workout-suggester/internal/routes/loops.go

package routes

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"math"
	"net/http"
	"net/url"
	"slices"
	"strconv"
	"strings"
	"sync"
	"time"
)

// BRouter's round trips come out about this many times longer than the
// roundTripDistance asked for (it's closer to a radius than a length).
const brouterStretch = 5.1

var brouterProfile = map[Kind]string{Foot: "hiking-mountain", Bike: "trekking"}

// Loops asks BRouter for round trips of about targetKm that start and end
// at p, heading out in three directions. The first heads out into the wind
// (windFrom, in degrees) so the way home is the easy part. The start point
// is rounded to ~100 m before it leaves the machine.
func (c *Client) Loops(ctx context.Context, kind Kind, p Point, targetKm float64, windFrom int) ([]Suggestion, error) {
	if c.BRouterURL == "" || c.BRouterURL == "-" || targetKm <= 0 {
		return nil, nil
	}
	p = Point{math.Round(p.Lat*1000) / 1000, math.Round(p.Lon*1000) / 1000}
	key := fmt.Sprintf("%s_%.3f_%.3f_%.1f_%d", kind, p.Lat, p.Lon, targetKm, windFrom/30)
	c.mu.Lock()
	c.initLocked()
	if hit, ok := c.loops[key]; ok && time.Since(hit.at) < cacheFor {
		c.mu.Unlock()
		return hit.s, nil
	}
	c.mu.Unlock()

	type result struct {
		r   *Route
		err error
	}
	results := make([]result, 3)
	var wg sync.WaitGroup
	for i := range results {
		wg.Go(func() {
			dir := (windFrom + i*120) % 360
			r, err := c.roundTrip(ctx, kind, p, targetKm, dir)
			results[i] = result{r, err}
		})
	}
	wg.Wait()

	var out []Suggestion
	var errs []error
	for _, res := range results {
		if res.err != nil {
			errs = append(errs, res.err)
			continue
		}
		out = append(out, Suggestion{Route: res.r, Plan: "loop", Laps: 1, TotalKm: res.r.LengthKm, JoinAt: p})
	}
	if len(out) == 0 {
		return nil, fmt.Errorf("brouter: %w", errors.Join(errs...))
	}
	// Closest to the target first; into-the-wind wins ties within 10%.
	slices.SortStableFunc(out, func(a, b Suggestion) int {
		da, db := math.Abs(a.TotalKm-targetKm), math.Abs(b.TotalKm-targetKm)
		if math.Abs(da-db) < 0.1*targetKm {
			return 0
		}
		if da < db {
			return -1
		}
		return 1
	})

	c.mu.Lock()
	c.loops[key] = cachedLoops{out, time.Now()}
	for _, s := range out {
		c.byID[s.Route.ID] = s.Route
	}
	c.mu.Unlock()
	return out, nil
}

type cachedLoops struct {
	s  []Suggestion
	at time.Time
}

func (c *Client) roundTrip(ctx context.Context, kind Kind, p Point, targetKm float64, dir int) (*Route, error) {
	q := url.Values{
		"lonlats":           {fmt.Sprintf("%.3f,%.3f", p.Lon, p.Lat)},
		"profile":           {brouterProfile[kind]},
		"alternativeidx":    {"0"},
		"format":            {"geojson"},
		"engineMode":        {"4"}, // round trip
		"roundTripDistance": {strconv.Itoa(int(targetKm * 1000 / brouterStretch))},
		"direction":         {strconv.Itoa(dir)},
	}
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, c.BRouterURL+"?"+q.Encode(), nil)
	if err != nil {
		return nil, err
	}
	req.Header.Set("User-Agent", userAgent)
	hc := c.HTTP
	if hc == nil {
		hc = http.DefaultClient
	}
	resp, err := hc.Do(req)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		msg, _ := io.ReadAll(io.LimitReader(resp.Body, 256))
		return nil, fmt.Errorf("%s: %s", resp.Status, strings.TrimSpace(string(msg)))
	}
	r, err := parseBRouter(resp.Body)
	if err != nil {
		return nil, err
	}
	r.Kind, r.Heading = kind, dir
	r.ID = c.nextLoopID()
	r.Name = fmt.Sprintf("%.1f km loop heading %s", r.LengthKm, compass(dir))
	return r, nil
}

// nextLoopID hands out negative ids so generated loops never collide with
// OpenStreetMap relation ids.
func (c *Client) nextLoopID() int64 {
	c.mu.Lock()
	defer c.mu.Unlock()
	c.loopSeq--
	return c.loopSeq
}

func parseBRouter(r io.Reader) (*Route, error) {
	var out struct {
		Features []struct {
			Properties struct {
				Length   string     `json:"track-length"`
				Ascent   string     `json:"filtered ascend"`
				Messages [][]string `json:"messages"`
			} `json:"properties"`
			Geometry struct {
				Coordinates [][]float64 `json:"coordinates"` // lon, lat, elevation
			} `json:"geometry"`
		} `json:"features"`
	}
	if err := json.NewDecoder(r).Decode(&out); err != nil {
		return nil, fmt.Errorf("decode brouter: %w", err)
	}
	if len(out.Features) == 0 || len(out.Features[0].Geometry.Coordinates) < 2 {
		return nil, errors.New("brouter: empty route")
	}
	f := out.Features[0]
	m, _ := strconv.Atoi(f.Properties.Length)
	ascent, _ := strconv.Atoi(f.Properties.Ascent)
	pts := make([]Point, len(f.Geometry.Coordinates))
	for i, c := range f.Geometry.Coordinates {
		pts[i] = Point{c[1], c[0]}
	}
	return &Route{
		LengthKm:  math.Round(float64(m)/100) / 10,
		Loop:      true,
		Generated: true,
		AscentM:   ascent,
		PavedPct:  pavedPct(f.Properties.Messages),
		Ways:      [][]Point{simplify(pts, 0.015)},
	}, nil
}

var (
	pavedSurfaces   = []string{"asphalt", "paved", "concrete", "concrete:plates", "concrete:lanes", "paving_stones", "sett", "metal", "wood"}
	unpavedHighways = []string{"track", "path", "bridleway"}
)

// pavedPct estimates how much of the route is paved from BRouter's per-
// segment way tags: the surface tag when there is one, else the road type.
func pavedPct(msgs [][]string) int {
	if len(msgs) < 2 {
		return -1
	}
	dist, tags := slices.Index(msgs[0], "Distance"), slices.Index(msgs[0], "WayTags")
	if dist < 0 || tags < 0 {
		return -1
	}
	var paved, total float64
	for _, row := range msgs[1:] {
		if len(row) <= max(dist, tags) {
			continue
		}
		d, _ := strconv.ParseFloat(row[dist], 64)
		kv := map[string]string{}
		for _, f := range strings.Fields(row[tags]) {
			k, v, _ := strings.Cut(f, "=")
			kv[k] = v
		}
		total += d
		if s, ok := kv["surface"]; ok {
			if slices.Contains(pavedSurfaces, s) {
				paved += d
			}
		} else if !slices.Contains(unpavedHighways, kv["highway"]) {
			paved += d
		}
	}
	if total == 0 {
		return -1
	}
	return int(math.Round(100 * paved / total))
}

func compass(deg int) string {
	dirs := []string{"north", "north-east", "east", "south-east", "south", "south-west", "west", "north-west"}
	return dirs[((deg%360+360)%360+22)/45%8]
}

// IntoWind reports whether a loop heads out roughly into the wind, so the
// way home has a tailwind.
func (r *Route) IntoWind(windFrom int) bool {
	if !r.Generated {
		return false
	}
	d := ((r.Heading-windFrom)%360 + 360) % 360
	return d <= 45 || d >= 315
}