package grib

import "math"

func lerp(a, b, t float64) float64 { return a + t*(b-a) }

// ghInterp returns interpolated geopotential height at given time/pressure/lat/lon
func (d *dataset) ghInterp(ti, pi int, y0, y1, x0, x1 int, wy, wx float64) float64 {
	g00 := d.cube.val(0, ti, pi, y0, x0)
	g10 := d.cube.val(0, ti, pi, y0, x1)
	g01 := d.cube.val(0, ti, pi, y1, x0)
	g11 := d.cube.val(0, ti, pi, y1, x1)
	return (1-wy)*((1-wx)*float64(g00)+wx*float64(g10)) + wy*((1-wx)*float64(g01)+wx*float64(g11))
}

// searchAltLevel uses geopotential height to find pressure level bracket for target altitude.
func (d *dataset) searchAltLevel(alt float64, ti, y0, y1, x0, x1 int, wy, wx float64) (int, float64) {
	levels := d.ds.Levels
	nLevels := len(levels)

	lo, hi := 0, nLevels-1
	for lo < hi-1 {
		mid := (lo + hi) / 2
		ghMid := d.ghInterp(ti, mid, y0, y1, x0, x1, wy, wx)
		if ghMid < alt {
			lo = mid
		} else {
			hi = mid
		}
	}

	ghLo := d.ghInterp(ti, lo, y0, y1, x0, x1, wy, wx)
	ghHi := d.ghInterp(ti, hi, y0, y1, x0, x1, wy, wx)

	wp := 0.0
	if ghHi != ghLo {
		wp = (alt - ghLo) / (ghHi - ghLo)
	}
	if wp < 0 {
		wp = 0
	}
	if wp > 1 {
		wp = 1
	}

	return lo, wp
}

// uv выполняет интерполяцию ветра по 4 измерениям (time, pressure, lat, lon).
func (d *dataset) uv(lat, lon, alt float64, tHours float64) (float64, float64) {
	if lon < 0 {
		lon += 360
	}

	inv := d.ds.InvResolution()

	// GRIB scan north→south: index 0 = 90°N
	iy := (90 - lat) * inv
	y0 := int(math.Floor(iy))
	if y0 < 0 {
		y0 = 0
	}
	if y0 >= d.cube.lat-1 {
		y0 = d.cube.lat - 2
	}
	y1 := y0 + 1
	wy := iy - float64(y0)

	ix := lon * inv
	x0 := int(math.Floor(ix)) % d.cube.lon
	x1 := (x0 + 1) % d.cube.lon
	wx := ix - float64(x0)

	// Время: tHours делим на шаг, чтобы получить индекс в кубе
	tIdx := tHours / float64(d.ds.TimeStep)
	it0 := int(math.Floor(tIdx))
	if it0 < 0 {
		it0 = 0
	}
	if it0 >= d.cube.t-1 {
		it0 = d.cube.t - 2
	}
	wt := tIdx - float64(it0)

	// ISA: высота → давление → индекс уровня
	levels := d.ds.Levels
	p := pressureFromAlt(alt)
	ip0 := 0
	for ip0+1 < len(levels) && levels[ip0+1] > p {
		ip0++
	}
	ip1 := ip0 + 1
	if ip1 >= len(levels) {
		ip1 = len(levels) - 1
	}
	wp := 0.0
	if levels[ip0] != levels[ip1] {
		wp = (levels[ip0] - p) / (levels[ip0] - levels[ip1])
	}

	fetch := func(ti, pi int) (float64, float64) {
		u00 := d.cube.val(1, ti, pi, y0, x0)
		u10 := d.cube.val(1, ti, pi, y0, x1)
		u01 := d.cube.val(1, ti, pi, y1, x0)
		u11 := d.cube.val(1, ti, pi, y1, x1)
		v00 := d.cube.val(2, ti, pi, y0, x0)
		v10 := d.cube.val(2, ti, pi, y0, x1)
		v01 := d.cube.val(2, ti, pi, y1, x0)
		v11 := d.cube.val(2, 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
}