rideaware-api/internal/export/fit_encoder.go

package export

import (
	"bytes"
	"fmt"
	"time"

	"github.com/muktihari/fit/encoder"
	"github.com/muktihari/fit/profile/filedef"
	"github.com/muktihari/fit/profile/mesgdef"
	"github.com/muktihari/fit/profile/typedef"

	"rideaware/internal/workout"
)

// EncodeFITWorkout generates a FIT workout file from a Workout's structured segments.
// userFTP is the user's Functional Threshold Power in watts, used to convert
// %FTP power targets into absolute watt values for the device.
func EncodeFITWorkout(w *workout.Workout, userFTP int) ([]byte, error) {
	if len(w.WorkoutData.Segments) == 0 {
		return nil, fmt.Errorf("workout has no segments")
	}
	if userFTP <= 0 {
		return nil, fmt.Errorf("user FTP must be greater than 0")
	}

	wktFile := filedef.NewWorkout()

	wktFile.FileId.
		SetType(typedef.FileWorkout).
		SetManufacturer(typedef.ManufacturerDevelopment).
		SetProduct(0).
		SetTimeCreated(time.Now()).
		SetSerialNumber(uint32(time.Now().Unix()))

	name := w.WorkoutData.Name
	if name == "" {
		name = w.Title
	}
	// Truncate name if too long (FIT spec supports up to 50 chars)
	if len(name) > 50 {
		name = name[:50]
	}

	wktFile.Workout = mesgdef.NewWorkout(nil).
		SetWktName(name).
		SetSport(typedef.SportCycling).
		SetSubSport(typedef.SubSportGeneric).
		SetNumValidSteps(uint16(len(w.WorkoutData.Segments)))

	steps := make([]*mesgdef.WorkoutStep, 0, len(w.WorkoutData.Segments))
	for i, seg := range w.WorkoutData.Segments {
		step := segmentToFITStep(seg, uint16(i), userFTP)
		steps = append(steps, step)
	}
	wktFile.WorkoutSteps = steps

	fit := wktFile.ToFIT(nil)

	var buf bytes.Buffer
	enc := encoder.New(&buf)
	if err := enc.Encode(&fit); err != nil {
		return nil, fmt.Errorf("failed to encode FIT workout: %w", err)
	}

	return buf.Bytes(), nil
}

func segmentToFITStep(seg workout.WorkoutSegment, index uint16, userFTP int) *mesgdef.WorkoutStep {
	step := mesgdef.NewWorkoutStep(nil).
		SetMessageIndex(typedef.MessageIndex(index)).
		SetDurationType(typedef.WktStepDurationTime).
		SetDurationValue(uint32(seg.Duration) * 1000) // seconds to milliseconds

	switch seg.Type {
	case "warmup":
		step.SetIntensity(typedef.IntensityWarmup)
		setPowerTarget(step, seg, userFTP)
		step.SetWktStepName("Warm Up")

	case "steadystate":
		step.SetIntensity(typedef.IntensityActive)
		setPowerTargetSteady(step, seg, userFTP)
		step.SetWktStepName("Steady State")

	case "interval":
		step.SetIntensity(typedef.IntensityInterval)
		setPowerTarget(step, seg, userFTP)
		step.SetWktStepName("Interval")

	case "cooldown":
		step.SetIntensity(typedef.IntensityCooldown)
		setPowerTarget(step, seg, userFTP)
		step.SetWktStepName("Cool Down")

	case "ramp":
		step.SetIntensity(typedef.IntensityActive)
		setPowerTarget(step, seg, userFTP)
		step.SetWktStepName("Ramp")

	case "freeride":
		step.SetIntensity(typedef.IntensityActive)
		step.SetTargetType(typedef.WktStepTargetOpen)
		step.SetTargetValue(0)
		step.SetWktStepName("Free Ride")

	case "rest":
		step.SetIntensity(typedef.IntensityRest)
		step.SetTargetType(typedef.WktStepTargetOpen)
		step.SetTargetValue(0)
		step.SetWktStepName("Rest")

	default:
		step.SetIntensity(typedef.IntensityActive)
		step.SetTargetType(typedef.WktStepTargetOpen)
		step.SetTargetValue(0)
		step.SetWktStepName("Active")
	}

	return step
}

// setPowerTarget sets a power range target using PowerLow/PowerHigh.
// Values are converted from %FTP fractions to absolute watts with +1000 offset.
func setPowerTarget(step *mesgdef.WorkoutStep, seg workout.WorkoutSegment, userFTP int) {
	if seg.PowerLow == 0 && seg.PowerHigh == 0 && seg.Power == 0 {
		step.SetTargetType(typedef.WktStepTargetOpen)
		step.SetTargetValue(0)
		return
	}

	step.SetTargetType(typedef.WktStepTargetPower)
	step.SetTargetValue(0) // 0 = custom range

	if seg.PowerLow != 0 || seg.PowerHigh != 0 {
		lowWatts := uint32(float64(userFTP) * seg.PowerLow)
		highWatts := uint32(float64(userFTP) * seg.PowerHigh)

		// Ensure we have a valid range (low < high)
		if lowWatts == highWatts {
			// Add a small tolerance range (±5W) around the target
			if lowWatts < 5 {
				lowWatts = 1
				highWatts = lowWatts + 5
			} else {
				lowWatts = lowWatts - 5
				highWatts = highWatts + 5
			}
		}

		step.SetCustomTargetValueLow(lowWatts + 1000)
		step.SetCustomTargetValueHigh(highWatts + 1000)
	} else if seg.Power != 0 {
		watts := uint32(float64(userFTP) * seg.Power)
		// Ensure low value doesn't go below 1000 (minimum is 1 watt)
		lowOffset := uint32(10)
		if watts < 10 {
			lowOffset = watts - 1
			if lowOffset == 0 {
				lowOffset = 0
			}
		}
		step.SetCustomTargetValueLow(watts - lowOffset + 1000)
		step.SetCustomTargetValueHigh(watts + 10 + 1000)
	}
}

// setPowerTargetSteady sets a power target for steady state segments using the Power field.
func setPowerTargetSteady(step *mesgdef.WorkoutStep, seg workout.WorkoutSegment, userFTP int) {
	if seg.Power == 0 {
		step.SetTargetType(typedef.WktStepTargetOpen)
		step.SetTargetValue(0)
		return
	}

	step.SetTargetType(typedef.WktStepTargetPower)
	step.SetTargetValue(0)

	watts := uint32(float64(userFTP) * seg.Power)
	// Ensure low value doesn't go below 1000 (minimum is 1 watt)
	lowOffset := uint32(10)
	if watts < 10 {
		lowOffset = watts - 1
		if lowOffset == 0 {
			lowOffset = 0
		}
	}
	step.SetCustomTargetValueLow(watts - lowOffset + 1000)
	step.SetCustomTargetValueHigh(watts + 10 + 1000)
}