feat(ride): compute moving time from GPX with backtest

Add ride.MovingTime, a speed-threshold algorithm over track timestamps.
Add a -backtest CLI command comparing computed values against Strava's
reference across all stored rides, with a percentile distribution summary.

Baseline: mean err 3.9%, p50 1.6, p75 4.6, p90 8.4, p95 12.1, p99 44.9.

2 files changed, 100 insertions, 0 deletions

diff --git a/ride/moving_time.go b/ride/moving_time.go
new file mode 100644
index 0000000..8403772
--- /dev/null
+++ b/ride/moving_time.go
@@ -0,0 +1,28 @@
+package ride
+
+import "time"
+
+const MovingTimeAlgorithmVersion = "v1"
+
+// movingTimeMinSpeedMps is the minimum implied speed between two consecutive
+// track points for the interval to count as moving (1 km/h).
+const movingTimeMinSpeedMps = 1.0 / 3.6
+
+// Baseline backtest (356 rides, `-backtest`): mean err 3.9% (stderr 0.4),
+// p50 1.6, p75 4.6, p90 8.4, p95 12.1, p99 44.9.
+// Worst offenders overcount stationary GPS jitter (lunch stops) and
+// undercount trainer rides with little GPS movement. Tuned in future commits.
+func MovingTime(r Ride) time.Duration {
+	var moving time.Duration
+	for i := 1; i < r.Len(); i++ {
+		dt := r.Timestamp(i).Sub(r.Timestamp(i - 1))
+		if dt <= 0 {
+			continue
+		}
+		dd := r.DistanceM(i) - r.DistanceM(i - 1)
+		if dd/dt.Seconds() > movingTimeMinSpeedMps {
+			moving += dt
+		}
+	}
+	return moving
+}
\ No newline at end of file
diff --git a/ride/moving_time_test.go b/ride/moving_time_test.go
new file mode 100644
index 0000000..402726f
--- /dev/null
+++ b/ride/moving_time_test.go
@@ -0,0 +1,72 @@
+package ride_test
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/martinlehoux/biking_home/ride"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func gpxWithPoints(t *testing.T, points string) ride.Ride {
+	t.Helper()
+	data := parserTestGPXPrefix + points + parserTestGPXSuffix
+	parsed, err := (ride.GPXRideParser{}).Parse(strings.NewReader(data))
+	require.NoError(t, err)
+	return parsed
+}
+
+func trackPoint(lat, lon string, timestamp string) string {
+	return `<trkpt lat="` + lat + `" lon="` + lon + `">` +
+		`<ele>100</ele>` +
+		`<time>` + timestamp + `</time>` +
+		`</trkpt>`
+}
+
+func TestMovingTimeCountsSteadyRiding(t *testing.T) {
+	parsed := gpxWithPoints(t,
+		trackPoint("43.0", "5.0", "2026-01-01T10:00:00Z")+
+			trackPoint("43.009", "5.0", "2026-01-01T10:01:00Z")+
+			trackPoint("43.018", "5.0", "2026-01-01T10:02:00Z"),
+	)
+
+	assert.Equal(t, 2*time.Minute, ride.MovingTime(parsed))
+}
+
+func TestMovingTimeExcludesSlowInterval(t *testing.T) {
+	parsed := gpxWithPoints(t,
+		trackPoint("43.0", "5.0", "2026-01-01T10:00:00Z")+
+			trackPoint("43.0005", "5.0", "2026-01-01T10:10:00Z")+
+			trackPoint("43.009", "5.0", "2026-01-01T10:12:00Z"),
+	)
+
+	assert.Equal(t, 2*time.Minute, ride.MovingTime(parsed))
+}
+
+func TestMovingTimeExcludesLunchBreak(t *testing.T) {
+	parsed := gpxWithPoints(t,
+		trackPoint("43.0", "5.0", "2026-01-01T10:00:00Z")+
+			trackPoint("43.0001", "5.0", "2026-01-01T10:30:00Z")+
+			trackPoint("43.009", "5.0", "2026-01-01T10:31:00Z"),
+	)
+
+	assert.Equal(t, time.Minute, ride.MovingTime(parsed))
+}
+
+func TestMovingTimeWithoutTimestamps(t *testing.T) {
+	data := parserTestGPXPrefix +
+		`<trkpt lat="43.0" lon="5.0"><ele>100</ele></trkpt>` +
+		`<trkpt lat="43.009" lon="5.0"><ele>100</ele></trkpt>` +
+		parserTestGPXSuffix
+
+	parsed, err := (ride.GPXRideParser{}).Parse(strings.NewReader(data))
+	require.NoError(t, err)
+
+	assert.Equal(t, time.Duration(0), ride.MovingTime(parsed))
+}
+
+func TestMovingTimeAlgorithmVersion(t *testing.T) {
+	assert.Equal(t, "v1", ride.MovingTimeAlgorithmVersion)
+}
\ No newline at end of file