wip: grib

internal/pkg/grib/cube.go

@@ -0,0 +1,43 @@
+package grib
+
+import (
+	"encoding/binary"
+	"math"
+	"os"
+
+	mmap "github.com/edsrzf/mmap-go"
+)
+
+type cube struct {
+	mm             mmap.MMap // read‑only, U followed by V (float32 LE)
+	t, p, lat, lon int
+	bytesPerVar    int64
+}
+
+func openCube(path string) (*cube, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	mm, err := mmap.Map(f, mmap.RDONLY, 0)
+	if err != nil {
+		return nil, err
+	}
+
+	const (
+		nT   = 17
+		nP   = 34
+		nLat = 361
+		nLon = 720
+	)
+
+	return &cube{mm: mm, t: nT, p: nP, lat: nLat, lon: nLon, bytesPerVar: int64(nT * nP * nLat * nLon * 4)}, nil
+}
+
+func (c *cube) val(varIdx, ti, pi, y, x int) float32 {
+	idx := (((ti*c.p+pi)*c.lat + y) * c.lon) + x
+	off := int64(varIdx)*c.bytesPerVar + int64(idx)*4
+	bits := binary.LittleEndian.Uint32(c.mm[off : off+4])
+	return math.Float32frombits(bits)
+}