wip: grib

internal/pkg/grib/extractor.go

@@ -0,0 +1,53 @@
+package grib
+
+import "math"
+
+func lerp(a, b, t float64) float64 { return a + t*(b-a) }
+
+// Interpolate 16‑point (time, p, lat, lon)
+func (d *dataset) uv(lat, lon, alt float64, tHours float64) (float64, float64) {
+	if lon < 0 {
+		lon += 360
+	}
+	iy := (lat + 90) * 2
+	y0 := int(math.Floor(iy))
+	y1 := y0 + 1
+	wy := iy - float64(y0)
+	ix := lon * 2
+	x0 := int(math.Floor(ix)) % d.cube.lon
+	x1 := (x0 + 1) % d.cube.lon
+	wx := ix - float64(x0)
+	it0 := int(math.Floor(tHours / 3.0))
+	wt := (tHours - float64(it0*3)) / 3.0
+	p := pressureFromAlt(alt)
+	ip0 := 0
+	for ip0+1 < len(pressureLevels) && pressureLevels[ip0+1] > p {
+		ip0++
+	}
+	ip1 := ip0 + 1
+	wp := (pressureLevels[ip0] - p) / (pressureLevels[ip0] - pressureLevels[ip1])
+	fetch := func(ti, pi int) (float64, float64) {
+		u00 := d.cube.val(0, ti, pi, y0, x0)
+		u10 := d.cube.val(0, ti, pi, y0, x1)
+		u01 := d.cube.val(0, ti, pi, y1, x0)
+		u11 := d.cube.val(0, ti, pi, y1, x1)
+		v00 := d.cube.val(1, ti, pi, y0, x0)
+		v10 := d.cube.val(1, ti, pi, y0, x1)
+		v01 := d.cube.val(1, ti, pi, y1, x0)
+		v11 := d.cube.val(1, ti, pi, y1, x1)
+		uxy := (1-wy)*((1-wx)*float64(u00)+wx*float64(u10)) + wy*((1-wx)*float64(u01)+wx*float64(u11))
+		vxy := (1-wy)*((1-wx)*float64(v00)+wx*float64(v10)) + wy*((1-wx)*float64(v01)+wx*float64(v11))
+		return uxy, vxy
+	}
+	u0p0, v0p0 := fetch(it0, ip0)
+	u0p1, v0p1 := fetch(it0, ip1)
+	u1p0, v1p0 := fetch(it0+1, ip0)
+	u1p1, v1p1 := fetch(it0+1, ip1)
+	uLow := lerp(u0p0, u0p1, wp)
+	vLow := lerp(v0p0, v0p1, wp)
+	uHig := lerp(u1p0, u1p1, wp)
+	vHig := lerp(v1p0, v1p1, wp)
+	u := lerp(uLow, uHig, wt)
+	v := lerp(vLow, vHig, wt)
+	return u, v
+}