workout-suggester/internal/routes/suggest.go

package routes

import (
	"encoding/xml"
	"fmt"
	"io"
	"math"
	"slices"
)

// Suggestion is a way to use a route for a workout of a given distance.
type Suggestion struct {
	Route     *Route
	Plan      string  // "loop", "laps", "out-and-back" or "section"
	Laps      int     // for "laps"
	TotalKm   float64 // what the user will actually cover on the route
	FromYouKm float64 // to the nearest point of the route
	JoinAt    Point
}

// Describe says in a few words how to use the route.
func (s Suggestion) Describe() string {
	switch s.Plan {
	case "laps":
		return fmt.Sprintf("%d laps of the %.1f km loop", s.Laps, s.Route.LengthKm)
	case "out-and-back":
		return fmt.Sprintf("out and back, %.1f km each way", s.Route.LengthKm)
	case "section":
		return fmt.Sprintf("a %.0f km stretch (out and back) of this %.0f km route", s.TotalKm, s.Route.LengthKm)
	}
	if s.Route.Generated {
		return "from your door and back"
	}
	if s.Route.Loop {
		return fmt.Sprintf("the full %.1f km loop", s.Route.LengthKm)
	}
	return fmt.Sprintf("the full %.1f km route", s.Route.LengthKm)
}

// maxApproachKm is how far we'd send someone to reach a route's start.
var maxApproachKm = map[Kind]float64{Bike: 6, Foot: 3}

// Suggest picks up to n routes of a kind that fit targetKm, starting from p.
func Suggest(a *Area, kind Kind, targetKm float64, p Point, n int) []Suggestion {
	type scored struct {
		Suggestion
		cost float64
	}
	var all []scored
	for i := range a.Routes {
		r := &a.Routes[i]
		if r.Kind != kind {
			continue
		}
		if r.LengthKm > 3*targetKm {
			continue // long-distance trails (Streek-GR & co) aren't a workout route
		}
		join, from := r.Closest(p)
		if from > maxApproachKm[kind] {
			continue
		}
		s := Suggestion{Route: r, Plan: "loop", Laps: 1, TotalKm: r.LengthKm, FromYouKm: from, JoinAt: join}
		switch {
		case r.Loop && r.LengthKm < 0.75*targetKm:
			s.Laps = min(4, max(1, int(math.Round(targetKm/r.LengthKm))))
			if s.Laps > 1 {
				s.Plan = "laps"
			}
			s.TotalKm = r.LengthKm * float64(s.Laps)
		case !r.Loop && 2*r.LengthKm <= 1.3*targetKm:
			s.Plan, s.TotalKm = "out-and-back", 2*r.LengthKm
		case r.LengthKm > 1.3*targetKm:
			s.Plan, s.TotalKm = "section", max(targetKm/2, targetKm-2*from)
		}
		// Getting to the start and back is part of the workout.
		cost := math.Abs(s.TotalKm+2*from-targetKm)/targetKm + 0.03*from
		if !r.Loop {
			cost += 0.3 // loops bring you home
		}
		if s.Plan == "section" {
			cost += 0.15
		}
		if s.Laps > 2 {
			cost += 0.1 // laps get dull
		}
		all = append(all, scored{s, cost})
	}
	slices.SortFunc(all, func(x, y scored) int {
		switch {
		case x.cost < y.cost:
			return -1
		case x.cost > y.cost:
			return 1
		}
		return 0
	})
	var out []Suggestion
	for _, s := range all[:min(n, len(all))] {
		out = append(out, s.Suggestion)
	}
	return out
}

// WriteGPX writes a route as a GPX track, one segment per OSM way, ready for
// a watch, bike computer or OsmAnd.
func WriteGPX(w io.Writer, r *Route) error {
	type trkpt struct {
		Lat float64 `xml:"lat,attr"`
		Lon float64 `xml:"lon,attr"`
	}
	type trkseg struct {
		Points []trkpt `xml:"trkpt"`
	}
	type gpx struct {
		XMLName xml.Name `xml:"gpx"`
		Version string   `xml:"version,attr"`
		Creator string   `xml:"creator,attr"`
		Xmlns   string   `xml:"xmlns,attr"`
		Name    string   `xml:"trk>name"`
		Segs    []trkseg `xml:"trk>trkseg"`
	}
	g := gpx{Version: "1.1", Creator: "workout-suggester (data © OpenStreetMap contributors, ODbL)",
		Xmlns: "http://www.topografix.com/GPX/1/1", Name: r.Name}
	for _, way := range r.Ways {
		var seg trkseg
		for _, p := range way {
			seg.Points = append(seg.Points, trkpt{p.Lat, p.Lon})
		}
		g.Segs = append(g.Segs, seg)
	}
	io.WriteString(w, xml.Header)
	enc := xml.NewEncoder(w)
	enc.Indent("", " ")
	return enc.Encode(g)
}