// Package trip manages trip-mode charging goals and session learning.
// A Goal records the user's intent ("car ready by HH:MM") and is persisted
// across restarts. Completed sessions are appended to a JSONL log so the
// charge rate estimate improves over time.
package trip

import (
	"bufio"
	"encoding/json"
	"math"
	"os"
	"path/filepath"
	"sync"
	"time"
)

const (
	sessionHistory         = 10             // sessions used for rate averaging
	minSessionsForLearning = 3              // below this: fall back to rated power
	chargingBuffer         = 15 * time.Minute
	minSessionKWh          = 0.1            // ignore sessions shorter than this
	minSessionDuration     = 5 * time.Minute
)

// Goal is a user-entered trip charging goal, persisted to disk.
type Goal struct {
	Wallbox           string    `json:"wallbox"`                       // "wallbox_a" or "wallbox_b"
	CarName           string    `json:"car_name"`                      // display label
	BatteryKWh        float64   `json:"battery_kwh"`
	CurrentSOC        float64   `json:"current_soc"`                   // % at time of entry
	Deadline          time.Time `json:"deadline"`                      // car must be ready by this time
	CreatedAt         time.Time `json:"created_at"`
	ChargingStartedAt time.Time `json:"charging_started_at,omitempty"` // set when trip mode activates the wallbox
}

// EnergyNeededKWh returns the energy required to charge from CurrentSOC to 100%.
func (g *Goal) EnergyNeededKWh() float64 {
	return (100.0 - g.CurrentSOC) / 100.0 * g.BatteryKWh
}

// ChargeDuration estimates the time required at the given charge rate.
func (g *Goal) ChargeDuration(rateKW float64) time.Duration {
	hours := g.EnergyNeededKWh() / rateKW
	return time.Duration(hours * float64(time.Hour))
}

// StartTime returns when charging must begin to meet the deadline (including buffer).
func (g *Goal) StartTime(rateKW float64) time.Time {
	return g.Deadline.Add(-(g.ChargeDuration(rateKW) + chargingBuffer))
}

// ShouldStartNow returns true if charging should begin immediately.
func (g *Goal) ShouldStartNow(now time.Time, rateKW float64) bool {
	return !now.Before(g.StartTime(rateKW))
}

// IsTight returns true if the start time has already passed (deadline at risk).
func (g *Goal) IsTight(now time.Time, rateKW float64) bool {
	return g.StartTime(rateKW).Before(now) && g.ChargingStartedAt.IsZero()
}

// Session records a completed charging session for rate learning.
type Session struct {
	Timestamp    time.Time `json:"ts"`
	Wallbox      string    `json:"wallbox"`
	DurationMin  float64   `json:"duration_min"`
	KWhDelivered float64   `json:"kwh_delivered"`
	AvgKW        float64   `json:"avg_kw"`
}

// Manager coordinates trip goals, session energy accumulation, and rate learning.
// All methods are safe for concurrent use (HTTP handler vs control loop).
type Manager struct {
	mu          sync.Mutex
	goal        *Goal
	goalFile    string
	sessionFile string

	// per-wallbox energy accumulation for the in-progress charging session
	accEnergy map[string]float64
	accStart  map[string]time.Time

	// previous active state per wallbox — used to detect active→inactive transitions
	prevActive map[string]bool
}

// New creates a Manager and loads any persisted goal from disk.
func New(goalFile, sessionFile string) *Manager {
	m := &Manager{
		goalFile:    goalFile,
		sessionFile: sessionFile,
		accEnergy:   make(map[string]float64),
		accStart:    make(map[string]time.Time),
		prevActive:  make(map[string]bool),
	}
	m.loadGoal()
	return m
}

func (m *Manager) loadGoal() {
	if m.goalFile == "" {
		return
	}
	data, err := os.ReadFile(m.goalFile)
	if err != nil {
		return // no file = no active goal
	}
	var g Goal
	if json.Unmarshal(data, &g) == nil {
		m.goal = &g
	}
}

// ActiveGoal returns a snapshot of the current goal, or nil if none is set.
func (m *Manager) ActiveGoal() *Goal {
	m.mu.Lock()
	defer m.mu.Unlock()
	if m.goal == nil {
		return nil
	}
	g := *m.goal
	return &g
}

// SetGoal persists and activates a new trip goal.
func (m *Manager) SetGoal(g Goal) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.goal = &g
	return m.saveGoalLocked()
}

// ClearGoal removes the active trip goal from memory and disk.
func (m *Manager) ClearGoal() error {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.goal = nil
	if m.goalFile == "" {
		return nil
	}
	err := os.Remove(m.goalFile)
	if os.IsNotExist(err) {
		return nil
	}
	return err
}

func (m *Manager) saveGoalLocked() error {
	if m.goalFile == "" {
		return nil
	}
	if err := os.MkdirAll(filepath.Dir(m.goalFile), 0755); err != nil {
		return err
	}
	data, err := json.MarshalIndent(m.goal, "", "  ")
	if err != nil {
		return err
	}
	return os.WriteFile(m.goalFile, data, 0644)
}

// MarkChargingStarted records when trip-mode charging began in the persisted goal.
func (m *Manager) MarkChargingStarted(now time.Time) error {
	m.mu.Lock()
	defer m.mu.Unlock()
	if m.goal == nil || !m.goal.ChargingStartedAt.IsZero() {
		return nil // already marked, or no goal
	}
	m.goal.ChargingStartedAt = now
	return m.saveGoalLocked()
}

// Tick must be called once per cycle. It:
//  1. Accumulates energy from active PM readings into the running session total.
//  2. Detects active→inactive wallbox transitions and finalises those sessions.
//
// Returns the list of wallboxes whose sessions just completed this cycle.
// The caller uses this to auto-clear a trip goal when its wallbox finishes.
func (m *Manager) Tick(
	activeStates map[string]bool,
	powerReadingsW map[string]float64,
	intervalMin float64,
) []string {
	completed := m.tickLocked(activeStates, powerReadingsW, intervalMin)
	for _, wb := range completed {
		m.finalizeSession(wb)
	}
	return completed
}

func (m *Manager) tickLocked(activeStates map[string]bool, powerW map[string]float64, intervalMin float64) []string {
	m.mu.Lock()
	defer m.mu.Unlock()

	for wb, active := range activeStates {
		if active {
			if _, ok := m.accStart[wb]; !ok {
				m.accStart[wb] = time.Now()
				m.accEnergy[wb] = 0
			}
			if pw := powerW[wb]; pw > 0 {
				m.accEnergy[wb] += pw / 1000.0 * (intervalMin / 60.0)
			}
		}
	}

	var completed []string
	for wb, active := range activeStates {
		if m.prevActive[wb] && !active {
			completed = append(completed, wb)
		}
		m.prevActive[wb] = active
	}
	return completed
}

// finalizeSession writes a completed session record to the log file.
// Called without m.mu held.
func (m *Manager) finalizeSession(wallbox string) {
	m.mu.Lock()
	start, ok := m.accStart[wallbox]
	kwh := m.accEnergy[wallbox]
	delete(m.accStart, wallbox)
	delete(m.accEnergy, wallbox)
	m.mu.Unlock()

	if !ok {
		return
	}
	duration := time.Since(start)
	if kwh < minSessionKWh || duration < minSessionDuration {
		return // too short / too little energy — ignore
	}

	s := Session{
		Timestamp:    time.Now(),
		Wallbox:      wallbox,
		DurationMin:  math.Round(duration.Minutes()*10) / 10,
		KWhDelivered: math.Round(kwh*100) / 100,
		AvgKW:        math.Round(kwh/duration.Hours()*100) / 100,
	}
	_ = m.appendSession(s)
}

func (m *Manager) appendSession(s Session) error {
	if m.sessionFile == "" {
		return nil
	}
	if err := os.MkdirAll(filepath.Dir(m.sessionFile), 0755); err != nil {
		return err
	}
	f, err := os.OpenFile(m.sessionFile, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0644)
	if err != nil {
		return err
	}
	defer f.Close()
	return json.NewEncoder(f).Encode(s)
}

// LearnedRateKW returns the average charging rate (kW) from recent sessions
// for the given wallbox. Falls back to fallbackKW if fewer than
// minSessionsForLearning exist.
func (m *Manager) LearnedRateKW(wallbox string, fallbackKW float64) float64 {
	sessions := m.recentSessions(wallbox, sessionHistory)
	if len(sessions) < minSessionsForLearning {
		return fallbackKW
	}
	var total float64
	for _, s := range sessions {
		total += s.AvgKW
	}
	return total / float64(len(sessions))
}

func (m *Manager) recentSessions(wallbox string, n int) []Session {
	if m.sessionFile == "" {
		return nil
	}
	f, err := os.Open(m.sessionFile)
	if err != nil {
		return nil
	}
	defer f.Close()

	var matched []Session
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		var s Session
		if json.Unmarshal(sc.Bytes(), &s) == nil && s.Wallbox == wallbox {
			matched = append(matched, s)
		}
	}
	if len(matched) <= n {
		return matched
	}
	return matched[len(matched)-n:]
}