feat: refactor

internal/elevation/elevation.go

@@ -0,0 +1,113 @@
+package elevation
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+	"os"
+
+	mmap "github.com/edsrzf/mmap-go"
+)
+
+// Dataset provides global elevation lookup, compatible with ruaumoko.
+// Binary format: int16 little-endian elevation values in metres, row-major (lat, lon).
+// Latitude axis: -90 to +90 (south to north), Longitude axis: 0 to 360 (wraps).
+// Resolution: 120 cells per degree (30 arc-seconds).
+const (
+	CellsPerDegree = 120
+	NumLats        = 180*CellsPerDegree + 1 // 21601
+	NumLons        = 360 * CellsPerDegree    // 43200
+	DataSize       = NumLats * NumLons * 2   // 1,866,326,400 bytes (~1.74 GiB)
+)
+
+// Dataset is a memory-mapped global elevation grid.
+type Dataset struct {
+	mm   mmap.MMap
+	file *os.File
+}
+
+// Open opens an existing elevation dataset file.
+func Open(path string) (*Dataset, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("open elevation: %w", err)
+	}
+
+	info, err := f.Stat()
+	if err != nil {
+		f.Close()
+		return nil, fmt.Errorf("stat elevation: %w", err)
+	}
+	if info.Size() != DataSize {
+		f.Close()
+		return nil, fmt.Errorf("elevation dataset should be %d bytes (was %d)", DataSize, info.Size())
+	}
+
+	mm, err := mmap.Map(f, mmap.RDONLY, 0)
+	if err != nil {
+		f.Close()
+		return nil, fmt.Errorf("mmap elevation: %w", err)
+	}
+
+	return &Dataset{mm: mm, file: f}, nil
+}
+
+// getCell reads the int16 elevation at grid indices (latIdx, lngIdx).
+func (d *Dataset) getCell(latIdx, lngIdx int) int16 {
+	// Clamp latitude
+	if latIdx < 0 {
+		latIdx = 0
+	}
+	if latIdx >= NumLats {
+		latIdx = NumLats - 1
+	}
+	// Wrap longitude
+	lngIdx = lngIdx % NumLons
+	if lngIdx < 0 {
+		lngIdx += NumLons
+	}
+
+	off := (latIdx*NumLons + lngIdx) * 2
+	return int16(binary.LittleEndian.Uint16(d.mm[off : off+2]))
+}
+
+// Get returns the interpolated elevation in metres at the given coordinates.
+// lat: -90 to +90, lng: 0 to 360 (or -180 to 180, will be normalised).
+func (d *Dataset) Get(lat, lng float64) float64 {
+	// Normalise longitude to [0, 360)
+	if lng < 0 {
+		lng += 360
+	}
+
+	// Convert to cell coordinates
+	latCell := (lat + 90.0) * CellsPerDegree
+	lngCell := lng * CellsPerDegree
+
+	lat0 := int(math.Floor(latCell))
+	lng0 := int(math.Floor(lngCell))
+	latFrac := latCell - float64(lat0)
+	lngFrac := lngCell - float64(lng0)
+
+	// Bilinear interpolation
+	v00 := float64(d.getCell(lat0, lng0))
+	v10 := float64(d.getCell(lat0+1, lng0))
+	v01 := float64(d.getCell(lat0, lng0+1))
+	v11 := float64(d.getCell(lat0+1, lng0+1))
+
+	return (1-latFrac)*((1-lngFrac)*v00+lngFrac*v01) +
+		latFrac*((1-lngFrac)*v10+lngFrac*v11)
+}
+
+// Close unmaps and closes the dataset.
+func (d *Dataset) Close() error {
+	if d.mm != nil {
+		d.mm.Unmap()
+		d.mm = nil
+	}
+	if d.file != nil {
+		err := d.file.Close()
+		d.file = nil
+		return err
+	}
+	return nil
+}