
| 63 | let cur = { ...sorted[0] }; |
| 64 | for (let i = 1; i < sorted.length; i++) { |
| 65 | const next = sorted[i]; |
| 66 | if (next.start - cur.end <= threshDeg) { |
| 67 | cur.end = Math.max(cur.end, next.end); |
| 68 | } else { |
| 69 | merged.push(cur); |
| 70 | cur = { ...next }; |
| 71 | } |
| 72 | } |
| 73 | merged.push(cur); |
| 74 | return merged; |
| 75 | } |