package activity

import (
	"bytes"
	"fmt"

	"github.com/muktihari/fit/decoder"
	"github.com/muktihari/fit/profile/basetype"
	"github.com/muktihari/fit/profile/mesgdef"
	"github.com/muktihari/fit/profile/typedef"
)

// ParseFIT decodes a FIT activity file and extracts session-level metrics.
func ParseFIT(data []byte) (*ParsedActivity, error) {
	dec := decoder.New(bytes.NewReader(data))

	if !dec.Next() {
		return nil, fmt.Errorf("empty or invalid FIT file")
	}

	fit, err := dec.Decode()
	if err != nil {
		return nil, fmt.Errorf("failed to decode FIT file: %w", err)
	}

	result := &ParsedActivity{}
	foundSession := false

	for i := range fit.Messages {
		if fit.Messages[i].Num != typedef.MesgNumSession {
			continue
		}

		foundSession = true
		session := mesgdef.NewSession(&fit.Messages[i])

		// total_elapsed_time: FIT stores as uint32 with scale 1000 (ms -> seconds)
		if session.TotalElapsedTime != basetype.Uint32Invalid {
			result.Duration = int(session.TotalElapsedTime / 1000)
		}

		// total_distance: FIT stores as uint32 with scale 100 (centimeters -> meters)
		// Convert to km for our model
		if session.TotalDistance != basetype.Uint32Invalid {
			result.Distance = float64(session.TotalDistance) / 100.0 / 1000.0
		}

		// Power (no scale)
		if session.AvgPower != basetype.Uint16Invalid {
			result.AvgPower = int(session.AvgPower)
		}
		if session.MaxPower != basetype.Uint16Invalid {
			result.MaxPower = int(session.MaxPower)
		}

		// Heart rate (no scale)
		if session.AvgHeartRate != basetype.Uint8Invalid {
			result.AvgHR = int(session.AvgHeartRate)
		}
		if session.MaxHeartRate != basetype.Uint8Invalid {
			result.MaxHR = int(session.MaxHeartRate)
		}

		// Calories (no scale)
		if session.TotalCalories != basetype.Uint16Invalid {
			result.CaloriesBurned = int(session.TotalCalories)
		}

		// Elevation gain (no scale, meters)
		if session.TotalAscent != basetype.Uint16Invalid {
			result.ElevGain = int(session.TotalAscent)
		}

		// Cadence (no scale)
		if session.AvgCadence != basetype.Uint8Invalid {
			result.AvgCadence = int(session.AvgCadence)
		}

		// Start time
		if session.StartTime.IsZero() {
			result.StartTime = session.Timestamp
		} else {
			result.StartTime = session.StartTime
		}

		break // use first session
	}

	if !foundSession {
		return nil, fmt.Errorf("no session data found in FIT file")
	}

	return result, nil
}