Initial

tests/unit/wind.test.ts

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+import { describe, it, expect } from 'vitest';
+import { decodeWindField, createWindInterpolator } from '$domain';
+import type { WindField, WindHeader } from '$domain';
+
+/**
+ * A 2×2 grid that reproduces the predictor's real conventions: longitudes in the
+ * 0..360 frame (358 → 0, i.e. -2° → 0°) and a north→south scan (la1 > la2) with
+ * a *positive* dy. Getting either wrong misplaces every vector.
+ */
+const header: WindHeader = {
+	parameterUnit: 'm.s-1',
+	parameterNumberName: 'wind',
+	nx: 2,
+	ny: 2,
+	lo1: 358,
+	la1: 10,
+	lo2: 0,
+	la2: 8,
+	dx: 2,
+	dy: 2,
+	refTime: '2026-01-01T00:00:00Z'
+};
+
+// Row-major: [ (10,-2) (10,0) ] then [ (8,-2) (8,0) ]
+const field: WindField = [
+	{ header, data: [10, 0, 0, 0] }, // U (eastward)
+	{ header, data: [0, 10, 0, 0] } // V (northward)
+];
+
+describe('decodeWindField', () => {
+	it('places vectors using the grid extent, not raw dx/dy', () => {
+		const v = decodeWindField(field);
+		expect(v).toHaveLength(4);
+		// First point: la1/lo1 wrapped from 358 into (-180, 180].
+		expect(v[0].lat).toBe(10);
+		expect(v[0].lng).toBe(-2);
+		// Second column steps east across the 0/360 seam.
+		expect(v[1].lng).toBe(0);
+		// Second row scans southward despite dy being positive.
+		expect(v[2].lat).toBe(8);
+	});
+
+	it('derives speed and the bearing the wind blows TO', () => {
+		const v = decodeWindField(field);
+		expect(v[0].speed).toBeCloseTo(10, 6);
+		expect(v[0].bearing).toBeCloseTo(90, 6); // pure U → eastward
+		expect(v[1].bearing).toBeCloseTo(0, 6); // pure V → northward
+	});
+});
+
+describe('createWindInterpolator', () => {
+	it('returns the exact grid value at a node', () => {
+		const at = createWindInterpolator(field);
+		expect(at(-2, 10)).toEqual([10, 0]);
+		expect(at(0, 10)).toEqual([0, 10]);
+	});
+
+	it('blends between nodes', () => {
+		const at = createWindInterpolator(field);
+		const mid = at(-1, 10)!;
+		expect(mid[0]).toBeCloseTo(5, 6);
+		expect(mid[1]).toBeCloseTo(5, 6);
+	});
+
+	it('returns null outside the grid', () => {
+		const at = createWindInterpolator(field);
+		expect(at(-2, 20)).toBeNull(); // north of la1
+		expect(at(-2, 0)).toBeNull(); // south of la2
+	});
+});