Implements ST_LineInterpolatePoint

[radi2015]

Linear Referencing is so useful

import (
	"github.com/peterstace/simplefeatures/geom"
)

const DISTMIN = 0.000001

type Segment struct {
	Start, End geom.XY
}

func (s Segment) Length() float64 {
	return s.End.Sub(s.Start).Length()
}

func distValid(dist float64) {
	if dist < DISTMIN {
		panic("dist can not < 0")
	}
}
func (s Segment) Interpolate(dist float64) geom.XY {
	distValid(dist)
	length := s.Length()
	if dist < length {
		rate := dist / length
		return s.Start.Add(s.End.Sub(s.Start).Scale(rate))
	} else {
		return s.End
	}
}

func Interpolate(linestring geom.LineString, dist float64) geom.XY {
	distValid(dist)
	seq := linestring.Coordinates()
	for i := 0; i < seq.Length(); i++ {
		segment := Segment{Start: seq.GetXY(i), End: seq.GetXY(i + 1)}
		length := segment.Length()
		if dist <= length {
			return segment.Interpolate(dist)
		} else {
			dist -= length
		}
	}
	return geom.XY{X: -9999, Y: -9999}
}

[peterstace]

Great suggestion, there are a bunch of other related functions that would be useful too: https://postgis.net/docs/reference.html#Linear_Referencing

I'll consider implementing these as the next major feature.

[radi2015]

This is a high-quality spatial geometry analysis package I have some immature suggestions

1. LineString with []Point may be easier to understand
2. Coordinates are literally multiple points, and Coordinate may be more appropriate
3. line composed of two points is called segment in JTS(https://github.com/locationtech/jts/blob/master/modules/core/src/main/java/org/locationtech/jts/geom/LineSegment.java). export with Line is also very useful

[peterstace]

The reason that LineString isn't implemented as []Point is that Point may optionally be empty (i.e. WKT POINT EMPTY). However, the coordinates that make up a LineString cannot be empty.

That's a good suggestion for exposing a Line object. There is an internal object that is very similar, I'll take a look to see what the implications of exposing it might be.

[peterstace]

This was fixed. by #446 a while ago.