93 lines
3 KiB
Go
93 lines
3 KiB
Go
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// Package geo provides the geographic distance calculations used to decide
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// whether two pictures were taken at the same place.
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package geo
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import (
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"errors"
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"math"
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"go-places-api/internal/domain"
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)
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// EarthRadiusMeters is the mean radius of the WGS-84 ellipsoid.
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const EarthRadiusMeters = 6371008.8
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var ErrInvalidCoordinate = errors.New("invalid coordinate")
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// DistanceMeters returns the great-circle distance between two points using the
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// haversine formula. It is accurate to roughly 0.5% which is far finer than the
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// radii this application compares against.
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func DistanceMeters(a, b domain.Coordinate) float64 {
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lat1 := degreesToRadians(a.Latitude)
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lat2 := degreesToRadians(b.Latitude)
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dLat := lat2 - lat1
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dLon := degreesToRadians(b.Longitude - a.Longitude)
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sinLat := math.Sin(dLat / 2)
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sinLon := math.Sin(dLon / 2)
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h := sinLat*sinLat + math.Cos(lat1)*math.Cos(lat2)*sinLon*sinLon
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return 2 * EarthRadiusMeters * math.Asin(math.Sqrt(math.Min(1, h)))
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}
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// WithinRadius reports whether two points are no further apart than radiusMeters.
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func WithinRadius(a, b domain.Coordinate, radiusMeters float64) bool {
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return DistanceMeters(a, b) <= radiusMeters
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}
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// BoundingBox is a coarse lat/lon window that fully contains a circle of the
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// given radius around a centre. It exists to let the store pre-filter rows in
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// SQL before the exact haversine test is applied.
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type BoundingBox struct {
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MinLat float64
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MaxLat float64
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MinLon float64
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MaxLon float64
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}
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// Around returns the bounding box of a circle of radiusMeters around center.
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func Around(center domain.Coordinate, radiusMeters float64) BoundingBox {
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// Latitude degrees are a constant distance apart, so the latitude delta is
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// a simple division. Longitude degrees shrink towards the poles, so the
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// delta is scaled by 1/cos(latitude) and clamped near the poles.
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latDelta := radiusMeters / EarthRadiusMeters * 180 / math.Pi
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cos := math.Cos(degreesToRadians(center.Latitude))
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var lonDelta float64
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if math.Abs(cos) > 1e-9 {
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lonDelta = latDelta / math.Abs(cos)
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} else {
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// At the poles every longitude is the same meridian; use everything.
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lonDelta = 180
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}
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// Clamp so the box stays within valid longitude range.
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if lonDelta > 180 {
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lonDelta = 180
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}
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return BoundingBox{
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MinLat: clamp(center.Latitude-latDelta, -90, 90),
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MaxLat: clamp(center.Latitude+latDelta, -90, 90),
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MinLon: clamp(center.Longitude-lonDelta, -180, 180),
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MaxLon: clamp(center.Longitude+lonDelta, -180, 180),
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}
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}
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// Contains reports whether the point falls inside the box.
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func (b BoundingBox) Contains(c domain.Coordinate) bool {
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return c.Latitude >= b.MinLat && c.Latitude <= b.MaxLat &&
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c.Longitude >= b.MinLon && c.Longitude <= b.MaxLon
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}
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// Round rounds a distance to the nearest metre so that JSON output and equality
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// assertions are stable.
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func Round(meters float64) float64 {
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return math.Round(meters)
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}
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func degreesToRadians(deg float64) float64 { return deg * math.Pi / 180 }
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func clamp(v, lo, hi float64) float64 {
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return math.Max(lo, math.Min(hi, v))
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}
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