perf(flights): make the fleet pass's ground-floor reads cheap

Every 80 ms the Flights and Military fleet passes read the ground floor for
each coasting aircraft. With the whole globe in view that is about 10,000
aircraft, and each read built three string keys through six toFixed calls and
rewrote the terrain cache to promote its entry. At 4x CPU throttle those reads
were 5.9 s of a 15.3 s profile.

- A coast above 9,000 m (no ground reaches it) skips the geodetic conversion
  and the floor read.
- cachedGroundFloor works its cell out once, as integers; mesh cells and the
  terrain cache's synchronous reads are found by numeric key.
- A synchronous terrain-cache read promotes its entry only when it has
  drifted halfway to eviction, not on every read.

Floors are unchanged: tests compare the reads against the string-keyed ones
on cell edges, near zero and out of range.

Co-Authored-By: Claude Fable 5.2 <[email protected]>

docs/CURRENT-STATE.md

@@ -2949,7 +2949,13 @@ visits to one place share a key but does not bound how many distinct places a
 session visits, and the cache previously had no eviction, so a long session
 retained every coordinate it ever resolved. Eviction is least-recently-used:
 a consumer read, including a warm hit in a batch lookup, promotes its entry, so
-the cell being watched is not dropped for having been resolved early. The bound sits far above a city-scale session, so
+the cell being watched is not dropped for having been resolved early. A
+synchronous read (`cachedEllipsoidalGround`, `cachedRealEllipsoidalGround`)
+promotes its entry only once half the bound's worth of stores and promotions
+have passed since the entry was last newest, so an entry read at least that
+often is still never evicted while the per-frame floor reads no longer rewrite
+the cache; those reads find entries through a numeric key and format no
+strings. The bound sits far above a city-scale session, so
 normal use never evicts and issues no additional proxy requests. A batch's
 results are assembled from what that call resolved or found warm, so eviction
 during a large batch cannot report one of its points as unresolved. Re:Earth and

src/data/motionModel.js

@@ -311,6 +311,10 @@ export const COAST_FLOOR_LIFT_M = 1.5;
  */
 export function floorCoastedPosition(position, cachedGroundFloor) {
   if (typeof cachedGroundFloor !== 'function') return position;
+  // Most coasting contacts are at cruise, and this runs for each of them on
+  // every fleet pass. One above COAST_FLOOR_CEILING_M cannot be lifted by any
+  // ground, so it needs neither the geodetic conversion nor the floor read.
+  if (clearOfAllGround(position)) return position;
   const carto = Cesium.Cartographic.fromCartesian(
     position,
     Cesium.Ellipsoid.WGS84,
@@ -334,6 +338,36 @@ export function floorCoastedPosition(position, cachedGroundFloor) {
 
 const _coastCarto = new Cesium.Cartographic();
 
+/**
+ * @constant {number} No ground reaches this height above the ellipsoid:
+ * Everest's summit is about 8,821 m (8,849 m above a geoid that sits 28 m
+ * below the ellipsoid there).
+ */
+export const COAST_FLOOR_CEILING_M = 9000;
+const _oneOverRadiiSq = Cesium.Ellipsoid.WGS84.oneOverRadiiSquared;
+const COAST_CLEAR_SCALED_SQ =
+  (1 + COAST_FLOOR_CEILING_M / Cesium.Ellipsoid.WGS84.minimumRadius) ** 2;
+
+/**
+ * Whether a position is certainly more than COAST_FLOOR_CEILING_M above the
+ * ellipsoid, from three multiplications. Scale the axes so the ellipsoid is
+ * the unit sphere: a point h metres up its surface normal is then at most
+ * 1 + h / (polar radius) from the centre, so one further out than
+ * 1 + ceiling / (polar radius) is above the ceiling. At the equator this
+ * answers yes from about 30 m above the ceiling; at the poles, from the
+ * ceiling itself.
+ * @param {Cesium.Cartesian3} position
+ * @returns {boolean}
+ */
+function clearOfAllGround(position) {
+  return (
+    position.x * position.x * _oneOverRadiiSq.x +
+      position.y * position.y * _oneOverRadiiSq.y +
+      position.z * position.z * _oneOverRadiiSq.z >
+    COAST_CLEAR_SCALED_SQ
+  );
+}
+
 /**
  * Seconds to coast past the last report: the elapsed time, bounded by the
  * source's coast limit and STALE_COAST_MAX_SEC; none for a contact that

src/data/motionModel.test.mjs

@@ -10,6 +10,7 @@ import {
   liftRepeatedGroundFix, synthesizeForwardKinematicsFix,
   projectGroundArcLatLon, corridorPathLatLon,
   COURSE_TRACK_ONLY_MPS, COURSE_CHORD_ONLY_MPS, COURSE_MIN_DPS, TURN_MIN_SPEED_MPS,
+  floorCoastedPosition, COAST_FLOOR_CEILING_M, COAST_FLOOR_LIFT_M,
 } from './motionModel.js';
 
 const AUSTIN = { lat: 30.2672, lon: -97.7431, alt: 9000 };
@@ -341,3 +342,79 @@ test('corridorPathLatLon returns nothing without a display position', () => {
     extrapolating: true, displayLat: NaN, displayLon: -97.66, courseDeg: 0, speedMps: 10, lookaheadSec: 60,
   }), []);
 });
+
+/** floorCoastedPosition as it was before the cruise skip: always converts and reads. */
+function floorCoastedReference(position, groundAt) {
+  const carto = Cesium.Cartographic.fromCartesian(position);
+  if (!carto) return position;
+  const ground = groundAt(
+    Cesium.Math.toDegrees(carto.latitude),
+    Cesium.Math.toDegrees(carto.longitude),
+  );
+  if (!Number.isFinite(ground) || carto.height >= ground + COAST_FLOOR_LIFT_M)
+    return position;
+  return Cesium.Cartesian3.fromRadians(carto.longitude, carto.latitude, ground + COAST_FLOOR_LIFT_M);
+}
+
+test('a coasting contact above any terrain is not looked up against the ground', () => {
+  let reads = 0;
+  const ground = () => {
+    reads += 1;
+    return 8821; // Everest's summit, the highest floor there is
+  };
+  // Just over the ceiling and at cruise, from pole to equator.
+  for (let lat = -90; lat <= 90; lat += 7.5) {
+    for (const lon of [0, 86.925, -78.8, 179.9]) {
+      for (const height of [COAST_FLOOR_CEILING_M + 35, 10_000, 12_500, 40_000]) {
+        const position = Cesium.Cartesian3.fromDegrees(lon, lat, height);
+        const before = Cesium.Cartesian3.clone(position);
+        assert.equal(floorCoastedPosition(position, ground), position);
+        assert.ok(Cesium.Cartesian3.equals(position, before));
+      }
+    }
+  }
+  assert.equal(reads, 0);
+});
+
+test('below the ceiling a coast is floored exactly as before', () => {
+  // Terrain that rises under the coast, known in some places and not others.
+  const ground = (lat, lon) => (Math.floor(lat * 10 + lon) % 3 === 0 ? null : 1500 + ((lat * 7 + lon * 3) % 900));
+  let lifted = 0;
+  let checked = 0;
+  for (let lat = -89; lat <= 89; lat += 5.5) {
+    for (let lon = -180; lon < 180; lon += 17) {
+      for (const height of [-40, 0, 800, 1499, 1501.4, 2399, 2402, 5000, 8990, 9000, 9010, 11_000, 30_000]) {
+        const position = Cesium.Cartesian3.fromDegrees(lon, lat, height);
+        const expected = floorCoastedReference(Cesium.Cartesian3.clone(position), ground);
+        const actual = floorCoastedPosition(position, ground);
+        assert.ok(
+          Cesium.Cartesian3.equals(actual, expected),
+          `coast at ${lat}, ${lon}, ${height} m`,
+        );
+        checked += 1;
+        if (!Cesium.Cartesian3.equals(expected, Cesium.Cartesian3.fromDegrees(lon, lat, height))) lifted += 1;
+      }
+    }
+  }
+  assert.ok(lifted > 500, `${lifted} of ${checked} coasts were lifted onto the ground`);
+});
+
+test('nothing at or below the ceiling skips the ground', () => {
+  let reads = 0;
+  const ground = () => {
+    reads += 1;
+    return null;
+  };
+  let asked = 0;
+  for (let lat = -90; lat <= 90; lat += 2.5) {
+    for (const height of [-400, 0, 4000, 8821, COAST_FLOOR_CEILING_M - 1, COAST_FLOOR_CEILING_M]) {
+      floorCoastedPosition(Cesium.Cartesian3.fromDegrees(33, lat, height), ground);
+      asked += 1;
+    }
+  }
+  assert.equal(reads, asked);
+});
+
+test('the ceiling clears the highest ground with room to spare', () => {
+  assert.ok(COAST_FLOOR_CEILING_M > 8849 + 100);
+});

src/services/groundFloor.js

@@ -45,6 +45,39 @@ export function createGroundFloor({ terrain, signal }) {
     return { lat: Number(lat.toFixed(3)), lon: Number(lon.toFixed(3)) };
   }
 
+  /**
+   * One coordinate's coarse cell as an integer: `Number(deg.toFixed(3))` in
+   * thousandths of a degree, without the trip through a string. The floor is
+   * read for every aircraft on every fleet pass, and `coarseFloorCoord`'s two
+   * `toFixed` calls and the string keys built from them were most of the cost.
+   * `toFixed` rounds the exact decimal value; `deg * 1000` is within 1e-10 of
+   * it for |deg| < 1000, so the two can only round differently right at a
+   * half, and there the string decides.
+   * @param {number} deg
+   * @returns {number}
+   */
+  function coarseCellIndex(deg) {
+    const scaled = deg * 1000;
+    if (
+      !(Math.abs(deg) < 1000) ||
+      Math.abs(scaled - Math.floor(scaled) - 0.5) < 1e-6
+    )
+      return Math.round(Number(deg.toFixed(3)) * 1000);
+    return Math.round(scaled);
+  }
+
+  /**
+   * Key of a coarse cell in `_meshCells`: the two indices packed into one
+   * number, or the text form for a longitude too far out of range to pack.
+   * @param {number} latIndex @param {number} lonIndex
+   * @returns {number|string}
+   */
+  function meshCellKey(latIndex, lonIndex) {
+    return Math.abs(lonIndex) < 500_000 && Math.abs(latIndex) < 1e9
+      ? latIndex * 1_000_000 + lonIndex
+      : `${latIndex},${lonIndex}`;
+  }
+
   /**
    * Pure floor clamp.
    *  - finite alt + finite ground → max(alt, ground + lift)
@@ -424,8 +457,8 @@ export function createGroundFloor({ terrain, signal }) {
   // reported here by meshFloorSampler.js. Design principle (owner): NEVER
   // below the visible surface; slightly above is always fine. The DEM stays
   // the instant global prior/fallback and the sanity gate for samples.
-  /** @type {Map<string, Array<{lat: number, lon: number, h: number}>>}
+  /** @type {Map<number|string, Array<{lat: number, lon: number, h: number}>>}
-   *  coarse cell key -> rendered-surface readings (m, ellipsoidal), each kept
+   *  coarse cell key (`meshCellKey`) -> rendered-surface readings (m, ellipsoidal), each kept
    *  at the point it was probed. A read answers from the reading nearest the
    *  asked point, so a viaduct and the street beside it in one cell keep their
    *  own heights. */
@@ -468,8 +501,7 @@ export function createGroundFloor({ terrain, signal }) {
       !Number.isFinite(heightM)
     )
       return;
-    const c = coarseFloorCoord(lat, lon);
+    const key = meshCellKey(coarseCellIndex(lat), coarseCellIndex(lon));
-    const key = `${c.lat},${c.lon}`;
     if (!_meshCells.has(key)) _meshCells.set(key, [{ lat, lon, h: heightM }]);
   }
 
@@ -497,7 +529,7 @@ export function createGroundFloor({ terrain, signal }) {
     const cell = coarseFloorCoord(lat, lon);
     const prior = cachedRealEllipsoidalGround(cell.lat, cell.lon);
     if (!meshFloorSampleWithinPrior(heightM, prior)) return false;
-    const key = `${cell.lat},${cell.lon}`;
+    const key = meshCellKey(coarseCellIndex(lat), coarseCellIndex(lon));
     const readings = _meshCells.get(key);
     if (!readings) {
       _meshCells.set(key, [{ lat, lon, h: heightM }]);
@@ -538,8 +570,16 @@ export function createGroundFloor({ terrain, signal }) {
   function cachedMeshFloor(lat, lon) {
     if (!_meshPreferred) return null;
     if (!Number.isFinite(lat) || !Number.isFinite(lon)) return null;
-    const c = coarseFloorCoord(lat, lon);
+    return meshFloorInCell(
-    const readings = _meshCells.get(`${c.lat},${c.lon}`);
+      lat,
+      lon,
+      meshCellKey(coarseCellIndex(lat), coarseCellIndex(lon)),
+    );
+  }
+
+  /** `cachedMeshFloor` for a caller that already has the point's cell key. */
+  function meshFloorInCell(lat, lon, key) {
+    const readings = _meshCells.get(key);
     if (!readings) return null;
     if (readings.length === 1) return readings[0].h;
     const k = Math.cos((lat * Math.PI) / 180) ** 2;
@@ -572,10 +612,14 @@ export function createGroundFloor({ terrain, signal }) {
    */
   function cachedGroundFloor(lat, lon) {
     if (!Number.isFinite(lat) || !Number.isFinite(lon)) return null;
-    const mesh = cachedMeshFloor(lat, lon);
+    // The cell is worked out once, as integers, for both reads.
-    if (mesh != null) return mesh;
+    const latIndex = coarseCellIndex(lat);
-    const c = coarseFloorCoord(lat, lon);
+    const lonIndex = coarseCellIndex(lon);
-    return cachedRealEllipsoidalGround(c.lat, c.lon);
+    if (_meshPreferred) {
+      const mesh = meshFloorInCell(lat, lon, meshCellKey(latIndex, lonIndex));
+      if (mesh != null) return mesh;
+    }
+    return cachedRealEllipsoidalGround(latIndex / 1000, lonIndex / 1000);
   }
 
   /**

src/services/groundFloor.test.mjs

@@ -0,0 +1,176 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { createGroundFloor } from './groundFloor.js';
+import { createTerrainHeights } from './terrainHeights.js';
+
+/** Counts Number.prototype.toFixed calls made while `run` executes. */
+function countToFixed(run) {
+  const original = Number.prototype.toFixed;
+  let calls = 0;
+  Number.prototype.toFixed = function (...args) {
+    calls += 1;
+    return original.apply(this, args);
+  };
+  try {
+    run();
+  } finally {
+    Number.prototype.toFixed = original;
+  }
+  return calls;
+}
+
+/**
+ * The floor read as it was before the cell became an integer: the coarse cell
+ * through `toFixed(3)`, mesh cells and DEM cells under string keys.
+ */
+function referenceFloor() {
+  const mesh = new Map();
+  const dem = new Map();
+  const coarse = (lat, lon) => ({
+    lat: Number(lat.toFixed(3)),
+    lon: Number(lon.toFixed(3)),
+  });
+  const demKey = (lat, lon) => `${lat.toFixed(5)},${lon.toFixed(5)}`;
+  return {
+    reportMesh(lat, lon, h) {
+      const c = coarse(lat, lon);
+      const key = `${c.lat},${c.lon}`;
+      if (!mesh.has(key)) mesh.set(key, [{ lat, lon, h }]);
+    },
+    storeDem(lat, lon, h) {
+      dem.set(demKey(lat, lon), h);
+    },
+    read(lat, lon, meshPreferred = true) {
+      if (!Number.isFinite(lat) || !Number.isFinite(lon)) return null;
+      const c = coarse(lat, lon);
+      const readings = meshPreferred ? mesh.get(`${c.lat},${c.lon}`) : null;
+      if (readings) return readings[0].h;
+      return dem.get(demKey(c.lat, c.lon)) ?? null;
+    },
+  };
+}
+
+/** A real terrain cache whose every resolved point takes the height asked for. */
+async function warmTerrain(points) {
+  const heights = new Map(points.map((p) => [`${p.lat},${p.lon}`, p.h]));
+  const terrain = createTerrainHeights({
+    source: {
+      async getHeights(chunk) {
+        return chunk.map((item) => ({
+          ellipsoid: heights.get(`${item.lat},${item.lon}`),
+        }));
+      },
+    },
+  });
+  await terrain.resolveEllipsoidalGround(
+    points.map(({ lat, lon }) => ({ lat, lon })),
+  );
+  return terrain;
+}
+
+/** Deterministic pseudo-random numbers in [0, 1). */
+function random(seed) {
+  let state = seed;
+  return () => {
+    state = (state * 1664525 + 1013904223) % 4294967296;
+    return state / 4294967296;
+  };
+}
+
+/** Coordinates that land on, and just either side of, a coarse-cell edge. */
+function edgeCoordinates() {
+  const out = [];
+  for (const base of [0, 0.0005, -0.0005, 12.3455, -12.3455, 89.9995, 179.9995])
+    for (const nudge of [0, 1e-13, -1e-13, 1e-9, -1e-9, 1e-7, -1e-7, 4e-4])
+      out.push(base + nudge);
+  // Thousandths whose doubles sit a hair either side of the half.
+  for (let k = -40; k <= 40; k += 1) out.push((k + 0.5) / 1000);
+  out.push(-0, 1.0005, 1.0015, 2.5, -2.5, 1e-7, -1e-7);
+  return out;
+}
+
+test('the floor read answers exactly as the string-keyed read did', async () => {
+  const next = random(20261113);
+  const cells = [];
+  for (let i = 0; i < 400; i += 1) {
+    cells.push({
+      lat: Number((next() * 170 - 85).toFixed(3)),
+      lon: Number((next() * 360 - 180).toFixed(3)),
+      h: Math.round(next() * 4000),
+    });
+  }
+  // Cells along the equator and the prime meridian, where a cell index is 0.
+  for (const edge of edgeCoordinates()) {
+    const lat = Number(edge.toFixed(3));
+    cells.push({ lat: Math.max(-89, Math.min(89, lat)), lon: 10, h: 77 });
+    cells.push({ lat: 10, lon: lat, h: 88 });
+  }
+  const reference = referenceFloor();
+  for (const c of cells) reference.storeDem(c.lat, c.lon, c.h);
+  const terrain = await warmTerrain(cells);
+  const groundFloor = createGroundFloor({ terrain });
+
+  // A rendered-surface reading in every seventh cell, taken off its centre.
+  cells.forEach((c, i) => {
+    if (i % 7) return;
+    const lat = c.lat + 0.0002;
+    const lon = c.lon - 0.0003;
+    reference.reportMesh(lat, lon, c.h + 3);
+    groundFloor.reportMeshFloorCell(lat, lon, c.h + 3);
+  });
+
+  const probes = [];
+  for (const c of cells) {
+    probes.push([c.lat, c.lon]);
+    probes.push([c.lat + (next() - 0.5) * 0.002, c.lon + (next() - 0.5) * 0.002]);
+  }
+  for (const a of edgeCoordinates())
+    for (const b of [10, 10.0004, -0, 0.0005, -0.0005]) {
+      probes.push([a, b]);
+      probes.push([b, a]);
+    }
+  probes.push([NaN, 1], [1, Infinity], [95, 400], [10, 720.1234]);
+
+  for (const meshPreferred of [true, false]) {
+    groundFloor.setMeshFloorPreferred(meshPreferred);
+    let hits = 0;
+    for (const [lat, lon] of probes) {
+      const expected = reference.read(lat, lon, meshPreferred);
+      assert.equal(
+        groundFloor.cachedGroundFloor(lat, lon),
+        expected,
+        `floor at ${lat}, ${lon} (mesh ${meshPreferred})`,
+      );
+      if (expected !== null) hits += 1;
+    }
+    assert.ok(hits > 400, `${hits} probes found a floor`);
+  }
+});
+
+test('reading a floor formats no numbers, hit or miss', async () => {
+  const cell = { lat: 51.47, lon: -0.454, h: 25 };
+  const terrain = await warmTerrain([cell]);
+  const groundFloor = createGroundFloor({ terrain });
+  groundFloor.reportMeshFloorCell(48.3537, 11.7861, 453);
+
+  const calls = countToFixed(() => {
+    // The DEM cell, a rendered-surface cell, and open ocean with neither.
+    assert.equal(groundFloor.cachedGroundFloor(51.4702, -0.4541), 25);
+    assert.equal(groundFloor.cachedGroundFloor(48.3538, 11.7862), 453);
+    for (let i = 0; i < 1000; i += 1)
+      assert.equal(groundFloor.cachedGroundFloor(30 + i * 0.011, -40.2), null);
+  });
+  // Before, each read made six toFixed calls (the cell twice, the DEM key).
+  assert.equal(calls, 0);
+});
+
+test('a reading reported at a point is found from anywhere in its cell', async () => {
+  const terrain = await warmTerrain([{ lat: 41.5, lon: -97.66, h: 120 }]);
+  const groundFloor = createGroundFloor({ terrain });
+  assert.equal(groundFloor.reportValidatedMeshFloorCell(41.5003, -97.6602, 129), true);
+  assert.equal(groundFloor.cachedMeshFloor(41.4996, -97.6598), 129);
+  assert.equal(groundFloor.cachedGroundFloor(41.4996, -97.6598), 129);
+  // The next cell over has no reading and no DEM entry.
+  assert.equal(groundFloor.cachedMeshFloor(41.5006, -97.6602), null);
+  assert.equal(groundFloor.cachedGroundFloor(41.5006, -97.6602), null);
+});

src/services/terrainHeights.js

@@ -86,6 +86,89 @@ export function createTerrainHeights({
    */
   const cache = new Map();
 
+  /**
+   * The same entries under a numeric key, for the synchronous reads. Those run
+   * for every aircraft on every fleet pass (about 94,000 a second with 7,500
+   * aircraft in view), nearly all of them misses, and building the string key
+   * (two `toFixed` calls and a concatenation) was most of what a read cost.
+   * Kept in step with `cache` by `cacheStore`, eviction and the abort handler.
+   * `at` is the value of `moves` when the entry last became the newest.
+   * @type {Map<number, {key: string, entry: object, at: number}>}
+   */
+  const cells = new Map();
+  /** Counts every move to the newest end of `cache`: stores and promotions. */
+  let moves = 0;
+  /**
+   * A read promotes its entry only once this many moves have passed since it
+   * was last the newest. An entry cannot reach the cold end in fewer than
+   * CACHE_MAX_ENTRIES - 1 moves, so one read at least this often is never
+   * evicted, without rewriting the Map on every read.
+   */
+  const PROMOTE_AFTER_MOVES = Math.floor(CACHE_MAX_ENTRIES / 2);
+
+  /**
+   * `value.toFixed(5)` as an integer count of 1e-5 steps, or null when the
+   * fast arithmetic cannot be trusted to match it. `toFixed` rounds the exact
+   * decimal value; `value * 1e5` is within 2e-8 of it for |value| < 1000, so
+   * the two can only round differently this close to a half. A negative value
+   * that rounds to zero is left to the string too: it formats as "-0.00000",
+   * a different key from "0.00000".
+   * @param {number} value
+   * @returns {number|null}
+   */
+  function fixed5Index(value) {
+    if (!(Math.abs(value) < 1000)) return null;
+    const scaled = value * 1e5;
+    if (Math.abs(scaled - Math.floor(scaled) - 0.5) < 1e-6) return null;
+    const index = Math.round(scaled);
+    return Object.is(index, -0) ? null : index;
+  }
+
+  /** Indices at or past this (500 degrees) do not pack into one integer. */
+  const CELL_INDEX_LIMIT = 5e7;
+
+  /**
+   * The numeric key for a coordinate: its two 5-decimal indices packed into
+   * one integer (below 2^53), or null when the coordinate has to go through
+   * its string key instead.
+   * @param {number} lat
+   * @param {number} lon
+   * @returns {number|null}
+   */
+  function cellKey(lat, lon) {
+    const latIndex = fixed5Index(lat);
+    const lonIndex = fixed5Index(lon);
+    if (
+      latIndex === null ||
+      lonIndex === null ||
+      Math.abs(latIndex) >= CELL_INDEX_LIMIT ||
+      Math.abs(lonIndex) >= CELL_INDEX_LIMIT
+    )
+      return null;
+    return latIndex * 1e8 + lonIndex;
+  }
+
+  /**
+   * The numeric key of a stored string key. The text is the rounded
+   * coordinate itself, so this is exact. Null for a key `cellKey` never
+   * produces; such a key is only ever read through its string.
+   * @param {string} key
+   * @returns {number|null}
+   */
+  function cellKeyOf(key) {
+    const comma = key.indexOf(',');
+    const latIndex = Math.round(Number(key.slice(0, comma)) * 1e5);
+    const lonIndex = Math.round(Number(key.slice(comma + 1)) * 1e5);
+    if (
+      !(Math.abs(latIndex) < CELL_INDEX_LIMIT) ||
+      !(Math.abs(lonIndex) < CELL_INDEX_LIMIT) ||
+      Object.is(latIndex, -0) ||
+      Object.is(lonIndex, -0)
+    )
+      return null;
+    return latIndex * 1e8 + lonIndex;
+  }
+
   /**
    * Store an entry and drop the coldest ones once past the cap. `Map` iterates
    * in insertion order, so deleting the first key evicts the least recently
@@ -101,10 +184,15 @@ export function createTerrainHeights({
   function cacheStore(key, entry) {
     cache.delete(key);
     cache.set(key, entry);
+    moves += 1;
+    const cell = cellKeyOf(key);
+    if (cell !== null) cells.set(cell, { key, entry, at: moves });
     while (cache.size > CACHE_MAX_ENTRIES) {
       const coldest = cache.keys().next().value;
       if (coldest === undefined) break;
       cache.delete(coldest);
+      const coldestCell = cellKeyOf(coldest);
+      if (coldestCell !== null) cells.delete(coldestCell);
     }
   }
 
@@ -119,9 +207,36 @@ export function createTerrainHeights({
     if (entry === undefined) return undefined;
     cache.delete(key);
     cache.set(key, entry);
+    moves += 1;
+    const cell = cellKeyOf(key);
+    const held = cell === null ? undefined : cells.get(cell);
+    if (held) held.at = moves;
     return entry;
   }
 
+  /**
+   * The synchronous read: the entry for a coordinate, or undefined on a miss.
+   * Same answer as `cacheTouch(cacheKey(lat, lon))`, without building the
+   * string, and promoting only an entry that has drifted towards the cold end
+   * (see PROMOTE_AFTER_MOVES).
+   * @param {number} lat
+   * @param {number} lon
+   * @returns {{ellipsoid: number, source: 'reearth'|'geoid-fallback', retryAt?: number}|undefined}
+   */
+  function cacheRead(lat, lon) {
+    const cell = cellKey(lat, lon);
+    if (cell === null) return cacheTouch(cacheKey(lat, lon));
+    const held = cells.get(cell);
+    if (held === undefined) return undefined;
+    if (moves - held.at > PROMOTE_AFTER_MOVES) {
+      cache.delete(held.key);
+      cache.set(held.key, held.entry);
+      moves += 1;
+      held.at = moves;
+    }
+    return held.entry;
+  }
+
   /**
    * Builds the rounded cache key shared between the in-memory cache and the
    * `points` string sent to the proxy (see file header, point 2).
@@ -142,7 +257,7 @@ export function createTerrainHeights({
    * @returns {number|null}
    */
   function cachedEllipsoidalGround(lat, lon) {
-    const entry = cacheTouch(cacheKey(lat, lon));
+    const entry = cacheRead(lat, lon);
     return entry ? entry.ellipsoid : null;
   }
 
@@ -159,7 +274,7 @@ export function createTerrainHeights({
    * @returns {number|null}
    */
   function cachedRealEllipsoidalGround(lat, lon) {
-    const entry = cacheTouch(cacheKey(lat, lon));
+    const entry = cacheRead(lat, lon);
     return entry && entry.source === 'reearth' ? entry.ellipsoid : null;
   }
 
@@ -370,7 +485,14 @@ export function createTerrainHeights({
     });
   }
 
-  signal?.addEventListener('abort', () => cache.clear(), { once: true });
+  signal?.addEventListener(
+    'abort',
+    () => {
+      cache.clear();
+      cells.clear();
+    },
+    { once: true },
+  );
   return {
     cachedEllipsoidalGround,
     cachedRealEllipsoidalGround,

src/services/terrainHeights.test.mjs

@@ -169,3 +169,151 @@ test('an out-of-range cache bound falls back to the default instead of disabling
     );
   }
 });
+
+/** Counts Number.prototype.toFixed calls made while `run` executes. */
+function countToFixed(run) {
+  const original = Number.prototype.toFixed;
+  let calls = 0;
+  Number.prototype.toFixed = function (...args) {
+    calls += 1;
+    return original.apply(this, args);
+  };
+  try {
+    run();
+  } finally {
+    Number.prototype.toFixed = original;
+  }
+  return calls;
+}
+
+test('a synchronous read finds exactly the entry its 5-decimal key names', async () => {
+  // Stored coordinates, and for each the reads that must and must not reach it.
+  // The expectation is the string key the cache has always used.
+  const stored = [
+    [0, 0.001],
+    [51.47, -0.454],
+    [-33.94611, 151.17722],
+    [0.00001, 0.00002],
+    [-0.00001, 0.00003],
+    [89.99999, 179.99999],
+    [-89.99999, -179.99999],
+    [12.345675, 98.765435],
+    [1e-7, 5],
+    [-1e-7, 6],
+    [7, -1e-7],
+    [200.5, 361.25],
+    [10, 500],
+    [10, 499.999999],
+  ];
+  const key = (lat, lon) => `${lat.toFixed(5)},${lon.toFixed(5)}`;
+  const heights = new Map(
+    stored.map(([lat, lon], i) => [key(lat, lon), 100 + i]),
+  );
+  const terrain = createTerrainHeights({
+    source: {
+      async getHeights(chunk) {
+        return chunk.map((item) => ({ ellipsoid: heights.get(item.key) }));
+      },
+    },
+  });
+  await terrain.resolveEllipsoidalGround(
+    stored.map(([lat, lon]) => ({ lat, lon })),
+  );
+
+  const probes = [];
+  const nudges = [
+    0, 1e-13, -1e-13, 1e-9, -1e-9, 4e-6, -4e-6, 4.9999e-6, 5e-6, -5e-6,
+    5.0001e-6, 1e-5, -1e-5,
+  ];
+  for (const [lat, lon] of stored)
+    for (const a of nudges)
+      for (const b of [0, 4e-6, -5e-6])
+        probes.push([lat + a, lon + b], [lat + b, lon + a]);
+  probes.push([0, 0], [-0, 0.001], [0, -0], [NaN, 1], [1, Infinity], [1e21, 2]);
+
+  let hits = 0;
+  for (const [lat, lon] of probes) {
+    const expected = heights.get(key(lat, lon)) ?? null;
+    assert.equal(
+      terrain.cachedEllipsoidalGround(lat, lon),
+      expected,
+      `plain read at ${lat}, ${lon}`,
+    );
+    assert.equal(
+      terrain.cachedRealEllipsoidalGround(lat, lon),
+      expected,
+      `real read at ${lat}, ${lon}`,
+    );
+    if (expected !== null) hits += 1;
+  }
+  assert.ok(
+    hits > 100 && hits < probes.length,
+    `${hits} of ${probes.length} reads hit`,
+  );
+});
+
+test('a synchronous read of an ordinary coordinate formats no numbers', async () => {
+  const terrain = createTerrainHeights({ source: stubSource() });
+  await terrain.resolveEllipsoidalGround([pointAt(1), pointAt(2)]);
+  const calls = countToFixed(() => {
+    assert.equal(
+      terrain.cachedRealEllipsoidalGround(0, lonAt(1)),
+      heightAt(lonAt(1)),
+    );
+    assert.equal(
+      terrain.cachedEllipsoidalGround(0, lonAt(2)),
+      heightAt(lonAt(2)),
+    );
+    // Misses are the common case: an aircraft over a cell nobody resolved.
+    for (let i = 10; i < 1010; i += 1)
+      assert.equal(terrain.cachedRealEllipsoidalGround(45.123, lonAt(i)), null);
+  });
+  // Before, every read built its key with two toFixed calls.
+  assert.equal(calls, 0);
+});
+
+test('an entry read again and again is not rewritten on each read, and still outlives the rest', async () => {
+  const terrain = createTerrainHeights({
+    source: stubSource(),
+    maxCacheEntries: 8,
+  });
+  for (let i = 0; i < 8; i += 1)
+    await terrain.resolveEllipsoidalGround([pointAt(i)]);
+
+  // Coordinate 0 is the coldest. Read it between stores, as a parked contact
+  // would; the seven newer ones are never read.
+  for (let i = 8; i < 15; i += 1) {
+    for (let read = 0; read < 50; read += 1)
+      assert.equal(
+        terrain.cachedEllipsoidalGround(0, lonAt(0)),
+        heightAt(lonAt(0)),
+      );
+    await terrain.resolveEllipsoidalGround([pointAt(i)]);
+  }
+  assert.equal(
+    terrain.cachedEllipsoidalGround(0, lonAt(0)),
+    heightAt(lonAt(0)),
+  );
+  for (let i = 1; i < 8; i += 1)
+    assert.equal(
+      terrain.cachedEllipsoidalGround(0, lonAt(i)),
+      null,
+      `unread coordinate ${i} was evicted`,
+    );
+});
+
+test('an evicted coordinate is gone from the synchronous read too', async () => {
+  const terrain = createTerrainHeights({
+    source: stubSource(),
+    maxCacheEntries: 2,
+  });
+  for (let i = 0; i < 5; i += 1)
+    await terrain.resolveEllipsoidalGround([pointAt(i)]);
+  for (let i = 0; i < 3; i += 1)
+    assert.equal(terrain.cachedRealEllipsoidalGround(0, lonAt(i)), null);
+  for (let i = 3; i < 5; i += 1)
+    assert.equal(
+      terrain.cachedRealEllipsoidalGround(0, lonAt(i)),
+      heightAt(lonAt(i)),
+    );
+});