package main

import (
	"encoding/binary"
	"fmt"
	"math"
	"os"

	mmap "github.com/edsrzf/mmap-go"
)

var pressureLevels = []float64{
	1000, 975, 950, 925, 900, 875, 850, 825, 800, 775,
	750, 725, 700, 675, 650, 625, 600, 575, 550, 525,
	500, 475, 450, 425, 400, 375, 350, 325, 300, 275,
	250, 225, 200, 175, 150, 125, 100, 70, 50, 30,
	20, 10, 7, 5, 3, 2, 1,
}

func main() {
	f, _ := os.Open("C:/tmp/grib/20260212_12.cube")
	mm, _ := mmap.Map(f, mmap.RDONLY, 0)
	defer mm.Unmap()
	defer f.Close()

	const (
		nT   = 97
		nP   = 47
		nLat = 721
		nLon = 1440
	)
	bytesPerVar := int64(nT * nP * nLat * nLon * 4)

	val := func(varIdx, ti, pi, y, x int) float32 {
		idx := (((ti*nP + pi) * nLat) + y) * nLon + x
		off := int64(varIdx)*bytesPerVar + int64(idx)*4
		bits := binary.LittleEndian.Uint32(mm[off : off+4])
		return math.Float32frombits(bits)
	}

	// Check gh values at lat=52.2N (y=(90-52.2)*4=151.2 → y=151), lon=0.1E (x=0.1*4=0.4 → x=0)
	// Time step 9 (9 hours into forecast)
	ti := 9
	y := 151
	x := 0

	fmt.Println("GH values at (52.25N, 0E), t=+9h:")
	fmt.Printf("%8s %8s %10s\n", "Level", "hPa", "GH(m)")
	for pi := 0; pi < nP; pi++ {
		gh := val(0, ti, pi, y, x)
		fmt.Printf("%8d %8.0f %10.1f\n", pi, pressureLevels[pi], gh)
	}

	fmt.Println("\nU-wind values at same point:")
	fmt.Printf("%8s %8s %10s\n", "Level", "hPa", "U(m/s)")
	for pi := 0; pi < nP; pi++ {
		u := val(1, ti, pi, y, x)
		fmt.Printf("%8d %8.0f %10.2f\n", pi, pressureLevels[pi], u)
	}
}