carpool-service/geoutils/distance.go

// Package geoutils provides utility functions to handler geographic calculations such as distance between geometries.
package geoutils

import (
	"math"

	"github.com/paulmach/orb"
	"github.com/paulmach/orb/geo"
)

// DistanceFromLineString provides the shorteds distance between a point and a linestring.
// It returns the distance in meters, and the index of the closest linestring segment to the point
func DistanceFromLineString(point orb.Point, lineString orb.LineString) (distance float64, closestIndex int) {
	minDistance := math.Inf(1)
	closest := -1

	for i := 0; i < len(lineString)-1; i++ {
		a := lineString[i]
		b := lineString[i+1]
		pointOnSegment := projectToSegment(point, a, b)
		dist := geo.Distance(point, pointOnSegment)
		if dist < minDistance {
			minDistance = dist
			closest = i
		}
	}

	// log.Debug().
	// 	Float64("min distance", minDistance).
	// 	Int("index", closest).
	// 	Any("point", point).
	// 	Msg("minimum distance to polyline")

	return minDistance, closest
}

// projectToSegment projects the point to the segment. This function is used in DistanceFromLineString for point-to-segment distance calculation
func projectToSegment(point, a, b orb.Point) orb.Point {
	x := a[0]
	y := a[1]
	dx := b[0] - x
	dy := b[1] - y

	if dx != 0 || dy != 0 {
		t := ((point[0]-x)*dx + (point[1]-y)*dy) / (dx*dx + dy*dy)

		if t > 1 {
			x = b[0]
			y = b[1]
		} else if t > 0 {
			x += dx * t
			y += dy * t
		}
	}

	return orb.Point{x, y}
}